Types of edible oils

Types of edible oils - sources - oilseeds

The main oilseed crops grown in India are groundnuts, rapeseed, mustard seed, sesame, sunflower seed, safflower seed, nigerseed, cottonseed and soybeans. Since the late 1980s, India has focussed its efforts on increasing oilseed production to meet its growing demand for vegoils. Increased production has been accomplished by expanding seeded area, increasing irrigation, improving crop production techniques, and developing higher yielding varieties of oilseeds.

The share of oilseed crops is expected to increase as India moves toward the commercialization of its agricultural sector. This includes the removal of price supports that have long favoured the production of sugarcane, paddy rice and wheat. The expectation is that if pricing of cereal crops is left to market forces, there will be a shift out of rice and wheat production into non-cereal crops such as oilseeds. The expected shift will help accommodate the growing demand for healthful products such as soyoil and canola/rapeseed oil, rather than the less expensive palm oil, as living standards in India continue to improve.

For 2004-2005, total oilseed production is forecast at 26.1 Mt, down from the record 28.6 Mt in 2003-2004. The decrease is due largely to lower rapeseed and soybean production in response to record-high imports of palm oil from Malaysia and Indonesia and soyoil from South America.

India, as the world's second largest consumer of vegoils, has seen per capita consumption increase from about 4 kilogram (kg) per annum to 10 kg per annum in the past four decades. The increase in vegoil consumption is due partially to a reduction in import barriers which helped to lower domestic prices and improve the availability of vegoils. India's vegoil consumption for 2004-2005 is forecast at a record 11.8 Mt, up from the previous record of 11.5 Mt in 2003-2004.

India is the world's second largest producer of vegoils, but that production only meets about half of its annual requirements. The shortfall is made up primarily with imports of palm oil from Malaysia and Indonesia, which have averaged 3.7 Mt annually during the past five years. The remainder is made up with imports of soyoil, sunflower seed oil, and canola/rapeseed oil.

Individually, the volumes of the soyoil, sunflower seed oil and canola/rapeseed oil fluctuate considerably from year to year, depending on availability and prices, but the total volume of the three oils imported averages 1.5 Mt annually. Currently, canola/rapeseed is the smallest component of India's vegoil imports, but that is expected to increase with increased urbanization, higher disposable incomes, and a heightened awareness of the health benefits of canola/rapeseed oil.

Linseed is an important oilseed and fibre crop grown both for its seed as well as fibre which is used for the manufacture of linen. The seed contains a good percentage of oil varying from 33 to 47 per cent in different varieties. The oil is edible and also due to its quick drying property is used for the preparation of paints, varnishes, printing ink, oilcloth, soap, patent leather, and waterproof fabrics. The oil cake left after the oil is pressed out is a most valuable feeding cake, perhaps the most favourite cattle feed. It is good in taste and contains 36 per cent protein, 85 per cent of, which is digestible. It is fed to both milch and fattening animals. It is also used as organic manure. It contains about 5 per cent nitrogen, 1.4 per cent phosphorus and 1.8 per cent potash. Straw from seed varieties are used in the manufacturer of upholstery two, insulating material, rugs, twine, and paper.

Table Oilseeds and products: Global supplies, trade and utilization (million tonnes)

1 Includes oils and fats of vegetable and animal origin.

2 Production plus opening stocks.

3 Residual of the balance.

4 Trade data refer to exports based on a common October/September marketing season.

5 All meal figures are expressed in protein equivalent. Meals include all meals and cakes derived from oilcrops as well as fish meal.

Note: Refer to footnote 1 in the text for further explanations regarding definitions and coverage.

Table World production of oilseeds (million tonnes)

Note: The split years bring together northern hemisphere annual crops harvested in the latter part of the first year shown, with southern hemisphere annual crops harvested in the early part of the second year shown. For tree crop, which are produced throughout the year, calendar year production for the second year shown is used.

Lecture 2

Oil content - coconut oil, palm oil, peanut oil, rice bran oil and

sunflower oil.

Lecture 3

Physical properties of fats and oils - colour, odour, consistency.

The following are some of the important physical properties of fats and oils

a) Relative Density (g/cm3; 20C/water at 20C)

b) Refractive Index (nD 40C)

c) Crismer Value 67 - 70

d) Viscosity (Kinematic at 20C, mm2/sec)

e) Cold Test (15 Hrs at 4C) Passed

f) Smoke Point (C)

g) Flash Point, Open cup (C)

h) Specific Heat (J/g at 20C)

i) Thermal Conductivity (W/mK)

Viscosity values estimate an oil's relative thickness or resistance to flow. Lang et al. (1992) and Noureddini et al. (1992a) found that the viscosity of canola and other vegetable oils, like other liquids, was affected by temperature and proposed an equation to calculate viscosity in the temperature range from 4 to 100C. Figure 2 shows the relation between temperature and viscosity for canola and selected vegetable oils.

Rapeseed oil exhibited a higher viscosity than canola, corn and soybean oils. This can be directly related to the contribution of saturated fatty acids (Noureddini et al., 1992a).

Figure 1. Effect of Temperature on Viscosity of Canola and Selected Oils.

As for other liquids, the density of vegetable oils is temperature dependent and decreases in value when temperature increases (Figure 1). The relative density of canola oil was first reported by Ackman and Eaton in 1977 and later confirmed by Vadke et al. (1988) and Lang et al. (1992). Noureddini et al. (1992) reported a density for high erucic acid rapeseed oil of 0.9073 g/cm3 while Appelqvist & Ohlson (1972) reported a range from 0.906 g/cm3 to 0.914 g/cm3. Ackman and Eaton (1977) indicated that a different proportion of eicosenoic (C20:1) and C18 polyunsaturated acids could be a major factor for the increase in relative density of canola oil. The higher specific gravity of 0.9193 g/cm3 observed for soybean oil can be attributed to the higher content of linoleic acid (Ackman and Eaton, 1977) than soybean oil (Figure 2).

Figure 2 . Effect of Temperature on Density of Selected Oils.

Smoke Point

Smoke point is the temperature at which a fat or oil produces a continuous wisp of smoke when heated. This provides a useful characterization of its suitability for frying. The Canadian Government specifications define that frying oil should have a smoke point above 200C. Table 1 indicates that canola oil fulfills this requirement. A similar smoke point was observed for rapeseed oil (Appelqvist & Ohlson, 1972). The heating technique used in the standard method for smoke point determination is well-defined (AOCS Method Cc 9a-48). Arens et al. (1977) reported that, when measured by different laboratories, the smoke point for the same oil can differ by 10%, causing 20C deviation. Therefore, caution should be exercised when comparing smoke points reported from different laboratories. These variations are related to the subjective determination by an observer as to when "a continuous stream of smoke" occurs, and is not matched with a reference.

Flash Point

Flash point defines the temperature at which the decomposition products formed from frying oils can be ignited (AOCS Method Cc 9b-55). This temperature ranges from 275C to 330C for different oils and fats. Canola oil falls within this range (Table 1).

The solid fat index (SFI) and dilatation curve for hydrogenated fat describe the amount of solid fat remaining at defined temperatures. Individual triglycerides differ in physical properties according to their fatty acid composition. Thus, when a fat is kept at a particular temperature those triglycerides containing unsaturated fatty acids melt first,

while those containing the more saturated and trans isomers of fatty acids melt last. An expansion of the solid fat component occurs as temperature increases, reaching a maximum when it melts completely. The expansion of the fat or dilatation can be monitored by measuring the increase in specific volume with temperature and establishing a dilatometric curve.

Cold Test

The cold test measures the resistance of an oil to formation of a sediment at 0C or 4C (AOCS Method Cc 6-25), and is generally used to measure the effectiveness of the winterization process. Compounds with high melting temperatures, mainly waxes and triglycerides with saturated fatty acids, usually cause sediment formation (Przybylski et al., 1993). The cold test reveals whether oil remains free of clouding when held at

4C or 0C for 15 hours. It has been observed that oil produced from seeds grown in dry conditions will develop sediment more quickly. This may be related to the higher content of saturated fatty acids formed as a response to dry stress conditions.

Estimation of Physical properties of oils.

Colour of the Oil

Pure glycerides are colour less except those derived from very short chain acids and they posses neither colour nor odour. The colour of the fat / oil is due to the presence of small percentages of fat soluble pigments such as carotinoids, xanthophylls, chlorophylls and some times due to oxidation and polymerization of fat and fatty acids.

Colour of the given oil is determined by using Lovibond Tintometer.

In this method the colour of the given oil is measured by taking the oil in a tube of known diameter and depth and compared with several glass standards. The glasses are graduated in three series namely, Yellow, red and blue. The analyst select the closest red glass which is easy to match and select the yellow colour (one among the ten divisions) and if it is required, select the blue too and makes colour match and express the colour as 2R3Y5B.

Fat Analysist Committee (FAC) of the American Oil Chemists Society (AOCS) uses a series of liquid colour standards sealed in glass tubes to be compared with the oil sample in a tube of same size. Solid glass standards are also available. This is normally used for darker colour oils.

Whiteness of shortenings and colour of peanut butter, margarine etc, can be measured by reflectance. Meters are available, which shine a beam of light on a smoothened surface of the sample and measure the quantity of light reflected back into a photosensitive tube. Peanut butter and margarine colours can be graded visually by comparision with colour standards consist of plastic or enamel sticks arranged in graduation from light to dark.

Viscosity Measurement

Cambridge Viscosity's patented technology sets us apart from other viscometer manufacturers. It uses only one moving part, a piston, driven electromagnetically through fluid in a small measurement chamber. A deflector, positioned over the piston, moves fluid into the measurement chamber.

Two coils move the piston back and forth at a constant force. Proprietary circuitry analyzes its two-way travel time to measure absolute viscosity. A built-in temperature detector senses actual temperature in the measurement chamber. Because all wetted parts are stainless steel and the piston is in constant motion, the sampling area is continually scrubbed and samples are kept fresh. There is no need for frequent calibration very little maintenance is required. Cambridge viscometers are rugged, highly accurate, and extremely reliable. The unique self-cleaning technology is employed in families of Cambridge Viscosity sensors, processors, and enclosures with measurement capabilities ranging from 0.2 to 20, 000 cp (centipoise).

Solid Fat Index (SFI)

Also correlates with consistency since the solid fat content of the shortenings is the main contributing factor to its solidity. SFI is an empherical method and deviates from true per cent solid fat content depending partly on the crystalline structure of the fat concerned. A beta prime fat creates a stiffer texture for a given weight of fat solids than does a beta type fat.

Chemical properties of fats and oils - iodine value, saponification value, melting

point, free fatty acids.

Numerous methods have been described for the quantitative measurement of fatty acids in biological materials. They can be classified into four categories:

Chemical titration methods

Thermometric titration method

Measurement of metal-fatty acid complexes

Enzymatic methods

Method using a fatty acid binding protein

Spectroscopic methods

Chemical titration methods

Fatty acid concentrations equal or higher than 1 mM may be easily determined by titrimetry even in the presence of other lipids. Titrimetry was classically used to determine the acid value (free fatty content) of vegetable oils and fats. This acid value is defined as the number of mg of KOH required to neutralize the fatty acids contained in 1 g of the fat. It is very easy to express the results in other units as mg fatty acids per g of sample or mmoles per kg

Macro-method
Reagents:
Solvent mixture (95% ethanol/diethyl ether, 1/1, v/v). 0.1 M KOH in ethanol accurately standardized with 0.1 M HCl (pure ethanol may be also used if aqueous samples are analyzed), 1 % phenolphthalein in 95% ethanol.

Procedure:

Weigh 0.1 to 10 g of oil or fat (according to the expected acid value) in glass vial and dissolve in at least 50 ml of the solvent mixture (if necessary by gentle heating).
Titrate, with shaking, with the KOH solution (in a 25 ml burette graduated in 0.1 ml) to the end point of the indicator (5 drops of indicator), the pink color persisting for at least 10s.The acid value is calculated by the formula: 56.1 x N x V / M where V is the number of ml of KOH solution used and N his exact normality, M is the mass in g of the sample. Other expressions can be easily calculated (concentration of fatty acids or their weight, considering an average molecular weight of 282).

Micro-method
When small amounts of fatty acids (less than 1 mmole per sample) must be determined it is convenient to use a titrimetric micro-method. Among the first reported method the most accurate and rapid is that of Dole where details on the extraction of non-esterified fatty acids from plasma are also given (Dole VP, J Clin Invest 1956, 35, 150).
We give below the part of the Dole's paper devoted to the accurate extraction and titration of fatty acids (down to about 20 nmoles per sample). When lipids are extracted into a heptane phase, the procedure can be considered specific for fatty acids, at least with respect to phospholipids or acidic sulfolipids.

2. Thermometric titration method (non-aqueous media)

This method permits rapid, accurate, automated analysis of the free fatty acid content of oils or fats with unprecedented precision and accuracy. The method uses a technique which may be referred to as Catalyzed Endpoint Thermometric Titrimetry or CETT (Smith TK, J Am Oil Chem Soc 2003, 80, 21-24). The basic titration resembles the current standard method where free fatty acids are titrated with a standard solution of KOH in iso-propanol. The first excess of hydroxyl ion after neutralization of fatty acids in a measured sample of lipids catalyzes a strong exothermic reaction between components of the solvent mixture (acetone/chloroform, 25/2, v/v) dissolving the fat or oil.
A simple thermistor is used to sense the change in solution temperature which signals the sharp and unequivocal titration endpoint. The modern thermometric titration system (multitrator) employs powerful algorithms to optimally condition the temperature signal and permit the computation of derivatives to accurately locate endpoints.

The time taken for analysis of free fatty acids by this method is typically 1-3 minutes and the Multitrator titration system allows for a full automation. Typical analytical precisions obtained have been 0.001 % fatty acids (as oleic acid) for vegetable oils. The same method may also see application for the determination of Total Acid Number (TAN) in lubricating oils. The thermometric probe requires no maintenance or calibration, and titrations are carried out under normal laboratory conditions in polypropylene beakers.

Acid Value (AV):

Measures the degree to which the triglycerides in the oil have broken down to release free fatty acids, by measuring the amount of alkali needed to neutralize a sample of oil.

Peroxide Value (PV):

PV is measured by titration with a solution of potassium iodide.

Anisidine Value (AnV):

AnV is measured by reacting the oil with a solution of p-anisidine in acetic acid, and measuring the increase in absorbance of light at a specific wavelength (350 nm).

Iodine Value: A measure of the degree of unsaturation of the oil. This measurement has been largely superseded by the fatty acid profile (see under) which gives a more accurate indication of oil quality, but it is still used in some places.

Saponification Value:

Measures the number of milligrams of potassium hydroxide needed to neutralize all the fatty acid molecules (both free and in triglycerides) in 1g of oil. It is an indication of the mean molecular weight of the fatty acids in the oil.

6. Estimation of chemical - property of oils.

Fatty Acids

Fatty acids are composed of a carboxyl group and a hydrocarbon chain. Individual fatty acids are distinguished from one another by the nature of the hydrocarbon chain This chain can vary in length from 4 to 24 carbon atoms and can be saturated, monounsaturated (one double bond, MUFA) or polyunsaturated (two or more double bonds, PUFA). The most common fatty acids in edible oils and fats are those containing 18 carbons. These include: stearic acid (a saturated fatty acid), oleic acid (a monounsaturated fatty acid), and linoleic and linolenic acids (polyunsaturated fatty acids containing two and three double bonds, respectively).

Fatty acid abbreviations are made according to the number of carbon atoms in the molecule and the number of cis ethylenic double bonds. The general assumption is that all multiple double bonds are methyleneinterrupted. The chemical nomenclature requires that carbon atoms be counted from the carboxyl end of the fatty acid. However, for biological activity carbon atoms are numbered from the terminal methyl group to the first carbon of the ethylenic bond. Such a classification is designated by the symbol .-x, .x, or n-x, nx, where x denotes the position of the double bond closest to the terminal methyl group. For example, linoleic acid with two double bonds, where one is located on the sixth carbon atom counted from the methyl group, will be abbreviated as C18:2n-6.

Geometric Isomerism

In the case of unsaturated fatty acids, the carbon chain is bent into a fixed position at the double bond, resulting in several possible geometric isomers. When the portions of the chain are bent towards each other they are called cis; and when bent away from each other, trans The natural configuration of fatty acids is cis, as shown for oleic acid. The corresponding trans configuration, elaidic acid, results in a straight chain. From a nutritional point of view the cis isomer is more desirable. However, fatty acids with trans configuration affect the texture and melting properties of fat or oil. Isomerization from cis to trans occurs mainly during the hydrogenation of an oil. Formation of trans isomers of linolenic and linoleic acids may also occur when harsh conditions are applied during refining. During processing of canola oil formation of trans isomers of linolenic and linoleic acids are observed. Oleic acid is less prone to isomerization, trans isomers were detected only when extreme parameters were applied. (Ferrari, 1996). Due to elevated temperatures, deodorization is the stage of processing where isomerization predominantly occurs. The effect of time and temperature on isomerization of linoleic and linolenic acids is presented in Figure 6 (Wolff, 1993). After heating for two hours at 260C about 22% of the linolenic acid was transformed into trans isomers. Measurement of the amount of isomers can be used as an assessment of the deodorization process, where a lack of vacuum is often "replaced" by an increase in temperature to obtain odourless oil. Properly optimized deodorization will produce oil that contains zero or very low amount of trans isomers of linolenic acid.

Tocopherols

The main nonsaponifiable components in vegetable oils are tocopherols and sterols, which are present in varying amounts depending on the oil. Tocopherols are natural antioxidants and their amount in the plant is probably governed by the content of unsaturated fatty acids. Tocopherols are present in different isomeric forms.

Iodine value

Iodine value is one of the important differentiating properties, and this is easily modeled from differentiating properties, and this is easily modeled from infrared measurements [1, 2]. Saponification value [2], free fatty acids [3], trans-unsaturation [4] and peroxide value [5] that are measurable from IR spectra are important parameters for the food industry. Internal Reflection is an excellent method, because it provides an ATR spectrum with ideal intensities for both qualitative and quantitative measurements, and the composite diamond IRE of the DuraSamplIR? system ensures that analyses can be obtained from essentially any sample matrix.

The general structure of a vegetable oil ester component is based on a glyceryl ester (glycerol backbone), with three associated carboxylic acid fragments. The properties and

characteristics of the oils are based on the individual carboxylic acids and their distribution. Examples of carboxylic acid composition of common vegetable oils are provided in Table . For many of the common oils, the variations in composition are

not large, and on first examination of the infrared spectra they appear to be similar, as indicated by the spectra overlayed in Figure 1. Upon close examination, differences can be determined, and can be correlated to the material composition, as seen in Figures 2A, 2B and 2C.

Table : Major constituent carboxylic acids of typical oils

Palmitic (Palm.) : CH3-(CH2)14-CO2H

Stearic (Stear.) : CH3-(CH2)16-CO2H

Oleic : CH3-(CH2)7-CH=CH-(CH2)7-CO2H

Linoleic (Diene) : CH3-(CH2)3-(CH2-CH=CH)2-(CH2)7-CO2H

Linolenic (Triene) : CH3- (CH2-CH=CH)3-(CH2)7-CO2H

1Ricinoleic : CH3-(CH2)5-CHOH-CH2-CH=CH-(CH2)7-CO2H

2Eleosteric : CH3- (CH2)3-(CH=CH)3-(CH2)7-CO2H

These indicate the presence of freefatty acids (A) and demonstrate variations in unsaturation (2B/C), in particular cisunsaturation (2B). As noted earlier, these differences may be quantified from the infrared spectra [1-5].

Some oils are unique, and are well differentiated by their IR spectra. Two examples are provided in Figures 3A and 3A, for castor oil (3A) and tung oil (3B). The major component in castor oil is ricinoleic acid, which contains a hydroxy component (bands

at 3400 cm-1, around 850 cm-1 and between 1100 and 1000 cm- 1), and tung oil, which contains a highly unsaturated carboxylic acid fragment, a conjugated triene (eleostearic acid), providing characteristic absorptions at 990 cm-1 and 963 cm-1.

Vegetable oils are well characterized by infrared analysis. The DuraSamplIR is an excellent tool for the standardization of measurements, and for handling all forms of materials and sample types containing these oils.

Important chemical reactions of oil - hydrolysis - hydrogenation - oxidation - polymerization.

Hydrolysis

Fat or oil is hydrolyzed with water and lipase effectively by supplying the fat or oil and water continuously each at a constant rate or semi continuously in portions and simultaneously withdrawing a solution containing fatty acid(s) and an aqueous solution containing glycerol formed by the enzymatic reaction from the reaction system continuously at the same rates as those of the supplied fat or oil and water respectively or semicontinuously in portions to thereby maintain the glycerol concentration in the aqueous phase of the reaction system constant within a range of 10 to 40% by weight.

A. Hydrolysis of Fats

Like other esters, glycerides can be hydrolyzed readily. Partial hydrolysis of triglycerides will yield mono- and diglycerides and free fatty acids. When hydrolysis is carried to completion with water in the presence of an acid catalyst, the mono-, di-, and triglycerides will hydrolyze to yield glycerol and free fatty acids. With aqueous sodium hydroxide, glycerol and the sodium salts of the component fatty acids (soaps) are obtained. In the digestive tracts of humans and animals and in bacteria, fats are hydrolyzed by enzymes (lipases). Lipolytic enzymes are present in some edible oil sources (i.e., palm fruit, coconut). Any residues of these lipolytic enzymes (present in some crude fats and oils) are deactivated by the elevated temperatures normally used in oil processing can complicate nutritional and biochemical studies because they can affect food consumption under ad libitum feeding conditions and also reduce the vitamin content of the food. If the diet has become unpalatable due to excessive oxidation of the fat component and is not accepted by the animal, a lack of growth by the animal could be due to its unwillingness to consume the diet. Thus, the experimental results might be attributed unwittingly to the type of fat or other nutrient being studied rather than to the condition of the ration. Knowing the oxidative condition of unsaturated fats is extremely important in biochemical and nutritional studies with animals.

B. Oxidation of Fats

1. Autoxidation. Of particular interest in the food arena is the process of oxidation induced by air at room temperature referred to as "autoxidation". Ordinarily, this is a slow process which occurs only to a limited degree. In autoxidation, oxygen reacts with unsaturated fatty acids. Initially, peroxides are formed which may break down into secondary oxidation products (hydrocarbons, ketones, aldehydes, and smaller amounts of epoxides and alcohols). Metals, such as copper or iron, present at low levels in fats and oils can also promote autoxidation. Fats and oils are normally treated with chelating agents such as citric acid to complex these trace metals (thus inactivating their prooxidant effect). The result of the autoxidation of fats and oils is the development of objectionable flavors and odors characteristic of the condition known as oxidative rancidity". Some fats resist this change to a remarkable extent while others are more susceptible depending on the degree of unsaturation, the presence of antioxidants, and other factors. The presence of light, for example, increases the rate of oxidation. It is common practice in the industry to protect fats and oils from oxidation to preserve their acceptable flavor and to maximize shelf life. When rancidity has progressed significantly, it becomes readily apparent from the flavor and odor of the oil. Expert tasters are able to detect the development of rancidity in its early stages. The peroxide value determination, if used judiciously, is oftentimes helpful in measuring the degree to which oxidative rancidity in the fat has progressed. It has been found that oxidatively abused fats can complicate nutritional and biochemical studies because they can affect food consumption under ad libitum feeding conditions and also reduce the vitamin content of the food. If the diet has become unpalatable due to excessive oxidation of the fat component and is not accepted by the animal, a lack of growth by the animal could be due to its unwillingness to consume the diet. Thus, the experimental results might be attributed unwittingly to the type of fat or other nutrient being studied rather than to the condition of the ration. Knowing the oxidative condition of unsaturated fats is extremely important in biochemical and nutritional studies with animals.

2. Oxidation at Higher Temperatures.

Although the rate of oxidation is greatly accelerated at higher temperatures, oxidative reactions, which occur at higher temperatures, may not follow precisely the same routes and mechanisms as the reactions at room temperature. Thus, differences in the stability of fats and oils often become more apparent when the fats are used for frying or slow baking. The more unsaturated the fat or oil, the greater will be its susceptibility to oxidative rancidity. redominantly unsaturated oils (i.e., soybean, cottonseed, or corn) are less stable than predominantly saturated oils (i.e., coconut oil). Dimethylsilicone is usually added to institutional frying fats and oils to reduce oxidation tendency and foaming at elevated temperatures. Frequently, partial ydrogenation is employed in the processing of liquid vegetable oil to increase the stability and functionality of the oil. Also, oxidative stability has been increased in many of the oils developed through biotechnological engineering, a technique which effects a change in the fatty acid composition of an oil. The stability of a fat or oil may be predicted to some degree by determining the oxidative stability index (OSI).

C. Polymerization of Fats

All commonly used fats and particularly those high in polyunsaturated fatty acids tend to form larger molecules (known broadly as polymers) when heated under extreme conditions of temperature and time. Under normal processing and cooking conditions, polymers are formed in insignificant quantities. Although the polymerization process is not completely understood, it is believed that polymers in fats and oils arise by formation of either carbon-to-carbon bonds or

oxygen bridges between molecules. When an appreciable amount of polymer is present, there is a

marked increase in viscosity. Animal studies have shown that polymers present in a fat or oil will be poorly absorbed from the intestinal tract and as such will be excreted in the feces.

G. Partial Hydrogenation/Hydrogenation

Hydrogenation is the process by which hydrogen is added to points of unsaturation in the fatty acids. Hydrogenation was developed as a result of the need to (1) convert liquid oils to the semi-solid form for greater utility in certain food uses and (2) increase the oxidative and thermal stability of the fat or oil. It is an important process to our food supply, because it provides the desired stability and functionality to many edible oil products. In the process of hydrogenation, hydrogen gas reacts with oil at elevated temperature and pressure in the presence of a catalyst. The catalyst most widely used is nickel, which is removed from the fat after the hydrogenation processing, is completed. Under these conditions, the gaseous hydrogen reacts with the double bonds of the unsaturated fatty acids as illustrated below:

The hydrogenation process is easily controlled and can be stopped at any desired point. As hydrogenation progresses, there is generally a gradual increase in the melting point of the fat or oil. If the hydrogenation of cottonseed or soybean oil, for example, is stopped after only a small amount of hydrogenation has taken place, the oils remain liquid. These partially hydrogenated oils are typically used to produce institutional cooking oils, liquid shortenings and liquid margarines. Further hydrogenation can produce soft but solid appearing fats which still contain appreciable amounts of unsaturated fatty acids and are used in solid shortenings and margarines. When oils are more fully hydrogenated, many of the carbon to carbon double bonds are converted to single bonds increasing the level of saturation. If an oil is hydrogenated completely, the carbon to carbon double bonds are eliminated. Therefore, fully hydrogenated fats contain no trans fatty acids. The resulting product is a hard brittle solid at room temperature. The manufacturer to meet certain physical and chemical characteristics desired in the finished product can vary the hydrogenation conditions. This is achieved through selection of the proper temperature, pressure, time, catalyst, and starting oils. Both positional and geometric (trans) isomers are formed to some extent during hydrogenation, the amounts depending on the

conditions employed. See Figure 4 for characterization of trans isomer formation as related to increase in saturated fat during hydrogenation. Biological hydrogenation of polyunsaturated

fatty acids occurs in some animal organisms, particularly in ruminants. This accounts for the presence of some trans isomers that occur in the tissues and milk of ruminants.

Oil extraction methods - pressing - prepressing operations - oil extraction by mechanical expressions - hydraulic press - screw press - ghani and power ghani.

Various small-scale techniques are available to enable people in rural areas to process their own oilseeds locally. Careful consideration is needed to select the system that will best suit the local circumstances. These circumstances include the scale of operation required, the availability of a power source, and a number of other factors. The options available for small-scale oilseed at levels of up to 100 kg seed/in include small powered expellers, manual- or animal-powered mechanical presses, and simple procedures using water to separate oil from oilseeds.

The following five basic oilseed processing methods are available and range from those suitable for use in domestic households, to those more suited to small-scale factories:

oil extraction methods using water;

manual methods using kneading;

manual presses;

ghanis; and

expellers.

The ghani is, in effect, a mechanized version of the kneading method

The general processing steps involved in oilseed extraction varies according to the nature of the seed. In this section these processes and the major items of equipment involved are discussed. Where possible, complete process descriptions are given in Chapter 4, which deals with individual oilseeds.

Some oilseeds have a hard outer shell which must be removed before processing. This process is called decortication. Palm kernel is an example of a seed that must be decorticated prior to processing. The extraction of oil from other oilseeds which can be processed without decorticating them first, such as sunflower, may be aided by removing a proportion of the hulls before processing.

It is essential to winnow and sieve oilseeds, prior to expelling, to remove as much dirt, dust, sand and small stones as possible. The presence of sand results in high wear on critical components of expellers such as cages, wormshafts and chokes. Using clean oilseed for expelling will greatly increase the time that the expeller can be used before replacement parts are needed.

Generally, small oilseeds (such as sesame or rapeseed) can be processed directly, while larger seeds (such as copra or shea nuts) need to be ground before processing. At the domestic level, grinding is usually carried out with a pestle and mortar (Plate I) while larger quantities may be ground in a village maize mill (Plate II). Hand-operated meat mincing machines can also be used in certain circumstances. The most common type of powered mill used for small-scale operations is the hammer mill.

Rolling a seed generally results in an improvement in oil extraction by increasing the surface area of the seed while at the same time retaining channels for the flow of oil. The flakes should be very fine and preferably thinner than 0.1 mm. Rolling before processing in a bridge press is said to increase oil yields by 10% for palm kernel, groundnut and sunflower (UNATA information sheet).

Conditioning or 'cooking' oilseeds involves heating the oilseed in the presence of water. The water may be that which is naturally present in the seed, or it may be added. The changes brought about by conditioning are complex but include the coalescence of the small droplets of oil, present in the seed, into drops large enough to flow easily from the seed. In addition, higher processing temperatures improve oil flow by reducing the viscosity of the oil.

Oilseeds are nearly always conditioned before large-scale expelling. Small-scale expellers minimize the need for pre-treatment by using a relatively fast wormshaft speed which shears the oilseed as it passes through the expeller and produces frictional heating within the expeller barrel. This assists oil expulsion by raising the temperature of the oilseed. However, even when using a small-scale expeller, oil extraction will be assisted by heating and/or steaming the oilseed before expelling. Heat treatment is essential for some seeds with a low fibre content such as groundnuts; they must be heated and moisturized before expelling or the machine will produce an oily paste instead of oil and cake.

Hydraulic press

A simple form of this type of press is shown in Figure was developed by KIT for processing shea nuts and was based on a 30 t lorry jack which exerted a maximum pressure of 125 kg/cm on the seed. The cage capacity is 8.1 kg

Hydraulic press (KIT design)

Ghani

A ghani (also known as a 'chekku' or 'kol') is a mortar and pestle device which grinds oilseed into fine particles and extracts the oil from it. Ghanis are used extensively in the Indian sub-continent to process mustard seed, sesame seed, copra and groundnuts. The mortar is fixed to the floor and is normally made from wood. The pestle can be made from either wood or stone. Usually the power source is a bullock harnessed to a long lever arranged to turn the pestle inside the mortar (see Plate V). A batch of oilseed is loaded into the mortar. As the bullock moves the lever around the mortar, the pestle grinds the oilseed inside. After the seed has been ground, a certain amount of water is added.

Figure 1 Power ghani with stationary pestle

. The water combines with the ground oilseed, releasing oil which is expelled by the kneading action of the pestle through a hole in the bottom of the mortar; the oil is collected in a container. When the ghani operator is satisfied that a good yield of oil has been extracted from the seed, the ghani is brought to a halt and the oil-cake is removed. Another batch of seed is placed in the mortar and the process is repeated. A typical bullock-driven ghani can process about 10 kg seed every 2 h. The bullock normally becomes fatigued after working the ghani for about 3-4 h and is replaced by another one.

Electrically-powered ghanis, known in India as 'power ghanis', are now replacing bullock-driven ghanis because bullocks are becoming increasingly costly to maintain. Either the pestle or the mortar is held stationary in power ghanis (see Figures 1 and 2 below) which are normally run in pairs so that one is always operating while the other is being discharged. About 100 kg seed/ day is the usual throughput.The advantages of the ghani are that it produces a reasonable oil yield of about 60%, it can be made locally, and it has low running costs. Oil produced in a ghani is usually valued for its quality. In addition, no pre-grinding equipment is needed for smaller oilseeds such as groundnuts, rapeseed, sesame and sunflower seeds, and it is suitable for use by small groups in villages.

Figure Power ghani with stationary mortar

Expeller
Expeller is capable of processing 100-130 kg seed/in. This expeller is also powered by a 7.5 kW motor. All the above expellers can be operated using diesel engines as well as electric motors. The barrels of expellers consist of steel bars assembled axially in line with the wormshaft. Cake is discharged as flakes. The wormshafts of the larger expellers are manufactured with removable flights so that when the end becomes worn after extended use, only that portion needs to be replaced and not the complete wormshaft.

De Smet Rosedowns, UK makes the Mini 40 expeller (see Figure 18) which has a throughput of 15-60 kg/in depending on the oilseed being processed. The Mini 40 barrel consists of 12 cast-iron rings arranged to fit on three bars running parallel to the wormshaft to form a barrel through which the wormshaft rotates. Spacer shims fit onto the bars and between each ring to form gaps through which the expelled oil is discharged. The size of the shims is adjusted to suit the seed being processed. The wormshaft supplied with the Mini 40 is of a single-piece construction. When this item becomes worn, the whole wormshaft needs to be replaced. The unit can be powered by a 4 kW electric motor or a 6.6 kW diesel engine. The electrically-powered version is fitted with an electromagnetic vibratory feeder.

Figure. Mini 40 expeller

The size of the shims is adjusted to suit the seed being processed. The wormshaft supplied with the Mini 40 is of a single-piece construction. When this item becomes worn, the whole wormshaft needs to be replaced. The unit can be powered by a 4 kW electric motor or a 6.6 kW diesel engine. The electrically-powered version is fitted with an electromagnetic vibratory

Mechanical expression of oil- ghani and rotory- construction, working

and maintenance

Traditional ghani technology

The oilseeds and subsequently the expressed oil are held in a scooped circular pit in the exact centre of a circular mortar made of stone or wood. In it works a stout, upright pestle, which descends from a top, curved or angled piece, in which the pestle rests in a scooped-out hollow that permits the pestle to rotate, eased by some soapy or oily lubricant. Today the single angled piece takes the form of two shorter pieces pinioned or chained together. The bottom of the lower angled piece is attached to a load-beam; one end of the load-beam rides around the outside of the barrel, while the other is yoked to the animal. The load-beam is weighted down with either heavy stones or even the seated operator. As the animal moves in a circular ambit, the pestle rotates, exerting lateral pressure on the upper chest of the pit, first pulverizing the oilseed and then crushing out its oil. Within India there are regional variations in ghani design (Patel, 1943;Chaudhuri and Selvaraj, 1985), which probably arose from the nature of the oilseeds that were regionally available for crushing. The large granite ghanis of southern India have a capacity of 35 to 40 kg, requiring two animals yoked side by side and two operators, one for the animals and the other near the mortar. The load-beam is very long and curved and rides on a strong outer groove on the mortar. These ghanis have a life of four to five years, after which the pit is too worn to be useful. The wooden ghani of western India has a capacity of 8 to 15 kg, has an oil outlet at the base of the pit (which is kept plugged during crushing) and frequently has the operator seated on the load-beam. The Bengal ghani has a small capacity of only 5 to 10 kg per charge and is usually used to crush a mixture of rape and mustard seeds to yield a bouquet of flavours. The pit is small and the pestle is tall and has a stout base. The operation is prolonged so as to permit slow enzymatic liberation of several pungent alkyl isothiocyanates from the precursor glucosinolates in the prevailing warm, moist conditions. Punjab ghanis are of similarly small capacity but generally carry a short pestle.

For the mortar, the trunks of hard woods such as the tamarind tree (Tamarindus indica), neem (Azadirachta indica), jack (Artocarpus heterophyllus), baheda (Terminalia bellirica), shirish (Albizia lebbeck) and sal (Shorea robusta) have been utilized regionally, all these being very large trees (Patel, 1943). Pit designs also vary with region, and could even take the form of a wooden sleeve that sits snugly in the cavity and is less expensive to replace (Patel, 1958). Even wooden strips laid radially in the pit cavity are in use. The pestle is generally made of baheda, shirish or babul (Acacia nilotica) wood, with a bulbous tip sometimes clad with lengthwise metal strips. The shape and design of the pestle end must match that of the pit to avoid excessive dead space (Nag, 1982). The curved or angled piece was once fashioned out of a single large piece of wood; a shortage of these has led to the use of two pieces, the top one angled or curved and the bottom one straight; these are tied, chained or pinioned together for easy detachment when the pestle has to be removed. The strong load-beam has to be designed so that lateral pressure on the animal does not force it to lean from a comfortable upright stance during ambulation. Trained male animals, cattle or buffalo, are generally used, usually blindfolded to avoid dizziness and distraction; however, on small ghanis in certain areas even human labour, both of men and women, is employed (Achaya, 1993).

Crushing oilseeds

In the crushing of 10 kg of sesame seed in a ghani, about three-fourths of the material is placed in the pit and the rest is evenly laid out all around the flat rim (Patel, 1943; Nag, 1982). The animal is prodded and allowed to perambulate for a few minutes until pulverized seed is found to climb the walls of the pit. The animal is halted, and 180 ml of water is sprinkled around the chest and 120 ml poured into the pit. A further 5 minutes of pestle rotation will cause about three-fourths of the seed to be pulverized, after which another 300 ml of water is poured evenly around the pithead. The material built up in the chest is raked using a crowbar, and the pieces are broken up by hand and cast into the pit. After the animal has resumed movement, the rest of the seed is evenly pushed in all around. The operator now tests the solid material by balling it in his or her palm; if it crumbles too easily, more water is needed. The layer of built-up material is again broken up, and brisk ambulation is resumed. After about 45 minutes, a sudden release of frothy oil floods the surface. Another 100 ml of water is sprinkled over the oil, the animal is stopped and the oil is allowed to settle. A final quantity of about 20 ml of water is now brushed over the compacted cake surface using the edge of the palm, after which the animal makes a few more rounds. The operation is stopped, the two curved pieces are detached and the pestle is lifted out and laid aside. If the ghani has a drainpipe, it is unplugged and the oil is drawn into a vessel. Otherwise the oil released into the pit is mopped up with a piece of cloth and wrung out by hand into vessels. While the cake is still hot and before it has set really hard, it is prised out as thick slabs from the chest using a crowbar.

Rape and mustard seeds need more water during crushing than sesame, and copra rather less. The oilcake is not raked during linseed crushing, but only at the very end. Safflower seeds are always very carefully decorticated by passage between grinding stones, sieving and winnowing: only practically pure meats are subjected to ghani crushing. During crushing of groundnuts at least part of the shells are retained in the ghani so as to ensure formation of a granular and compact cake.

At the point of maximum contact, the pressure in a ghani is about 10 kg/cm2 (Gujarathi, 1982), about one-third of that in a small screw-press and about one-tenth to one-hundredth of that in a large modern expeller. The pressure in the ghani is largely determined by the weight placed on the load-beam, usually 115 to 160 kg, which is transmitted by way of the curved piece to the top of the pestle.

The fit of the pestle within the pit is important. Experiments have shown that an inclination that exceeds 21 from the perpendicular causes so much lateral pressure that the mass will not climb the walls of the pit. Too much dead space in the pit will have the same effect.

The phased additions of about 7, 5 percent water during ghani operation have a major role. The first addition provides the pestle with a grip on the dry oilseed, and the friction produces heat. The second portion, with the heat present, cooks the ground seed. This is analogous with what happens in the stack precooker in modern screw-press operation. Protein is denatured and coagulated, and as the moisture level reaches a critical point, oil is rather suddenly displaced from the cells. (In the Russian Skipin process, in which oil is extracted by displacement with hot water, this critical moisture level has been ascertained to lie within the limits of 14 to 18 percent (Alderks, 1948)). The cake at this stage turns granular and cohesive, and will not reabsorb the expelled oil. After the oil has appeared, the third addition of water serves to hydrate and coagulate gums and phospholipids. This phase is analogous to modern oil degumming. The last brushing with water serves to clear surface oil on the cake and give it a sheen.

Oil yield

An oil-rich seed such as sesame seed or groundnut yields about 5 percent less oil in a ghani than in a modern expeller, mainly because of insufficient pressure. Ghani oilcake carries about 15 percent residual fat, about twice that of screw-press oilcake (Achaya, 1993). In fact, in modern commercial Indian practice oilcake produced by crushing rape and mustard seeds in the ghani is put through screw-presses to obtain about 2 percent oil; this is added to the pungent ghani oil already obtained, to raise total yield.

Advantages and disadvantages of ghani crushing

When ghani crushing was widespread, fresh oil was in greater demand than it is today. Flavour, which was traditionally an important attribute of all oils, and particularly of rape and mustard, coconut, groundnut and safflower oils, was best in oils produced from mild ghani crushing. Both storage quality and nutritive value were perceived as being high, although this is not borne out by modern studies. Today homemakers, especially in urban areas, demand bland and refined packaged oils. Since vegetable oils are naturally sterile, problems of hygiene in ghani oil are unlikely. Turnover of oil in the home is so rapid, and usage of oil in India so low, that oxidative and lipolytic deterioration resulting from storage is also insignificant. Ghani cake is known to be exceptionally hard and dry and is not prone to mould infestation unless inadvertently wetted. However, the ghani has disadvantages which are mainly economic in nature. Traditional ghanis have a maximum capacity of about 50 kg per day, and modern powered units only about twice that much. As a result, running costs are disproportionately high. If animals are used, they need to be trained, and they are expensive to feed. Artisan training is also essential. Ghani oilcake as prised out of the unit after crushing is extremely hard and is not accepted by the trade for further solvent extraction, as are expeller oilcakes.

In ancient times, ghani crushers in India were recorded as being a separate caste, and this distinction still persists (Bose, 1975). Since the start of the twentieth century, as the demand for ghani oil has dropped sharply, younger people have shifted to more remunerative occupations, turning away from ghani crushing as they have from many other artisanal activities in a rapidly changing social, technological and economic environment. Use of ghani crushing in India has probably stabilized at the current level of subsidized operations. In the future, power-driven devices are certain to displace traditional ghanis worked by animal traction. There may still be room for powered ghanis in India and perhaps even in other developing countries with limited local supplies of raw materials for oilseed extraction, and there may be a place for batteries of power ghanis to multiply oil output from a common shaft in factory operations.

Fig. Traditional Ghani in ancient sculptures.

Fig. Traditional ghani

Lecture 10

press) - construction, working and maintenance.

Theoretical considerations

1. Principles of screw press operation

The oil, in the form of oil globules, is present in the cells of the oilseed at dierent locations along with other constituents such as proteins, globoids and nucleus. A tough membrane called cell wall surrounds these. Oilseed medium is fed continuously into the screw press, where it is compressed under high pressure (435 MPa) which ruptures the cell walls so that the oil globules can escape, and forces oil through the slits provided along the barrel length. The compressed solids are simultaneously discharged through a choke provided at the end of the barrel. Ward (1976) and Singh, Singh, Bargale and Shukla (1990) have discussed in detail the various components of screw presses, their functions and some design criteria.

2. Design considerations

The literature reveals that screw presses designs and modications have largely come from screw press manufacturers who have used their experience in design of these expellers (Tindale & Hill-Hass, 1976; Stainsby, 1988). Mostly, the theory of Newtonian fluid flow in an extruder, wherein pressure is built-up continuously along the barrel length, has been adopted. Contrary to the flow of material in the extruder, the cage bar of an expeller is not continuous and radial flow of oil takes place as the material moves forward in the barrel. Thus, the analysis and design of an expeller becomes complicated.

For this reason, specific theoretical considerations for screw press designs have not been applied. Fig.1 shows the material flow through a small section of the worm channel (Vadke, Sosulski & Shook, 1988). As the mixture of the oil and solids passes through

several such sections, it is subjected to a radial pressure. This pressure is generated due to volumetric compression along the screw barrel, and is exerted by the shaft. According to Ward (1976), the maximum radial pressure is generated at the feed end and the axial pressure follows a similar trend. This pressure causes flow of oil in the radial direction through the oilsolid matrix and oil flows out through the barrel slits. Such oil flow,

Fig. 1. Flow of material through a section of worm channel (Vadke, Sosulski & Shook, 1988)

in turn, changes the flow rate of mixture inside the barrel in the axial direction. Expellers work on the principle of a pressure dierential applied to the incoming oilseeds versus that applied to the discharge material. The compression ratio of the screw/worm is therefore one of the most important criteria influencing the performance of a screw press. It is defined as the ratio of volume of material displaced per revolution of the shaft at the feed section to the volume displaced at the choke section. In practice, compression ratios higher than the theoretical compression ratios of high oil content seeds are used to

compensate for slip and rotation of meal with respect to the shaft. For example, a compression ratio of 10:1 is normally used for groundnut compared to a theoretically

calculated ratio of 4.3:1.0 (Singh & Agarwal, 1988). As for extruder design, the following assumptions are made for design of screw press:

The maceration of oilseed mass is complete in the feed section, leaving a homogeneous mixture of oil and solids in the ram section.

No pressure development would take place in the feed section. The pressure development and the expression of oil starts at the beginning of the ram section.

The temperature of oilseed mass remains constant in the ram section (while in reality, the temperature increases along the ram section due to shearing action of the shaft).

Fig. 3 shows the volume change pattern as well as the theoretical variation of the

Fig. 4. Semi-sectional view of the screw press (oil expeller) developed based on the concept of single feed double stage compression. (1) hopper, (2) speed reduction unit, (3) frame, (4) cake collecting tray, (5) worm-shaft, (6) barrel, (7) spacers, (8) oil collection tray, (9) clearance adjustment, (10) electric motor, (11) choke mechanism.

radial and axial pressures in various sections of the developed screw press (based on Ward, 1976). The developed screw press consisted of a screw, barrel (circular barrel rings inside which the screw rotates) and a cone mechanism for adjustment of clearance and

regulation of pressure on fed oilseeds (Fig. 4). The power transmission system consisted of a motor (5.6 kW/3Ph/1440 rpm), a gear reduction unit (25:1) and two four-step pulleys selected to provide four worm speeds of 29, 58, 96 and 115 rpm. To provide a consistent

uniform feeding in the screw press, a regulatory feeder was provided. This was driven through the gear reduction unit such that the feeder speed was synchronized with that of the screw press. Other components included thrust bearings, and oil and cake pan. Major specifications of the developed screw press are given in Table 1.

The barrel was made up of a number of single circular mild steel plates joined together using two hollow mild steel rods which went through these plates. Using a pump, water may be circulated through these rods to check the increasing barrel temperature during operation of the press. Between the plates, 0.025 mm thick spacers/shims were provided (15 in number) to facilitate the flow of expressed oil during operation of the press. A relatively large number of shims were provided in the plug section followed by the ram section. This was necessary due to maximum applied pressure in this section, a higher quantity of oil was expressed and its quicker escape was necessary through wider spaced slits to avoid its accumulation and possibility of back-flow towards the feed section. Provision of a slightly larger area for these barrel plates facilitated faster dissipation of heat during operation of the screw press.

4. Operation mechanism

As the oilseed fed through a regulated feeder enters into the feed section (refer Fig) of the primary section of the screw (1), it is conveyed to the ram section (2) where it disintegrates into small particles, thereby exposing a larger surface area to the pressure application in the forthcoming primary plug section (3). The pressure in the primary plug section is generated due to restriction created by an intermediate choke. This creates axial and thereby radial compression on the disintegrated oilseeds, rupturing the cell-walls, and facilitating removal of oil from the oilseed. The extracted oil flows through slits provided in the barrel through shims in the primary plug section.

Meanwhile, as oilseed is continuously fed through the feeder, the material in the primary plug section moves forward to the secondary section where it enters the ram section directly. In this section, it is subjected to a gradually increasing pressure, which remains however; lower than that in the preceding-primary plug section. This provides an all-important breather to the material, which then enters the secondary plug section and is

Compressed to the maximum pressure designed for the press before finally exiting from the expeller. This second compression is more effective since the clearance between the barrel and screw can be reduced considerably towards the feed section. Provision of a slightly larger considerably with the help of the end cone clearance. To accomplish

this, the end part of the worm is of conical form. The backward movement of the worm (i.e. movement towards the feed hopper) increases the clearance and thereby reduces the pressure on oilseed present inside while its forward movement does it otherwise. Using this mechanism, a clearance in the range of 0.80.4 mm was attainable.

Prior to each experiment, the screw press was warmed

up to a temperature of about 50C by processing raw

rapeseed kernels. Once this temperature was obtained,

the experiment was started and pretreated rapeseed

samples, in the batch sizes of 5 kg each, were fed to the

screw press. Each experiment was replicated ve times.

During the process of oil expression, barrel temperature,

specic energy consumption and pressing time were

measured using a digital temperature indicator (resolution

0.1C; Model-660, Omega Engineering Stamford,

CT, USA), energy meter (Make HFD, Havells Electronics,

New Delhi, 3 Phase; 30 Amp; 50 cps; 60 rev/

kWh) and a digital stop watch (resolution 0.01 s, LCD

quartz watches, Smart, Japan), respectively. The expressed

oil was collected in measuring cylinders (resolution

1 ml). The cake was collected in a weighing pan

connected to a digital balance (resolution 0.1 g, Make

Avery, India). For the rst pass, the clearance was set

at 0.8 mm while for the second pass it was reduced to 0.4

mm. The feed rate to the screw press feed section was

controlled and regulated through a feeder. A screw

speed of 96 rpm was used based on the preliminary tests

to ensure least choking/jamming of the screw press. For

long-term test, the press was operated ve times continuously

for 16 h duration to evaluate its suitability for

longer durations without choking/jamming and excessive

heating.

The collected oil also contained small fractions of

solid particles (`foots') in suspension. These particles

which are known as foots that had owed out through

the barrel slits along with the oil. The oil was allowed to

settle for 24 h. In most of the cases, half of the weight of

the settled foots was found to be oil when separated in a

centrifuge. Hence, this quantity was added to the oil

expressed from the oil outlet for the purpose of percent

oil recovery, dened for the purpose of this study as the

ratio of amount of oil expressed to the amount of oil

initially present in the pressed samples. Oil content of

each of the sample was expressed on a moisture free

basis. This was to ensure uniformity in oil recovery

on a mass basis, per kilogram of fed samples

calculations from samples having dierent initial oil

content because of dierent moisture contents. Initial oil

content for the sample was determined using the laboratory

scale solvent extraction method. The soxhlet apparatus

(Make Tecator, Sweden, Model-1020) and the

standard procedure (AACC, 1995) were used for this

purpose.

pressed at selected moisture contents. The residual oil

content in the cake after the rst pass (Table 2; column

9) was calculated as the ratio of quantity of the oil remaining

in the cake (Table 2, column 4, pass-II) to the

total weight of the sample after removal of net oil

(column 7). For this purpose, one-half of the weight of

the foots was added to the weight of the deoiled cake

that exited from the cake outlet.

The maximum oil recovery was obtained at a moisture

content of 7.5% (w.b.) when a total of 90.2% of the

available oil in the sample could be recovered (Table 2).

The throughput and the eective capacity of the screw

press were found to be the maximum for this moisture

content. The values of the throughput capacities at 7.5%

moisture content were 28.2 kg of feed/h for rst pass and

23.9 kg of feed/h for the second pass, for an overall effective

capacity of 15 kg of feed/h in two passes. The

quantity of foots was also minimum (22.4 g) at this

moisture content. The specic energy consumption did

not vary much with moisture contents with values from

0.048 to 0.056 kWh/kg of feed, although specic energy

consumption did decrease with increasing moisture

content. This may be due to the plasticizing eect of

water which decreases the frictional coe...cient of the

material. The measured values of maximum temperature

recorded also decreased with increasing moisture contents

from 5.1% to 11.1% (w.b.). The maximum barrel

temperature was 70.3C at a moisture content of 5.1%

and decreased to 61.4C at a moisture content of 11.1%

(w.b.).

The developed screw press operated smoothly

throughout these experiments without any observed

choking or jamming. This may also be reected in the

recorded range of maximum barrel temperatures (61.4

70.3C) during the operation of the press. During the

long-term tests of 16 h duration, it was found that the

press capacity was relatively higher than that reported

for shorter duration tests of about 2-h duration. This

may be due to reduction in loss of time for larger

from the Farm Section of the Central Institute of Agricultural

Engineering, Bhopal. The water sprinkled

samples were then thoroughly mixed manually, packed

in an air-tight metal container and stored for about 48 h

for equilibration. The container was shaken at regular

intervals to distribute moisture uniformly throughout

Lecture 11

Expression of oil using expellers - types - construction of screw press - working and maintenance.

Principle of operation

Oilseed expellers produce oil and oil-cake from oilseed continuously, unlike bridge presses which operate on a batch system. The essential components of a typical small-scale expeller are shown in Figure 13. The expeller is driven either by an electric motor or by a diesel engine. At the heart of the machine is a powered wormshaft which rotates inside a closely fitting cage. The oilseed is fed continuously into the press through a hopper and is crushed as it is transported through the cage by the wormshaft. Pressure is exerted on the system by restricting the gap at the end of the cage through which the oil cake is discharged from the press. The expelled oil drains out of the cage through small gaps.

The friction generated inside the expeller barrel will eventually result in the wearing down of the wormshaft end portion, barrel bars or rings, and choke. Replacement of these parts will be required at intervals depending on the type and amount of oilseed processed and the degree of dirt contaminating the seed. Rapid wear is a particular problem when expelling undecorticated, dirty sunflower seed. The availability and cost of wearing parts are important considerations when setting up a small-scale expeller facility.

General method of operation for expellers having an adjustable choke

Starting up the press

1. Before starting up the drive to the press, check that all the safety guards are in good order and the machine has been lubricated according to the manufacturer's instructions. Ensure the choke control is adjusted so oil-cake can be discharged from the press cake outlet.

2. Start the press drive and check that the wormshaft is turning over correctly. Then begin feeding the oilseed very gradually by hand into the hopper. The screw press will not expel oil satisfactorily until the barrel is hot. The operating temperature required varies according to the type of oilseed processed, but it is normally between 60 and 100C. Some oilseeds (such as copra) have a fibre content which provides the friction needed to heat the barrel. Softer oilseeds (such as groundnuts and sesame seed) first need to be heated and conditioned in order to reach a satisfactory operating temperature (see Chapter 4 for details of oilseed conditioning). When expelling the softer seeds, the time taken to reach the operating temperature can be reduced considerably by feeding the press cautiously by hand with crumbled oil-cake of the seed being processed. As the barrel temperature increases seed should be mixed with the oil-cake in progressively increasing proportions so that finally only oilseed is being fed to the press. In some circumstances it is beneficial to use a mixture of oilseed and oil-cake all the time.

3. When the seed material has been discharging freely from the press cake outlet for a few minutes, the choke may be reduced gradually to increase the pressure within the press and improve the quality of the oil-cake and the oil flow from the press barrel. The optimum cake thickness for a small-scale expeller is usually about 1-2 mm.

4. The choke is adjusted by the wormshaft regulator at the feed end of the machine. Turning the handles anti-clockwise moves the taper plug section of the wormshaft axially further into the taper bore of the choke ring, thus reducing the thickness of the cake. Turning the wormshaft regulator clockwise withdraws the shaft and increases the cake thickness. The locknut (if fitted) has to be released to allow the operating screw to move and should be relocked after each adjustment.

5. Until the barrel reaches the operating temperature, a large amount of sediment 'fools' may be produced with the oil. To limit this, oilseed should be fed slowly to the press during the warming-up period. However, some oilseeds do not produce a large amount of sediment with the oil during the start-up period. In this case, the rate of feed may be increased more quickly until optimum operating conditions are reached. Once uniform operating conditions have been reached, the sediment can be gradually mixed in with the oilseed fed to the press.

Normal operation of the press When uniform operating conditions have been reached, the objective should be to produce the best quality of oil-cake possible, consistent with the required throughput of the press. It is possible to process some oilseeds without needing to restrict the rate at which they are fed to the press. In other words, with the feed control slide on the hopper fully open, the press will continue to operate in a uniform and steady manner, drawing oilseed from the full hopper.

There are occasions, however, when the feed has to be metered to the press. Metering can be necessary for processing the more fibrous seeds such as palm kernels and copra. This is because the power required can be much greater than that needed to process softer seeds, and so the feed rate has to be restricted to keep the power required within the power limitation of the driving unit. In this event, if laborious manual feeding is to be avoided, the press may be fed by some metering device such as a variable delivery vibrator or screw feeder from a hopper or storage bin.

Metering is also necessary when palm kernels, copra and other large seeds are broken, or coarsely ground, before feeding to the press. In this condition they will not easily flow steadily into the press feed area and therefore may have to be metered either by hand or, preferably, from a vibratory feed. Some seeds need to be pressed a second time to obtain optimum extraction of the oil. Again, the cake from the first pressing may not flow satisfactorily from a full feed hopper and may need to be metered to the press.

Palm kernels are described above as usually being broken or coarsely ground before processing but, with caution, they can be first-pressed as whole kernels from a full open feed. However, a metered feed may be needed to recycle the oil-cake from the first pressing.

When feeding from a full feed hopper, the flow characteristics of some materials through the press may be exceptionally good and result in a press capacity higher than required. If this condition occurs, the resulting residual oil in the cake tends to be high. A lower, more acceptable oil-in-cake result may usually be obtained by metering the seed to the press at a reduced rate.

Theoretical considerations

1. Principles of screw press operation

The oil, in the form of oil globules, is present in the cells of the oilseed at dierent locations along with other constituents such as proteins, globoids and nucleus. A tough membrane called cell wall surrounds these. Oilseed medium is fed continuously into the screw press, where it is compressed under high pressure (435 MPa) which ruptures the cell walls so that the oil globules can escape, and forces oil through the slits provided along the barrel length. The compressed solids are simultaneously discharged through a choke provided at the end of the barrel. Ward (1976) and Singh, Singh, Bargale and Shukla (1990) have discussed in detail the various components of screw presses, their functions and some design criteria.

2. Design considerations

The literature reveals that screw presses designs and modications have largely come from screw press manufacturers who have used their experience in design of these expellers (Tindale & Hill-Hass, 1976; Stainsby, 1988). Mostly, the theory of Newtonian fluid flow in an extruder, wherein pressure is built-up continuously along the barrel length, has been adopted. Contrary to the flow of material in the extruder, the cage bar of an expeller is not continuous and radial flow of oil takes place as the material moves forward in the barrel. Thus, the analysis and design of an expeller becomes complicated.

For this reason, specific theoretical considerations for screw press designs have not been applied. Fig. shows the material flow through a small section of the worm channel (Vadke, Sosulski & Shook, 1988). As the mixture of the oil and solids passes through

several such sections, it is subjected to a radial pressure. This pressure is generated due to volumetric compression along the screw barrel, and is exerted by the shaft. According to Ward (1976), the maximum radial pressure is generated at the feed end and the axial pressure follows a similar trend (Fig. 3). This pressure causes flow of oil in the radial direction through the oilsolid matrix and oil flows out through the barrel slits. Such oil flow,

Fig. 2. Flow of material through a section of worm channel (Vadke,

Sosulski & Shook, 1988)

in turn, changes the flow rate of mixture inside the barrel in the axial direction. Expellers work on the principle of a pressure dierential applied to the incoming oilseeds versus that applied to the discharge material. The compression ratio of the screw/worm is therefore one of the most important criteria influencing the performance of a screw press. It is defined as the ratio of volume of material displaced per revolution of the shaft at the feed section to the volume displaced at the choke section. In practice, compression ratios higher than the theoretical compression ratios of high oil content seeds are used to

compensate for slip and rotation of meal with respect to the shaft. For example, a compression ratio of 10:1 is normally used for groundnut compared to a theoretically

calculated ratio of 4.3:1.0 (Singh & Agarwal, 1988). As for extruder design, the following assumptions are made for design of screw press:

The maceration of oilseed mass is complete in the feed section, leaving a homogeneous mixture of oil and solids in the ram section.

No pressure development would take place in the feed section. The pressure development and the expression of oil starts at the beginning of the ram section.

The temperature of oilseed mass remains constant in the ram section (while in reality, the temperature increases along the ram section due to shearing action of the shaft).

3. Design of the developed oil expeller

The expeller developed in the present study is a prototype design based on the principle of single-feed of provided additional length, the compression ratio of the worm configuration was changed such that it was reduced to almost half. Hence, instead of a single stage compression ratio of 10:1 typical of conventional screw presses, a compression ratio of 5:1 was used for the primary section and a ratio of 3:1 for the secondary section, for a theoretical overall effective compression ratio of 15:1 in one pass. This compression was achieved through increasing the root diameter of the worm while the pitch and helix angle of the screw and the barrel diameter were kept constant. Fig. 3 shows the volume change pattern as well as the theoretical variation of the

Fig. 4. Semi-sectional view of the screw press (oil expeller) developed based on the concept of single feed double stage compression. (1) hopper, (2) speed reduction unit, (3) frame, (4) cake collecting tray, (5) worm-shaft, (6) barrel, (7) spacers, (8) oil collection tray, (9) clearance adjustment, (10) electric motor, (11) choke mechanism.

radial and axial pressures in various sections of the developed screw press (based on Ward, 1976). The developed screw press consisted of a screw, barrel (circular barrel rings inside which the screw rotates) and a cone mechanism for adjustment of clearance and

regulation of pressure on fed oilseeds (Fig. 4). The power transmission system consisted of a motor (5.6 kW/3Ph/1440 rpm), a gear reduction unit (25:1) and two four-step pulleys selected to provide four worm speeds of 29, 58, 96 and 115 rpm. To provide a consistent

uniform feeding in the screw press, a regulatory feeder was provided. This was driven through the gear reduction unit such that the feeder speed was synchronized with that of the screw press. Other components included thrust bearings, and oil and cake pan. Major specifications of the developed screw press are given in Table 1.

The barrel was made up of a number of single circular mild steel plates joined together using two hollow mild steel rods which went through these plates. Using a pump, water may be circulated through these rods to check the increasing barrel temperature during operation of the press. Between the plates, 0.025 mm thick spacers/shims were provided (15 in number) to facilitate the flow of expressed oil during operation of the press. A relatively large number of shims were provided in the plug section followed by the ram section. This was necessary due to maximum applied pressure in this section, a higher quantity of oil was expressed and its quicker escape was necessary through wider spaced slits to avoid its accumulation and possibility of back-flow towards the feed section. Provision of a slightly larger area for these barrel plates facilitated faster dissipation of heat during operation of the screw press.

4. Operation mechanism

As the oilseed fed through a regulated feeder enters into the feed section (refer Fig) of the primary section of the screw (1), it is conveyed to the ram section (2) where it disintegrates into small particles, thereby exposing a larger surface area to the pressure application in the forthcoming primary plug section (3). The pressure in the primary plug section is generated due to restriction created by an intermediate choke. This creates axial and thereby radial compression on the disintegrated oilseeds, rupturing the cell-walls, and facilitating removal of oil from the oilseed. The extracted oil flows through slits provided in the barrel through shims in the primary plug section.

Meanwhile, as oilseed is continuously fed through the feeder, the material in the primary plug section moves forward to the secondary section where it enters the ram section directly. In this section, it is subjected to a gradually increasing pressure, which remains however; lower than that in the preceding-primary plug section. This provides an all-important breather to the material, which then enters the secondary plug section and is

Compressed to the maximum pressure designed for the press before finally exiting from the expeller. This second compression is more effective since the clearance between the barrel and screw can be reduced considerably towards the feed section. Provision of a slightly larger considerably with the help of the end cone clearance. To accomplish

this, the end part of the worm is of conical form. The backward movement of the worm (i.e. movement towards the feed hopper) increases the clearance and thereby reduces the pressure on oilseed present inside while its forward movement does it otherwise. Using this mechanism, a clearance in the range of 0.80.4 mm was attainable.

Lecture 12.

Steam kettle - oilseed cooking - construction and operation

Lecture No. 18. Production of Palm oil and Peanut oil

Condensate Steam

(9.1%) Steam 2 - 3 kg / cm2

40 - 60 min.

Empty Fruit Bunch

( 35.7 % )

Hot water

90 - 950 C (15 % )

1 1 16.4 %

Hot water 40% Steam

								Crude Palm Oil ( 18.2 % )
					Sludge & Water ( 54.2 % )

Peanut Oil Production Process

				Peanut Pods

Non-fatty

matter

Lecture No. 19. Production of Rice bran & Soya bean oils

Rice Bran Oil

Steam cooking &

Drying

Solvent Extraction

Meal Miscella

Desolventized bran Distillation

Cooling

Solvent (Liquid)