14.4: Food Microbiology
Microorganisms have been used for centuries for food preservation and to improve or change its taste. Evidence exists that yogurt, which is milk fermented by bacteria, has been around for over 4000 years. Today, many of the foods we eat are the result of microorganisms acting on foods for a specific and desired effect. Examples of other fermented foods include wine/beer, sausage, sauerkraut/kimchee, bread, and cheese. The industrial use of microorganisms in food production began in earnest in the late 19th century, when pure cultures of bacteria were grown specifically for that purpose.
Bacteria and Health Hazards
While there are a wide number of foods that are produced with microorganisms, the introduction of harmful organisms to food presents a serious health hazard. Even bacteria that normally colonize the human digestive system (such as E. coli) can cause severe illness or even death if ingested. The most commonly encountered microbial pathogens are Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Bacillus anthracis, Clostridium botulinum, Clostridium perfringens, members of the Salmonella and Campylobacter genus, Escherichia coli (especially type O157:H7), hepatitis and Rota viruses, prions, different species of tapeworms and roundworms, and protozoa. Food-borne illness can be limited to a single person who ingests the contaminated food, or can lead to a more widespread outbreak of the same illness when multiple people consumed the same contaminated product. Many outbreaks are local in nature, such as when food from a restaurant is not held at the proper temperature, allowing bacteria to grow to disease-causing levels. Increasingly there have been food-borne illness outbreaks that affect many geographical areas due to improper food handling. On May 23, 2019 5 lb
bags of Baker’s Corner All purpose flour was recalled due to potential E. Coli contamination. The CDC reported 17 infected individuals from 8 states.
https://www.cdc.gov/ecoli/2019/flour-05-19/index.html
Bacteria and Fermentation
Fermentation is important for the production of a variety of dairy products. Cheeses are typically made using lactic acid-producing bacteria that aid the coagulation of the milk protein casein in curd. The curd is then further treated with a different bacteria (depending on the desired final cheese) to produce distinctive tastes and aromas. The characteristic holes in Swiss cheese are produced by specific bacteria that generate carbon dioxide which create gas bubbles in the cheese.
Yogurt is made by the fermentation of lactose (milk sugar) by bacteria. Lactose is a disaccharide that consists of two simple sugars. During the making of yogurt, the lactose is broken down by the enzyme lactase into glucose and galactose. These sugars are then fermented generating lactic acid and acetaldehyde. These two products lower the pH of the milk giving it a sour tart taste. The lowered pH also effects the milk proteins (caseins) causing coagulation, precipitating the proteins into a solid curd that forms the yogurt. The left over watery liquid is the whey. There are a variety of bacteria that may be utilized in this process, the two most commonly used to are Lactobacillus bulgaris and Streptococcus thermophilus
In this experiment we will start with milk. To increase the shelf-life milk that is sold commercially is pasteurized. Pasteurization does not sterilize the milk, but will kill most of the microorganisms present. To manufacture yogurt these organisms must be destroyed so that they will not compete with the added organisms essential for the fermentation process. Commercially available yogurt containing live bacterial cultures will be used to inoculate the milk after heating.