Microbiological Concerns in Raw Material Handling and Storage

Every food product starts with raw materials – grains, oilseeds, pulses, spices. But before these materials even reach a processing facility, they face an invisible threat: microorganisms. Moulds, bacteria, and yeasts can silently colonize raw materials during harvesting, transport, and storage, leading to spoilage, quality loss, and sometimes dangerous toxin contamination. Understanding these microbiological concerns is essential for anyone working in food processing, agriculture, or supply chain management.

Why microorganisms matter in raw materials 🔗

Raw agricultural materials are living biological systems. They carry a natural microbial load from the field, including bacteria on the surface and fungal spores embedded in crevices. Under favourable conditions – warmth, moisture, and poor ventilation – these microorganisms multiply rapidly. The result? Spoilage signs like off-odours, discolouration, caking, and heating of stored goods. In worst-case scenarios, certain fungi produce mycotoxins, which are toxic secondary metabolites that pose serious health risks to humans and animals.

According to the World Health Organization, mycotoxins are naturally produced by certain types of moulds and can be found in a wide range of foodstuffs including cereals, dried fruits, nuts, and spices. Mould growth can occur before harvest or after harvest during storage, particularly under warm, damp, and humid conditions. What makes mycotoxins particularly dangerous is that most of them are chemically stable and survive food processing.

Field fungi vs. storage fungi 🔗

Not all fungi associated with raw materials behave the same way. Scientists broadly classify them into two categories: field fungi and storage fungi.

Field fungi 🔗

These fungi infect crops while they are still growing in the field. Common genera include Fusarium, Alternaria, and Cladosporium. Fusarium species, for example, are well-known plant pathogens that attack cereal grains before harvest and produce mycotoxins like deoxynivalenol (DON), zearalenone, and fumonisins. Field fungi typically need high moisture levels (above 20%) and are most active during the growing season.

Storage fungi 🔗

Once crops are harvested and placed in storage, a different set of fungi takes over. The most important storage fungi belong to the genera Aspergillus and Penicillium. As noted in a FAO manual on post-harvest grain losses, storage fungi need a relative humidity of at least 65% (water activity of 0.65), which corresponds to an equilibrium moisture content of about 13% in cereal grain. They thrive at temperatures between 10°C and 40°C, and each species has its own optimum climatic requirements.

Storage fungi are far more common in lots infested by stored-product insects, because insects generate heat and moisture and distribute fungal spores throughout the commodity.

Key storage fungi: Aspergillus and Penicillium 🔗

Among all storage fungi, two genera dominate the conversation when it comes to raw material quality and food safety.

Aspergillus species 🔗

Aspergillus is a large genus with over 100 recognized species, several of which are capable of producing mycotoxins. The most significant species include:

Aspergillus flavus and A. parasiticus – These are the primary producers of aflatoxins, among the most potent naturally occurring carcinogens. Crops frequently affected include cereals such as corn, sorghum, wheat, and rice, as well as oilseeds like soybean, peanut, sunflower, and cotton seeds. According to the WHO, high doses of aflatoxins can cause acute liver damage (aflatoxicosis), and they have been shown to be genotoxic – capable of damaging DNA and causing cancer in both animals and humans.

Aspergillus ochraceus – This species produces ochratoxin A, a toxin primarily associated with kidney damage. It is commonly found in dried foods such as peanuts, beans, and dried fruit. In Scandinavian and Baltic regions, ochratoxin contamination in barley and wheat is a particular concern, often caused by the related species Penicillium verrucosum.

Penicillium species 🔗

Penicillium species are equally widespread in stored grains and oilseeds. Nearly 100 Penicillium species have been identified as toxin producers. Key toxins from this genus include citrinin (a kidney toxin commonly found in rice, wheat, and corn), ochratoxin A (co-produced with Aspergillus), and patulin (most commonly associated with mouldy apples and apple products but also found in grains). As described in research published in IntechOpen, chemical preservatives like propionic acid can inhibit the growth and toxin production of certain Penicillium species, though effective concentrations vary.

Types of damage caused by storage fungi 🔗

When storage fungi colonize raw materials, the consequences go well beyond a mouldy appearance. Here are the main types of damage:

Nutrient loss – Fungi feed on the carbohydrates, proteins, and lipids in grains, reducing their nutritional value. Oilseeds are particularly vulnerable because their high fat content provides an ideal nutrient source for fungal growth.

Discolouration – Fungal growth causes visible changes in the colour and appearance of grains, making them unsuitable for milling, baking, or sale.

Heating and spontaneous combustion – As fungi metabolize grain components, they generate heat. In severe cases, this metabolic heat can build up in grain bulks, creating “hot spots” that further accelerate fungal activity. In extreme situations, temperatures can rise high enough to cause spontaneous combustion.

Caking – Fungal mycelium binds grain kernels together, creating clumps that make handling and processing difficult.

Off-odours and off-flavours – Mouldy, musty, or sour smells and tastes are among the most noticeable indicators of fungal spoilage. These off-flavours can render entire batches commercially unacceptable.

Reduced germination – For seeds intended for planting, fungal damage drastically reduces germination rates, affecting future crop yields.

Mycotoxin production – The most serious consequence. As the FAO notes, mycotoxins are extremely stable and cannot be destroyed by boiling, pressing, or processing. Once contaminated produce cannot simply be mixed with clean grain or diverted to animal feed, because the toxins accumulate in animal tissues and enter the human food chain through milk, meat, and eggs.

Mycotoxins: the hidden danger 🔗

Mycotoxins deserve special attention because of the scale of the problem and the severity of health consequences.

How widespread are mycotoxins? 🔗

The Food and Agriculture Organization (FAO) estimates that 30 to 50% of food commodities are lost during pre-harvest or post-harvest stages globally. Fungal bio-deterioration is a chronic problem, particularly in tropical hot and humid climates. Research published in Frontiers in Sustainable Food Systems confirms that harvested food grains can be contaminated by multiple genera of fungi including Aspergillus, Alternaria, Fusarium, Cladosporium, Penicillium, Mucor, and Rhizopus.

Major mycotoxins of concern 🔗

The five mycotoxin groups of greatest agricultural importance are:

Aflatoxins – Produced mainly by A. flavus and A. parasiticus. Aflatoxin B1 is the most toxic of all known fungal metabolites and is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). Commodities at highest risk include maize, groundnuts, rice, and oilseeds with high fat content.

Deoxynivalenol (DON) – Likely the most widely distributed mycotoxin in food, produced by Fusarium species in wheat, maize, and barley.

Fumonisins – Very common contaminants of maize-based food and feed, produced by Fusarium moniliforme and related species.

Ochratoxin A – Produced by both Aspergillus and Penicillium species, affecting barley, wheat, and other cereals. It is associated with kidney damage and is a suspected carcinogen.

Zearalenone – An oestrogen analogue produced by Fusarium species that interferes with mammalian reproductive hormones, primarily affecting maize and wheat.

Health effects in humans and animals 🔗

Mycotoxin exposure can cause a wide range of adverse health outcomes. The FAO highlights that even at very low levels, certain mycotoxins are linked to fertility issues, weakened immunity, stunted growth in children, and cancer. In livestock, effects include reduced growth, reproductive problems, and suppressed immune function. Aflatoxin B1 is especially concerning because it can be metabolized into aflatoxin M1 in the bodies of dairy animals, which then passes into milk – a direct risk for infants and young children.

Factors that promote microbial growth in storage 🔗

Several environmental and handling factors influence whether stored raw materials will develop fungal problems:

Moisture content – This is the single most critical factor. Different fungal species have different minimum moisture thresholds. For example, Aspergillus restrictus can grow at grain moisture levels as low as 13.5%, while A. flavus requires at least 18%. Storing grain above the safe moisture limit is the primary cause of fungal spoilage.

Temperature – Most storage fungi grow between 10°C and 40°C, with optimum growth typically between 25°C and 35°C. Warm storage conditions significantly accelerate fungal development.

Physical damage – Grain that has been cracked, broken, or bruised during harvesting, threshing, or handling is far more susceptible to fungal invasion. Damaged kernels provide easy entry points for mould spores.

Insect infestation – Insects generate heat and moisture as they feed and respire inside grain stores. They also physically damage kernels and carry fungal spores, creating a synergy between insect and fungal attack.

Poor aeration – Without adequate ventilation, moisture can accumulate in pockets within the grain bulk, creating localized “hot spots” where temperatures rise and fungi proliferate.

Preventive measures to minimize microbial contamination 🔗

Since mycotoxins cannot be destroyed once they form, prevention is the only effective strategy. Measures need to be applied across the entire chain – from field to storage.

Pre-harvest practices 🔗

Implementing Good Agricultural Practices (GAPs) is the first line of defence. This includes selecting crop varieties suited to local conditions, practising crop rotation to reduce fungal pressure in soil, managing irrigation to avoid drought stress (which predisposes plants to fungal attack), and controlling insect pests in the field. The PMC research review highlights that the most effective strategies for preventing mycotoxin formation involve implementing GAPs and Hazard Analysis and Critical Control Points (HACCP) at both pre- and post-harvest stages.

Harvesting and drying 🔗

Timely harvesting is critical. Crops left too long in the field after maturity are exposed to rain, insects, and fungal infection. After harvest, rapid and uniform drying to safe moisture levels (generally below 13-14% for cereals) is the single most important step. The FAO guidance on mycotoxin prevention notes that drying is proven effective for preventing mycotoxin contamination, though access to drying equipment can be a challenge in many developing regions.

Grain should be handled carefully during threshing and transport to minimize physical damage to kernels, which creates entry points for fungi.

Storage conditions 🔗

Proper storage focuses on keeping moisture, temperature, and insect populations under control. Key practices include:

Maintaining low moisture – Store only adequately dried grain. Prevent moisture re-entry through leaks, condensation, or improper ventilation.

Temperature management – Keep storage facilities cool. Regular aeration or mechanical cooling can help maintain low temperatures and prevent hot spots.

Insect and pest control – Since insects accelerate fungal development, effective pest management is essential. This includes clean storage structures, fumigation where appropriate, and regular inspection.

Regular monitoring – Check stored grain frequently for signs of heating, off-odours, visible mould, or insect activity. Early detection allows corrective action before significant damage occurs.

Clean storage structures – Remove residual grain and debris from previous storage seasons before loading new produce. Old residue can harbour fungal spores and insect populations.

Biological control approaches 🔗

Some countries are now using non-toxigenic strains of Aspergillus flavus as a biological control method. When applied to crops, these harmless strains compete with toxin-producing strains for space and nutrients, reducing aflatoxin contamination. This technique, known as competitive exclusion, has shown promising results in maize and groundnuts in warm, humid climates.

The role of regulations and monitoring 🔗

Governments worldwide have established maximum residue limits (MRLs) for mycotoxins in food and feed. The Codex Alimentarius Commission, based on scientific risk assessments by the Joint FAO/WHO Expert Committee on Food Additives (JECFA), sets internationally recognized standards. For aflatoxins, the Codex maximum levels in nuts, grains, dried figs, and milk range from 0.5 to 15 µg/kg – extremely low limits reflecting the potency of these toxins.

Rapid testing methods such as ELISA kits, immunoaffinity cartridges, and minicolumn screening are increasingly available for field-level mycotoxin detection, enabling quicker decisions about whether grain lots are safe for consumption or trade.

Summing it up 🔗

Microbiological contamination of raw materials – particularly by storage fungi like Aspergillus and Penicillium – is a persistent challenge in the food and agriculture sector. The consequences range from simple quality defects like off-odours and discolouration to life-threatening mycotoxin contamination. Since mycotoxins are chemically stable and cannot be removed through processing, the emphasis must always be on prevention: proper drying, clean and cool storage, insect control, regular monitoring, and adherence to Good Agricultural Practices from the field to the warehouse.

What do you think? In regions where farmers lack access to mechanical dryers and modern storage facilities, what practical low-cost strategies could be most effective in reducing mycotoxin contamination? And how can consumer awareness about invisible threats like mycotoxins be improved in developing countries?

References

1. https://www.who.int/news-room/fact-sheets/detail/mycotoxins
2. https://www.fao.org/4/x5065e/x5065E0c.htm
3. https://www.intechopen.com/chapters/42603
4. https://www.fao.org/one-health/highlights/mycotoxins–a-silent-risk-to-plants–people-and-animals/en
5. https://www.frontiersin.org/journals/sustainable-food-systems/articles/10.3389/fsufs.2023.1162595/full
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC10706277/
7. https://www.fao.org/4/x5036e/x5036e0w.htm