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T.P. Ojha. Post-Harvest Technology Centre, Indian Institute of Technology, Kharagpur, India
Harvesting and threshing operations
Drying and curing operations
Milling of grain
Grain storage and preservation
The need for improving the high yielding varieties and also the application of fertilizers, irrigation water coupled with multiple cropping systems appeared a far more attractive approach to achieve higher yields during the 1970s. Hence, not enough attention was given to the post-harvest operations. But the limitations of the pre-harvest operations became quite evident in a short span of time resulting in that of an appropriate attention towards the reduction of losses in the post-harvest operations. The postharvest losses of food grains in developing countries are enormous. When the consequences of such losses are evaluated in terms of human sufferings and economic cost, they represent an international challenge. Successful prevention of these losses can be a tremendous help to the human race all over the world. The quantum of losses vary from country to country and also in the same country for different crops due to variations in environmental factors and human efforts utilized for prevention. Under Indian conditions, the post-harvest losses amount to about 10 per cent of the total grain yield. In a systematic study carried out on paddy crops, the losses appeared to be contributed by the following major post-harvest factors. (a) harvesting, 1 to 5 per cent; (b) transportation, 2 to 7 per cent; (c) threshing, 2 to 6 per cent; (d) storage, 2 to 6 per cent; (e) drying. 1 to 6 per cent; and (f) milling, 2 to 10 per cent.
Thus, these losses cumulatively could amount to as high as 40 per cent by weight and are normally accompanied by loss of quality also. Quality loss of grains can take place at all stages but more severely at the stage of drying, storage, and milling. These factors indicate the urgency of attaching the highest importance to the management of post-harvest problems, if the gains of higher production are to be fully exploited for the benefit of the community and the country.
Quality prevention during post-harvest operation in general and the same during storage in particular is also important for preserving the nutritive value of food grains. The grains undergo some chemical changes that alter the flavour and nutritive values.
The insect and fungi not only consume the grain but also accelerate the harmful chemical changes. Some fungi produce poisonous substance or toxins in grains. Aflatoxins are the most dangerous toxins produced by Aspergillus.
The quantitative losses can be caused by mechanical agents such as birds, animals, hailstorms, rains, over drying, and shattering in the fields during harvesting. Biological agents such as birds, rodents, mites, and insects are believed to be the most destructive agents of grains and seeds in the stores. The losses occurring due to other factors such as (a) changes in moisture content; (b) dust and broken grains; (c) reduction in germination power, (d) loss of palatibility; (e) heating and caking, etc. can render the grains unacceptable for use as human food or as animal feed.
The results obtained from various studies presented in the paper support the idea that substantial increase in nutritional food availability can be achieved through postharvest loss prevention at farm level as well as storage points in trade system.
Developing high-yielding varieties was definitely the most attractive route to higher production in the early 1970s. Not enough attention was attached to post-harvest operations, which were considered less significant. However, the limitations of pre-harvest systems became quite evident shortly thereafter. Staggering losses to the extent of 40 per cent or more were reported in many countries, and the importance of preventing post-harvest losses are now fully recognized.
Post-harvest losses were long considered to be contributed by storage alone, and not much attention was paid to losses occurring during harvesting, transport, drying, threshing, milling and pre-milling treatments. Quantitative loss of food grains was the main concern of the producers and the consumers. Qualitative losses at various stages were not fully recognized, but systematic studies by different organizations revealed that these losses are enormous and should be prevented.
Thus in paddy crops, the following major factors contribute to post-harvest losses (Ojha 1978b): harvesting, 1 to 5 per cent; drying, 1 to 6 per cent; transportation, 2 to 7 per cent; storage, 2 to 5 per cent; threshing, 2 to 6 per cent; milling and pre-milling, 2 to 10 per cent.
These losses cumulatively could be as high as 40 per cent and are normally accompanied by loss of quality also. Quality loss of products can take place at all stages but more particularly during drying or curing of harvested crops, storage, and milling and pre-milling treatments. Post-harvest attention is essential if the grains of higher production are to be fully exploited. Also the quality loss occurring in food grains can be extremely serious if the farming community is not aware of them. To illustrate the point, freshly harvested high-moisture paddy, bajra, or groundnut crops, if not dried to a safe moisture level, can become poisonous by the development of toxins through mould growth.
Harvesting is a short-period operation and a labour-intensive one. It takes about 170 to 200 person hours to harvest one hectare of paddy field. If harvesting is not initiated at an appropriate moisture content, the matured paddy or wheat grains are likely to shatter in the field or during subsequent movement to the threshing years. Such losses may be as high as 25 per cent depending on variety and stage of harvesting.
Harvesting Stage for Quality and Quantity
Farmers in general are not well aware of the advantages of early harvesting of field crops. Conventionally, most farmers in the mono-culture system of paddy cultivation harvest the paddy crop at about 16 per cent moisture content, and suffer a loss of 10 to 15 per cent of the expected field yield. A definite correlation between grain moisture and days after flowering has been established through field studies (Bal and Ojha 1977). About 0.36 per cent moisture was observed to be lost every day from the Jaya variety between the fifteenth and forty-fifth day after flowering. Figure 1 (see FIG. 1 a. Effect of Date of Harvest on Moisture Content at Harvest and Field Yield and FIG. 1 b. Effect of Date of Harvest on Total Yield and Head Yield of Paddy ) a shows a continuous moisture reduction from 16 to 46 days after flowering; beyond the forty-eighth day, moisture loss becomes negligible and the crop comes into equilibrium with the environment. Normally about a week would pass during which the grains are at a moisture content between 20 and 24 per cent. If the crop is harvested during that period, the losses are considerably reduced and the field yields are comparatively high. Maximum yields were actually recorded from the harvest carried out on the thirty-first and thirty-second day after flowering, all field yields being corrected to a fixed moisture content of 14 per cent. An appropriate period for harvesting paddy crops may therefore lie between the twenty-eighth and thirty-sixth day after flowering. This provides about nine days to complete the operation without sacrifice of either the yield or quality of the grain. If the crop is over-dried, sun-cracks develop on the kernel and cause breakage during threshing and milling.
Variation in Milling Quality
Milling quality of paddy is defined by the terms "total yield" and "head yield" (yield of full-size rice), both expressed as percentage weight of the original paddy. The total yield of the Jaya variety increased from 54 to 73 per cent between the sixteenth and thirtieth day after flowering. The poor total yield on early harvesting was a result of immature grains on the panicles. The total yield beyond the thirty-sixth day after flowering again showed a reducing trend because of cracks which had developed in the kernels. Thus study again supports the contention that the paddy crop should be harvested within 28 to 36 days after flowering. This would be the optimum harvest period for most high-yielding paddy varieties of India. Pillaiyar (1977) reported that the maximum yield could be obtained by harvesting paddy crops 15 days after ripening of the tip grain of the primary earhead. According to Khan (1976), harvesting should be carried out when the hulled grains on the upper portion of the panicles are clean and firm, when the grains at the base of the panicles would be in the hand dough stage.
Grain damage can be caused not only by delayed harvesting but also by improper handling subsequent to harvesting. Devnani (1976) and Chabra and Singh (1977) reported that over-dried grains are prone to fracture during threshing, whereas moist grains are likely to bruise through improper cylinder-concave clearance and high thresher cylinder speed. If not carried out properly, harvesting and threshing by machines can be more injurious to seeds than manual methods. Of course, threshing by beating with sticks can crack or break the grains into pieces and should be discouraged as a means of seed production.
The desirable moisture content at harvest appears to be between 20 and 24 per cent for paddy and 15 to 20 per cent for wheat. As the grain has to be stored for an extended period, it must be dried either in the sun or in a mechanical dryer utilizing heated air.
Harvesting and drying during rainy weather in the coastal areas, and in the eastern regions of high precipitation, have always posed a serious problem of quality deterioration in paddy crops. Mechanical drying is the only acceptable solution.
The continuous spell of humid and rainy weather prevailing during the harvesting period of the aus or ahu varieties of the north-east region and the kuruvai varieties of the Cauvery delta conduces to the growth and proliferation of microbes on paddy grains. Subrahmanyan (personal communication 1978) reported that the higher levels of micro-organisms observed in the flowering and milk stages of paddy might be due to the presence of a higher level of readily available nutrients in free form, like reducing sugars and amino acids. The chances of increased infestation and rapid multiplication, leading to heating and discoloration, become more acute where immature grains are not separated after threshing. Those organisms multiply rapidly when grains are moist and germinate. Such fungal forms as Mucor, Aspergillus flavus, A. niger, A. fumigatus and Penicillium are generally observed in paddy harvested in the wet season. The vegetative growth of these fungi ramify the grains and cause webbing. Fungi webbing and grain germination result in caking when moist paddy is bagged in gunny bags.
Recognizing the need for better drying facilities for both freshly harvested and parboiled paddy, mechanical drying systems were introduced in the late 1960s in India. These dryers were of Louisiana State University (LSU) design, with a holding capacity of 6 tonnes or more. As the heat-generating fuel was furnace oil, these dryers tended to be uneconomical. Later, a few portable dryers of small holding capacity that employed rice husk as fuel were developed and are being used commercially (Ojha 1978a). Among these, the RPEC recirculatory dryer and LSU dryer coupled with a husk-fired furnace gave good performance (see FIG. 2a. Pictorial View of Recirculating Batch Dryer Coupled with Husk-fired Furnace for Paddy Drying (Rice Process Engineering Centre (ITT), Kharagpur)). Bose et al. (1980) developed a system utilizing a fixed-bed LSU dryer coupled with a solar-cum-husk fired heating unit for drying high-moisture paddy. Since these dryers were of about one tonne holding capacity, they were adopted by smaller millers and others. The circulatory dryers can be successfully used to dry high-moisture (24 per cent moisture content) paddy to about 16 per cent m.c. in four passes of 30 minutes each. Paddy so dried gave a high head yield due to uniform drying of individual grains. Table 1 presents yield data after drying at various temperatures in mechanical dryers (Rama Rao 1974). Drying of high-moisture grain, either for seed or for human food, is essential. A typical cross-flow recirculatory drying system is shown in figure 2b (see FIG. 2b. Cross-flow Drier with Forced-air Drying and Cooling). Grain flows from the wet grain holding bin at the top, down the columns and is discharged at the bottom. The column of grain is held between perforated sheets and air passes through the column perpendicular to the grain flow. In one design, the entire column receives the heated air for removal of moisture, whereas in another, the upper portion of the columns are drying sections, and in the lower portion the grain is cooled. The flow rate is regulated by a metering device at the outlet of the column. The metering device can be either automatically or manually controlled depending upon the particular requirement.
TABLE 1. Performance Data for Drying Parboiled Paddy with Recirculatory Batch Dryer without Tempering
|Drying air temperature (°C)||100||90||80||70||60|
|Average total yield (%)||72.8||72.7||72.7||73.0||72.3|
|Average head yield (%)||52.0||50.0||55.0||58.5||61.6|
Source: Rama Rao 1974.
A multipass system is used in India for drying. Each pass takes about 30 minutes with the temperature of the heated air being 50 °C for raw paddy and as high as 90 °C for parboiled paddy. About 2 per cent loss of moisture occurs in each pass. Moisture removal can be accelerated by tempering the grain in a heap or in a silo at the end of each pass. The most common cause of cracking is believed to be either rapid drying or rapid moistening, or both. Cracks are a result of stresses developed in the grains, and they are more frequent in kernels of large sizes.
Soaking in cold water and drying in sun, soaking in hot water followed by sun drying or steaming, soaking, steaming, and drying are the pre-milling combinations practised in India to improve the milling, nutritional, and keeping qualities of rice. Some varieties of paddy possess poor milling quality for many reasons. The weather prevalent during field maturity and harvesting, or unscientific post-harvest practices in handling and storage, render the paddy grains fragile and lead to breakage during milling. Proper hydrothermic treatments, as earlier described, can improve the milling qualities of paddy. One such treatment is parboiling, which is given to about 60 per cent of all the paddy produced in India. Improved methods of parboiling include the three steps of hot-soaking, steaming, and drying. Parboiled paddy is produced either as a small scale, family trade, or at a commercial level in rice mills. It would not be feasible to discuss here the various parboiling methods practised in India. However, the quantitative and qualitative improvements rendered by parboiling will now be mentioned.
Nutritive Value of Parboiled Rico
The rice grain is composed of a surface layer called bran (itself consisting of the pericarp and aleurone layers), the starchy endosperm, and the germ. The surface layers are rich in protein, oils, and vitamins compared to the endosperm. Parboiling augments the nutritive value, through first translocating vitamins from the peripheral layers to the central core, and oil from the endosperm to the bran layers, followed by heat sealing them.
Milling Quality of Parboiled Rico
It is well known that parboiling reduces breakage during milling. Thus improvement is usually attributed to increased hardness, or the healing of sun-cracks and other defects resulting from biochemical changes. The percentage of brokers is drastically reduced and the head rice outturn is remarkably increased, as shown in table 2. Parboiling leads to an increase in total yield as well as head yield. The performance study report on rice mills reports an increase of about 6.6 per cent in total yield and over 15 per cent in head yield for a few varieties (Mathrani 1971). Varieties that give poor yields in raw milling often show excellent milling quality after parboiling.
TABLE 2. Head and Total Yields or Raw and Parboiled Rice of Mixed Varieties
|Type of rice||
|Raw||71.3||52.1||Modern mill of the Dandekar|
|Parboiled||73.0||53.9||type was used for milling.|
Source: Mathrani 1971
Bran from parboiled rice contains less starch and more oil than raw rice bran. The oil content of parboiled bran varies from 15 to 25 per cent, about 5 per cent higher than in raw bran.
Cooking Quality of Parboiled Rice
The cooking quality of rice may be defined as the ability of the grain to be cooked to the required texture or tenderness without losing shape. A grain that cooks to a fluffy, non-cohesive, tender texture, and retains its shape, while increasing in size, is said to have good cooking quality.
Losses in Weight during Perboiling and Drying
Large-scale field experiments carried out on commercial plants have indicated that weight losses during parboiling and drying varied from 0.85 to 1.75 per cent of the total input. The lowest percentage losses were observed with the modern parboiling methods, such as the CFTRI and pressure parboiling methods in which hot soaking of paddy is completed in less than four hours.
The term milling generally refers to the size-reduction of granular material, but for food grains the term has different connotations. Wheat milling means wheat grinding to prepare flour, maida, suji, etc. Rice milling includes operations like dehusking and polishing; dhal milling also involves processing operations such as husk separation, splitting of kernels and polishing, or polishing only.
TABLE 3. Evaluation of the Yields of Rice Obtained From Different Mills
|Items||Parboiled rice||Raw rice|
|Disc Sheller (%)||Rubber Roll Sheller (%)||Disc Sheller (%)||Rubber Roll Sheller (%)|
|Additional recovery over huller mill||0.8||1.6||4.1||66|
|Lesser incidence of brokers compared to huller||3.0||4.1||8.9||15|
Source: Mathrani 1971.
The traditional methods employed for milling are wasteful, non-uniform, and do not yield useful byproducts. Modern and the traditional methods can be compared using the outputs obtained, which are shown in table 3. About 7 per cent extra yield can be derived by employing modern rice milling machines. On an all-India basis, an additional quantity of 5 million tonnes of rice and Rs 6,000 million worth of pure rice bran can be achieved. A better quality of rice is produced through modern mills. The degree of polish and the percentage of brokers in the final product can be very precisely controlled to suit the specific needs either of industry or of the consumer. The most impressive gain that rice mill modernization has brought about is the efficient utilization of by-products of rice. If the total quantity of available bran is solventextracted, it will yield about 800,000 tonnes of bran oil for the country.
Movement of harvested field crops to the threshing yards, and of the threshed grains to various storage and consumption points, as indicated in figure 3 (see FIG. 3 Losses of Food Grains in Trade Channels), results in a partial loss of food grains through handling, shipments, pilferage, predation by birds and rodents, and weather hazards. Conservative estimates of such losses are noted in the figure. Most of these losses occur through careless handling of gunny bags by the labourers or porters, and leaky bags and containers employed by user agencies. Movement of food grains from warehouses of the Food Corporation of India (FCI) or the Central Warehousing Corporate (CWC) using road transport frequently results in considerable spoilage of grains on the roadside. Food grains being moved from Punjab to West Bengal, a distance of 1,500 km, in open rail-wagons frequently are subject to heavy showers on the way, causing partial or total spoilage through caking, discoloration, and sprouting. Loss of food grains from railway yards through birds. rodents, and miscreants is a common occurrence in commercial centres and metropolitan towns. Singh (1974) indicated the following range of transit losses for food grains in various modes of transport: rail, 0.5 to 3.5 per cent; road, 0.25 to 1.25 per cent; and river, 0.5 to 1.0 per cent. In 1970/71 the losses of food grains in medium to long-distance rail transit were estimated by FCI at about 1 per cent. Safe, and less-wasteful, transport is needed for movement of food grains.
In India about 30 to 40 per cent of the grain produced in the country is handled by the Government and private agencies, while the remaining 60 to 70 per cent is retained by village farmers for household use and consumption. The storage and warehousing facilities built by the FCI and CWC were located, mostly, in or near urban centres. Storage at the village level was neglected until village co-operatives initiated the building of improved storage facilites with the financial support of the National Co-operative Development Corporation (NCDC). though the total storage facilities thus provided by the co-operatives is very meagre (Prasad 1980). The loss of food grains stored in FCI and CWC godowns is less than 1 per cent, but in the village storage systems it can be as high as 5 per cent as a national average. Wimberly (1972) placed storage losses at 5 to 10 per cent. Undoubtedly there is a high loss each year that is still to be controlled.
Public agencies handle only about 10 per cent of the total produce. The other 90 per cent is handled by farmers and the private trade both for seed and family consumption purposes. Even a conservative 3 per cent loss in farm storage would work out to the loss of 4 million tonnes of food grain, annually. However, a study by Khare et al. (1972) indicated maximum and minimum weight losses of 4.5 and 1.7 per cent, depending on the store type and climate. In a similar study on wheat storage in villages around Delhi, Zutshi (1976) noted that farmers suffered a weight loss of only 0.8 per cent by storing in earthen pots as against 15 per cent in bulk in a room.
Quality Control and Nutrition
Prasad (1980) points out that good-quality grains have a natural attraction for the consumer. Quality depends on storage, and includes the nature of grain, period of storage and method of preservation. Storability differs from grain to grain. All types of grain deteriorate through the action of moisture and pests. The effects of pests are relatively greater in raw rice than in parboiled rice. Paddy is less damaged by pests than is rice. Of course, though the keeping quality of wheat is better than that of rice, even wheat products have a shelf life of not more than a few weeks. Pulses are prone to greater insect damage in storage than cereals, and split pulses or dhals can be stored longer than the whole materials. Coarse grains such as maize, bajra, and jowar are softer in texture than rice and wheat, and, therefore, relatively more susceptible to damage by pests as well as moisture. They also tend to become rancid after 18 to 24 months because of a high-percentage oil content.
Stored grains undergo chemical changes that alter their flavour and nutritive value. Insects and fungi not only consume the grain but can also accelerate harmful chemical changes. Some fungi produce poisonous toxins in food grains. The discovery of the poisonous metabolite aflatoxin in groundnuts brought to light the hazards of consumption of mouldy food material. Insect infestation causes loss of carbohydrates and thiamin. Fats present in food grains tend to break down during improper storage, and this is accelerated by insects, mould attack, high moisture and temperature. Breakdown of fats causes an increase in free fatty acids, and oxidation brings on rancid flavours. Infested food material becomes unpalatable through the presence of excrete and live or dead insects, and even results in poor growth of the body.
There is a very close relationship between the quality maintenance and the nutritive value of food grain. Quality is manifested in many aspects, such as appearance, freshness, mustiness, insect or fungus attack, moisture content, freedom from foreign matter, pesticide residues, filth, uric acid content, and hidden infestation.
Bal, S., and T.P. Ojha. 1977. "Determination of Biological Maturity and Effect of Harvesting and Drying Condition on Milling Quality of Paddy." RPEC Reporter, 3 (2): 62-69.
Bose, A.S.C., T.P. Ojha, and R.C. Maheshwari 1980. "All-weather System for Crop Drying with Solar Collector and Agricultural Wastes." Paper presented at ISAE Convention, New Delhi, 6 8 Feb.
Chabra, S.D., and K.N. Singh. 1977. "Effect of Cylinder Speed and Peg Spacing of Axial Flow Thresher on Wheat Threshing." J. Agric. Engg., ISAE, 14 (4): 141 144.
Devnani, R.S. 1976. "Development of Pulses Crop Thrushers." Annual Report 1975-76. College of Technology and Agricultural Engineering, Udaipur.
Khan, A.U. 1976. Harvesting and Threshing Equipment and Operations Rice Post-harvest Technology, 1st ed. IDRC, Canada.
Khare, B.P., C.S. Sengar, K.N. Singh, R.K. Agrawal, and H.H. Singh. 1972. "Losses in Grain Due to Insect Feeding. l-Wheat." Indian J. Agric. Res., 6 (29): 125 133.
Mathrani, K.P. 1971 "Modern Versus Traditional Rice Mills - A Performance Study." Dept. of Food, Ministry of Agriculture, Govt. of India
Ojha, T.P. 1978a "Solving the Food Problem through Post Harvest Technology." Productivity. 19 (2); 193201
___. 1978b. "Role of Harvest and Post Harvest Technology of Rice Production, Harvesting, Drying and Storage of Rice." National Symposium on Increasing Rice Yield in Kharif, held at CRRI. Cuttack, from 8 11 Feb.
Pillaiyar, P. 1977. "Causes and Presentation of Pre-harvest Rice Losses, Benefits of Pre-harvest Spray for Early Ripening and Harvest." Paper presented at the Workshop held in Malaysia, 12-30 March.
Prasad, Kamala. 1980. "Storage and Preservation of Food Grains in India." Bulletin of Grain Technology, 18 (2): 135-158.
Rama Rao, V.V. 1974 "Some Studies on Drying of Parboiled Paddy." Unpublished Ph. D. thesis. IIT, Kharagpur.
Singh, K.P. 1974. "Transportation of Food Grains in India " Training Manual: Post-harvest Prevention of Waste and Loss of Food Grains Asia Productivity Organization.
Wimberly, J.E. 1972. "Review of Storage and Procuring of Rice in Asia." Paper presented at the International Rice Research Institute, Manila. Philippines.
Zutshi, M.K. 1976. "Storage of Wheat by Farmers in Delhi " Bull. Grain Technology, 4 (3): 143-145.
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