Aflatoxins in Rice and Other Staple Grains: An Invisible Hazard Hidden in Storage
This is the 4th part from the original 'let food be thy medicine and medicine be thy food' series, first posted on:
Sunday, July 12, 2026
“Let Food Be Thy Medicine”: Wisdom, Misquotation, and the Limits of Turning Medicine into Food (Part 1)
https://scientificlogic.blogspot.com/2026/07/let-food-be-thy-medicine-wisdom.html
The last part (Part 3) was on:
Wednesday, July 22, 2026
Food Toxicology: Merging Biological Pathogens, Secondary Fungal Metabolites, Pyrolytic Carcinogens, and Analytical Quality Control
https://scientificlogic.blogspot.com/2026/07/food-toxicology-merging-biological.html
When most people hear the word aflatoxin, they immediately think of peanuts or groundnuts.
I have briefly mentioned about aflatoxin in my last article on:
The Multi-Dimensional Matrix of Food Toxicology: Merging Biological Pathogens, Secondary Fungal Metabolites, Pyrolytic Carcinogens, and Food Quality Control
Few realise that these same toxic compounds can also contaminate rice, the staple food consumed daily by billions of people throughout Asia.
Surprised?
Most people are. Rice has long been regarded as one of the safest foods because it is eaten every day by so many people. Fortunately, the risk to consumers is generally low. Rice is a fast-moving commodity that is usually milled, marketed, and consumed relatively quickly. Commercial producers also monitor their products carefully to comply with national and international food safety standards.
Nevertheless, under warm and humid tropical conditions, rice can become contaminated if it is inadequately dried before storage or kept in damp warehouses or containers. Under these favourable conditions, moulds such as Aspergillus flavus and Aspergillus parasiticus may grow on the grains and produce aflatoxins—among the most potent naturally occurring liver carcinogens known.
Perhaps even more surprising is another fact that challenges common perception.
Most people believe that brown rice is invariably healthier than polished white rice because it retains more fibre, vitamins, minerals, essential fatty acids, and antioxidants. Nutritionally, this is indeed true. However, from the standpoint of aflatoxin contamination, brown rice may carry a higher risk if contamination has occurred, because the toxins are concentrated mainly in the grain's outer layers.
The reason becomes clear when we examine the structure of the rice grain.
Brown rice retains its bran and germ, whereas polished white rice has these outer layers removed during milling.
Brown Rice
→ Bran and Germ Retained
→ Rich in oils, fats, proteins, vitamins and minerals
→ Provides an ideal surface for fungal growth
→ Greater retention of any aflatoxins present
White Rice
→ Bran and Germ Removed by Milling
→ Mostly starchy endosperm remains
→ Less favourable for fungal growth
→ Milling removes a large proportion of surface contamination and accumulated toxins.
Several biological factors explain this difference.
1. The Nutrient-Rich Bran Layer
The bran and germ of brown rice contain abundant lipids, fatty acids, proteins, vitamins, and minerals. These nutrients make brown rice nutritionally superior, but they also provide an excellent food source for Aspergillus moulds. Once fungal spores settle on the grain under moist conditions, they can colonise these nutrient-rich tissues more readily than the relatively nutrient-poor starchy endosperm of polished rice.
2. The Protective Effect of Milling
Fungal contamination usually begins on the outer surface of the grain. Since aflatoxins accumulate mainly within the bran and germ, the commercial milling process that converts brown rice into polished white rice removes much of the contaminated outer layer. Numerous studies have shown that polishing can substantially reduce aflatoxin levels, although it cannot guarantee complete elimination.
3. Surface Structure and Moisture
The rough, textured bran surface of brown rice traps moisture more readily than the smooth surface of polished rice. Increased moisture encourages fungal growth during prolonged storage, particularly in tropical climates where heat and humidity remain high throughout the year.
This does not mean that brown rice is unhealthy. On the contrary, under normal conditions it remains an excellent whole grain with many nutritional advantages. The important point is that when aflatoxin contamination occurs, brown rice generally retains more of the toxin than polished white rice because its protective outer layers remain intact. In short, it is not always true that brown rice is more nutritious and health-protective than white rice. Surprised?
An additional concern is that contamination is virtually impossible to recognize by sight alone. Aspergillus mould may not always produce obvious visible growth, while aflatoxins themselves are completely invisible, odourless, and tasteless.
It is unfortunate there is no way of knowing if a food is contaminated with aflatoxin without sending the sample to an analytical chemistry laboratory for analysis such as using TLC (Thin Layer Chromatography).
Other methods requires specialized laboratory methods such as high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), or enzyme-linked immunosorbent assay (ELISA).
They may be some simple 'dip stick' method that can be done at home, but I am unsure how reliable it is? Normally I used TLC or the HPLC which is far more complicated for analytical chemistry and in food quality control, but more reliable .
Even more troubling is the remarkable stability of these toxins. Unlike many bacteria, aflatoxins are highly heat-stable and are not destroyed by normal cooking temperatures. Washing or boiling rice may remove some surface dust but cannot reliably eliminate the toxins once they have formed.
Reducing Your Risk
Fortunately, simple precautions can greatly reduce exposure.
Purchase rice from reputable manufacturers that routinely monitor their products for aflatoxins and comply with national and international food safety regulations.
Store rice properly in a cool, dry, well-ventilated place using airtight containers to minimize moisture absorption.
Avoid storing rice for prolonged periods, particularly in warm and humid environments.
Maintain a varied diet rather than depending exclusively on a single grain source, thereby reducing long-term exposure should contamination occur.
What About Maize, Wheat and Flour?
Rice is by no means the only cereal vulnerable to aflatoxins. Maize (corn), wheat, barley, sorghum, millet, and cereal flours can also become contaminated when crops are grown or stored under warm, humid conditions that favour the growth of Aspergillus species.
Among these cereals, maize is considered one of the most susceptible worldwide. The open structure of the maize ear allows insects to damage the kernels, creating entry points for fungal spores. Heat stress, drought, and improper post-harvest drying further encourage fungal growth and stimulate aflatoxin production. Consequently, maize is one of the commodities most frequently monitored in international food safety programmes.
Wheat is generally less susceptible because it is commonly cultivated in cooler temperate climates. Nevertheless, contamination can still occur if heavy rainfall delays harvesting, if grain moisture remains high after harvest, or if wheat is stored under damp conditions for prolonged periods.
Flour deserves special attention because milling does not destroy aflatoxins already present in contaminated grain. In addition, flour readily absorbs moisture from the surrounding air. If stored in humid conditions, mould growth may develop within the flour itself, further compromising its quality, although fresh toxin production depends on the presence of viable moulds and suitable environmental conditions.
The important lesson is that aflatoxin contamination is not restricted to peanuts alone. It represents a potential hazard for many staple foods. Fortunately, modern food safety regulations, proper agricultural practices, adequate drying, good storage conditions, and routine laboratory monitoring have greatly reduced this risk in most commercially produced foods. For consumers, buying from reputable suppliers and storing food correctly remain the simplest and most effective safeguards against these invisible toxins.
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