Legume Processing Techniques and Applications

Legumes belong to the family Leguminosae and are the second most important food crop after cereals in the tropics. They serve as sources of low-cost dietary vegetable proteins and minerals compared to animal products such as meat, fish, and eggs.

Indigenous legumes are a vital source of affordable alternative protein for resource-poor people in many tropical countries, particularly in Africa and Asia, where they are predominantly consumed. In developing countries, research focuses on improving legume utilization to address protein malnutrition and enhance food security.

Classification of Legumes

Legumes can be classified as:

1. Pulses or Grain Legumes: These include various peas and seeds low in fat content, such as cowpea, pigeon pea, and bambara groundnut.

2. Oilseeds: Examples include soybean and groundnut.

3. Forage Leguminous Crops: These include Mucuna pruriens and Psophocarpus tetragonolobus (winged bean).

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Overview of Legume Processing

One major way of utilizing legumes is through food processing, which involves techniques to convert raw materials into semi-finished or finished products suitable for consumption or storage.

Food processing occurs at different levels, including home-based and industrial processing, with industrial processing ranging from cottage to large-scale operations. The advantages of legume processing include:

1. Transformation of raw produce into edible forms.
2. Improving digestibility of foods.
3. Enhancing the nutritional quality of foods.
4. Reducing and eliminating anti-nutritional factors.
5. Improving consumer appeal and acceptability of foods.
6. Destroying food enzymes that cause spoilage, thus extending shelf life.
7. Deactivating spoilage and pathogenic microorganisms in food products.
8. Serving as a means of income generation.

Common Legumes and Their Processing

Common legumes grown in the tropics include cowpea, soybean, pigeon pea, African yam bean, bambara groundnut, kidney bean, and lima bean. From the harvesting of pods in the field, legume seeds undergo common post-harvest processes to obtain dried seeds.

These dried seeds are further processed into semi-finished or finished products through several processing steps, referred to as unit operations, each serving a specific purpose.

1. Primary Unit Operations in Legume Processing

i. Sun-Drying: Raw mature grains at harvest have approximately 20% moisture content and are prone to spoilage unless dried. They are enclosed in husks or pods, which are removed manually or mechanically.

In some regions, grain legumes are steeped in water for 2–8 hours before sun-drying. Seeds are dried on raised platforms. In certain cases, grains are treated with oil before drying to aid the dehusking process.

ii. Husking: Also known as hulling, this process can be performed using dry or wet methods. In African and Asian countries, the dry method traditionally involves pounding dried grains in a mortar with pestles or using hand-operated wooden or stone shellers.

Improved power-operated shellers and abrasive hulling machines have been developed to enhance the hulling process. The wet method involves soaking grains before drying, with conditioning techniques through moisture adjustment to facilitate easier husking.

iii. Winnowing: Separated husks are removed from cotyledons by winnowing, which can be done manually—a time-consuming and laborious process—or with improved abrasive hulling machines that separate husks from cotyledons.

iv. Separation: This process removes whole grains from split, broken, or powdery ones, either manually using sieves or mechanically with machines equipped with sieving devices. Manual sieving is laborious and time-consuming.

v. Storage: Proper post-harvest handling prevents both qualitative and quantitative losses. Legumes must be dried to a safe moisture level of 12–14% to ensure proper storage. Table 1 shows the safe moisture content for some tropical legumes.

Dried seeds with high moisture content are susceptible to mold attack and infestation. Dehulled seeds are dried to a safe moisture level, and milled seeds are packaged and stored under dry conditions to prevent moisture absorption, which could lead to spoilage.

Safe Moisture Content of Tropical Legumes

2. Secondary Processing Techniques for Legumes

i. Sorting and Cleaning Before Use: Legumes are sorted and cleaned to remove dirt, stones, chaff, broken or spoiled seeds, and other foreign materials. Sorting can be done by hand, which is laborious and time-consuming, or through mechanical or electronic sorting devices.

Cleaning is performed using dry or wet methods. Dry cleaning is used for grain legumes intended for storage, while wet cleaning involves washing with water.

ii. Soaking and Blanching: Seeds are soaked in water for varying periods to absorb water and reduce or eliminate anti-nutritional factors. However, prolonged soaking can reduce nutritional quality by leaching nutrients into the soak water.

Blanching involves a mild heat treatment by soaking seeds in hot water or boiling them for a few minutes, which destroys food enzymes and some anti-nutritional factors, and aids dehulling.

iii. Boiling/Cooking and Roasting: Boiling at 100°C for several minutes tenderizes seeds through water absorption, eliminates heat-labile anti-nutritional factors (e.g., trypsin inhibitors), and improves sensory properties.

Traditionally, beans are cooked using firewood, but pressure cooking pots reduce cooking time. Roasting, done on an open frying pan with or without salts or ash, improves taste, edibility, and reduces anti-nutritional factors, giving roasted legumes unique flavors that enhance sensory appeal.

iv. Fermenting and Germinating: Fermentation increases protein digestibility, reduces anti-nutritional factors (e.g., phytate), and enhances flavor, color, and texture, making it a key process in developing fermented legume products like condiments (e.g., fermented locust bean, known as iru).

Germination improves digestibility, reduces anti-nutrients like trypsin inhibitors, and enhances nutritional quality by hydrolyzing proteins into absorbable polypeptides and essential amino acids.

Germinated or malted legumes are consumed as sprouts, which are richer in vitamins B and C and have reduced polyphenol content. Chickpea and broad beans are commonly germinated before eating, cooking, or use in salad dressings.

v. Milling and Sieving: Dehulled legumes may be wet-milled or dry-milled. Wet-milling produces a paste, while dry-milling results in flour. Various equipment has been designed for household or industrial milling.

Wet-milled legumes may be mixed with other ingredients and steamed in leaves to produce pudding (moinmoin) or fried in hot oil to make bean cake (akara). Rehydrated flour can also be used for these products. Sieving removes unwanted materials from ground legumes.

Wet sieving, used in soymilk production to filter ground soybean paste and remove residue (okara), employs cheesecloth or muslin cloth. Dry sieving, using local or standard sieves, achieves different particle sizes, and some milling equipment incorporates sieving devices.

vi. Frying and Canning: Wet-milled legumes are mixed with ingredients to prepare local or oriental dishes. Frying improves appeal, eating quality, digestibility, and reduces anti-nutritional factors. Canning, a sophisticated technology, packages cooked beans in brine, sugar, or tomato puree, ensuring year-round availability and preservation, though it is expensive.

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Household Utilization of Tropical Legumes

1. Cowpea:

i. Cooked Beans: Whole or dehulled cowpea beans are cooked, with whole beans requiring 45–60 minutes on a stove or gas cooker, depending on hull hardness. They are eaten whole, mashed, or combined with foods like bread, gari, boiled yam with vegetable soup, or fish/meat sauce.

ii. Bean Soup: Beans are washed, soaked, dehulled, boiled, mashed, and sieved, then cooked with palm oil, pepper, spices, seasoning, and fresh or dried fish to produce gbegiri, a soup eaten with reconstituted yam flour product (amala).

iii. Bean Cake and Pudding: Beans are washed, soaked, dehulled, and milled into a paste. For bean cake (akara), the paste is mixed to a fluffy texture, incorporating air, with milled onions and pepper, then fried in oil. For pudding (moinmoin), the paste is mixed with vegetable oil and other ingredients, traditionally packaged in leaves, and steamed.

iv. Soybean: Soybean is an excellent source of protein, fat, and minerals, particularly calcium, and is processed into various products, serving as a means of income generation and food security for many economically challenged families in Nigeria.

v. Soymilk: Processed by soaking soybean in water, milling, sieving, boiling, and adding sugar and desired flavors, soymilk is rich in protein, fat, and minerals. Its beany flavor can limit consumption.

vi. Soy Cheese: Known as tofu in the Orient and soya-wara or soy warankasi in Nigeria, this highly digestible product is ideal for lactose-intolerant individuals. It is made by preparing soymilk and precipitating it with coagulants such as enzyme-based Calotropis procera leaf water extract, acidic lime or lemon juice, or fermented maize water liquor (the most common). Some processors use alum.

vii. Tempeh: Originating from Indonesia, tempeh is made from soaked, dehulled, and partly cooked whole soybean seeds mixed with Rhizopus oligosporus spores, spread thinly on a tray, and fermented for 24–36 hours at 30°C. Good tempeh has a firm texture, can be cut, soaked in brine or salty sauce, and fried. It can also be made from other beans or mixed with whole grains.

viii. Soy Yoghurt: A fermented product made from soybean, cheaper than milk-based yoghurt, it is a good source of protein and minerals, produced using Lactobacillus bulgaricus and Streptococcus thermophilus cultures.

ix. Soy Sauce: A condiment common in East and Southeast Asia, it is processed by fermenting soybean seeds with Aspergillus oryzae and Aspergillus sojae molds in the presence of salt and water, yielding moromi, which is pressed to obtain soy sauce, also called miso (sometimes made from rice or barley).

2. Groundnut:

• i. Boiled and Roasted Groundnut: In West African countries, groundnuts are cooked with pods to produce boiled groundnuts, while shelled or unshelled groundnuts are roasted with or without salt for direct consumption. Roasted groundnuts can be ground into powder for sauces or as ingredients in other dishes.
• ii. Peanut Butter: Dry-roasted groundnuts (peanuts) are ground into a smooth paste, with stabilizers (partially or fully hydrogenated vegetable oil), sweeteners, spices, emulsifiers, and salt added, commonly used as a sandwich spread.

3. African Locust Bean:
i. Dawadawa: Fermented African locust bean, known as dawadawa or iru, is a traditional Nigerian condiment. The hard seeds are cooked, dehulled, spread thinly in containers (often calabash lined with leaves), and fermented for 24–36 hours.

Traditionally cooked overnight over firewood, dawadawa has a characteristic ammoniacal smell and unique flavor, used as a natural seasoning in soups, stews, and traditional delicacies. It can also be processed from soybean, bambara groundnut, or pigeon pea seeds.

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