Microbial Spoilage in Pulses and Grains: Causes and Prevention

Pulses and grains are the backbone of global nutrition. From rice and wheat to lentils and chickpeas, these staples feed billions of people every day. But there’s a hidden challenge that threatens their quality long before they reach your plate – microbial spoilage. Bacteria, fungi, and yeasts quietly colonize these foods during harvesting, transport, and storage, leading to nutritional losses, off-flavours, and even serious health risks like mycotoxin poisoning. Understanding what drives this spoilage – and how to stop it – is essential for farmers, food processors, and consumers alike.

Why are pulses and grains vulnerable to microbial spoilage? 🔗

Despite being classified as non-perishable foods due to their low moisture content, pulses and grains are far from immune to microbial attack. Their rich nutrient profile – packed with carbohydrates, proteins, and fats – makes them an attractive substrate for microorganisms once conditions become favourable. The key difference between grains spoiling and staying safe often comes down to how well moisture is managed.

Grains and pulses carry a natural microbial load right from the field. Soil, irrigation water, wind, and contact with harvesting equipment all introduce bacteria, fungal spores, and yeasts onto the surface of seeds. According to research published in Frontiers in Microbiology, water serves as a carrier for numerous bacteria and parasites that contaminate foods during both pre-harvest and harvest stages. Wind also carries mould spores that settle on grain surfaces. Once these microorganisms find the right combination of moisture and warmth, they multiply rapidly.

Types of microorganisms involved in spoilage 🔗

Not all microbes cause the same type of damage. The spoilage of pulses and grains involves distinct categories of organisms, each with different environmental preferences and impacts.

Fungi – the primary culprits 🔗

Fungi are the most significant spoilage agents in stored grains and pulses. They are broadly grouped into field fungi and storage fungi based on when they infect the crop.

Field fungi include species of Fusarium, Alternaria, and Cladosporium. These organisms infect crops while they are still growing and typically need high moisture levels to remain active. Once grains are harvested and dried, field fungi usually become dormant.

Storage fungi are primarily species of Aspergillus and Penicillium. These are the real troublemakers during post-harvest storage. As noted by FAO’s mycotoxin prevention guidelines, storage fungi can thrive at relatively low moisture levels and are responsible for producing dangerous mycotoxins. Aspergillus flavus and A. parasiticus produce aflatoxins, while various Penicillium species generate ochratoxins and other harmful compounds.

There is also a third category – advanced deterioration fungi – such as Rhizopus, Mucor, and Absidia. These species attack grains that are already damaged or have very high moisture levels, further accelerating the breakdown process.

Bacteria 🔗

While fungi dominate stored grain spoilage, bacteria also play a role – particularly in grains with higher moisture content or in processed pulse products. Common bacterial genera found on stored grains include Bacillus, Pseudomonas, Enterobacter, and Pantoea. According to a study published in PMC on wheat grain microbiota, bacterial communities comprising Bacillus and Pseudomonas species are consistently found on stored wheat grains. Bacterial contamination becomes a bigger concern when grains are processed into flour, dough, or cooked products where moisture levels are higher.

Yeasts 🔗

Yeasts like Cryptococcus, Rhodotorula, and Candida species are commonly isolated from stored grain environments. While they are generally less damaging than fungi, yeasts can cause fermentation, off-flavours, and contribute to quality degradation in pulse and grain products that contain higher sugar or moisture levels.

Key factors that drive microbial spoilage 🔗

Microbial growth in stored pulses and grains doesn’t happen randomly. It is governed by a set of environmental and intrinsic factors that either promote or restrict microbial activity.

Moisture content and water activity 🔗

Moisture content is the single most critical factor determining whether stored grains remain safe or become a breeding ground for microbes. Most grains are considered safe for storage when their moisture content stays below 14%, while pulses should ideally be maintained below 15-16%. Storage fungi like Aspergillus species require a minimum water activity (a_w) of about 0.65-0.70 to begin germinating, according to research from ScienceDirect. Even small increases in grain moisture – from, say, 12% to 16% – can trigger rapid fungal proliferation within days.

Water activity (a_w) is actually more important than total water content because it measures the availability of water for microbial metabolism. Properly dried grains have low water activity, making it difficult for most spoilage organisms to survive.

Temperature 🔗

Temperature acts as an accelerator for microbial activity. Most storage fungi grow best between 25°C and 40°C, which means tropical and subtropical climates face especially high spoilage risks. However, some fungi can grow slowly even at cooler temperatures. Temperature fluctuations during storage are particularly dangerous because they cause moisture migration – warm air carries moisture to cooler grain surfaces, creating localised “hot spots” where fungal growth intensifies. Research has shown that fungal hot spots in grain stores can raise the temperature to 50-60°C, which then allows thermophilic bacteria to take over and cause even further damage.

Physical damage to grains 🔗

Broken, cracked, or insect-damaged grains are far more susceptible to fungal invasion than intact ones. The seed coat of grains and pulses serves as a natural barrier against microbial entry. When this barrier is compromised – during mechanical harvesting, rough handling, or insect feeding – the starchy endosperm and nutrients inside become directly accessible to moulds. The World Health Organization advises that avoiding damage to grains before and during drying, and in storage, is important because damaged grain is more prone to mould invasion and mycotoxin contamination.

Insect infestation 🔗

Insects and microbial spoilage go hand in hand. Insects such as grain weevils, flour beetles, and pulse beetles bore into seeds, creating entry points for fungal spores. Their metabolic activity also generates heat and moisture within the grain mass, further encouraging mould growth. According to FAO research on grain drying and chemical treatment, insect damage is often followed by mould because insects produce heat and water that push the storage microclimate into a high-deterioration zone. Approximately 10% of harvested food is destroyed in storage by insect pests alone.

Storage duration and conditions 🔗

The longer grains are stored, the greater the risk of spoilage – especially if conditions are not well controlled. Poorly ventilated storage structures, leaking roofs, or direct ground contact allow moisture to seep in. In many developing countries, farmers store grains in traditional structures made of mud, bamboo, or jute bags that offer limited protection against humidity and pests.

The mycotoxin threat 🔗

Microbial spoilage of grains and pulses is not just about losing food quality. One of the most dangerous consequences is mycotoxin contamination. Mycotoxins are toxic secondary metabolites produced by certain fungal species, and they pose severe risks to human and animal health.

The WHO reports that mycotoxins are chemically stable compounds that survive food processing, meaning once they contaminate grain, they are extremely difficult to remove. The most concerning mycotoxins include aflatoxins (from Aspergillus flavus and A. parasiticus), ochratoxin A (from Aspergillus and Penicillium species), fumonisins (from Fusarium species), and deoxynivalenol (also from Fusarium).

Aflatoxin B1 is considered one of the most potent liver carcinogens known. Chronic exposure to low levels of aflatoxins has been linked to liver cancer, immune suppression, and growth impairment in children. According to the research published in PMC, global crop products may have a mycotoxin contamination rate as high as 25%, with roughly 2% of contaminated products losing their nutritional and economic value entirely.

Mycotoxin contamination can occur at any stage – from pre-harvest stress and insect damage in the field to improper drying and storage. This makes a comprehensive, multi-stage prevention approach essential.

Signs of microbial spoilage in pulses and grains 🔗

Detecting spoilage early is crucial. Here are the main indicators to watch for:

Visual changes: Mould growth appears as fuzzy patches or discolouration (green, black, white, or yellow) on grain surfaces. Caking or clumping of grains often signals excess moisture and fungal activity.

Off-odours: A musty, sour, or fermented smell is one of the earliest warning signs of spoilage, often appearing before visible mould. This is caused by volatile metabolic compounds released by fungi and bacteria.

Heating of grain mass: Unusual warming within a grain store indicates active microbial or insect respiration. This self-heating can escalate quickly, potentially leading to grain charring in severe cases.

Changes in nutritional quality: Fungal contamination leads to a decrease in carbohydrate and oil content, reduced germination ability, increased free fatty acid levels, and degradation of storage proteins. These changes may not be visible but significantly reduce the value and safety of the grain.

Spoilage of processed pulse and grain products 🔗

Processing pulses and grains into products like flour, bread, pasta, dal, or fermented foods changes the spoilage dynamics significantly. These products generally have higher moisture content and altered pH levels compared to whole dry grains, making them more susceptible to bacterial spoilage alongside fungal issues.

For example, wheat flour can harbour Bacillus spores that survive milling and become active when moisture is added during dough preparation. Cooked rice left at room temperature provides ideal conditions for Bacillus cereus to multiply and produce emetic toxins. Fermented pulse products like idli batter or tempeh rely on controlled microbial activity, but if undesirable organisms dominate, the products can spoil rapidly.

Bread and bakery products made from grain flour are prone to mould growth – particularly Penicillium, Aspergillus, and Rhizopus species – especially in humid conditions. Proper packaging and the use of approved preservatives like propionic acid (commonly used in bread) help control these issues.

Prevention and control of microbial spoilage 🔗

Effective prevention requires a multi-layered approach addressing the entire chain from harvesting to consumption.

Proper drying 🔗

Drying grains and pulses to safe moisture levels immediately after harvest is the single most effective step against spoilage. For most cereals, this means reaching below 14% moisture content; for pulses, below 15%. Sun drying is the most common and affordable method, though mechanical dryers offer faster, more consistent results – especially critical in humid or rainy climates. As emphasized by the FAO’s grain storage guidance, drying to moisture levels that ensure safe storage is especially important when grains are stored in tropical climates.

Optimal storage conditions 🔗

Once dried, grains must be stored in clean, dry, well-ventilated structures that protect them from moisture re-absorption, pests, and temperature extremes. Key practices include using moisture-proof containers or silos, ensuring storage facilities are cleaned and disinfected before loading new grain, avoiding direct contact between grain bags and floors or walls, and monitoring temperature and moisture regularly throughout the storage period.

Insect and pest control 🔗

Since insects are major vectors of fungal contamination, controlling infestations is crucial. This includes using approved insecticides or fumigants (such as phosphine), maintaining storage hygiene, and using physical barriers like hermetic storage bags that limit oxygen availability to both insects and moulds.

Controlled atmosphere storage 🔗

Reducing oxygen levels in storage environments slows down both microbial and insect metabolism. Hermetic storage – where grain is sealed in airtight containers – has proven highly effective, particularly for smallholder farmers in developing countries. As oxygen depletes and carbon dioxide builds up inside the sealed space, fungal growth is significantly inhibited.

Chemical and biological control 🔗

Food-grade organic acids such as propionic acid and acetic acid can inhibit mould growth when applied to stored grain. Diatomaceous earth provides physical protection against insects. Newer approaches include biological control agents – beneficial microorganisms like non-toxigenic strains of Aspergillus or lactic acid bacteria that compete with harmful fungi and reduce mycotoxin production. Research published in PMC has shown that yeast strains like Rhodotorula glutinis isolated from wheat grains can reduce aflatoxin B1 levels by up to 65%.

Good agricultural practices (GAPs) 🔗

Prevention starts in the field. Timely harvesting, minimizing mechanical damage during threshing, avoiding contamination with soil or debris, and not mixing clean grain with dirty grain are all foundational steps. Crop rotation and the use of fungal-resistant varieties also help reduce the microbial load that grains carry into storage.

The global impact of grain spoilage 🔗

Microbial spoilage of pulses and grains is not just a storage inconvenience – it is a major global food security and public health concern. According to Frontiers in Sustainable Food Systems, an estimated 30-50% of food commodities are lost during pre-harvest or post-harvest stages globally. In developing countries, where storage infrastructure is often inadequate, spoilage losses can be devastating – particularly for smallholder farmers who depend on stored grain for both food and income.

The economic costs are staggering. Mycotoxin management alone costs billions of dollars annually across major grain-producing regions. Beyond economics, mycotoxin-contaminated grain that enters local food supplies can cause chronic health problems, especially among vulnerable populations like children and pregnant women.

Addressing this challenge requires investment in better storage infrastructure, farmer education on post-harvest handling, development of affordable moisture-testing tools, and stronger regulatory frameworks for mycotoxin monitoring.

What do you think? How can smallholder farmers in tropical regions be better supported to reduce post-harvest grain losses? What role should governments and international agencies play in making affordable storage technologies more accessible?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10325786/
2. https://www.fao.org/4/x5036e/x5036E0q.htm
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC6533538/
4. https://www.sciencedirect.com/science/article/abs/pii/S0926669017304259
5. https://www.who.int/news-room/fact-sheets/detail/mycotoxins
6. https://www.fao.org/4/x5036e/x5036e0w.htm
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC10972482/
8. https://www.fao.org/4/t1838e/T1838E0c.htm
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC8470753/
10. https://www.frontiersin.org/journals/sustainable-food-systems/articles/10.3389/fsufs.2023.1162595/full