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Approaches to the assessment of food losses

S.K. Majumder
Central Food Technological Research Institute, Mysore, India

Introduction

Tropical and subtropical regions have a greater potential for food production because of climate and can grow multiple crops annually. In spite of the high potential for increasing crop yields under existing agro-climatic conditions, however, pests, diseases, and other agents compete against humans in their struggle to achieve higher outputs. Therefore, efforts to control pests have a vital role to play in maintaining food production throughout the world.

TABLE 2. Fungal Species Isolated from Storage Insects

Origin of food losses

Insect pests are abundant and ubiquitous in the tropics and subtropics. Although post-harvest losses of food grains have been recognized, analysis of losses in overall quality, specific nutrients, and energy have been scanty. These losses are caused by insects, moulds, mites, and enzymes activated by moisture. The latter include lipase and tyrosinase activity during germination, oxidative rancidity, the browning reaction, and other destructive changes that reduce the quality of food grains. Insect infestation may deplete vitamin content through preferential consumption of seed tissue or grain by various taxonomic groups of insects.

Apart from physical loss in weight, there are qualitative depletions in nutrient contents, caloric value, and other adverse transformations during storage that must be considered. The growth of insects, moulds, and mites in stored grain also results in the elaboration of toxins and allergens. No insect, mite or mould attacks grain in isolation. They act concomitantly with organisms of different taxonomic groups. Damaye and biodeterioration of the grain are caused by their combined activities. Fungal species isolated from storage insects are given in table 2. Damage caused by these agents is reflected in qualitative and quantitative changes in the grain.

Drying grain reduces enzymatic and microbial activities so that they remain dormant until higher temperature and humidity increase the moisture content to a level optimal for their growth and development. In tropical and humid environments, insect activities remain high in relatively dry grain. In some cases, freshly harvested grain will carry infestation from field to threshing floor. Eggs are laid by stored-product insects on the panicle in the field before harvest. Such field infestations are quite frequent on paddy, maize, sorghum, legumes, oilseeds, spices, and even some root crops, and lead to post harvest losses. In addition to insects, fungi can also invade crops in the field. Invasion begins before harvest and continues through every stage of post-harvest handling, processing, storage, transportation, and marketing because of the crop's inherent biodegradability.

Since insects are uricotelic, they deposit crystalline uric acid on the grain as they multiply. Additional side effects of insect activity involve changes in the kernel, altering the quantity and quality of carbohydrates, proteins, amino acids, fatty acids, and vitamins. These more subtle changes are difficult to measure on a quantitative basis, and it has so far not been possible to establish criteria for measurement.

TABLE 3. Fungi Isolated from Common Staples in India

Source: Ref. 3.

Rice, wheat, sorghum, maize, ground-nuts, and other grains contain epiphytic and endophytic microflora by the time they are harvested (table 3). Mycotoxins may be associated with these microflora, and at moisture contents of 9 per cent in oilseeds and 12 per cent in cereals, fungi grow and elaborate mycotoxins at normal tropical temperatures.

In Sudan, where atmospheric relative humidity is very low and peanuts are easily dried to below the critical moisture level, Aspergillus Aspergilus flaws is found even before harvest, attributable to the activities of soil arthropods (Majumder, unpublished records, 1971). The carrying of fungal inoculum by field insects in the pre-harvest period and distribution of the inoculum by stored-product insects may be responsible for the presence of many toxin-producing fungi in grains.

During storage in rural and urban structures, moisture migration occurs to different degrees, depending on the thermal gradient created within the bulk as a result of ambient temperature fluctuations (2, 4, 5). Moisture condenses when it reaches the dew point, the result of contact with the colder periphery of metal storage walls, and this initiates the growth of moulds.

Even in dry grains, insect activity creates a moist microclimate within the infested kernel or cotyledon. The inoculum carried by stored-product insects and the epiphytic and endophytic micro-flora on and in the grains begin to consume the food part of the grain, produce

Fig. 1. Longitudinal section of infested grain (left), showing loss of food and accumulation of filth, and sound grain (right) with no loss of food constituents

toxins, and degrade quality. The adverse changes in grain quality that are inflicted by various agents and the damage they bring about are listed in table 4 (see Fig. 1.)

Insects produce uric acid, inoculate fungi and bacteria, and leave faecal matter and cast-off skins on the grain, creating a foul odour. Quinone and other harmful substances may be produced. Many fungi form dangerous mycotoxins.

Although mycotoxins, such as islandotoxin, citrinin, and aflatoxins, are common enough in stored products to be recognized by food quality regulatory personnel, the potential harmful effects of fungal and bacterial metabolites on consumers are not really appreciated.

The loss estimates shown in tables 5 and 6 only reflect weight loss. Thus, the grain weight includes insect bodies, mould, debris, moisture absorbed, and good as well as damaged portions of grain. In spite of this deterioration, for statistical purposes, weight is interpreted as available food. However, when the grain reaches the retailer or the home for processing, bad grain is discarded as filth and foreign matter. Therefore, the weight of the grain as originally purchased is not an indicator of caloric and food value. Although attempts have been made to quantify losses in quality, the extent of nutrient loss and toxic effects has not been established.

TABLE 4. Quality Changes in Cereal Grains Induced by Various Agents

* Eggs, larvae, pupae, area adults

TABLE 5. Examples of Food Losses in India

Source: Ref. 1.

TABLE 6. Possibility of Extending Food Availability from Existing Production

The losses cited above are only the quantitative losses The loss in quality is proportionately much greater when food standards are applied for the assessment of edible quality.

Source: Ref. 1.

Insects contain chitin, moulds produce mycotoxins, mites excrete guanine, and moisture and enzymes combined produce free amino acids or sugars or fatty acids from substrates. They react with each other and may undergo a Maillard change or form other harmful substances.

Uric acid is obviously of very considerable physiologic significance, since it is the main nitrogenous excretory product of many animals and is excreted in lesser amounts by other living organisms. It is generally regarded as a product of direct oxidation of other purine derivatives, e.g., nucleic acid in the cells, and may be oxidized further to allantoin or similar molecules.

Control of infestation is essential for maintaining quality in the field. Potential yield increases afforded by such inputs as improved seed, adequate irrigation, and highquality fertilizers applied in the correct amounts at the right time, will be negated and possibly fall far below expectation if plants are not protected from pests. The higher-yielding varieties require more of all the above inputs to be productive, and new methods exist to control insects, fungi, bacteria, viruses, and nematodes. They are pivotal to increasing crop quantity and quality.

Nature and magnitude of crop losses

Agro-climatic conditions also effect post-harvest quality and storage characteristics of food grain, fruits, and vegetables. The higher the environmental temperature and humidity are during crop production, the lower will be resistance to disease and infestation during the postharvest period. The pro- and post-harvest interfaces that influence final quantity and quality of crops are listed in table 7.

Since most estimates of post-harvest losses have been restricted to evaluating weight and yield losses, the data available in the world literature do not reflect actual nutritional quality, energy balance, and food safety of food grains around the globe.

Analytical methods for determing food quality changes and losses

Early methods of uric acid analysis involved the isolation of uric acid or its salts in crystalline form. A modified procedure for determining urinary uric acid level consists of precipitation as silver urate, obtained by treatment with magnesium, followed by decomposition with hydrogen sulfide and measurement of liberated uric acid by direct weighing or titration with permanganate.

The calorimetric procedure developed by Folin and Wu and also by Benedict uses an arsenic-phosphoric tungstic acid reagent. This technique can be used directly on urine. Tungstomolybdic acid in the presence of cyanide has been used in both macro- and micro-modifications, and the photoelectric calorimeter has been employed successfully with these methods. Newton introduced a chromogenic arseno-tungstate method (6).

Ultraviolet absorption of urine was suggested for direct determination of uric acid, and a combined calorimetric method using the enzyme uricase was introduced by Leucke and Pearson (2,3)

The Benedict and Franke method (8) is widely used for uric acid analysis. It is based on the fact that solutions containing uric acid develop a blue colour, the intensity of which is proportional to the uric acid concentration, when arsenophosphotungstic acid (prepared from reagents like sodium tungstate, arsenic trioxide, and phosphoric and hydrochloric acids) and sodium cyanide are added. Uric acid in blood has been analysed using a lithium carbonate remnant

TABLE 7. Pre- and Post-harvest Influences on Food Quality and Food Losses

Uric acid level has only limited value as an indicator of insect-caused biodegradation. It is

concentrated in the frass material and is released in powder form. This slips down and settles at the bottom of the grain pile, introducing another factor in determining the real quantity of uric acid in the grain. Sampling systems must therefore include sources of error in uric acid analysis.

Analytical methods to determine grain and grain product quality should measure physical, chemical, and biochemical changes occurring in the grain during the entire period of post-harvest handling and storage. Most deterioration in storage is the result of insect and fungal damage and internal infestations from both pre- and post-harvest sources.

Previous workers have used different criteria as indication. of biodegradation. Christensen and Gordon considered moisture level as an index (9); Venkatarao et al. measured uric acid content (8); Linko and Sogn measured glutamic acid (10); Nicholson measured kernel damage (11); Howe and Oxley measured carbon dioxide level (12); and Harris and Knudson counted insect fragments (13). However, many of these criteria have not so far been applied to grading the grain.

Work by Venkatarao et al. (8), Farn and Smith (14), and Sen (15) has indicated a good correlation between degree

of insect infestation and uric acid content of flour. Moisture content is an important factor because lower levels limit the activities of insects and fungi. However, by itself it is not an index of deterioration that has already occurred. It is used as an index to define grades of cereal grains. Most of the other causes of damage described above are measures of existing levels of destruction and may also be used as predictors of future damage. The biodegradation caused by insects, fungi, and excess moisture is reflected in the level of free fat acidity, mould count, insect population and fragment count, kernel damage, frass content, and organoleptic qualities (2, 3).

The "Benedict reaction complex" has a significant correlation with the extent of total damage caused by insectmould-moisture-enzyme systems. The Benedict-Franke reagent is non-selective for uric acid and gives colour reactions with many ultraviolet fluorescent molecules. On thin layer chromatography (TLC), the Benedict reaction complex includes true uric acid, apparent uric acid, and related substances. The largest spot on TLC had Rf values of 0.7 and a UV absorption pattern of 280 x 340 mp. Both insect and fungal infestation of grain resulted in UV fluorescent spots on TLC, and, depending on the type of grain, the Benedict reaction substances were 7 to 13 in two-dimensional TLC runs (2). Uninfested grain did not yield these substances within detectable limits. The Benedict reaction complex is elaborated by individual or combined activities of insects or mould. Insect counts, mould counts, true uric acid, apparent uric acid, kernel damage, and frass content show direct correlations with the Benedict reaction complex (2). In the Benedict-Franke analysis of uric acid, the reagents used also give colour reactions.

Currently in the United States a chemical technique is being used for standard determinations of the degree of insect infestation in stored flour by means of an enzymatic ultraviolet-absorption assay to measure uric acid content (16). T. confusum produces 18 per cent excrete in the form of uric acid (17). The results of this study suggest that insect infestation of flour can be detected early, even when the insect population is relatively low. The fluorometric method offers many advantages over other chemical and physical methods, particularly because it provides an accurate measurement of uric acid in flour, and therefore degree of insect infestation.

Suggested additional indices for grain-loss assessment

The tissues in food grains consist of histochemically active constituents. The bran and seed coat are comparatively less active, while the germ, endosperm, and cotyledons are physiologically active even during storage. Biodeterioration occurs in grain because of the accelerated activities of the enxymes and chemical activities of the tissue components. These are only intrinsic factors and related to the constituents of the food grains. During storage, the grain should be viable and not undergo denaturation or adverse histochemical changes. Therefore, efforts are made to store grain under sound conditions by reducing the moisture content and also by controlling extrinsic factors such as the insects, moulds, and related organisms. Low temperature retards the activities of intrinsic and extrinsic factors and, therefore, the histochemical property of the seeds is maintained almost in the original condition.

The biological activity of the grain is reflected in the respiratory rate. A high rate of oxygen consumption and consequent evolution of carbon dioxide and release of energy are the measures of biodeterioration. The respiratory activity can be measured by gas analysis, change in temperature, formazon test (tetrazoleum chloride reaction), and related criteria. From time to time several marketing standards have been proposed based on bulk density, specific gravity, organoleptic quality, and related characteristics. Losses attributed to weight reduction alone have been found to be extremely deceptive. Since the equilibrium moisture content of a grain bulk changes with the degree of deterioration, it can most often be characterized by an increase in the sorption capacity for moisture. The dry matter loss is not accounted for, as it is compensated for by moisture uptake from the atmosphere and also insect and mould remnants. The deteriorated grain, therefore, may not show actual total weight change, although weight might have been lost in dry matter because of biodeterioration.

Although nutritional characteristics and biochemical criteria such as amino acid composition, free fatty acid, glutamic acid content, and nutritive value are objective criteria, it is difficult to adopt any of these in practice. Insects, mould, and moisture acting on the grain can be measured by physical and chemical procedures. Insect count, including life stages, insect fragment count, and mould count can give indices of the degree of insect infestation and mould damage. Damage to the kernel caused by insects brings about qualitative and quantitative changes in carbohydrates, protein, amino acids, fatty acids, and vitamins. Still, it has not been possilbe to measure these subtle changes and adopt them as quantitative criteria.

Food grains are biological entities. The degradation of the kernels or cotyledons occurs mostly from the activities of insects, fungi, and excess moisture favoured by tropical temperatures and humidity. Their activities bring about physical and biochemical changes. Histological and bio chemical analyses indicate incipient changes in the kernels during storage that might have originated at the pre-harvest stage. Organoleptic and other visual qualities are employed for grading and marketing. Weight/volume ratios and changes in total weight are deceptive criteria and do not truly reflect soundness of grain and nutritive qualities.

An index of deterioration based on moisture level, uric acid and glutamic acid content, kernel damage, carbon dioxide production, insect fragment count, microbial counts, tetrazoleum reaction, biological value, nutritive value, energy, and related criteria have been proposed from time to time.

A recent measure of fungal invasion in grains is determination of ergosterol. According to Seitz et al. (18), ergosterol (a) is a constituent of nearly all fungi, (b) is not a native consituent of grain, and (c) can be reliably and rapidly determined by the high-pressure liquid chromatography (HPLC) method. Adequate comparisons of ergosterol and chitin as measures of fungal growth are not available, but tests for these factors are being studied currently in some laboratories around the world. The extent of pre-harvest fungal invasion in grain sorghum, wheat, and corn samples measured by an ergosterol assay was compared to results from plating the surface of disinfected seeds. Ergosterol content and percentage of kernels with fungi increased in sorghum harvested at successively later dates. Assays of wheat samples showed a close relation between ergosterol level and invasion by field fungi and/or with weather conditions known to favour fungal invasion.

The ergosterol assay gave a quantitative estimate of previous invasion by fungi even though the fungi were no longer viable. Corn kernels with obvious fungal damage had as much as 200 µg/g ergosterol, while freshly harvested, sound kernels contained as little as 0.2 µg/g. Ergosterol was quantified by high-pressure liquid chromatography and could alos be estimated from ultraviolet spectroscopy of sterols obtained by thin-layer chromatography.

One of the symptoms of changes occurring in grain stored under conditions promoting the growth of micro-flora is the appearance of off-flavour. As a rule, the off-flavour is caused by the production of certain volatile metabolites by moulds. Kaminski et al. (19) proposed the application of gas chromatography by a headspace technique to detect metabolites such as 3-methylbutanol, 3-octanone, 3-octanol and 1-octan-3-ol. Identification of these componets can be effected by the retention method and chemical modification of the sample. This is a non-destructive test that can be employed for monitoring the progress of biodeterioration in grains store-d in bulk in silos and bins.

Basic information on toxicological and nutritional impairment in relation to the deteriorative changes has Yet to be obtained. There is a significant relationship between the physical criteria and biochemical changes taking place in grain: Moisture can be correlated with free fatty acid, fungi with apparent uric acid, kernel damage has a positive correlation with the insect count, fragment count, and true uric acid. The results have indicated that a high correlation exists between fungi and apparent uric acid-like substances that give a positive reaction to Benedict's reagents and produce colour reaction complexes (2).

The development of insects that live hidden inside grain kernels remained somewhat obscure until the availability of X-ray inspection apparatus. Ashman's amino acid test for internal infestation to some extent also shows a quantitative relationship between visible kernel damage (2) and internal infestation. These tests reveal internal infestation in grains that show no visible external damage. The true losses, therefore, must be reckoned with the probable changes that the grain will suffer between the time of oviposition of the insect and the ultimate emergence of the adult from the infested kernel. The ratio of kernel damage to internal infestation is 1:5. Therefore, kernel damage with a multiplication factor will indicate the extent of infested grain in a particular lot.

The weight loss generally adopted as a criterion of grain damage does not give a picture of actual loss, as the grains containing internal infestations are not detected in such analyses. Basic research is needed to find the relationship between weight loss and true loss in quality brought about by internal infestations.

References

1. S.K. Majumder, "Infestation Control," in IIT, Madras, Encyclopaedia ('Indian Institute of Technology, Madras, India 19791, pp. 16.1 - 16.36.

2. S. K. Majumder, Control of Microflora in Stored Grains (Central Food Technological Research Institute, Mysore, India, 1970).

3. S.K. Majumder, in Ann. de technol. agricole, 1973, p. 485.

4. T.A. Oxley, Principles of Grain Storage (Northern Publn., 1948).

5. S.K Majumder, KS. Narasimhan, and H.A.B. Parpia, in Wogan, ea., Mycotoxins (MIT Press, Cambridge, Ma ss., USA, 1964).

6. E.B. Newton, in J. Biol Chem., 120:315 (1937).

7. R.W. Leucke and P.B. Pearson, in J. Biol. Chem., 153: 259 (1944).

8. S. Venkatarao, R.N. Nuggehalli, S.V. Pingale, M, Swaminathan and v. Subrahmanyan, in Cereal Chem., 37: 97 (1959)

9. C.M. Christensen and D.R. Gordon, in Cereal Chem., 25: 40 (1948).

10. Pekka Linko and Lars Sogn, in Cereal Chem., 37: 489 (1960).

11. K.L. Nicholson, in JAOAC, 36: 150 (1953).

12. R.W, Howe and T.A. Oxley, in Bull Ento. Research, 35: 11 (1944).

13. A.M. Harris and L Knudson, in Bull Econ. Entomol, 39: 64 (1948).

14. G. Earn and D.M. Smith, in JAOAC, 46: 522 (1963).

15. N.P. Sen, in JAOAC, 51: 785 (1968)

16. AOAC Manual, 13th ed.(1980), P. 178.

17. P.D. Gupta and R.N. Sinha, in Ann. Entomol. Soc. Amer., 53: 632 (1960).

18. LM. Seitz, H.E, Mohr, R. Burroughs, and D.B. Saver, in Cereal Chem., 54: 1207 (1977).

19. E. Kaminski, S. Stawichi, E. Wasowicz, and R. Przybylski, in Pol Pismo Entomol, 44: 441 (1975).

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