Food microbiology

(food engineering) The science that deals with the microorganisms involved in the spoilage, contamination, and preservation of food.

A subdiscipline in the field of microbiology concerned with the study of bacteria, fungi, and viruses that grow in or are transmitted by foods. While bacteria are frequently associated with food spoilage and food poisoning, some species preserve foods through fermentation or produce food ingredients. Food microbiology is a broad field that can include not only microbiology but also sanitation, epidemiology, biochemistry, engineering, statistics, and mathematical modeling.

Most food-related illnesses have historically been attributed to one of five major groups of pathogenic bacteria. These five groups are Salmonella and Shigella; Clostridium botulinum; Clostridium perfringens and Bacillus cereus; and Staphylococcus aureus. These have been joined by the emerging pathogens Yersinia enterocolitica, Escherichia coli, Listeria monocytogens, and Campylobacter jejuni. See also Bacteria; Food poisoning.

When certain bacteria grow in foods, they produce desirable flavors and textures, and may also inhibit pathogenic organisms. Most of these bacteria belong to the genera Streptococcus, Lactobacillus, Leuconostoc, Pediococcus, or Micrococcus. They are used to make fermented dairy products, meats, and vegetables, and to preserve food by converting the sugars needed by competing microbes to lactic acid, which inhibits their growth. Acetobacter and Gluconobacter are used in the production of vinegar. Yeasts, usually Saccharomyces, which produce ethanol and carbon dioxide, are used in the processes of brewing and baking.

Modern food microbiology views foods as habitats where different organisms compete for survival. The fact that there are 250 genera of bacteria and that only 25 of these (8 pathogenic) are found in foods suggests that foods provide unique ecological niches. Viruses do not reproduce in foods and are not competitors in this sense (the food acts only as a carrier). Yeasts and molds usually grow more slowly than bacteria and are rarely a problem in foods that support bacterial growth. See also Fungi; Yeast.

Bacteria reproduce by binary fission; it takes only 20 doublings for one cell to yield more than 1 million cells. In environments where the doubling time is short, this occurs quite rapidly. Many preservation methods alter foods' environmental conditions, such as temperature, acidity, and water and oxygen availability, in order to slow microbial growth. See also Bacterial growth.

Microbial analysis of foods frequently requires “zero defects” in the absence of 100% testing. Legally, ready-to-eat foods must be free of Salmonella. This demands that the food microbiologist be able to detect one Salmonella among millions of innocuous bacteria in a pound of food. Moreover, all of the food cannot be tested because microbial analysis is destructive. Therefore, statistical sampling plans determine how many samples must be tested to have confidence that the whole lot is free of Salmonella.

In the classical methods for counting microorganisms, a food or its hemogenate is highly diluted so that only 30–300 cells are transferred to growth media. After 2–10 days, each cell grows into a colony, and these are counted and multiplied by the dilution factor to estimate the number of cells in the food. Automated methods have been developed that measure growth products, bacterial deoxyribonucleic acid (DNA), or specific toxins; these methods dramatically reduce the analysis time and are rapidly replacing the petri-dish method.

A procedure known as hazard analysis critical control points (HACCP) can replace much postproduction testing. This technique examines a food, its ingredients, and its processing to identify points critical to safety. These points are then heavily monitored during production; if they are maintained, a safe product results.

Advances in molecular biology have generated interest in applications to food processing. The most important contribution of biotechnology to food microbiology is the production of probes that detect pathogenic organisms much faster than conventional methods. For example, conventional methods require 5 days to confirm the presence of Salmonella in foods; probes that detect Salmonella-specific DNA or antigens can give similar results in 2 days.

The dairy industry has benefitted from advances in biotechnology by acquiring the ability to determine the genetic basis for the bacterial metabolism of lactose in milk and to stabilize it. In addition, enzymes that accelerate the aging of cheese have become commercially available, making it possible to produce a cheese with the taste of 9-month-old cheddar in just 3 months. See also Bacteria; Biotechnology; Enzyme; Food engineering; Food manufacturing; Food preservation; Genetic engineering; Virus.