Preventing Spoilage in Processed Pulses and Grains Products

Processed pulses and grains – products like kheer, daliya, bread, pasta, and ready-to-eat cereal mixes – are dietary staples across the globe, especially in South Asia. But despite their widespread consumption, these products are surprisingly vulnerable to spoilage. Whether it’s a packet of daliya turning stale or a batch of kheer going sour within hours, spoilage in processed pulse and grain products is a real challenge for manufacturers, retailers, and consumers alike. The causes of this spoilage fall into two broad categories: abiotic factors (chemical and physical changes) and biotic factors (microbial growth). Understanding both is essential for keeping these foods safe, nutritious, and appealing.

What makes processed pulses and grains prone to spoilage? 🔗

Pulses and grains in their raw, dry form are relatively stable. Their low moisture content – typically below 14% for grains and 16% for pulses – keeps microbial activity in check. However, once these commodities are processed into products like cooked porridges, milk-based desserts, flours, batters, or ready-to-eat meals, the game changes entirely. Processing introduces moisture, alters pH, and exposes new surfaces to contamination. Cereals generally contain 70-75% carbohydrates along with 8-15% protein, fat, fibre, vitamins, and minerals at a near-neutral pH, making them highly susceptible to microbial colonisation once moisture is available.

Products like kheer (a milk-and-rice dessert) combine the high moisture and protein of dairy with the carbohydrate-rich grain base, creating an environment that practically invites spoilage organisms. Daliya (cracked wheat porridge), especially when cooked and stored improperly, offers similar conditions. The key point is this: processing increases the water activity (a_w) of these products, and water activity is the single most critical factor determining whether microorganisms can grow.

Abiotic spoilage: chemical and physical changes 🔗

Abiotic spoilage refers to deterioration caused by non-living factors – primarily chemical reactions and physical changes that occur during processing, packaging, or storage. No microorganism is involved here, but the damage to quality can be just as severe.

Lipid oxidation and rancidity 🔗

Fats and oils present in grain products (especially those containing wheat germ, rice bran, or added ghee/butter as in kheer) are vulnerable to oxidative rancidity. This occurs when unsaturated fatty acids react with atmospheric oxygen, producing aldehydes, ketones, and free radicals that give food a stale, unpleasant taste and odour. Oxidation can affect a broad range of foods, including those containing nuts, seeds, and dairy fats, and it reduces nutritional value by degrading essential fatty acids and fat-soluble vitamins like vitamin E.

In processed grain products, the wheat germ is particularly problematic. It is nutrient-rich but also high in lipids, and once milled, the exposed germ deteriorates rapidly through lipase enzyme activity, producing rancid off-flavours. Light exposure and high storage temperatures accelerate this process significantly.

Non-enzymatic browning (Maillard reaction) 🔗

The Maillard reaction is a chemical interaction between amino acids (from proteins) and reducing sugars. While this reaction is desirable during cooking – it creates the golden crust on bread, for instance – it becomes a spoilage problem during prolonged storage. In products such as dried milk, dry whole eggs, and breakfast cereals, this reaction causes unwanted colour darkening and flavour changes over time. It accelerates at higher temperatures and lower moisture levels, which is why stored grain-based products like daliya mixes or cereal powders can gradually turn darker and develop off-flavours even without microbial involvement.

Additionally, advanced Maillard reaction products can contribute to undesirable sensory qualities such as bitterness and astringency, and may interact with oxygen to produce stale flavours.

Staling 🔗

Staling is a physical spoilage phenomenon most commonly seen in baked cereal products like bread, cakes, and biscuits. It involves the retrogradation of starch – a process where gelatinised starch molecules slowly re-crystallise during storage, causing the product to become dry, hard, and crumbly. This is not caused by microorganisms or moisture loss alone; it is fundamentally a structural change in the starch molecules. Staling reduces the palatability of many processed grain products and is a major quality concern in the bakery industry.

Moisture absorption and texture changes 🔗

Dry processed grain products – think instant daliya, breakfast cereals, or pulse-based snacks – can absorb moisture from the surrounding environment if packaging is poor. Even a small rise in moisture content from around 12% to 15-17% can dramatically change texture, making crispy products soggy and creating favourable conditions for subsequent mould growth. Temperature fluctuations during storage can also cause moisture condensation inside packages, accelerating this problem.

Biotic spoilage: the role of microorganisms 🔗

Biotic spoilage is driven by the growth and metabolic activity of bacteria, moulds, and yeasts. In processed pulse and grain products, the type of microorganism responsible depends largely on the product’s moisture content, pH, and composition.

Mould growth and mycotoxin production 🔗

Moulds are the primary spoilage organisms in cereal and pulse products. Mould is considered the primary organism causing spoilage in cereals, and the most commonly implicated genera include Aspergillus, Penicillium, Fusarium, Rhizopus, and Mucor. These organisms can grow at relatively low water activity levels, which is why even semi-dry products are at risk.

Beyond quality loss, mould contamination poses a serious health hazard through mycotoxin production. Species like Aspergillus flavus and A. parasiticus produce aflatoxins – potent carcinogenic compounds that can contaminate grains, pulses, and their processed products. Spoilage of plant-based commodities, including cereals, occurs worldwide due to a combination of environmental factors, and mycotoxin contamination remains a significant global food safety concern.

Bacterial spoilage 🔗

Bacteria play a more significant role in high-moisture processed products. In kheer and other milk-based cereal desserts, the combination of protein, sugar, and moisture provides an ideal growth medium for bacteria like Bacillus, Lactobacillus, Pseudomonas, and Micrococcus species. The most commonly associated bacterial families with cereals are Bacillaceae, Micrococcaceae, Lactobacillaceae, and Pseudomonadaceae.

A particularly concerning form of bacterial spoilage in baked grain products is ropiness, caused by Bacillus subtilis and related species. Rope-forming bacteria produce heat-resistant spores that survive the baking process. When conditions become favourable after baking (warm, humid storage), these spores germinate and produce sticky, stringy threads within the bread crumb, along with a characteristic fruity or putrid odour. This renders the product completely inedible.

In cooked pulse products, acid-forming bacteria can initiate fermentation, producing lactic acid and making the product sour. If yeasts are also present, this can progress to alcoholic fermentation, followed by acetic acid production by Acetobacter species – a cascade of microbial activity that rapidly degrades the product.

Yeast spoilage 🔗

Yeasts are generally a less common cause of spoilage in cereal and pulse products compared to moulds and bacteria. However, in products with higher sugar content – such as sweetened kheer, pastry fillings, or sugar-coated cereal snacks – yeasts like Saccharomyces, Pichia, and Zygosaccharomyces can cause fermentation, producing carbon dioxide (leading to package swelling), alcohol, and off-flavours.

Spoilage in specific processed products 🔗

Kheer and milk-based cereal desserts 🔗

Kheer is highly perishable because it combines the spoilage vulnerabilities of both dairy and cereal. The high moisture, near-neutral pH, and rich nutrient profile make it a perfect substrate for rapid bacterial growth. At room temperature, kheer can spoil within a few hours. Souring (from lactic acid bacteria), curdling (from acid or protease activity), and off-odour development are the most common signs. If improperly canned or packaged, Clostridium species can also pose a risk in hermetically sealed versions of such products.

Daliya and cooked cereal porridges 🔗

Once cooked, daliya has high water activity and is susceptible to both bacterial and fungal spoilage. Leftover cooked daliya stored at room temperature can develop sour flavours from lactic acid bacteria within hours. If stored longer, mould growth becomes visible. Dry daliya, on the other hand, is mainly vulnerable to insect infestation, moisture absorption, and lipid oxidation during extended storage.

Bread and bakery products from pulses and grains 🔗

Bread spoilage is dominated by mould growth. The surface of freshly baked bread is sterile, but it quickly picks up mould spores from the air during cooling and packaging. Common bread moulds include species of Penicillium, Aspergillus, Rhizopus, and Cladosporium. As noted above, ropiness from Bacillus species is the second major concern. Staling in starchy baked products also reduces palatability through starch retrogradation, producing dryness and leathery textures even when microbial counts are still low.

Pasta and dried pulse products 🔗

Pasta and dried pulse-based products are generally stable due to their low moisture content. Spoilage typically occurs only if they absorb moisture during storage. Swelling of moist pasta due to gas production by bacteria has been reported in the literature. During the drying process, moulds can occasionally cause discolouration if drying is slow or uneven.

Prevention: processing, packaging, and storage 🔗

Preventing spoilage in processed pulse and grain products requires a multi-pronged approach addressing both abiotic and biotic factors simultaneously.

Controlling moisture and water activity 🔗

Keeping moisture content below critical thresholds is the single most effective strategy. For dry grain products, maintaining moisture below 13% prevents virtually all microbial growth. Using moisture-barrier packaging materials, including multilayer films and aluminium-lined pouches, protects products from environmental humidity. Desiccant sachets inside packages provide an additional layer of protection.

Temperature management 🔗

Storage temperatures of around 4-7°C are recommended for perishable processed products like kheer and cooked porridges. For dry products, cool and consistent storage temperatures slow down both chemical reactions (especially oxidation and Maillard browning) and any residual microbial activity. Avoiding temperature fluctuations prevents condensation inside packages.

Modified atmosphere and vacuum packaging 🔗

Packaging technologies such as vacuum-sealed bags, modified atmosphere packaging (MAP), and oxygen-absorbing sachets help minimise oxidation and limit the growth of aerobic spoilage organisms. Replacing air inside packages with nitrogen or carbon dioxide is particularly effective for extending the shelf life of grain-based snacks, bakery items, and cereal mixes.

Heat treatment and aseptic processing 🔗

Proper thermal processing destroys vegetative microbial cells, yeasts, and mould spores. For products like kheer, UHT (ultra-high temperature) treatment combined with aseptic packaging can extend shelf life from hours to months without refrigeration. For bakery products, the baking process itself eliminates most organisms, though heat-resistant spores of rope-forming Bacillus species may survive and require additional preventive measures.

Chemical preservatives and natural antimicrobials 🔗

Commonly used preservatives in cereal products include calcium propionate and sodium diacetate (effective against mould and rope bacteria in bread), as well as sorbic acid and its salts. Acidification of dough with acetic acid or vinegar is a traditional and effective way to inhibit rope formation. Adding organic acids such as acetic, citric, lactic, or propionic acid to tempering water has been shown to significantly reduce microbial contamination in cereal processing. Natural antimicrobials and antioxidants (like rosemary extract or vitamin E) can also help delay lipid oxidation and extend shelf life.

Hygienic processing practices 🔗

Maintaining sanitary conditions throughout the processing chain is critical. Improperly sanitised equipment can be a source of rope bacteria, acid-forming organisms, and mould spores. Post-baking contamination is a major risk for bread – products should be cooled rapidly in clean environments and wrapped promptly to minimise exposure to airborne spores. For pulse-based products, proper cleaning and dehusking steps reduce the initial microbial load carried from field to factory.

Irradiation and emerging technologies 🔗

Food irradiation using gamma rays or electron beams is effective at reducing microbial contamination on cereal grains and flours without significantly affecting nutritional quality. Irradiation has been used to control microorganisms and extend the storage life of grain products in several countries. Newer technologies like pulsed electric fields, cold plasma treatment, and high-pressure processing are also showing promise for controlling spoilage in minimally processed cereal and pulse products while preserving sensory quality.

Why it all matters 🔗

Spoilage of processed pulses and grains is not just an economic problem – it has direct implications for food safety, nutrition, and food security. Mycotoxin contamination alone is responsible for significant health burdens globally, especially in tropical and subtropical regions where storage conditions are often suboptimal. Losses of cereal grains during storage due to moulds and mycotoxins are estimated to be substantial, particularly in developing countries where proper infrastructure may be lacking.

For consumers, understanding that even “dry” products can spoil – through oxidation, Maillard reactions, or moisture absorption – helps make better choices about storage and shelf life. For the food industry, investing in appropriate processing technologies, packaging systems, and quality control measures is essential for delivering safe, high-quality products consistently.

What do you think? How can small-scale food processors in developing regions adopt better spoilage prevention practices without significantly increasing costs? And in your experience, what is the most common sign of spoilage you notice in stored grain or pulse products at home?

References

1. https://microbenotes.com/spoilage-cereal-products/
2. https://www.alliedacademies.org/articles/how-chemical-reactions-cause-food-spoilage-and-how-to-prevent-it-32255.html
3. https://foodsafety.institute/food-microbiology/types-causes-food-spoilage/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/
5. https://www.sciencedirect.com/science/article/pii/S2772502222000828
6. https://blog.aemtek.com/chemical-food-spoilage-10-mechanisms
7. https://www.researchgate.net/publication/342618332_A_review_on_microbial_contamination_of_Cereal_grains