Prolonged storage impairs the industrial quality of wheat: Measures to control it
Gupta O.P.*1, Venkatesh K.2, Gupta R.K.1, Sharma I.3
With the increasing ever growing global population, maintaining the nutritional quality of wheat is emerging as a crucial step forward. Nutritional deterioration starts right from the field before harvesting the crops to the storage and the final consumption (processing loss). The wheat grain contains 2-3% germ, 13-17% bran and 80-85% mealy endosperm. The wheat bran is mainly rich in fiber (53%), vitamin B complex and minerals which are lost from the starchy endosperm during the first stage of milling. The endosperm mainly contains energy-yielding starch. The germ lies at one end of the grain which is rich in proteins (25%) and lipids (8-13%).
The production of wheat varies from year to year therefore the produce should be stored strategically from years of over production for the use in year of under production. The primary factors affecting the grain quality in storage are moisture, temperature and humidity of storage environment. The safe moisture content for storage of wheat is 12-14% at 25°C and 75% relative humidity. Other factors responsible for deterioration are poor containers/warehouses, feeding by rodents, insect pests and microorganisms. Improper post harvest storage of wheat grains can cause losses in both quantity and quality viz. depletion in seed viability, hardness, color, size and shape, grain weight and various biochemical parameters viz. protein, carbohydrate and vitamins.
Effects of prolonged storage on protein quality
About ~85% of wheat protein is comprised of gluten which is responsible for development of extensibility and elasticity in dough. High storage temperature (>30°C) leads to degradation of gluten and therefore limiting its functionality. Wheat grains maintain wet gluten content in 1st year of storage which starts degrading in the 2nd and subsequent years. Lysine is one of the essential amino acid required by human beings. Lysine is a major amino acid of gluten and is one of the limiting amino acid of the cereals. Total available lysine in freshly harvested wheat grains is 2.92% which decreases to various extents upon storage. According to a report around 10% of total available lysine was decreased at 25°C and 45°C after 3 and 2 month of storage which at same temperature further decreased to 18.0% and 22.6% after 6 months of storage respectively. Bioavailability of protein and starch when consumed depends on their digestibility. Prolonged storage duration (> 6 month) and elevated temperature (>40°C) leads to reduction in total digestibility of protein and starch thereby reduced bioavailability which leads to starvation/malnutrition.
Effects of prolonged storage on enzymatic activity
Amylase is a major enzyme responsible for mobilization of stored food reserve during seed germination present in the aleurone layer of wheat kernels. . Amylase activity in wheat grains decreases to various extents during storage at different temperatures. Decrease in amylase activity is extremely slow at 10°C whereas it is comparatively higher at 45°C than at 25°C during six months of storage.
Effects of prolonged storage on carbohydrates
Soluble sugars are important component because they protect the grain’s membrane integrity during dry condition. The grains stored at 25°C tend to increase the total soluble sugars by 12% which at 45°C tend to decrease drastically up to 37 %. The increase in the soluble sugars is a result of amylolytic activity of the endogenous amylases whereas the decrease in soluble sugars at 45°C is due to their involvement in Maillard reactions. Crude fiber is greatly considered undesirable in wheat products although required in human diet for many beneficial effects. The modern milling process reduces to minimize the crude fiber content of flour. Storage duration has no apparent effect on the availability of crude fiber.
Effects of prolonged storage on vitamins and ash
Thiamine is an important vitamin as it acts as a coenzyme in various energy transfer biochemical reactions during metabolism. Storage of wheat grains for 6 month at 25°C and 45°C results in reduction of thiamine contents by 21.4 % and 29.5% respectively. Ash is composed of minerals like, calcium, copper, iron, potassium, sodium and zinc. Besides providing nutrients in human diet, ash increases fermentation by providing nutrients to yeast, thus helping in baking. On the other hand minerals may act as co-factor for enzymes responsible for metabolism of food. Ash content of wheat flour does not differ significantly during the storage.
Effects of prolonged storage on lipids
Although a very small amount of lipids/fats (1-1.8%) is present in wheat, these are necessary for proper gluten development of flour. Duration of storage and elevated temperature of the storage has significant effect on the crude fat as a result of lipase activity in the grains which reduces total crude fat over the period of time. High grain moisture also decreases the availability of total crude fat. The organic material burnt after ignition of pre-digested flour in dilute acid & alkali, is called crude fiber. This includes 50-80% of cellulose, 10-50% of lignin and 20% of hemicelluloses. Fat acidity is important for baking quality of flour. Unsaturated fatty acids released during long term storage reduce baking quality of wheat flour by decreased water absorption and gluten swelling. The increase in the acidity in stored wheat grains is attributed to the increasing concentration of the free fatty acid because of lipase activity and phosphates which resulted from increased grain deterioration.
Fungal activity causes undesirable effects in grains including discoloration, contribute to heating and losses in nutritional value, produce off‐odours, losses in germinability, deterioration in baking and milling quality, and result in contamination by mycotoxins (Hocking, 2003). The most common storage fungus in the stored wheat grain samples is Alternaria alternata (14.6%). Maintaining the nutritional value and quality of various end products of wheat grains is of prime importance. Proper care should be taken right from the harvesting up to final consumption to minimize the nutritional losses. Storage of grains should be done at ambient temperature, moisture and relative humidity condition. The safe moisture content for storage of wheat is 12-14% at 25°C and 75% relative humidity.
Important measures to combat nutritional loss during storage:
There are a number of precautions, which should be followed during grain storage to minimize nutritional losses of grain.
- Optimum moisture content should be maintained in the storage.
- Proper pest control measures should be done prior and during storage.
- Bins, air ducts, perforated floors, etc. should be clean or vacuumed.
- Spoiled grain in and around storage and harvest facilities should be destroyed.
- Holes and leaks in bins should be patched to prevent grain loss, keep out the rain, and limit the access of rodents.
- Storage should be sprayed for insects well before harvest starts, especially if there is a history of insect problems in the storage facility.
- Proper sampling of the grain should be done before storing it which will help to keep track of grain condition before and after storage to identify potential problems.
- Amount of foreign material in the grain should be kept to a minimum level to avoid insect-pest infestation.
- Care should be taken to avoid mixing of newly harvested grain with grain that has been in storage which will reduce insects-pests.
- Regular observation of grain should be done to know the temperature and moisture level. Inspection every two weeks during the winter and once a week during the rest of the year is recommended to allow the opportunity for corrective action to be taken before significant grain spoilage occurs.
- Move grain that has started to heat. This will temporarily cool the grain. It can also be used as a temporary measure for the control of insects and mites, especially if the grain is moved during the winter when outside air temperature is cold. The grain should be cleaned and sold, treated with an insecticide or fumigated if insects are detected.
- Keep detailed records on the condition of the grain in storage. Accurate records can help in identifying potential storage problems and in planning preventative action.
Gupta O.P.*1, Venkatesh K.2, Gupta R.K.1, Sharma I.3
1Division of Quality and Basic Sciences, 2Division of Crop Improvement, 3Project Director,
Directorate of Wheat Research, Karnal-132 001, Haryana-India