Food Microbiology

Food microbiology is the study of the microorganisms that inhabit, create or contaminate food. This includes the study of microorganisms causing spoilage; pathogens that may cause disease (especially if the food is improperly cooked or stored); microbes used to produce fermented foods such as cheese, yogurt, bread, beer, and wine; and microbes with other useful roles, such as producing probiotics.

In the study of bacteria in food, important groups have been subdivided based on certain characteristics. These groupings are not of taxonomic significance:

• Lactic acid bacteria are bacteria that use carbohydrates to produce lactic acid. The main genera are Lactococcus, Leoconostoc, Pediococcus, Lactobacillus, and Streptococcus thermophilus.
• Acetic acid bacteria like Acetobacter aceti produce acetic acid.
• Bacteria such as Propionibacterium freudenrencii that produce propionic acid are used to ferment dairy products.
• Some Clostridium spp. Clostridium butyricum produce butyric acid.
• Proteolytic bacteria hydrolyze proteins by producing extracellulat proteinases. This group includes bacteria species from the Micrococcus, Staphylococcus, Bacillus, Clostridium, Pseudomonas, Alteromonas, Flavobacterium, and Alcaligenes genera, and more limited from Enterobacteriaceae and Brevibacterium.
• Lipolytic bacteria hydrolyze triglycerides by production of extracellular lipases. This group includes bacteria species from the Micrococcus, Staphylococcus, Pseudomonas, Altetomonas, and Flavobacterium genera.
• Saccharolytic bacteria hydrolyze complex carbohydrates. This group includes bacteria species from Bacillus, Clostridium, Aeromonas, Pseudomonas, and Enterobacter genera.
• Thermophilic bacteria are able to thrive in high temperatures above 50 Celsius. Thermoduric bacteria, including spores, can survive pasteurization. Bacteria that grow in cold temperatures below 5 Celsius are called psychotropic.
• Halotolerant bacteria can survive high salt concentrations greater than 10%. Aciduric bacteria survive at low pH.
• Osmophilic bacteria, while less osmophilic than yeasts and molds, can tolerate a relatively higher osmotic environment. Aerobes require oxygen, while anaerobes are inhabited by it. Facultative anaerobes can grow with and without oxygen.
• Some bacteria can produce gases during metabolism of nutrients, others produce slime by synthesizing polysaccharides.
• Spore producing bacteria are further divided into groups of aerobic, anaerobic, flat sour, thermophilic, and sulfide-producing.
• Coliforms, including fecal coliforms (such as e. coli) are used as a level of sanitation. Enteric pathogens can cause gastrointestinal infection and may be included  in this group.

Food safety is a major focus of food microbiology. Numerous agents of disease and pathogens are readily transmitted via food which includes bacteria and viruses. Microbial toxins are also possible contaminants of food. However, microorganisms and their products can also be used to combat these pathogenic microbes. Probiotic bacteria, including those that produce bacteriocins can kill and inhabit pathogens. Alternatively, purified bacteriocins such as nisins can be added directly to food products. Finally, bacteriophages, viruses that only infect bacteria can be used to kill bacterial pathogens. Thorough preparation of food, including proper cooking, eliminates most bacteria and viruses. However, toxins produced by contaminants may not be liable to change to non-toxic forms by heating or cooking the contaminated food due to other safety conditions.

Fermentation is one of the methods to preserve food and alter its quality. Yeast, especially Saccharomyces cerevisiae, is used to leaven bread, brew beer, and make wine. Certain bacteria, including lactic acid bacteria, are used to make yogurt, cheese, hot sauce, pickles, fermented sausages, and dishes such as kimchi. A common effect of these fermentations is that the food product is less hospitable to other mictoorganisms, including pathogens and spoilage-causing microorganisms, thus extending the food's shelf-life. Some cheese varieties also require molds to ripen and develop their characteristic flavors.

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