Food spoilage from microorganisms, whether bacteria, fungi, viruses, or other microorganisms, can be divided into 2 types: expired due to safety, i.e., the growth of disease-causing pathogens such as food poisoning, or microorganisms producing toxins harmful to the body and expiring due to food deterioration such as changes of color and texture, off flavour, loss of vitamins until less than stated on the label, etc. Although the food is still safe to consume, and it does not cause disease or abnormalities when eaten, consumers can no longer accept or are not satisfied with it (read more: Factors Affecting Shelf Life of Foods, Determining the Shelf Life of Foods). These quality deteriorations may be caused by microorganisms such as acid-producing bacteria resulting in unpleasantly sour taste, pigment-producing fungi that discolor food, or in the case where microorganisms grow and produce various substances causing the properties of food to change, such as enzymes, volatile substances with odor, etc. Each type of food will have different microorganisms, depending on the conditions of the food are suitable for the growth of which type of microorganisms, such as microorganisms that are acid resistant, salt resistant, heat resistant, cold resistant, resistant to anaerobic conditions, resistant to dehydration, etc., including food components such as carbohydrates, proteins, fats, which are food sources for different microorganisms as well.

What microorganisms spoil which food? (1)

            •Meat

            Deterioration of meat occurs when an animal dies, shutting down enzymes and the immune system, leading to external contamination such as dirt on surfaces, excrement, butchering equipment and contamination from workers’ hands. Contaminant microorganisms thrive on meat because blood and protein are a rich source of food. Microbial spoilage of meat is as follows:

Souring: Lactobacillus, Leuconostoc, Alcaligenes, Pseudomonas spp.

Discoloration) such as red spots, blue, yellow: Serratia marcescens, Pseudomonas syncyanea, Micrococcus, Flavobacterium spp.

Slime: Micrococcus spp., Lactobacillus, Bacillus, Leuconostoc spp.

Putrefaction: Clostridium, Alcaligenes spp.

Bone taint: Clostridium spp.

Spongy texture: Bacillus spp.

            •Milk and dairy products

            Microbial contamination that spoils milk and dairy products can be from the udders of animals that are inflamed, sick or from unhygienic milking processes. Common spoilage characteristics and related microorganisms are as follows:

Souring: Lactobacillus spp., Micrococcus, Bacillus cereus

Off-flavor: Pseudomonas spp., Staphylococcus spp.

Bitter: Pseudomonas, Flavobacterium, Bacillus spp.

Discoloration: Pseudomonas syncyanea with L. lactis, Lactobacillus, Leuconostoc spp.

Ropiness: Micrococcus spp., Alcaligenes viscolactis

Rancidity: Micrococcus, Serratia, Pseudomonas spp.

Sweet curdling: Alcaligenes, Proteus spp.

            •Vegetables and Fruits

            Vegetables and Fruits are sources of carbohydrates, especially sugar, microorganisms, vitamins, and minerals, including high humidity, which is necessary for the growth of microorganisms. Each type of fruits has different microbial spoilage due to factors such as acidity, carbohydrate accumulation in tuber crops, moisture content, the likelihood of ground contact (e.g., underground roots or stems or shoots or fruits on plants), nature and structural composition, etc.

Souring: From lactic acid-producing bacteria such as: Lactobacillus spp.

Soft rot: Usually found in tubers or underground roots such as potatoes) from bacteria that digest pectin structures to soften the structure such as Erwinia spp. Pseudomonas marginalis, Bacillus, Clostridium spp.

Off flavor: Acidic fruit juices by microorganisms that can tolerate acids such as Alicyclobacillus spp.

Fermented odor: From yeast

Surface mold (2): from mold species of Penicillium, Aspergillus, Rhizopus, Alternaria, and Fusarium spp.

            •Grains and bakery products

            As this group of products has low moisture content, they are less likely to be spoiled by microorganisms, but rather by mold. However, in the bakery food production process, moisture is often added and there is fermentation or resting the dough, which may cause the growth of Leuconostoc and Lactobacillus spp. microorganisms that cause spoilage. For baked products that are slowly cooled in high humidity conditions, B. subtilis microorganisms may grow and cause abnormal odor and ropiness in the products.

Reference

1.Robinson, R. K. (2014). Encyclopedia of food microbiology. Academic press.

2.Lasztity R. 2009. Food Quality And Standards – Volume III. EOLSS Publications