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8. Vegetables


Nutritional significance
Processing
Packaging and storage
Suitability for small-scale production

There is a universal demand for vegetable oil due to its use in domestic cooking, as an ingredient for other food production (in baked goods and fried snack foods), and as a raw material for the manufacture of soap, body/hair oils, and detergents.

Major oils and their uses

Raw material

Oil content (per cent)

Use

Oilseeds


Castor

35-55

Paints, lubricants


Cotton

15-25

Cooking oil, soapmaking


Linseed

35-44

Paints, varnishes


Niger

38-50

Cooking oil, soapmaking, paint


Rape/mustard

40-45

Cooking oil


Sesame

35-50

Cooking oil


Sunflower

25-40

Cooking oil, soapmaking

Nuts


Coconuts

64 dried copra
35 fresh nut

Cooking oil, body/hair cream, soapmaking


Groundnuts (peanuts)

38-50

Cooking oil, soapmaking


Palm kernel nuts

46-57

Cooking oil, body/hair cream, soapmaking


Shea nuts

34-44

Cooking oil, soapmaking

Mesocarp


Oil palm

56

Cooking oil, Soapmaking

Oil can be extracted from many raw materials, but not all oil-bearing seeds, nuts, and fruit contain edible oil. Some contain poisons or unpleasant flavours and these are only used for paints; others such as castor oil need very careful processing in order to make them safe. Such oils are not suitable for small-scale processing.

Oils from certain crops such as maize are extracted by using solvents which dissolve the oil. This method of extraction is not suitable for small-scale operation due to the high capital costs of equipment, the need for solvents which may not be easily available, and the risk of fire or explosions.

Aflatoxins are a poisonous group of compounds produced by certain moulds which grow on seeds and nuts. Their occurrence has caused much concern recently, as they are poisonous to both humans and animals (causing liver damage, cancer, and death), if consumed over a prolonged period. The mould may grow either before or after harvest but this growth can be prevented (thereby preventing the production of aflatoxins) by drying the crop correctly. Aflatoxins are not destroyed or removed by heating, or during the subsequent processing stages and are difficult and expensive to remove if they occur - therefore grading is of particular importance in removing potentially dangerous produce. Seeds and nuts can be easily identified if there are visible signs of mould growth. There is, however, a danger that afflicted produce may not look mouldy after drying, and in these cases, produce contaminated with aflatoxins may only be detected by signs of discolouration or a shrivelled appearance.

If properly stored, vegetable oil has a shelf-life ranging from 6-12 months. Heat applied during processing destroys enzymes in raw materials, and also any contaminating micro-organisms which would cause rancidity. Additionally, the oil may be heated after extraction to remove as much water as possible. This lessens the occurrence of microbial spoilage during storage. Correct packaging and storage conditions slow down chemical changes caused by light and heat which may lead to rancidity.

Nutritional significance

Oil provides twice as much energy as the same quantity of carbohydrate and is therefore considered to be a valuable part of a well-balanced diet. Oil also contains a range of fat-soluble vitamins (A, D, E, and K) and essential fatty acids, both of which are necessary for the healthy functioning of the body.

The process of oil extraction produces a by-product known as oilcake. This is very nutritious, and can be used either for animal feed or as an ingredient in the production of other food products.

Processing

Oil is contained in plant cells, and its release depends on these cells being ruptured. Methods for achieving this depend on the composition of the raw material. For example, seeds, beans, and some nuts are processed in a dry state, whereas palm fruits are processed wet.

Principles of preservation:

· to destroy enzymes in the raw material and contaminating micro-organisms by using heat during processing

· removal of as much water from the oil as possible to prevent microbial growth during storage.

In addition, correct packaging and storage are important factors in preventing rancidity.

Girl collecting groundnuts

FAO photo

There are four main stages in the extraction of oil:

· preparation of the raw material
· extraction
· clarification
· packaging and storage

The following table outlines the stages involved in the processing of oils and also points out the type of equipment needed.


Oilseeds (e.g. sunflower, safflower, mustard and sesame)

Groundnuts

Coconut (wet method)

Palm kernel

Palm fruit

Decorticate/dehusk

*

*




Crack



*

*


Grind/grate


*

*

*


Pulp





*

Heat/condition

*

*


*


Press/expell

*

*

*

*

*

Clarify

*

*

*

*

*

Pack

*

*

*

*

*

Equipment required

Processing stage

Equipment

Section reference

Decorticate/dehusk

Decorticators

19.0

Crack

Hand hammer


Hammer mill

41.3

Kernel cutters

17.3

Winnow

Winnowers

67.0

Pulp

Pestle and mortar or pulping machine

55.2

Grind/grate

Roller mill for groundnuts

41.2

Hammer mill for palm kernels 41.3


Heat/condition


Open pan for heating

48.0

Heat source

36.0

Thermometer

63.0

Measuring and weighing equipment

64.1 and 64.2

Press/expell

Oil presses

53.2

Expellers

26.0

Raw material preparation

Decortication

Some raw materials have a fibrous husk or seed coat, and this must be removed prior to processing. The removal is known as decortication and a range of decorticating machines are available which are suitable for small-scale production.

Winnowing

Winnowing takes place after decortication, and is the separation of the husks or seed coat from the oil-bearing material. Traditionally, this operation is achieved by gently throwing the seeds into the air and letting the air blow away the husks. This method of winnowing requires skill and experience. For higher rates of production, it is possible to use either a manual or a powered winnowing machine.

Winnowing

Cracking

Palm kernels and coconuts need to be cracked and the shell removed before processing can begin. This can be achieved manually using simple tools such as a hammer or a heavy knife. Motorized hammer mills are available for cracking but for coconuts it seems that the manual method is presently more efficient.

Pulping

Palm fruits can be pulped manually with a pestle and mortar, or more quickly, with a motorized pulper.

Grinding/flaking

Traditional hand-pounding methods using a pestle and mortar or more sophisticated roller mills, may be employed to grind groundnuts into a coarse flour. Flakers are used for sunflower seeds, and hammer mills are applicable for palm kernels. Coconut flesh needs to be grated, and a wide range of manual graters are available.

Heating/conditioning

Many raw materials, such as groundnuts and sunflower seeds, are heated with water prior to extraction. This processing stage is known as 'conditioning' and has the following effects:

· it assists in the rupturing of the oil-bearing cells
· it decreases the thickness of the oil, allowing it to flow more easily.

The desired moisture content will differ, according to the raw material being used. For example, groundnut flour needs an addition of 10 per cent water and is heated at a temperature of 90°C. The heating is traditionally carried out over open fires, although seed scorchers, which are basically pans fitted with stirrers, are now available to mix the nuts/seeds better.

Assessing the correct amount of moisture and degree of heating is often quite difficult, and therefore a simple hand-feel test is used. For example, in the case of groundnuts, the heating process is complete when the mixture stops sticking together and forms a free-flowing flour again.

Conditioning groundnut flour

Oil extraction

One traditional method for extracting oil involves the use of a 'ghani'. Ghanis originated in India, but their use is now more widespread.

A ghani in operation

A ghani consists of a wooden mortar and pestle. The mortar is fixed to the ground, and the pestle is located in the mortar, where the raw material is crushed by friction and pressure. An animal is required to move the pestle and as this continues the oil is pressed out, runs through a hole at the bottom of the mortar, and the residue (cake) is then scooped out. Ghanis are limited in that two animals are required, since any one animal will tire after 3-4 hours.

Improved technologies for extracting oil include:

· motorized ghanis
· oil presses
· oil expellers.

Motorized ghanis

These are becoming increasingly popular and are fast replacing animal-powered equipment.

Oil presses

Many types of oil press are available, but all work on a similar principle. Raw materials are placed in a heavy perforated or slotted metal cage and a metal plunger is used to press out the oil. The main differences in design are as follows:

· The method used to move the plunger.
· The amount of pressure in the press.
· The size of the cage.

The plunger can be moved either manually or with the aid of a motor. The motorized method is faster but more expensive. Different designs use either a screw-thread or a hydraulic system.

A manual oil press

In most cases, oil presses can be manufactured locally. If it is a screw-press, however, a lathe is needed to manufacture the screw. Hydraulic presses may be manufactured locally if lorry jacks are available. It is important that the mineral oil used with either the screw or the hydraulic press does not contaminate the oil.

Oil expellers

Expellers use a horizontally-rotating screw which feeds oil-bearing raw material into a barrel-shaped outer casing with perforated walls. The raw material is continuously fed to the expeller, which grinds, crushes, and presses out the oil as it passes through the machine. Oil flows through the perforations in the casing and is collected underneath. The residue, or oilcake, is pushed out of the end of the unit.

Most small expellers are power-driven. Due to the wear and tear, it is likely that the screw will need to be repaired and replaced at frequent intervals. Therefore, it is essential that the skills and resources required for maintenance are available locally.

Clarification

Crude oil contains a suspension of fine pulp and fibre from plant material. It also contains smaller quantities of water, resins, colours, and bacteria which makes it darker in colour. These contaminants are removed by clarifying the oil, either by allowing the oil to stand undisturbed for a few days and then removing the upper layer, or by using a clarifier. If further clarification is needed, the oil may be filtered through a plastic funnel which has been fitted with a fine filter cloth. Finally, the oil is heated to boil off the traces of water and destroy any bacteria. For those raw materials which are processed wet (such as coconut), heating is applied prior to clarification in order to break the emulsion. When these impurities are removed, the shelf-life of the oil can be extended from a few days to several months, provided it is stored properly.

Tinytech oil expeller

Packaging and storage

Rancidity can cause the oil to deteriorate and develop 'off' flavours during storage. This may be prevented by using clean, dry containers which exclude light and heat, and prevent contact with metals such as iron or copper.

Sealed glass or plastic bottles are adequate packaging materials. It is preferable if bottles are made from coloured glass and kept in a dark box. Metal cans may be used provided the metal is tin-coated. Alternatively, glazed ceramic pots sealed with a cork and a wax stopper are also suitable.

Re-used bottles must be well-cleaned to ensure that there is no film of old rancid oil on the inside of the container. If not, it will quickly turn new oil rancid. The containers should be properly dried after cleaning to remove all traces of water.

If the oil is packaged adequately and kept away from heat and sunlight, the shelf-life can be expected to be 6-12 months.

Suitability for small-scale production

Only seeds, nuts, and fruits which contain considerable amounts of edible oil are used for small-scale extraction. Other types may contain edible oil but small-scale equipment cannot extract enough of the oil to be economically viable.

Oilseeds have a long shelf-life if kept sufficiently dry (thereby preventing the growth of mould and the subsequent occurence of aflatoxins), and oil processing can therefore continue throughout the year, and make better use of the equipment than more seasonal products.

The low volume of oil, compared to oilseeds, makes transport and distribution easier and more effective. Distribution to a wider area creates more potential markets, and producers are able to receive an income throughout the year.

The residual oilcake is often highly nutritious and can be used for either animal or human food. For example, the cake produced from peanut oil extraction can be fried and made into a snack food.

There are also disadvantages to oil extraction. These include the higher value of oilseeds compared to other crops, the higher financial risk from losses, the need for fuel, and the time-consuming hard work entailed. The equipment can be expensive, and year-round production needs a large working capital to buy and store the seasonal crops. There is also the risk of competition from large-scale producers who are able to market high-quality oils at a lower cost because of economies of scale.

Due to competition from larger-scale manufacturers and the importation of cheaper oils in some areas, it is often more profitable for small producers to use the oil as part of the secondary processing of other products, such as soap.

Small-scale extraction processes produce crude oil. This has a different appearance and flavour compared to commercially-refined oils. It is therefore necessary to test market the crude oil for acceptability before embarking on a processing enterprise.


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