Tropical grain storage

by Carl Reed
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Maintaining grain quality in consistently warm climates is much more difficult than in temperate zones. Grain managers in tropical and sub-tropical areas confront very different challenges depending on whether the grain is imported or local, whether it is rice or wheat, and whether the climate is arid, lowland tropics or upland tropics.

Therefore, there is no single set of “expert recommendations.” But as a stored-grain scientist with a couple of decades of site visits and short course experiences in countries with warm climates, I have formulated some potentially useful ideas.


Some of the most common questions asked by grain industry businessmen about tropical climate grain storage include:

• Is a concrete bin or steel bin better?

• What about upright versus flat stores?

• What is the ideal grain moisture and temperature for storage?

• Should the aeration fans push or pull?

Unfortunately, the answers depend on many factors, most of which are beyond the manager’s control. Under the extreme conditions of tropical storage, a more practical approach is to identify technologies and/or practices that fit your operation and budget, and that work well enough and long enough to meet your requirements.


The useful questions are related to risks. Front-office managers of grain-related businesses today are thinking about things such as markets, transportation, currencies, or weather. In addition to those big-ticket items, they also are concerned about risks related to grain quality deterioration.

Every grain manager with more than a few months experience in his company can write a list of the grain deterioration risks to which his company is exposed. Typically, some “risk-list” items result from painful experience whereas others are concerns about a catastrophic event that, thankfully, has not yet occurred. A sampling of the useful questions of this type includes:

• What were the root causes of the previous, expensive events related to grain deterioration?

• What is the probability they will recur?

• Are the feared events likely to happen?

• How can these risks be controlled in an efficient manner, given the conditions of our operation?



The grains that move in international commerce are considered durable commodities. They retain their valuable attributes over very long periods of time. Stored grain does not deteriorate by itself. Something must cause it to deteriorate, and the list of culprits is short. It includes:

• Physical forces. Forces exerted on kernels in harvesters and grain handling equipment damage the seed coats and other protective layers, destroying the natural protective mechanism of the seed. By the time most grain arrives at the storage or processing site, it is easily attacked by molds and insects, and contains broken kernels.

• Insects. Stored-grain insects cause grain contamination and loss, and are likely to create “bad neighbor” issues in tropical climates.

• Molds. In addition to the kernel damage, odors, caking, and heating that molds sometimes cause in stored grain, the misinformation and misunderstandings associated with this topic cause much fretting and hand-wringing, especially in tropical areas of the world. If grain handlers in temperate zones tested for mycotoxins as frequently as some of their counterparts in the tropics, owners of test kit companies would be some of the wealthiest men in the world.

This short list, plus potential contamination by a variety of substances, is the same for grain stored in warm climates as elsewhere. But the rate of deterioration is faster in hot, moist environments, and interventions often are less effective.


The Hazard Analysis and Critical Control Point (HACCP) approach does not fit stored grain perfectly, but many of the same principles apply. Risk control in stored grain must start with and be centered on:

• information that provides insight and understanding of deteriorative processes;

• information about the grain condition;

• information to document good practices;

• information passed in a timely manner to decision-makers;

• information to objectively evaluate performance of employees and risk control programs.

Risks in stored grain are controlled through information, not hardware and chemicals. Certainly one must have appropriate equipment, and chemicals for controlling molds and insects are valuable. But they can not be used efficiently without information and understanding.

The issue of controlling the impact of breakage and fine material is of particular importance in imported grain, though segregation based on differences in particle size occurs whenever bulk grain is handled. Fine material usually builds up in a column beneath each spout position where a column of falling grain contacts the surface of the grain already in the bin.

Accumulations of foreign material and/or fine material can be avoided through pre-cleaning, or detected through sampling. Over time, the grain manager can learn to manage this issue to minimize the impact on processes and profits, but upper management must assign the task and provide the incentives.

When it comes to insect control, many grain managers – even those with many years of experience – do not advance beyond the “gas ’em” mentality. Often, this is because they can’t tell what is working and what is not. Unless you have sampling procedures that provide real answers, you cannot know whether the incoming grain is infested, whether sanitation is adequate, whether the fumigation was successful, or whether insects are mostly invading from the neighboring plant.

In processing plants and finished products warehouses, various types of insect traps are helpful in describing the insect risk. But in grain handling facilities, we are stuck with grain sampling, and that is the basis of the problem.

It is important to remember that: only about 20% of the typical grain beetle population is adults; only the adults are seen in the samples; and the probability of capturing an insect — adult or otherwise — is about one-third what you think. As a result, the probability of detecting insects in a 1-kilogram (kg) sample is only about one in 15 when each tonne of grain contains 1,000 insects, and it is only about one in 150 samples if the density is one insect per 10 kg. To improve the probability of detecting insects, you must take larger samples at a greater frequency.

To control molds, you must control the environment within the grain mass. Unfortunately, that’s not as easy as it sounds. First, most people have little understanding of microorganisms. The fact that a high mold count often is associated with recently harvested grain in good condition whereas mold-deteriorated grain may test low for mold presence is counterintuitive to us. Secondly, the properties of air – the most important attribute of the storage environment for the microorganism – constitute a similar mystery. The grain type, temperature and moisture content determine the equilibrium relative humidity of air within the grain mass. This parameter, often expressed as water activity, the decimal form of the percent relative humidity, determines which mold species can grow and how fast. Often, a water activity of 0.72 or greater is taken as an indicator of grain at risk.

That does not mean that you must measure this elusive parameter, and it certainly does not mean that water activity should be used to accept or reject grain shipments. Equations to estimate water activity are readily available in agricultural engineering handbooks. To use them, all you need to know is the grain temperature, the grain moisture content and some high school algebra.

One such equation generated the numbers shown in Table 1 (see page 59). In this example, a grain sample collected from a ship’s hold and tested in a laboratory reflected neither the water activity in the ship nor the hazard in the storage bin.

Similar confusion about the equilibrium between air and grain has caused some managers to develop grain aeration practices that create more problems than they solve, especially in warm, humid climates. In most warm-climate

situations, the only reason to aerate is to keep the grain as cool as possible. Yet, it is common in tropical areas of the world that fans are operated in the hot afternoons because the relative humidity is considered too high during other parts of the day.

Efficient aeration practices for this situation begin with grain temperature monitoring. Only if the interior grain temperature increases over time, and only if the warmest grain in the bin becomes3 or 4 degrees C warmer than the daily average ambient temperature will aeration be useful. If these conditions are met, and if the grain will be kept for an additional three or four weeks, aeration with the coolest air available — regardless of the relative humidity — just long enough to pass the thermal front through the grain mass will be useful.


Grain managers in warm or tropical climates must be better prepared than their counterparts in temperate climates. My experience has been that a search for the latest or best equipment or chemical is of limited value.

The approach that incorporates data driven decision-making, values a well trained staff, and focuses on collecting and using the right information in the right program that will pay dividends in the future.

Carl Reed is a grain storage specialist who recently retired from the Department of Grain Science and Industry at Kansas State University, Manhattan, Kansas, U.S. He can be contacted through the website:  .