Microbial spoilage of food

Food can be considered bad when it’s no longer edible or satisfactory to consumers or doesn’t meet the grade characteristics that its patron or manufacturer deems optimum. Foods spoil as a consequence of both natural biochemical changes and changes caused by microorganism growth and metabolism. This ultimately involves the deterioration of proteins, carbohydrates, and fats by microorganisms or microbial enzymes. Signs of decomposition include appearance, flavour, odour, and texture changes. The kind of spoilage depends on the initial microflora of a product, its physical and chemical characteristics, and how it’s processed, packaged, and reposited. This article aims to discuss and understand microbial spoilage of food: microbial spoilage definition, and types of microbial spoilage.

Microorganisms are generally present in foods. Depending on their characteristics and growth eventuality, they can play different purposes commensals (neither metabolic nor adverse health effects), food cultures (technological benefit), pathogens (adverse health effects on the host), probiotics (health advantage on the host) and spoilers (adverse metabolic conditioning on the food). Food microbiology focuses on the association of habitat to the occurrence of microorganisms, the result of surroundings on the growth of varied microorganisms in food, the microbiology of food decay and food manufacture, the physical, chemical, and natural loss of microorganisms in foods, the microbiological test of foodstuffs, and public wellness and sanitation bacteriology.

Microbial spoilage of food is a metabolic process that causes undesirable or bad for human consumption due to alteration in sensory characteristics. Decayed foods may be safe to consume, i.e. they may not prompt illness as there is no toxins or pathogens present, but inconsistencies in texture, taste, smell or appearance cause them to be disapproved. Some ecologists have inferred microbes produce these noxious odours to repulse voluminous creatures, thereby keeping the food resource for themselves.

Types of microbial spoilage

Chemical reactions that cause unpleasant and delicate changes in food are mediated by various microorganisms that use food as a source of carbon and energy. 

• These organisms include unicellular organisms, prokaryotes (bacteria) and eukaryotes for which the nucleus and other organelles are not defined, unicellular (yeast) and multicellular (mould) with nuclei and separate organelles. 
• Some microbes are commonly found in many types of degraded foods, while others are more likely to be found in the foods they consume. 
• Many species are often found in a single spoilage food, but there may be one species (specific spoilage organisms, SSOs) that is originally involved in the production of compounds that cause unpleasant odours and tastes.
• There is constant competition for different populations, rising and falling as different nutrients become available or depleted. Some microorganisms, such as lactic acid bacteria and mould, secrete compounds that block competitors.

Bacteria in food

Foods can grow under a variety of conditions and are most commonly damaged by bacteria. However, bacteria are used for the beneficial fermentation of pickles, dairy products, and some fermented vegetable products. Bacteria require a neutral pH of 6.57 to cause food spoilage (e.g. milk, meat, green vegetables, fruits, etc.). Some bacteria can form spores, make them very heat resistant, and some can produce heat resistant toxins. Eating such decomposed foods leads to food poisoning. The most common bacteria that cause food spoilage are Staphylococcus aureus, Clostridium spp., Bacillus spp., Lactic Acid Bacteria (LAB),  Streptococcus spp., Leuconostoc spp., Brochothrix spp., Mycobacterium Bovis, Weissella spp. and other gram-positive bacteria. Salmonella spp., Aeromonas spp., Shigella, Campylobacter jejuni, Plesiomonas shigelloides, Vibrio spp., Escherichia coli,   Coxiella burnetii, Brucella spp., Yersinia enterocolitica, and so on. These bacteria cause stinks, off-flavours, discolouration, gas formation, mucus formation, and low food pH.

Fungi in food

Fungi are the most common group of microorganisms that play an important role in food spoilage. Fungi are osmotic and acquire nutrients by absorption. Fungi can be divided into mould and yeast. Mould is the most common microorganism that causes food spoilage. Mould grows on the surface of food (it requires free oxygen to grow) and grows at a wide range of pH values ​​(2 to 8.5), most of which prefer acidic pH values. Mould can grow on dry foods (grains, beans, peanuts, some spices, etc.) that have very low water activity (0.7-0.8). The most common moulds that cause food spoilage are Mucor and Aspergillus spp., Rhizopus spp., Penicillium spp., Alternaria spp., Botrytis, Byssochlamys, Fusarium spp. am. This mould causes stink, mycotoxin contamination, discolouration, spoilage and are visible from the outside of food. Compared to bacteria and mould, yeast plays a minor role in food spoilage. Yeast can grow with or without oxygen and is used for beneficial fermentation in the fermentation of bread and alcoholic beverages. It often spoils sugar- and salt-rich foods (maple syrup, pickles, jams, soy sauce, sauerkraut, etc.). Yeast requires a water activity of 0.90 to 0.95 to grow and can grow in a wide pH range (3 to 8). , But in general, they prefer acidic pH (4.55.5). The most common food spoilage yeast is Zygosaccharomyces spp., Saccharomyces spp., Candida spp., Dekkera spp. These yeasts cause colour changes, texture changes, unpleasant odours, or unwanted tastes in food.

Prions in food

Prions are infectious pathogens, normal proteins in the brain that are misfolded, lack the genome, and result in pathological infectious conformation. If accidentally folded, other normally folded proteins can be accidentally folded. Prion disease can affect both humans and animals. Infected meat and meat products can also be transmitted from animals to humans. Examples of prion disease include bovine spongiform encephalopathy (BSE), scrapie, chronic wasting disease (CWD), and Creutzfeldt-Jakob disease (CJD).

Protozoa in food

Protozoa are unicellular microorganisms with hard cell walls, and the virulent form of these microorganisms is called a cyst. Protozoan parasites are deeply involved in developing food poisoning and water-borne diseases. Water and food serve as carriers for the transmission of unicellular parasites from one host to another. The most common food poisoning parasites are Giardia lamblia, Amoebiasis erythematosus, Cyclospora cayetanensis, Toxoplasma gondii, and Trichinella.

Conclusion

If certain precautionary and preventive measures are not taken, food spoilage can have economic implications. The food industry uses natural preservatives, new processing systems, refrigeration, packaging materials, and, more recently, management systems to minimise spoilage. However, these techniques cannot control spoilage unless the incoming material is of the highest quality and handled in good hygiene. In all cases, the shelf life of many foods can be extended if the food is prepared to minimise the level of bacterial contamination before final processing.