By Prof. B. M. Devani, Prof. B. L. Jani and Dr. S. P. Cholera*
Food, one of the basic needs for human comprising of various categories like grains, fruits, vegetables, dairy products,is the important part of daily diet and supplies all the macro and micro nutrients for the growth and survival if taken in balanced way. Grains, commonly referred to as cereals (maize, wheat, millet, rice, millets), pulses (beans, peas, cowpeas), oilseeds (soybean, sunflower, linseed) are the edible seeds of specific grasses belonging to the Poaceae family with or without outer layer while the grains like amaranth, buckwheat and quinoa are known as pseudo-cereal grains as they do not belong to the same family but nutritionally they are at par or superior to the grains.
Grain: A nutri-rich treasure
Nature has designed the grains in a way that balance diet along with fruits and vegetables and supply the requisite nutrients to our body. Cereals are dominant in low cost energy source carbohydrates while pulses are predominant in protein and oilseeds are cherished with lipids which are important macro nutrients of human diets.
What is the grain processing?
Processing of agricultural commodity expressly grains is an important practice for their further use asraw or ingredient. Grains undergo various post-harvest pre-treatments and processing steps to make them available for final consumption. The post-harvest processing of grains falls in different phases. In the primary processing the grains are cleaned, graded, sorted, dehusked/dehulled, milled, and converted into edible form. The secondary processing uses the primary processed grains using various steps to convert it into value added edible food products.
Grains are naturally covered with cellulosic materials known as hull or husk which are non-edible fibers for human consumption. To make the grain edible and ready for the further processing, onset the grains are passed through various cleaners and graders with the aim of removing impurities and grading the grains into different quality fractions. The sound grains are dehusked for further processing through conventional or advanced mechanical methods. The grains are then dried to obtain optimum recommended moisture content and packed carefully in suitable packaging material for storage in warehouse/godownsuntil further use for consumption or processing. During the storage period, the suitable care has to be taken to protect the grains from external or environmental factors such as rodents, mice, moisture, temperature, etc. GrainPro Cocoon is one of the advanced solutions (made of flexible UV-resistant Polyvinyl chloride) to safely store the dry grains like agricultural commodities without need of chemicals. Also, the advanced techniques of grain storage involve use of silo having the capacities of around or more than 100 tones.
Traditional and modern grain handling
Traditionally, the grains are stored in bags without proper cleaning and drying, and handled manually. The advancement in the technologies, benefits the handling and storage life of grains. The grains are now processed scientifically and stored either in godowns or in silo and conveyed using conveying mechanism. The wide varieties of grain handling mechanisms are available namely bucket elevator, belt, auger or screw conveyor, pneumatic conveyor, etc. to convey the grains.
Post-harvest management of grains
Post-harvest management of grains encompasses the usages of steps reducing post-harvest losses during handling, transportation, storage, turning the grains in to value added preserved products with modern scientific technology. Thermal, non-thermal, chemical and biological technologies, together with other technologies are used to increase the storage life of grains. General steps for processing of grains after harvesting, transportation and threshing includes drying, cleaning, storage, milling into various fractions and value addition in to different products.
The grains are harvested at around 20-25% moisture content at which they are highly susceptible to fungal contamination and deterioration. For safe storage and increased shelf life it has to be dried to within the range of 13-15% moisture content. Storage temperature and grain moisture content have strong influence on grain quality and shelflife. The conventional method of drying under sun is also in use at some corner which has disadvantages like time consuming, losses during drying, non-uniformity in drying, poor end quality, weather dependent, etc. The grains are dried before storage to retard microbial and pest growth in a continuous flow or batch drying process. Moisture can also be controlled at the storage silo throughin-storage dryer or aeration drying. Different directions of air flows in relation to the grain are used in Continuous flow dryers like cross-flow (e.g. screen dryers), mixed-flow (e.g. rack dryers) and concurrent-flow /counter-flow (e.g. tower/column dryers) before storage. Bin drying is a process where the grains are dried while they are stored in silo which is known as in-storage or aeration drying.
All the methods of drying aim to maintain moisture content at desired level however they use specific control systems. Control of moisture content can be performed manually or automatically such as feed-forward controllers, later being expensive but accurate. In advanced grain drying system, commonly used sensors include thermocouples and resistance thermometers (for controlling air temperature); infra-red pyrometers (for controlling product surface temperatures); and wet-bulb and dry-bulb thermometers, resistance sensors and absorption capacitive sensors (for de-humidifying the air).
Milling is the process separating different pieces that makes up the grain. The grains are milled in two ways, i.e. dry milling and wet milling. Dry milling consisting of grinding and shifting is the oldest way of milling grains to obtain milled fractions of the grains. Wet milling of grains comprises of milling of soaked grain, followed by separation of starch, protein, fiber and oil. Corn, rice, wheat, and other grainsare processed by dry milling and yield into important products such as flour, grits, semolina, etc. depending up on particle size. First, the grains are cleaned using various types of cleaners or separators to remove impurities from it. The various cleaning machineries used are magnetic separator that removes ferrous metal particles, disc orsieve separator that removesimpurities size based (i.e., straw), an aspirator to remove lighter impurities (i.e., dust), destoner that separates materials based on density difference (e.g., stones) but of the same size as thedesired grain and also color sorters.
After cleaning, the grains are conditioned or tempered using controlled amount of water for achieving moistened kernel, to soften the inner endosperm and harden the outer bran to improve the gradual separation during milling and sieving efficiency. Generally, thesoaking time and temperature of grain kernels can vary depending on the type ofgrain, the variety, and also the initial moisture level. Wheat is milled to separate endosperm from the bran and germ to obtain various products such wheat flour, refined flour, semolina, bran, germs, etc. This process uses break rollers which break wheat kernel and remove the endosperm and germfrom the pericarp.The break material consists of bran, sizings (the coarsest part of the endosperm), middlings (finer particles of endosperm that require further reductionto yield the flour), and break flour fractions by reduction rollers. Then different fractions are separated using plan shifters and purifiers. Paddy is milled to obtain brown rice, white rice. The parboiled paddy is dehusked, polished, ground and white rice, rice bran etc. are obtained. The rice bran is important by-product of paddy milling process and source of good quality oil.
Storage of grains take many forms, ranging from piles of unprotected grains on the floor, underground pits or containers, and piles of sacked grain, to storage bins of many sizesand shapes. Consumer packages for grain commonlyconsist of heat sealed pouches made from LDPE provides better moisturebarrier supports required shelf lives for the grains. Bags, made from cotton twill or paper, have been used successfully since a long for consumer packs of flour. Kraft paper bags with an LDPE liner provide additional protection, thus, a longer shelf life.
Through primary processing, the sound grains are processed and milled to various fractions as per the consumers’ requirements. As moisture content and temperature are the major factors affecting shelf lives of grains, causing lipid oxidation, growth of insects and deterioration, to avoid it, they must be stored at optimum storage conditions. Flours, dals, intermediates are the end products which will be utilized as base materials for daily diets and various processed food products for delicious dishes, bakery and confectionary items, and such food articles.
* The author is from Department of Processing and Food Engineering, College of Agricultural Engineering and Technology, Junagadh Agricultural University, Junagadh and can be reached at firstname.lastname@example.org