Different Methods of Processing Crop Products
The common element in all sectors of the food processing is conversion of raw material into crop products or products of higher value. In some situations, processing is a one-step conversion of raw material to a consumer product.
The history of food processing lay emphasis on the role of establishing and maintaining microbial safety in foods, as well as the desire to establish and maintain economic shelf-life for foods.
Processing methods are used worldwide as they improve the digestibility and nutritional quality of the grains.
1. Methods of Processing Crop Products
Ineffective or inappropriate food processing technologies, careless harvesting and inefficient post-harvest handling practices, bad roads, moribund rail systems, bad market practices and inadequate or complete lack of storage facilities, packing houses and market infrastructures are some of the factors responsible for high post-harvest food losses.
1. Cereals
Cereals can be separated or subdivided into their components during processing into other forms of foods or industrial uses. The cereal grain may be de-husked (hulling) to remove the outer coat or hull of cereal grains in order to obtain the endorsperm. De-hulling separates the bran or seed chart and the germ.
After de-hulling, the grain may be orulled (i.e. grinding of grain into smaller particles).This increases the digestion of the grain and makes it possible to use the product in various ways. Usually, cereal grains are processed in the dry form to make other products.
Some cereal grains may be prepared as whole cereal meal, de-corticated or production is geared towards the production of whole-wheat flours, employing a simplified process flow sheet, but most demand and effort is directed towards the production of white flour.
White flour is the ultimate product of flour milling. The aim of white flour milling is to extract a maximum amount of endosperm from the wheat berry in as pure a form as possible. The outer bran layers become the co-product of the process called wheat feed.
Many operations also separate the embryonic part of the berry, known as the germ. This is a high value co-product when a market exists. Where a market for germ does not exist, it is sold for animal feed with the wheat feed produced.
These co-products contribute significantly to the financial viability of milling operations. One of the keys to the success of a flour milling operation is the efficient, economical separation of starchy endosperm from the rest of the berry. The process has developed along very specific lines towards achieving this goal.
There is just one accepted manner in which flour is produced globally. This is known as the gradual reduction system. The gradual reduction system of flour milling is the process of taking the whole wheat berry and, via a series of grinding and sieving stages, producing white flour of the desired quality and yield.
The gradual reduction system has enabled the production of flours of low ash content and high yield. Specialist, high quality flours, is produced by extracting high purity sub-products from within the process.
There are three principal divisions within the process. These are known as the breaking system, the purification system and the reduction system. The purification system is not favoured by many millers and may be absent from processes.
It is often replaced by what is known as the Sizing system. However, the other two blocks are present in all gradual reduction flour mills in operation today. The breaking block or break system is the area of the process where most endosperm separation is achieved.
This work is performed principally on roller mills whose surfaces have a saw tooth profile. The rollers run at different speeds towards each other.
2. Whole Meal Cereal
This is obtained by grinding the cereal whole. The meal produced is not passed through a sieve to remove any parts of the grain such as the hull or the germ particles meals produced this way are 97–100 percent extraction and is known as a Whole meal‖.
The name of the product reflects the cereal grain type used in producing it e.g. whole maize meal‖, whole Wheat Meal for meals produced from maize and wheat respectively.
If the cereal is coarsely ground, the product is known as cereal grits, and if finely ground, the product is called cereal flour. The particle size of cereal meal is between cereal grinds and cereal flour. Maize or sorghum rice or produced by milling either maize or sorghum to rice sized particles.
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This product is used in the same way as rice different process extraction rates may be use but the higher the extraction rate, the higher the nutrient component of the product.
3. Decorticated Meal
This is cereal meal which is also referred to as bolted meal or flour depending on the particles size. It is obtained by grinding the whole cereal into a fine meal which is passed through a sieve to remove portions of the full, leaving most of the germ in the meal.
Some of the germ is taken out during the sieving to remove the hull giving the product 90–96% extraction rate.
4. De-germinated
This is a highly refined cereal meal produced by grinding whole grain cereal into fine flour. Most of the hull and the germ is removed by passing the flour through different sizes of mesh sieve.
The meal of flour produced is 85% or less extraction, depending on the rate at which the hall and the germ have been removed from the final flour product. Coarsely mulled cereal grain cleaned of bran or hull and germ is called de-germed grits.
The germ or the embryo portion of the cereal which is recovered during milling is called cereal germ. This is very rich in oil. Corn or maize oil is extracted from corn germ using organic solvents.
The oil is recovered by distilling off the solvent, cleaned and refined. The residues left over after oil extraction is rich in protein and is used in animal feed formulations. The hull or the outer coating of the cereal which is removed during milling is known as cereal bran.
During milling, the cereal brain obtained is attached to parts of the endosperm. Cereal bran may be added to cereal porridges or to cereal pastes. Cereal bran provides roughage which keeps the bowls free and therefore eliminates constipation
5. Polished Cereal
This is produced when the hull and the layer underlying it are removed (together with the germ) during the dry milling of whole grains (e.g. polished rice).
The appearance of the remaining meal of flour is considered attractive or more appealing to consumers even though it is less nourishing. However, the processing used renders the products more stable when stored.
2. Industrial Processing of Cereal Grains
Industries cereal grains (e.g. wheat) are separated into portions of different qualities by a series of crushing, pulverizing and sifting processes which separate fractions of differing fineness. The grain is cleaned, before milling and moistened or conditioned (to cause the seed coat or the bran to absorb moisture and to toughen).
It is then fed through a series of corrugated rollers placed far enough apart to crush the kernels into coarse fragments and largely separates the seed coat from the endosperm. This process is known as breaking. At each crushing, some of the endosperm becomes sufficiently ground to pass through flour sieves as break flour.
The coarse particles of endosperm left behind are called middlings. These are small enough to pass through sieves that remove most of the bran. Some bran particles, small enough to go through with the middlings are removed by air currents, a process called purification.
Middlings may be bagged and sold as farina for use as breakfast cereal in creamed cereal porridge, or sold as semolina used in macaroni production.
2. Processing of Roots and Tubers
These foodstuffs are packed or pared to remove the outer skin and cut into large or small pieces or sliced. Yams, sweet potatoes, Irish potatoes, cocoyams, all darken when their cut surface are exposed to the air.
The darkening (or browning affect) is caused by enzymes acting on compounds in the raw food tissue in the presence of oxygen. The action of these enzymes may be checked by preventing the cut surface from coming into contact with air (oxygen).
Water, weak salt solutions, sugar solutions or an acid solution in the form of diluted lemon or lure juice, vinegar or other fruit juice (e.g. pineapple juice) may all be used to cover cut pieces to prevent their surfaces from coming into contact with the air. Acid solutions are very effective; when the acidity is increased, the rate of darkening is decreased.
Another way of preventing contact with the oxygen in the air is by covering the cut surface with a compound or agent which reacts with or attracts oxygen. This makes the oxygen unavailable to the cut surface and hence the discolouring does not occur.
These compounds known as reducing agents (e.g. sulphure dioxide and ascorbic acid) are used industrially to prevent darkening in roots and tubers, also in fruits and vegetables during processing.
1. Cassava
A great diversity of processing techniques has developed in different regions. Some of the processes are used in all areas either because they are standard methods for preparing starchy food, such as boiling or roasting or because they were introduced with the cassava when it was introduced from South America.
The simplest techniques used to prepare cassava for immediate consumption are boiling, roasting or baking. Peeled roots are boiled whole or sliced and served in a variety of ways
Roasting the roots is less popular with the Amerindians than boiling and generally only resorted to when no cooking utensils are available. Roots are roasted by placing them whole in the ashes of a fire.
Frying is believed not to have been used traditionally but to have been introduced by European. slices of peeled root are fried in oils of various kinds according to availability and taste.
In Vanuatu grated cassava is wrapped in banana leaves and baked in an oven. The more complex technique involving pounding of the cassava roots into a paste is particularly popular on the African continent where it is a very widespread method, The resulting paste from all these crops is generally known in West Africa as fufu, also foofoo, fuifai, foufou, foutou and vou-vou depending on the locality.
Fufu can be prepared by boiling or steaming peeled cassava roots and then pounding them in a wooden pestle and mortar until a homogeneous paste is obtained which is eaten with soups or stews of meat or fish
The simplest method used and probably the most widespread certainly in Africa or Asia, for preparing flour from cassava is by sun-drying slices or chips of peeled roots which can then be stored as dried chips and ground into a flour when needed
A method for extending the storage life of chips to up to 12 months and also speeding up the drying process is by parboiling them before drying, a technique often used in India and West Africa
Amongst the most important products from Cassava roots are the coarse meals known as gari in West Africa
2. Yam
Yam, possibly the oldest cultivated food plant in West Africa, is of major importance to the economy of the sub-region that accounts for the bulk of world production of the crop. By far the most important product derived from white yam (Dioscorea rotundata Poir) is fufu or pounded yam, popular throughout West Africa.
Traditionally, pounded yam is prepared by boiling peeled yam pieces and pounding using a wooden mortar and pestle until a somewhat glutinous dough is obtained.
Production of Instant Yam Flour
Arising from the need to have a convenience food and reduce the drudgery associated with the preparation of pounded yam, various brands of instant yam flour are now available in West
Africa since the introduction of poundo yam, which is no longer in the market, by Cadbury Nigeria Ltd in the 1970s. Instant yam, on addition of hot water and stirring, reconstitutes into a dough with smooth consistency similar to pounded yam.
The product is so popular that considerable quantities are exported to other parts of the world, especially Europe and North America, where there are sizable African populations. Commonly, instant yam flour is produced by sulfiting peeled yam pieces, followed by steaming, drying, milling and packaging in polyethylene bags.
Instant yam flour can also be produced by drum drying cooked, mashed yam and milling the resultant flakes into a powder using a process similar to that used for production of dehydrated mashed potato.
3. Production of Fruit Juices
Fruits are normally eaten fresh when mature but for long preservation and wider distribution, they are processed into juice. The few fruits that can be processed into juice are pineapple, mango, guava, citrus fruits, etc.
1. Orange/Grape-fruit Processing
Fruits are sorted by hand to remove rotten, broken, diseased fruits. The sorted fruits are washed and may be de-oiled by passing the washed fruits through another metal column consisting of a series of revolving graters which rasp the thin skin layer under jets of water.
The oil/water emulsion is then collected, screened and centrifuged to recover the oils. The juice is extracted by pressing; and screened to obtain a cleaner juice. Granulated sugar may be added if necessary. The juice is pasteurized, filled into sterilized cans, closed, cooled and dried for packing and storage.
Stored commodity includes crop produce/products, dried and smoked fish, meat, tides and skins, tuber products and household goods e.g. clothes, carpets, books etc. Crop produce obtained from field and horticultural crops are grouped into four classes:- durables, semi-durables, perishables and others.
2. Oil fruits
Since the oil palm gives the economically most important tropical oil fruit, the technologies for its extraction can serve as an example in this category.
In the traditional process, the fruit is first removed from the bunches generally after the bunches have fermented for a few days.
The fruit is then cooked and pounded or trampled. The mashed mass is mixed into water. The oil and oil containing cell material is separated from the fibre and the nuts by rinsing with excess water and pressing by hand.
The oil-containing mass, now floating on the top, is collected and boiled. In this step, the oil separates from the rest and collects on the surface. It is skimmed off and finally dried.
The actual execution of the process may vary somewhat from area to area; most traditional processes, however, have in common the superfluous use of water. Using this process, generally not more than 50% of the oil is obtained.
The problems are:
The digestion by means of pounding or trampling,
The separation of the oil and oil containing material from the fibres and the nuts by means of water and the liberation of the oil by cooking afterwards.
The potential for improvement of this technology and thereby the development of small scale extraction equipment in principal depends on;
Better cooking by means of steam,
Better digesting using a reheating step with steam and
Effective pressing in a batch press or continuously working screw press.
The modern process of extracting palm oil, used on a larger scale, starts with the steam sterilization of the bunches. The bunches are threshed and the fruit is digested mechanically, while heated with steam.
The mass is then pressed in hydraulic presses or continuously in screw presses. The oil is separated from the press fluid by heating and is finally dried.
3. Oil seeds
In addition to the distinction made between traditional and modern methods, the processes for oil seeds should also be divided into so-called wet and dry extraction methods. Of the traditional wet processes, the extraction of coconut oil from fresh coconuts is the best known.
It starts with grating the meat, after which the oil as well as the proteins and impurities are extracted as milk from the fibrous residue by pressing (by hand or foot) and rinsing with fresh water.
The milk is left to stand to form an oil rich cream on top. The cream is boiled to separate the oil from water and other impurities. The oil can be skimmed off. It still contains a protein- rich residue that can be filtered off after drying and used for human consumption.
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Other oil seeds, like groundnuts, palm kernels and Shea nuts are roasted and crushed as fine as possible (e.g. first by pounding, followed by crushing between stones or a stone and an iron bar).
The crushed mass is mixed with water, and the oil is obtained by cooking the mixture, causing the oil to float. The oil is finally skimmed off and dried by heating. Sheanut oil is often obtained by beating air into a mixture of crushed seeds with some water using a hand-operated butter making process.
The milk or cream floating on top of the beaten mass at the end of the process is then cooked to evaporate the water and dry the oil.
The weak points of these processes are the grating or crushing steps. They are time consuming and exhausting work, yet crushing is generally not fine enough. Thorough crushing can improve the oil recovery considerably. In many areas, engine-driven disc mills are used by women in small commercial enterprises to get their seed crushed.
In summary, ineffective or inappropriate food processing technologies, careless harvesting and inefficient post-harvest handling practices, bad roads, moribund rail systems, bad market practices and inadequate or complete lack of storage facilities, packing houses and market infrastructures are some of the factors responsible for high post-harvest food losses.
The common element in all sectors of the food processing is conversion of raw material into a product of higher value. In some situations, processing is a one-step conversion of raw material to a consumer product.
The history of food processing lay emphasis on the role of establishing and maintaining microbial safety in foods, as well as the desire to establish and maintain economic shelf-life for foods. Processing methods are used worldwide as they improve the digestibility and nutritional quality of the grains.
Read Also: Characteristics of Hazardous Wastes
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