Post-harvest Handling of Soybean and Groundnut

1Soybean: from field to store

1. Harvest on time: when most leaves have yellowed and dropped and about 95% of the pods are brown and dry. Harvest early in the morning if pods shatter easily in the afternoon heat.
2. Cut or pull the plants and carry them to a clean drying area.
3. Dry the plants on a tarpaulin, mat or raised rack in the sun, not on bare soil.
4. Thresh by beating the dry plants in bags or on a tarpaulin with sticks, or with a small threshing machine. Avoid threshing on bare ground, which adds soil and stones and splits seeds.
5. Winnow and clean: remove chaff, pods, stones and dust.
6. Sort and grade: remove broken, shrivelled, mouldy and discoloured seeds.
7. Dry to a safe moisture content of about 12% or less for storage. A dry seed cracks when bitten and rattles in the bag.
8. Store in clean bags on wooden pallets, off the floor and away from walls, in a dry, cool, ventilated store; or in hermetic (airtight) bags that suffocate storage insects.

Processing soybean: raw soybeans contain substances (trypsin inhibitors) that block protein digestion, so soybean must always be cooked, roasted or fermented before it is eaten. Products include soy flour (for porridge and baking, often mixed with maize or sorghum flour), soy milk and tofu, roasted soy snacks, soybean oil, and soybean cake, a protein-rich animal feed left after oil extraction.

2Groundnut: from field to store

1. Harvest at maturity: leaves turn yellow, and when a few pods are opened the inside of the shell is darkened and the seed coat has its full colour. Early varieties are often ready about 90–130 days after planting. Too early: many pods are immature. Too late: pods break off and stay in the soil.
2. Lift the plants carefully with a hoe, loosening the soil so pods are not left behind or damaged.
3. Dry in the field: turn the plants upside down (pods facing up) in small heaps or on racks so air and sun reach the pods and they do not touch damp soil.
4. Strip the pods from the dried plants.
5. Dry the pods quickly on tarpaulins or raised racks until the kernels rattle in the shells.
6. Sort: remove damaged, broken, shrivelled, mouldy and discoloured pods and kernels. This single step removes most of the aflatoxin risk.
7. Store in the shell, fully dry, in clean bags on pallets or in hermetic bags, in a cool, dry store protected from rats and insects. Shell only shortly before sale or use.

3Aflatoxin: why groundnuts must be dried and sorted

Aflatoxins are poisons produced by the moulds Aspergillus flavus and A. parasiticus, which grow on groundnuts (and maize) that are damaged, stressed by drought, dried slowly or stored damp. Aflatoxin cannot be seen, smelled or tasted, and it is not destroyed by cooking or roasting. Eaten regularly, it damages the liver, can cause liver cancer, and is linked to stunted growth in children. Many countries destroy or reject groundnut lots above a legal limit, so contaminated nuts also lose their market.

Prevention: harvest on time; avoid damaging pods; dry quickly and never on bare soil; sort out damaged and mouldy nuts; store dry and cool; never feed mouldy nuts to people or animals.

Processing groundnuts: roasted nuts, groundnut paste (peanut butter) used in sauces, groundnut oil, and groundnut cake for animal feed.

Worked example: drying soybean

Question: A cooperative harvests 500 kg of soybean grain at 18% moisture. It must be dried to 12% moisture for storage. (a) What will the grain weigh after drying? (b) How much water must be removed?

(a) The dry matter does not change. Dry matter = (100% − 18%) of 500 kg = 0.82 × 500 = 410 kg. After drying, dry matter is 100% − 12% = 88% of the mass, so mass = 410 ÷ 0.88 ≈ 465.9 kg.

(b) Water removed = 500 − 465.9 ≈ 34.1 kg.

Interpretation: the cooperative "loses" about 34 kg of weight, but it is only water. Selling or storing wet grain would risk mould and weevils and could lose the whole lot.

Practice