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Storage and the quality of grain: village-level studies


P. Pushpamma and V. Vimala. Faculty of Home Science. Andhra Pradesh Agricultural University. Hyderabad. India


Abstract
Introduction
Qualitative changes in storage of cereals and millets
Qualitative changes in storage of grain legumes
Conclusion
References


Abstract

Insect pests, aflatoxins, nutritive value. and cooking quality of rice, sorghum, millets, and legumes stored in 100 rural households of Andhra Pradesh for family consumption over a period of three to twelve months were studied. Regional differences in the level of infestation and associated quality changes were noticed. Uric acid levels were found to be within permissible levels in rice, sorghum, and millets, but higher in legumes. A progressive decrease in starch content (of 3 to 11 per cent) and an increase in reducing sugars /of 30 to 70 per cent) were observed in all grains, but varied depending on the grain. In vitro carbohydrate digestibility increased during storage of bajra and groundnut (20 per cent), but decreased by 1 to 5 per cent in legumes. Protein content of the grains was invariably reduced, though the magnitude varied (2 to 11 per cent) depending on the grain and length of storage. Extensive loss of thiamine and considerable losses of niacin occurred in storage of all the grains. Both desirable and undesirable changes in the cooking quality of rice and legumes were noticed, but no such perceivable changes are found in sorghum and millets.


Introduction

Food grains after harvest are usually stored till the next harvest season for home consumption. It is well documented that about 80 per cent of the food grains produced in India are stored by rural families for their own consumption. Bulk storage is used for the remaining 20 per cent of the grain, which goes to urban markets. Proper storage of the food grains in rural families then would not only help in bridging the existing marginal food gap, but would also contribute to health and nutrition by conserving grain quality. Despite its importance, information on grain quality changes during storage, especially in home storage, is scarce.

Reported storage losses vary widely between 5 and 50 per cent (Swaminathan 1977). Most of these estimates are based on studies conducted in bulk storage structures. A study conducted by Boxall et al. (1979) on farm-level storage of paddy in coastal Andhra Pradesh is one of the few attempts to assess losses in farm-and home-level storage. Information on storage losses - both quantitative and qualitative - of food grains other than wheat and rice, such as sorghum, millet, and legumes, is extremely limited. And even this limited knowledge is based more on laboratory experiments than on field study.

Reduction of weight losses in bulk storage of grain is directly summable in financial terms, and the cost-benefit ratio is a highly convincing factor for research and development in improved storage structures. In contrast to the problems of commercial storage, those of farm and home storage have received low priority because the damage is insidious and often difficult to quantify.

Grain deterioration in storage is caused mainly through (a) big-deterioration, (b) insects and pests, and (c) moulds and fungi. Bio-deterioration is due to the activity of enzymes present in the seed. The extent of deterioration depends upon the level of enzyme activity, which in turn is determined by moisture and temperature.

Insect pests are a greater problem in regions where the relative humidity is high, but temperature has the greatest influence on insect multiplication. At temperatures of about 32° C, the rate of multiplication is such that a monthly compound increase of fifty times the original number is theoretically possible. Thus fifty insects at harvest could multiply to 312 million after four months. Growth of insect pests and moulds raises both temperature and moisture, and thereby accelerates the activity of the enzymes, which would remain at a low level if conditions of storage were favourable.

Fungal attack in storage generally occurs where drying has been inadequate, or where large numbers of insects are present, thereby causing a temperature rise in the grains, or where the stored crop is exposed to high humidity or actual wetting.

All these circumstances lead to grain deterioration during storage resulting in weight loss. In addition to weight loss, enhanced big-deterioration results in an enhanced loss of nutrients and contamination with anti-nutritional factors. In populations where most of the dietary needs are derived from cereals and legumes, nutritional impairment in these grains during storage will be of great consequence to the health and nutrition of the people.

Several efforts were made in the recent past to summarize the available information on post-harvest losses (Shulten 1982; Tyler 1982) to identify location, causes, and magnitude, and to deduce appropriate strategies for conservation of food grains. These reports clearly indicate that qualitative changes in storage of these grains, especially at farm and home levels, are the main grey patches in our knowledge of post-harvest damages occurring in food grains. In assessing damage, emphasis is frequently on weight loss followed by kernel damage. Other forms of damage, such as reduction in quality and nutritive value, viability of seeds, microbial spoilage, and contamination with substances harmful to health or unacceptable for edible purposes, which could be of greater importance than weight loss, are often ignored or given low priority. Even when these factors are given importance, lack of approved and standardized methodology for assessing qualitative changes is the main constraint. Recognizing the need for such information, an attempt has been made by the Department of Foods and Nutrition in Andhra Pradesh Agricultural University to study the qualitative changes in farm-and home-level storage of grain, mainly sorghum, millets, and legumes, in selected villages of Andhra Pradesh.


Qualitative changes in storage of cereals and millets

Uric Acid and Aflatoxins

In one study (Pushpamma and Chittemma Rao 1981), samples of sorghum, bajra and ragi were collected periodically from households that stored these grains over a minimum period of nine months. Insect damage was maximum in sorghum, leading to maximum weight losses (2.4 per cent) in this grain compared with bajra and ragi (see TABLE 1. Major Constituent Fractions, Weight Loss, and Increase in Uric Acid Content of the Stored Sorghum and Millets). Moisture content also went up as the level of insect infestation and duration of storage increased. In the three grains maximum increase in uric acid content was observed in sorghum, but the level was still below the specified safe limit (10 mg/100 g).

In another study (Pushpamma and Uma Reddy 1979) on the changes in the quality of rice and jowar stored for up to one year in three different agro-climatic regions of Andhra Pradesh, insect infestation increased progressively in both grains during storage. Jowar samples were more frequently infested than were rice samples stored under similar conditions. Weight losses ranged between 3.9 and 5.10 per cent at the end of twelve months of storage. Samples from the Andhra coastal region had a higher level of insect infestation and higher uric acid levels. But, except for rice samples stored for one year in the coastal region, all the jowar and rice samples had uric acid contents below the safe level.

Though freshly harvested samples of rice and jowar were free of aflatoxin, after three months of storage, six samples of rice were contaminated with aflatoxin and two were above the safety levels. After twelve months of storage no increase was observed in either the number of samples bearing toxins, or the level of aflatoxin in infected rice samples. But in jowar, both the number of samples and the levels of aflatoxin increased up to six months of storage. This could be because of the higher prevalent moisture content during storage of jowar, which increased even more through insect infestation. A greater number of coastal area samples were contaminated with aflatoxin than those from the other two regions.

 

Carbohydrates

In the same study it was observed that length of storage had no significant effect on the starch content of either rice or jowar. Under normal conditions of storage the starch content of rice was unaffected. The reducing sugar content of rice and jowar increased significantly (P < .05) during storage. The increase was greater in the first six months than in the next five months.

In vitro digestibility of the carbohydrates of bajra and groundnuts increased on storage for six months, the mean values being 24.2 and 26.1 per cent respectively (Sathyavathi Devi 1978).

 

Proteins

Data presented in table 2 (see TABLE 2. Moisture, Protein, and Non-protein Nitrogen Content of Fresh and Stored Sorghum and Millets(a)) show that during nine months storage maximum protein losses were observed (11.0 per cent) in bajra followed by sorghum (10.6 per cent) and ragi (5.3 per cent). There was a progressive increase in non-protein nitrogen content, which could be due both to the presence of insect excrete and protein degradation during storage (Pushpamma and Chittemma Rao 1981). In another study Pushpamma and Uma Reddy (1979) reported a loss of 0.59 to 0.70 9 protein/100 9 in rice and of 0.51 to 0.74 g in jowar.

Many investigators have reported a decrease in free amino acids and free amino nitrogen during storage of rice and jowar (Pushpamma and Uma Reddy 1979; Iwasaki and Tani 1967), which appears to be dependent on moisture and storage temperature.

There was a gradual decrease in limiting amino acids in stored paddy and jowar, the decrease being greater in lysine content than in other amino acids even when stored under controlled conditions (Uma Reddy 1981). A varietal difference was also observed (paddy 5-10 per cent, sorghum 25-30 per cent). Thus the grain itself appears to utilize some amino acids for its own metabolic activities, and also to consume them through non-enzymatic Maillard browning. Sorghum is naturally deficient in lysine content, and the drastic loss observed following insect infestation will affect the overall biological quality. Local varieties retained a higher amount of amino acids during storage than did hybrid varieties.

Both ß-amylase and protease activity also decreased during storage for 12 months of both rice and jowar in rural Andhra Pradesh (Pushpamma and Uma Reddy 1979). The decrease in ß-amylase activity was maximum in the first 3 months of storage. where the loss was nearly 60 per cent. Similar results in rice have been reported by many investigators (Srinivasan 1939; Narayana Rao et al. 1954).

Changes in the Biological Quality of Proteins

Assessment of various food grains through the net protein ratio and digestibility coefficient indicated a gradual decrease during storage, Among the cereals, rice retains its protein quality better than does sorghum. The length of storage had a significant effect on the net protein ratio and digestibility of all varieties of sorghum. In varieties CHS-1 and CSH-6, where infestation is heavy, the net protein ratio decreased drastically from 1.91 to 0.41 and from 2.00 to 0.33 respectively in 12 months of storage. The decrease was comparatively less in local varieties.

 

Vitamin Losses

A study conducted in rural areas in Andhra Pradesh (Pushpamma and Chittemma Rao 1981) revealed that B-vitamins of all grains were reduced under storage (see TABLE 3. B-Vitamin Content of Fresh and Stored Sorghum and Millets(a)), but the percentage decrease varied from grain to grain depending upon the level of insect infestation. Maximum loss in thiamine (43.2 per cent) was observed in Eleusine at the end of nine months of storage, followed by Pennisetum (39.4 per cent). The percentage loss of thiamine was observed to be higher during storage than that of riboflavin and niacin. The rate of decrease of B-vitamins was observed to be highest between five and nine months of storage after harvest. Factors other than insect infestation also contribute, such as moisture, temperature, and oxygen fluctuations, which lead to thiamine losses (Bayfield and O'Donnell 1945; Williams 1952).

 

Cooking Quality of Stored Cereals and Millets

Cooking quality is one of the important aspects of food quality. However nutritious a food material may be, it will not be accepted by the consumer unless it satisfies specific culinary characteristics. Thus the grains of cooked rice should be non-sticky and well separated, qualities that can be judged by swelling number, water uptake, and amylose content. An improvement in the cooking quality of rice was found after six months of storage, but not after three months (Pushpamma and Uma Reddy 1979). It is interesting to note that while the swelling number increased, the amylose content of rice remained constant. Storage decreased the amylose dispersion, which is responsible for greater absorption and retention of water, and thus the expansion of rice grain. The cooking quality of jowar did not improve under storage as markedly as rice. No relationship was noticed between the swelling number and amylose content of jowar during storage.


Qualitative changes in storage of grain legumes

Uric Acid and Aflatoxin

A study of rural families conducted in Andhra Pradesh had revealed that after nine months of storage, the percentage weight loss was 1.5 to 2.1 for pigeon-pea, green gram, and black gram, but only 0.5 per cent in chick-pea (Pushpamma and Chittemma Rao 1981). The greatest proportion of damaged grain was observed in pigeon-pea, followed by green gram and black gram with chick-pea the least, both in terms of percentage by weight and percentage by number. Compared to cereals and millets, increase in the true uric content in legumes was higher after a nine-month storage period, and the uric acid level exceeded the safe limit in all the legumes except the chick-pea.

A similar type of study conducted by Vimala (1982) showed that weight losses in four legumes ranged from 1.79 to 5.9 per cent in 12 months of storage, being maximum in green gram and minimum in chickpea, while the true uric acid content ranged from 11.92 to 26.53 mg/100 g. After up to four months of storage, uric acid levels were below 10 mg (fit for human consumption), but after eight months of storage, the content increased to levels (10-20 mg) at which it is unfit for human consumption in all the legumes, except for the chick-pea, in which the level is very close to the permissible range even after 12 months of storage. Coastal area samples of green gram contained the highest concentration of uric acid, namely 31 mg/100 g. Other weight losses reported in legumes are much higher than the two present studies (Venkat Rao et al. 1960; Kapoor et al. 1972; Swaminathan 1977; Adams 1977; Rajyalakshmi 1978). Periodical drying and cleaning of the gram during home storage seems to reduce the damage considerably.

The initial moisture content of the stored legumes ranged between 9 and 10 per cent. There was either a slight increase or decrease (+ 4-14 per cent) in moisture content of legumes stored for nine months. The change in moisture content could have been caused by a change in atmospheric temperature and relative humidity, by periodic sun-drying, by insect infestation, or by a combination of these factors.

A study of the aflatoxin contamination in red gram samples from three agro-climatic regions of Andhra Pradesh showed that no freshly harvested samples were positive for aflatoxin (Rajyalakshmi 1978). After three months of storage, 20.8 per cent of the pigeon-pea samples contained toxin; 4 per cent of these were at a level considered unsafe. After six months the frequency of aflatoxin contamination had further increased.

 

Carbohydrates

Vimala (1982) has provided further information on changes occurring in carbohydrates in legumes stored in the home for 12 months by rural families of Andhra Pradesh. The starch content of all the legumes decreased as the period of storage increased, but reduction in the starch content was not directly related to infestation (see TABLE 4. Starch Content (g/100 g) of Fresh and Stored Legumes from Rural Areas of Andhra Pradesha). Though insect infestation was highest in green gram, the reduction in starch content after 12 months was maximum in pigeon-pea (7.24 per cent), followed by green gram (6.19 per cent), black gram (3.49 per cent), and chickpea 12.92 per cent).

Reducing sugars in four legumes increased at the expense of non-reducing sugars after twelve months of storage, in accordance with the observations of many other investigators (Bottomley et al. 1950; Pingale et al. 1956; Houston et al. 1957; Glass et al. 1959; Lynch et al. 1962; Premvalli et al. 1979; Bhat et al. 1975; Pushpamma and Uma Reddy 1979). The greatest increase in reducing sugars was observed in pigeon-pea 170.22 per cent) followed by green gram (47.75 per cent), chick-pea (34.40 per cent), and black gram (28.87 per cent). This marked increase in reducing sugars could be due to la) changes in moisture, relative humidity, and temperature of the surrounding atmosphere as well as of grains, (b) insect activity, and (c) seed activity. A similar pattern of decrease occurred in non-reducing sugars - from 26.14 to 44.61 per cent in the four legumes.

Amylose content (see TABLE 5. Amylose (g/100 g of starch) Content of Fresh and Stored Legumes from Rural Areas of Andhra Pradesha) decreased in the four legumes as the storage period increased, but the decrease was not found to be significant (P > .05).

Indigestible residue content of the four legumes increased with the level of insect infestation because of the increase in the husk portion (see TABLE 6. Percentage of Indigestible Residue Content of Fresh and Stored Legumes from Rural Areas of Andhra Pradesha). The greatest percentage increases were observed in green gram (3.65 per cent), followed by pigeon-pea (3.5 per cent), black gram (2.33 per cent), and chick-pea (1.44 per cent). No such increase in indigestible residue content occurred in samples free of infestation.

Decrease in in vitro digestibility of carbohydrates was noticed in all the four legumes (see TABLE 7. In vitro Digestibility of Carbohydrates (g maltose/100 g) of Fresh and Stored Legumes from Rural Areas of Andhra Pradesha). The decrease in digestibility was greatest in green gram (3.87 per cent) followed by pigeon-pea (3.07 per cent), black gram (2.80 per cent), and chick-pea (0.91 per cent). Starch degradation and increase in indigestibte residue content might be the causes of this.

Studies of carbohydrate changes in stored legumes are too scanty for comparison with the present data.

 

Proteins

A study on home-level storage of legumes for nine months was conducted in rural areas in Andhra Pradesh (Pushpamma and Chittemma Rao 1981). Samples were drawn from families storing legumes under natural conditions and methods of storage. A progressive decrease in protein content occurred during storage of all legumes with a maximum loss in green gram (11 per cent) after nine months of storage. There was an increase in non-protein nitrogen with longer storage, with varying figures for different legumes.

A decrease in free amino nitrogen was seen in both treated and untreated samples in all varieties of legumes during storage. In infested samples this decrease was related to the degree of infestation, showing that free amino acids were utilized for the growth of insects (Uma Reddy 1981).

Uma Reddy (1981 ) reported a decrease in three amino acids in stored chick-pea, green gram, and pigeonpea. The decrease was higher in lysine than in methionine and tryptophan. Amino acid loss is relatively less in chick-pea than in pigeon-pea and green gram where the degree of insect infestation is high. In green gram, the loss was as high as 50 per cent. Apart from Maillard reactions resulting in the decreased availability of amino acids, the consumption by insect of the germ, which is rich in lysine, could have caused the striking fall in lysine level.

Biological Quality of Proteins

Gradual decrease in the protein biological quality during storage of legumes, as assessed by net protein ratio (NPR) and digestibility coefficient, was reported in the same study. At the end of one year's storage, only chick-pea retained most of its growth promoting value and digestibility, because it is relatively resistant to insect infestation even when not treated. A significant decrease in NPR and digestibility value was observed during storage of all varieties of both treated and untreated pigeon-pea samples (Uma Reddy and Pushpamma 1980). Similar results have been reported by other researchers (Daniel et al. 1977; Yanni and Zimmerman 1970; Mitchell 1944).

 

Vitamin Losses

The losses ranged from 13 to 25 per cent for thiamine, 7 to 11 per cent for riboflavin, and 7 to 14 per cent for niacin in all the four legumes after five months of storage, and these increased to 21 to 38 per cent, 14 to 23 per cent, and 20 to 42 per cent after nine months of storage. Of these three, ß-vitamins, riboflavin losses were least in all the legumes.

Thiamine losses were greatest in green gram (42.09 per cent), followed by pigeon-pea (33.01 per cent), black gram (27.71 per cent), and chick-pea (27.14 per cent) in a similar type of study on home-level rural storage of legumes in Andhra Pradesh (Vimala 1982). Similar losses of thiamine and niacin during storage of various food grains have been reported by others (Chitra et al. 1955; Daniel et al. 1977; and Morgan et al. 1945).

 

Cooking Quality of Stored Legumes

Storage under unfavourable conditions is known to adversely affect legume cooking quality, leading to the hard-to-cook defect, a result both of chemical and enzymatic changes.

The cooking quality of legumes is judged by the amount of water absorbed during soaking as measured by the hydration coefficient, increase in weight and volume after cooking, cooking time, and solids dispersed in the cooked medium.

The cooking time for grain legumes was found to increase with storage. The largest increase in cooking time was observed in stored green gram (52 per cent) followed by black gram (35 per cent), chick-pea (31 per cent), and pigeon-pea (29 per cent). The initial water uptake at a given standard time for legumes also differed. Green gram had the highest water uptake (2.03 9/9) but required the least cooking time, whereas chick-pea showed the minimum water uptake (0.81 9/9) but required the longest cooking time. With storage a progressive decrease in water uptake was observed in ail the legumes Except for the cooking time and water uptake of legumes initially, no relationship was found after storage between the percentage decrease in water uptake and percentage increase in cooking time of legumes (Vimala 1982).

Uma Reddy (1981) believed that the cooking quality of legumes deteriorated on storage: they took a longer time to cook, absorbed less water during soaking, and gave a lower yield after cooking.


Conclusion

The information available on grain quality changes, though limited, does definitely indicate that deterioration in the quality of food grains stored for a period of six to twelve months is of a magnitude that deserves the attention of researchers working on the post-harvest problems of food grains. So far reduction in losses was attempted mainly in terms of weight loss . Considering the extent of nutrient losses as well as the lowered biological quality of the grain during storage, attempts to minimize qualitative losses would improve the per capita availability of nutrients to the rural population.


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