13.4: Food Microbiology - Biology

13.4: Food Microbiology - Biology

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Microorganisms have been used for centuries for food preservation and to improve or change its taste. 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.

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.

Spoilage of Food: 2 Factors | Food Microbiology

Temperature and oxygen are considered two most important factors that invite microbial contamination resulting in spoilage of foods in storage conditions.

1. Role of temperature:

Foods stored at below -17°C remain free from microbial growth and a slow decrease in their population may even take place. Above this temperature, the presence and multiplication of microorganisms in food is usually recorded.

This is the reason why refrigerated foods are subject to spoilage by microorganisms. Food and food items stored at room temperature or in warm conditions remain open for spoilage by mesophilic and thermophilic microorganisms.

Aerobic and anaerobic conditions play an important role in determining the kinds of microorganisms which can multiply and spoil various food and food-items in storage conditions. If oxygen is available, various aerobic bacteria and moulds cause spoilage chiefly surface spoilage, whereas if the conditions are anaerobic the spoilage is caused by anaerobic, bacteria like Clostridium spp., etc.

Factor # 2. Food’s Own Chemical Properties:

The chemical conditions of foods influence the type of microorganisms which can grow over and within it and hence determine the nature of changes that would be brought by the spoilage action of contaminating microorganisms. Four major chemical conditions of food, e.g., composition, acidity, moisture, and osmotic concentration are of major importance in this type of spoilage.

1. Chemical composition:

(i) Foods rich in proteins are degraded by proteolytic microorganisms. Proteins are degraded into its various components due to the action of especially gram-negative, spore forming bacteria, e.g., Proteus, Pseudomonas, some cocci, etc.

Protein foods + Proteolytic microbes → Amino acids + Amines + Ammonia + Hydrogen sulfide

(ii) Foods rich in carbohydrates are degraded by carbohydrate fermenting microorganisms, particularly yeasts and moulds. Bacteria like Micrococcus, Leuconostoc, and Streptococcus can also degrade carbohydrates.

Carbohydrate foods + Carbohydrate fermenting microorganisms → Acids + Alcohols + Gases

(iii) Foods rich in fats are attacked by relatively few microorganisms such as moulds and some gram- negative bacteria. These microorganisms are, therefore, lipolytic in nature.

Fatty foods + Lipolytic microorganisms →Fatty acids + Glycerol

Generally the fruits are acid foods (pH below 4.5) while nearly all vegetables, fish, meats, and milk-products are non-acid (pH above 4.5). Since the pH of the acid foods (fruits) is sufficiently low, they do not allow bacterial growth and subsequent spoilage. They are spoiled mainly by yeasts and moulds. Contrary to this, non-acid foods have sufficiently high pH and are spoiled mainly by bacteria.

3. Moisture and osmotic concentration:

Average 13% free water is required in food for usual microbial growth. This is the reason why the foods of high sugar and salt concentrations do not allow most of the microorganisms to grow. But, specific microbial growths cannot be over-ruled. 65-70% sugar concentration is required to prevent mould-growth and 50% to prevent bacterial and yeast growth.


Food Microbiology focuses on a wide variety of current research on microbes that have both beneficial and deleterious effects on the safety and quality of foods, and are thus a concern of public health. Food Microbiology publishes significant basic and applied research in the following areas:

  • Food-borne microbes and their interactions with various foods and food chain environments including their adaptation and response mechanisms to food-processing and food-handling stresses.
  • Industrial and biotechnological exploitation of microbial diversity and versatility for the improvement of quality, safety and healthy properties of processed foods.
  • Microbiology of fermented foods.
  • Evolutionary dynamics of food-borne pathogenic microbes and those of value to food processing and safety in their different ecological contexts.
  • Genomics and functional genomics of pathogenic and value-adding technological microbes.
  • Molecular methods for the identification, typing and characterization of food-associated microbes and complex microbial communities.
  • Development of probiotics as food supplements and their effects on human health including effects on host gut microbiota.
  • Predictive microbiology and its application to food and process optimization and risk assessment.

One ultimate goal is to facilitate and accelerate communication among food microbiologists and those active in pertinent other areas of the sciences, and stimulate new perspectives and innovation. Another very important goal is to contribute to a constructive dialog between scientists and the lay public on issues of food safety, food quality and healthy lifestyles.

Watch the video: Intro to Food Microbiology (August 2022).