The common factor across all sectors of food processing is the conversion of raw materials into products of higher value. In certain cases, processing involves a single-step transformation of raw materials into consumer products.
The history of food processing emphasizes establishing and maintaining microbial safety in foods and ensuring economic shelf-life. These processing methods are used globally because they improve the digestibility and nutritional quality of grains.
Factors Affecting Crop Product Processing and Post-Harvest Losses in West Africa
Inefficient or inappropriate food processing technologies, careless harvesting, poor post-harvest handling practices, bad roads, non-functional rail systems, poor market practices, and lack of storage facilities, packing houses, and market infrastructure contribute significantly to high post-harvest food losses in West African countries.
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Processing Methods for Cereal Crops
Cereals can be divided into components during processing for food or industrial use. The grain may be de-husked (hulling) to remove the outer coat or hull, exposing the endosperm. De-hulling separates the bran or seed coat and the germ. After de-hulling, grains may be ground (milled) into smaller particles, improving digestibility and expanding usage. Typically, cereal grains are processed in dry form to produce other products.
Some cereals are prepared as whole cereal meals or de-corticated forms. Whole-wheat flour production uses a simplified process, but most demand targets white flour production. White flour is the primary product of flour milling.
The goal is to extract the maximum amount of endosperm from the wheat berry in a pure form. The bran layers become a co-product called wheat feed.
The germ, an embryonic part, is also separated and, if a market exists, is sold as a high-value co-product; otherwise, it joins wheat feed as animal feed. These co-products enhance milling profitability. Efficient separation of starchy endosperm from other parts of the grain is key to milling success.
The global standard flour milling process is the gradual reduction system. This system transforms whole wheat berries through a series of grinding and sieving steps to produce white flour of desired quality and yield.
It allows for flours with low ash content and high yields. Specialty high-quality flours result from extracting pure sub-products.
The process has three main divisions: the breaking system, the purification system, and the reduction system. Some millers omit the purification system, replacing it with a sizing system, but the breaking and reduction systems are present in all operational gradual reduction mills.
The breaking system achieves most endosperm separation using roller mills with saw-tooth profiles. The rollers rotate at different speeds toward each other.
Types of Cereal Meals and Flours
Whole Meal Cereal: Obtained by grinding the entire cereal grain without sieving to remove parts such as the hull or germ. The resulting meal has 97–100% extraction and is called whole meal, named according to the cereal used (e.g., whole maize meal, whole wheat meal).
Coarsely ground products are cereal grits, while finely ground products are cereal flour. Maize or sorghum “rice” is made by milling grains to rice-sized particles, used similarly to rice. Higher extraction rates retain more nutrients.
1. Decorticated Meal: Also known as bolted meal or flour, depending on particle size. This meal is ground finely and passed through a sieve to remove some hull portions, retaining most germ in the meal, with extraction rates of 90–96%.
2. De-germinated Meal: A highly refined cereal meal where most hull and germ are removed by passing through mesh sieves. Extraction rates are 85% or less, depending on removal level. Coarsely milled cereal cleaned of bran or hull and germ is called de-germed grits. The germ recovered is rich in oil, from which corn oil is extracted using organic solvents. Residues after oil extraction are protein-rich and used in animal feed. The hull removed during milling is cereal bran, which provides dietary roughage to prevent constipation and may be added to porridges or pastes.
Polished Cereal Products
Polished cereals result when the hull, underlying layers, and germ are removed during dry milling (e.g., polished rice). Although less nutritious, polished products appear more attractive to consumers and have improved storage stability.
Industrial Processing of Cereal Grains
Industrial processing of cereal grains (such as wheat) involves separation into fractions of different qualities by crushing, pulverizing, and sifting. Grains are cleaned and conditioned by moistening to toughen the bran and facilitate separation.
The “breaking” process uses corrugated rollers to crush kernels into coarse fragments, separating seed coats from endosperm. The fine particles passing through flour sieves are called break flour, while larger endosperm fragments left behind are middlings.
Purification uses air currents to remove small bran particles mixed with middlings. Middlings may be sold as farina (breakfast cereal) or semolina (used in macaroni production).
Processing of Roots and Tubers
Roots and tubers such as yams, sweet potatoes, Irish potatoes, and cocoyams are often prepared by peeling or paring to remove the outer skin, then cut into large or small pieces or sliced.
These crops tend to darken (brown) when their cut surfaces are exposed to air due to enzymes reacting with compounds in the raw tissue in the presence of oxygen.
This enzymatic browning can be prevented by stopping the cut surfaces from contacting oxygen. Covering the pieces with water, weak salt solutions, sugar solutions, or acidic solutions like diluted lemon juice, vinegar, or fruit juices (pineapple juice) helps reduce browning.
Acidic solutions are especially effective because increased acidity slows down the browning reaction. Another method involves applying reducing agents like sulphur dioxide or ascorbic acid, which chemically react with oxygen, making it unavailable to the cut surface and preventing discoloration. These agents are widely used industrially for roots, tubers, fruits, and vegetables during processing.
Cassava
Cassava processing varies widely by region, but basic methods like boiling, roasting, or baking are common everywhere. Peeled cassava roots can be boiled whole or sliced for direct consumption. Roasting, usually done by placing whole roots in fire ashes, is less common than boiling.
Frying cassava slices in oil was introduced by Europeans and is now practiced in some areas. In places like Vanuatu, grated cassava is wrapped in banana leaves and baked.
A popular African method involves pounding boiled or steamed cassava roots into a smooth paste known as fufu (or foofoo, fuifai, foufou, foutou, vou-vou depending on locality). This paste is typically eaten with soups or stews.
The simplest way to produce cassava flour is by sun-drying peeled root slices or chips, which can be stored and ground when needed.
Parboiling chips before drying improves drying speed and extends storage life up to 12 months, a practice common in India and West Africa. One important cassava product in West Africa is gari, a coarse meal made from processed cassava roots.
Yam
Yam is a major crop in West Africa and is likely the oldest cultivated food plant in the region. The main product derived from white yam (Dioscorea rotundata) is pounded yam or fufu, a staple food popular across West Africa. Traditional preparation involves boiling peeled yam pieces and pounding them into a glutinous dough using a wooden mortar and pestle.
Production of Instant Yam Flour
To reduce the labor-intensive process of pounding, instant yam flour was developed and is now widely available in West Africa. This product, introduced in the 1970s by Cadbury Nigeria Ltd (formerly “Poundo Yam”), can be reconstituted with hot water into a smooth dough similar to pounded yam.
It has gained popularity internationally, especially in regions with African populations like Europe and North America. Instant yam flour production involves sulfiting peeled yam pieces, steaming, drying, milling, and packaging in polyethylene bags.
Another method uses drum drying of cooked mashed yam, followed by milling the dried flakes into powder, similar to dehydrated mashed potato production.
Production of Fruit Juices
Fruits are usually eaten fresh but are processed into juices for longer preservation and wider distribution. Common juice fruits include pineapple, mango, guava, and citrus fruits.
Orange and Grapefruit Processing
Fruit processing starts with sorting by hand to remove rotten or diseased fruits, followed by washing. The fruits may be de-oiled by passing through revolving graters under water jets, removing essential oils which are then recovered by centrifuging.
Juice extraction is done by pressing, and the juice is screened for cleanliness. Sugar may be added to taste. Finally, juice is pasteurized, canned in sterilized containers, sealed, cooled, and dried for storage.
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Oil Fruits
The oil palm is the most economically important tropical oil fruit, and its extraction techniques serve as a model in this category.
Traditional Palm Oil Extraction
The fruit is removed from bunches, often after fermentation. It is cooked, pounded or trampled, mixed with water, and the oil-containing mass is separated by rinsing and pressing.
The floating oil is collected, boiled to separate it fully, skimmed off, and dried. This method uses excessive water and yields about 50% oil recovery. Problems include manual digestion and inefficient oil separation.
Modern Palm Oil Extraction
Modern processes use steam sterilization, mechanical digestion with steam heating, and hydraulic or screw pressing. Oil is separated from press fluid by heating and drying. This process improves efficiency and oil recovery.
Oil Seed Processing
Oil seeds like coconut, groundnut, palm kernel, and shea nuts are processed by wet or dry extraction methods.
- Wet extraction: Coconut meat is grated, pressed or rinsed with water to obtain oil-rich milk, which settles and is boiled to separate oil.
- Dry extraction: Groundnuts, palm kernels, and sheanuts are roasted and crushed, mixed with water, cooked to release oil which is skimmed and dried. Shea nut oil is extracted by beating crushed seeds with water, then cooking the cream to dry the oil.
The main weaknesses of traditional methods are the time-consuming and inefficient grating or crushing steps. Using engine-driven mills improves crushing and oil recovery in small commercial enterprises.
Ineffective food processing technologies, careless harvesting, poor post-harvest handling, inadequate storage facilities, poor transportation infrastructure, and inefficient market practices all contribute to high post-harvest food losses in West African countries. Improving these areas is essential to reduce losses and increase food availability and farmer incomes.
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