Food spoilage is a metabolic process that results in changes in sensory properties that make food unsuitable or unpleasant for human ingestion. Spoiled foods may still be safe to eat—that is, they may not make us sick since no infections or toxins are present—but people may reject them because of changes in their texture, flavor, aroma, or appearance.
A food’s shelf life is the time it is stable and maintains the required characteristics. Food spoilage causes large-scale losses in terms of economics and the occurrence of food-borne illnesses. The problem of microbial food deterioration has a long history. For instance, lactic acid bacteria can ruin beer by producing turbidity, acidity, and unpleasant odors (1).
Causes of food spoilage
The cause of food spoilage can be due to various reasons, such as:
- The presence of microorganisms, especially bacteria, yeast, and mold, cause food spoilage.
- Insects carry different kinds of microorganisms to the food.
- The presence of indigenous enzymes such as catalase and proteinase in animal and plant products also destroys food quality.
- Physical harm destroys food, such as radiation, drying, temperature, and pressure.
- The presence of high moisture, high protein, high fat, and unhygienic handling can occur food spoilage (2).
Food Spoilage by Microorganisms
Microorganism plays a fundamental role in a living organism. Some microbes spread positive impacts, but some of them harm. Microbes like yeast, molds, and bacteria are usually responsible for spoiling food items.
Yeast, a eukaryotic organism, is a subgroup species of organism known as fungus. Yeasts are well recognized for their beneficial fermentations that produce bread and alcoholic beverages and can grow with or without oxygen.
In addition, they are used to increase the surface of cheese and meat as well. They generally colonize foods with maximum sugar or salt content, such as sauerkraut, pickles, maple syrup, low pH fruits, and liquids, and deteriorate them.
The four main yeast species that cause yeast spoilage are Zygosaccharomyces, Debaryomyces hansenii, Brettanomyces, and Candida. Zygosaccharomyces species spoil the foods such as honey, dry fruits, jams, and soya sauce by releasing the off odors or flavors and carbon dioxide that sometimes causes containers to swell and burst.
Debaryomyces hansenii can grow in high salt concentration (approx. 24%) so, are frequently isolated from salt brines used for cured meats, cheeses, and olives. Saccharomyces spp., and Candida, spoils fruits, some vegetables, and dairy products.
Dekkera/Brettanomyces only spoil fermented foods such as alcoholic beverages and dairy products by producing phenolic compounds which cause off-flavors on food.
Molds are microscopic fungi that are characterized by multicellular filaments known as hyphae. It is an organism that helps in decaying dead animals and plants. They also ruin the wide range of food products.
Some molds that cause food spoilage are Zygomycetes, a primitive fungus that proliferates on fruits and vegetables rich in fruits like strawberries and potatoes. Some of them also grow in bread. The Zygomycetes that are commonly observed in spoiled products are Mucor and Rhizopus. Penicillium and related genera (Furcatum) are found in soil and plant debris. Many of these species produce antibiotics used as medicine by humans.
Still, some species cause harm by releasing mycotoxin (spatulin, ochratoxin, citreoviridin, penitrem) in different fruits and vegetables ( pear, citrus fruits, and apples).
Byssochlamys are the genera that cause spoilage to pasteurized juices due to heat-resistant spores. Aspergillus typically develop more quickly, are much more resistant to high temperatures and low water activity, and predominate spoiling in warmer areas.
This mold affects a wide range of food products (grains, dried beans, peanuts, tree nuts, and some spices) and non-food products (paper, leather) by releasing the mycotoxins such as aflatoxins, ochratoxin, territrems, and cyclopiazonic acid.
Fusarium spp. does not spoil the food, but the mycotoxins released in harvested grains can cause a health threat.
The bacteria-producing spore is present in heat-treated food due to their abilities to sustain life even in high temperatures. Spore-producing bacteria are Gram-positive, thermophiles anaerobes. Some of them can be facultative.
These bacteria produce hydrogen sulfide (e.g., Desulfotomaculum), and others release hydrogen and carbon dioxide (e.g., Thermoanaerobacterium) when kept at high temperatures (like soups sold in vending machines).
Psychrotolerant spore-producer organisms such as Clostridium spp release gas and sickly odors in food products (meats and brine-cured hams), whereas Bacillus spp produces off-fragrances and gas in chilled and milk products.
Lactic Acid Bacteria (LAB) such as Lactobacillus, Pediococcus, Leuconostoc, and Oenococcus help to form fermented foods such as yogurt and pickles. Still, they damage the foods (meat, wine, or juices) in low oxygen, low temperature, and acidic conditions producing off flavors (mousy, cheesy, malty, sour, buttery).
These organisms secrete gas and vast amounts of extra-polysaccharides resulting in slime on meat and ropy spoilage in some food products.
Pseudomonas generally need maximum water activity for growth and are inhibited by a pH below 5.4. Some species (psychrophilic) require refrigeration to grow, whereas others can thrive and grow at warmer ambient temperatures.
The Pseudomonas groups of four are mainly responsible for causing food to spoil, i.e., P. fluorescens, P. fragi, P. lundensis, and P. viridiflava. P. fluorescens, P. fragi, P. lundensis, and S. putrefaciens ruin foods originating from animals (meat, fish, and milk) by secreting lipases and proteases that result in the release of sulfides and trimethylamine (off-odors), as well as by producing biofilms (slime) on surfaces.
Some strains ruin the meals in the refrigerator since they can grow at low temperatures, whereas X. campestris, P. fluorescens, and P. viridiflava secrete enzymes such as pectic lyase enzymes that break down the pectin in the plant-derived food and produce musky or slime layer within it (3).
Other than these microorganisms, other factors can also cause food deterioration. The insects such as flies, mites, moths, beetles, and weevils are the leading cause of spoilage of stored-food products like flour, dried fruits, grains, nuts, cheese, corn, and dried vegetables. These insects carry disease-causing microorganisms or toxins produced in their body.
The indigenous enzymes such as catalase, proteinase, and lipase amylase in plants and animals continue to function. In addition, they deteriorate the quality of food if not appropriately destroyed during storage. For example, the peroxidase enzyme causes off-flavors during storage in green vegetables.
The temperature is an essential factor that may cause food spoilage as well. For instance: the food kept at a low temperature makes it partially frozen, which leads to the breakage of cells and damages food quality.
The foods are exposed to irradiation to kill the deteriorating microorganism, increasing the shelf-life of food. However, prolonged exposure of food to rays cause a loss in micronutrients (mostly vitamin A, B, C, and E), affecting the nutritional value of food (5).
Prevention of Food Spoilage by Organisms
Food preservation is a technique to keep food safe and unharmed from the metabolic activities of microorganisms. The best way of preserving the food depends on the abilities of microorganism metabolism. The most common food preservative methods are:
- The use of weak organic acids such as acetic acid, lactic acid, benzoic acid, and sorbic acid reduces the chance of deteriorating the food products (pickles, sauce, jam, jelly) by preventing the growth and development of various species of bacteria and fungi.
- The addition of sugar concentration in about 68% -70% helps to prevent food like jam and jelly by exerting osmosis and limiting the water activity for the multiplication of microbes.
- 15%-20% of salt concentration in food products hinders growth by exerting osmotic pressure that creates plasmolysis of cells and restricts abilities dissolving the oxygen.
- The heating and freezing also increase the shelf life of food (4)
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