Moisture problems in food aid grain

by Teresa Acklin
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   Considerable quantities of food are frequently lost because grain from temperate regions is shipped in a condition unsuitable for storage in tropical climates. Cargoes may deteriorate during the voyage, and serious mold damage often occurs during storage at the receiving port. Losses can be reduced if grain is shipped at lower moisture content, and it is incumbent upon donor organizations to ensure that this is done. The additional costs of drying are likely to be more than offset by the benefits to the consumer and the improved keeping quality of grain.

   National food security in many developing countries depends upon the availability of imported grain making up for deficiencies in local production. According to estimates by the Food and Agriculture Organization (F.A.O.), cereal imports by the food-deficit developing countries are running at about 120 million tonnes annually and, of this, approximately 10% to 15% is food aid grain. Reports from the International Wheat Council and the World Food Program (F.A.O. 1991) indicate that in the period from 1983 to 1991, between 9.8 million and 13.2 million tonnes of food aid grain, principally wheat and maize from the United States, Canada and Europe, were shipped annually to vulnerable countries.

   Transport costs in developing countries increase the value of grain imports by two or three times on average and up to five times when the destination is a remote rural area. Any loss, particularly of food aid, may be of enormous economic importance and even life threatening where rapid replacement of damaged or inedible food is difficult or impossible.

      Grain losses

   In the last decade, the Natural Resources Institute (N.R.I.) has responded to requests for assistance with food aid grain distribution from more than 25 countries in Africa, Asia and Latin America. It is evident that significant losses occur, particularly when grain is imported into regions where ambient temperatures are frequently high. Grain deteriorates through insect attack and mold damage, and it can become poisonous if mycotoxins form. Although N.R.I.'s experience is with aid shipments, similar problems are likely to occur with commercial grain imports.

   Many of the problems associated with food aid shipments arise because the standards used for purchase of grain are commonly based upon those used for intervention operations in Europe or the commercial grades offered for sale in Europe, the U.S. and Canada. The standards are not always appropriate for safe storage in temperate regions and are quite unsuitable for grain stored in tropical climates. The normally acceptable levels of foreign matter and broken or other defective grains increase the potential for damage by insects and mold. But, the most important factor is the grain moisture content.

   Commercial grain contracts usually specify a maximum permitted moisture content at which grain can be traded without penalty. The moisture content values are often assumed to represent safe levels for storage, but they are really no more than the average values at which deterioration is prevented under the average storage conditions in a particular country or region.

   Grains are hygroscopic and at a constant temperature will take up moisture from, or release it to, the surrounding atmosphere until a state of equilibrium is reached, at which no further moisture exchange will take place. The relative humidity of the air in this condition is known as the equilibrium relative humidity (e.r.h.). The “safe” storage moisture content for grain is usually taken to mean that which corresponds to an e.r.h. of 70%, which is the lower limit for the development of molds that cause spoilage of grain.

   The relationship between grain moisture content and e.r.h. is complex, but the effect of temperature is critical. It is known that for a fixed grain moisture content, the e.r.h. will rise by about 3% when the temperature increases 10°C (Pixton 1982). This apparently small increase in e.r.h. is important because grain is usually dried to the e.r.h. that is only just safe for storage. Even when the e.r.h. is only marginally safe, clean, insect-free grain may store satisfactorily in temperate regions because the temperature is sufficiently low. When such grain is subjected to a large increase in temperature — for example, when it is shipped from temperate to tropical regions — the e.r.h. may rise above 70%, and molds will grow.

   The losses occurring in individual shipments of grain, once landed, are extremely variable and may be as high as 100% in exceptional circumstances. The following cases serve to illustrate the scale of the problem.

   In 1981, a cargo of maize shipped from North America to West Africa was found to have deteriorated soon after unloading. Of the original 10,000 tonnes shipped, approximately 5,000 tonnes were considered acceptable for distribution as food. A further 1,500 tonnes were also classified as fit for consumption, despite the presence of considerable mold damage, but only because of the severe shortage of food grain in the country concerned. The remaining 3,500 tonnes were downgraded and used for animal feed.

   Between 1982 and 1985, four cargoes of food aid wheat and maize shipped to East and West Africa suffered similar deterioration. The total amount of grain involved was 150,000 tonnes, of which 11.5% was lost as food through outright rejection or downgrading to animal feed. The loss in individual consignments varied from 4% to 46%.

   A whole shipment of 4,500 tonnes of maize received in South America in 1989 had to be downgraded to animal feed following the discovery of severe mold damage and a patchy distribution of aflatoxin throughout the consignment.

   The reports of such losses usually refer to deterioration identified at, or soon after, unloading at the destination port. However, the problem is probably more widespread with potentially serious situations remaining undetected. A consignment of grain with a moisture content that makes it unsuitable for long-term storage may appear to be in good condition when unloaded. Even if molds have already started to grow, detection will require a microscopic examination. Once in store, the surface of a grain bulk or the outer bags in a stack may remain mold-free, but deep within the bulk or stack, mold growth may spread rapidly.

   Even when this mold damage is detected, recipients of food aid may be reluctant to complain or may do so belatedly. The commodities may be seen as gifts, which they do not wish to reject. Or the late detection of a problem may be accepted as a result of a shortcoming in local storage management procedures. The view may be taken that an alternative use can always be found for such damaged grain.


   During the period from 1988 to 1991, N.R.I. conducted a series of shipboard and warehouse storage trials to investigate the causes of loss and deterioration in shipments of European wheat to East Africa and Asia and of North American maize to West Africa. In each case, the grain was shipped at the standard moisture content usually applied to aid grain. Although in the final trial, observations were made on a small additional parcel of maize that had been specially dried by about 1.5%.

   Instruments were placed in the ships' holds to record grain moisture content and temperature during the voyage. At the destination port, stacks of the discharged grain were stored in warehouses close to the port, and the grain was similarly monitored until it was issued. Samples of grain collected at the beginning and end of the voyage and before and after storage were analyzed for the presence of molds.


   The sea trials were conducted during northern latitude winter or early spring, so all grain was cold at loading. During each voyage, the grain moisture content and temperature remained relatively constant. When unloaded, the cargoes were exposed to substantially higher ambient temperatures, and the condition of the grain, especially of the maize, was expected to change from safe to unsafe, allowing molds to grow.

   The average moisture content of a shipment of North American maize to West Africa was 15.2% (equivalent to an e.r.h. of 70% at 5°C), but mold growth was kept in check during the voyage because of the low temperature. However, with an average daily temperature of about 28°C at the destination port, the e.r.h. of the maize was expected to rise to 77%, quite unsatisfactory for storage.

   Congestion at the port meant the vessel had to lay at anchor for a week. During this time, the temperature of the grain at the top and sides of the hold gradually increased to about 25°C. The temperature increased further to 30°C because of long delays in removing grain from the quayside, and a distinct odor of heating maize could be detected.

   During storage, the air circulating in the warehouse cooled the maize in the outermost layers of the stacks, but deep inside, the temperature rose rapidly to 50°C. The maize then began to cool, but even after 14 weeks of storage, some of the maize was warm (30°C), about 5°C above the temperature of the surrounding air. The maize in the bags at the edges of the stacks remained in good condition, but heavy mold damage was found when the top bags were turned over and the stacks dismantled.

   A second shipment of maize, also from North America to West Africa, was loaded at 10°C. It consisted mostly of standard-grade maize with an e.r.h. of 70%, but included a second lot dried to a moisture content corresponding to an e.r.h. of 65%. The ambient temperature at the destination port was expected to be about 28°C. While there was a high risk of deterioration in the standard grade maize, the drier maize would be only just over the limit for safe storage.

   The maize was stored for three months and, as expected, the standard grade deteriorated rapidly; the temperature within the stack rose to more than 50°C within the first 10 days, and mold damage was extensive. The drier maize also deteriorated but to a lesser extent, and heating was much slower.

   The advantage of storing maize at a lower moisture content was more clearly demonstrated by a consignment imported to West Africa from Zimbabwe and stored at the same site as the North American maize. After three months of storage, the maximum temperature recorded was 28.5°C, even deep within the interior of the stack.. The average moisture content was 12.2% (which equates to an e.r.h. of about 60% at 28°C), and no mold damage occurred.

      Reducing the problem

   The moisture content of temperate food aid grain is often too high for safe storage at the tropical destination, but donor organizations and grain exporters responsible for arranging food aid shipments may be unaware of the technical requirements for management of grain in tropical climates.

   In commercial grain contracts, importers may specify particular requirements relating to safe storage and pay a premium, if necessary. Recipients of food aid are rarely able to make demands and may be morally or politically obliged to accept whatever is offered. Some may be able to make their requirements known to donor agencies, but since most decisions on food aid are made on short notice by administrators, the technical issues often are overlooked.

   Three options are available to reduce food losses associated with grain shipped from temperate to tropical regions:

   1— Modify the handling procedures at the receiving ports to reduce grain moisture content and allow rapid distribution to consumption areas where climatic conditions may be less hostile for grain storage.

   2— Manipulate shipping dates, ports of origin and destinations to avoid combinations that will result in a high risk of deterioration.

   3— Dispatch the grain in a condition that will inhibit the effects of climatic change and the anticipated handling procedures.

      Changing practices

   The first option requires the construction of small stacks with “pigeon-hole” stacking for maximum ventilation or the use of portable stack ventilation equipment. However, in the majority of receiving countries, storage space is at a premium; “pigeon-hole” stacking may not be possible, and the cost of providing and maintaining the portable ventilation units may be prohibitive.

   Drying grain at the import point has been considered, but the complexity and cost of even a modest bulk handling and drying plant is beyond the financial and technical resources of many developing countries.

   Rapid movement of grain to inland areas where climatic conditions are less hostile would help, but shortage of suitable transport is often a constraint. Problems are increased when imports of large consignments, arranged by donors to achieve economies of scale, outstrip the capacity of local transport.

   Because of the lack of skilled manpower in many of the countries receiving food aid, only very basic management techniques can be applied. The World Food Council recognizes the need to provide technical assistance and proper training of staff in the management and handling of food commodities, but as yet there is little sign that this extends beyond the most elementary training.

   The second option involves manipulating shipping dates and ports of origin and destination. This practice would seem possible since sufficient data exists to predict the likelihood of cargoes deteriorating. However, the routing and timing recommendations may be inconvenient to donors and recipients alike. Forecasting of potential problems may be of value in planning grain shipments, but this option is really only applicable for destinations where there is a distinct cool season.

   The third option of shipping grain at predetermined “safe” moisture content seems quite straightforward. However, the view among donor agencies appears to have been that the collection of shipments of up to 20,000 tonnes for special treatment would impose such serious logistical constraints on commercial operations that the trade would be reluctant to supply drier grain, or the additional costs would be prohibitive.

      Value of food aid

   Much concern is often expressed over minimizing the costs of food aid shipments, which are generally conducted as normal commercial operations. However, it can be argued that aid shipments should be regarded differently, and alternate methods should be used. Costs are normally calculated on a wet-weigh f.o.b. (load port) basis, but it would be more appropriate to use a dry-weight, c.i.f. (destination port) basis. In the latter approach, it is the food value of the grain shipped that matters, not the amount of water transported. Viewed in this way, the extra costs incurred in drying are unlikely to appear excessive, even before account is taken of the additional benefits of a reduction in the amount of food lost or downgraded as a result of mold damage.

   Furthermore, the point of assessment should be c.i.f. (target population) rather than c.i.f. (destination port). The value of the grain c.i.f. (target population) is many times higher than the f.o.b. value. In Ethiopia, for example, the value of European wheat delivered to beneficiaries in remote areas can be as much as five times the f.o.b. value. In these situations, the costs of supplying drier grain would be far outweighed by the benefits.

      Moisture contents

   Experience has shown that for temperate exports of maize, the maximum moisture content should be 13.5%, and for wheat, 14%. Moreover, moisture content should be determined by methods prescribed by the International Standards Organization (ISO). Exporters sometimes use other methods, not all of which may be internationally recognized. Some have shown to produce results between 1% and 1.5% lower than the ISO readings.

   The moisture content limits refer to the average for a consignment as a whole, but the moisture in the individual lots making up the consignment is equally important. Grain from separate lots will approach but rarely attain a common moisture level. As a result, the grain from a particular lot, despite the final average, may be acceptable. All grain should, therefore, be dried to a safe level rather than over-drying in an attempt to achieve an acceptable average. It is sometimes suggested that a maximum limit should be imposed for each 500-tonne lot to limit the variation in moisture throughout a consignment. This would impose an unacceptable burden upon suppliers, especially of food aid, because contracts will require that the moisture content be determined by standard methods that would delay loading. A compromise may be needed, with limits equal to the permitted average for the consignment being imposed on lots of between 1,000 tonnes and 2,000 tonnes or on the quantity loaded each day.

      Drying maize

   Serious difficulties in obtaining low-moisture wheat are rare, but there are special problems with maize, particularly in North America. Drying at high temperatures can produce stress cracks in the grain during handling. Two-stage drying with a tempering stage will reduce stress cracks. But at harvest, the dryers are often worked to capacity, and there is not time for two-stage drying.

   The level of damage increases when dry maize is handled in bulk and is severe when grain from the silo strikes the steel work of the ship. The overall quality of the grain decreases because the grain may not conform to specifications and becomes more susceptible to insect and mold damage. The amount of damage will be less in bagged cargoes, because bagging is invariably carried out before the grain is delivered to the port. The cost per tonne of bagged maize is likely to be about 20% higher than for bulk shipments, but this is offset by the better keeping qualities (lower moisture content and fewer brokens) and the faster unloading at destination, particularly in those developing countries lacking adequate port bulk-handling facilities.


   Another solution to the moisture problem in food aid maize imports is the greater use of triangular transactions, whereby a donor purchases grain from surpluses in a neighboring developing country. The climatic conditions in the two countries may be similar. The grain, being warm and dry, will store well in the receiving country, exhibiting none of the problems associated with cold moist grain imported from temperate regions. Other advantages are lower transport costs and shorter transit times. The commodity is likely to be more suited to the local diet of beneficiaries, and the supplying country receives much needed foreign exchange.

   However, this is unlikely to be a complete answer in the near future. While grain production in many developing countries remains erratic and demand for food aid is high, large quantities of grain will have to be imported from temperate regions. Therefore, donor agencies have a special responsibility to ensure that such cargoes are shipped at appropriate moisture contents and that suppliers are made aware of the reasons for the special requirements.


   Food and Agriculture Organization (1991) Food Outlook 9, F.A.O., Rome, 1991.

   Pixton, S.W. (1982). ‘‘The importance of moisture and equilibrium relative humidity in stored products.'' Tropical Stored Products Information 43, p. 16-29.