Understanding Food Microbiology and Contamination

Food microbiology focuses on the general biology of microorganisms found in foods, including their growth characteristics, identification, and pathogenesis. Key areas of interest include food poisoning, food spoilage, food preservation, and food legislation. Pathogens in food products, or harmful microorganisms, result in major public health problems worldwide and are the leading causes of illnesses and death.

Parameters Affecting Microbial Growth in Food

There are two primary parameters that affect the growth of microorganisms in food products: extrinsic and intrinsic. Extrinsic parameters are properties of the environment (processing and storage) that exist outside of the food product, affecting both the foods and their microorganisms.

Intrinsic parameters, on the other hand, are properties inherent to the food product itself, such as tissues that may, under certain conditions, promote microbiological growth. Below is a list of these parameters that may either result in multiplication or inhibition of microbial growth in food products.

Examples of Intrinsic Parameters

1. pH: It has been well established that most microorganisms grow best at pH values around 7.0 (6.6 – 7.5), whereas few grow below a pH of 4.0. Bacteria tend to be more fastidious (complex nutritional or cultural requirements for growth) in their relationships to pH than molds and yeasts, with pathogenic bacteria being the most fastidious. Most meats have a final pH of about 5.6 and above, making these products susceptible to bacteria as well as mold and yeast spoilage.

2. Moisture Content (Water Activity): One of the oldest methods of preserving foods is drying or desiccation. The preservation of foods by drying is a direct consequence of the removal or binding of moisture, without which microorganisms do not grow. It is now generally accepted that the water requirements of microorganisms should be described in terms of water activity (a_w) in the environment.

In pure liquid water, water molecules are loosely oriented and can easily rearrange. When a solute, like salt, is added to water, the water molecules orient themselves on the surface of the solute (e.g., Na⁺ and Cl⁻ ions), and the properties of the solution change dramatically.

The microbial cell must then compete with solute molecules for free water molecules. The water activity of pure water is 1.00; the addition of solute decreases a_w to less than 1.00. Most foodborne pathogenic bacteria require a_w greater than 0.9; however, Staphylococcus aureus may grow in a_w as low as 0.86.

3. Oxidation-Reduction Potential: Microorganisms display varying degrees of sensitivity to the oxidation-reduction potential (O/R or EH) of their growth medium or environment. Aerobic microorganisms require more oxidized environments (more oxygen), whereas anaerobic organisms require more reduced environments (lacking oxygen).

4. Nutrient Content: To grow and function normally, microorganisms of concern in the food industry require water, a source of energy, a source of nitrogen, vitamins and related growth factors, and minerals.

5. Antimicrobial Constituents: The stability of some foods against microbial attack is due to the presence of certain naturally occurring substances that have been shown to have antimicrobial activity. Nisin and other bacteriocins are good examples.

6. Biological Structures: The natural covering of some food sources provides excellent protection against the entry and subsequent damage by spoilage organisms. Examples of such protective structures are the hide, skin, and feathers of animals.

Examples of Extrinsic Parameters

1. Storage Temperature: Microorganisms, individually and as a group, grow over a wide range of temperatures. Knowing the temperature growth ranges for organisms of importance in foods is critical for selecting the proper temperature for product storage.

2. Relative Humidity: The relative humidity of the storage environment is important both from the standpoint of water activity (a_w) within foods and the growth of microorganisms at the surfaces. Humidity can also be a critical factor when producing certain types of products.

3. Presence/Concentration of Gases: Carbon dioxide (CO₂) is the single most important atmospheric gas used to control microorganisms in foods. It has been shown to be effective against a variety of microorganisms. Because of its effectiveness, CO₂ is used as one of the methods for modified-atmosphere packaging (refer to FDA Food Code).

4. Presence/Activities of Other Microorganisms: The inhibitory effect of some members of the food microbiota on other microorganisms is well established. Some foodborne organisms produce substances that are either inhibitory or lethal to others, including antibiotics, bacteriocins, hydrogen peroxide, and organic acids (such as lactic acid).

General microbial interference refers to nonspecific inhibition or destruction of one microorganism by other members of the same habitat or environment; the mechanism for this interference is not fully clear.

Some possibilities include competition for nutrients, competition for attachment/adhesion sites, unfavorable alteration of the environment, or combinations of these.

Read Also: Historical Development of Crop Production

Understanding Food Contamination

Contamination is the occurrence of any objectionable matter in food. Food is said to be contaminated when foreign bodies, materials, or harmful substances are introduced, making it dirty or impure.

These objectionable matters are called contaminants and may be organic or inorganic in nature, e.g., microorganisms, chemicals, or adulterants, which are not part of the food. Food can become contaminated in a multitude of ways, with contaminants entering food accidentally or incidentally from the source or farm to the table when food is consumed.

Sources of Food Contamination

1. Agricultural Contamination: Many crops are treated with insecticides, fungicides to prevent fungal growth, and weed killers or growth regulators to kill weeds selectively. Pesticides and fungicides are invaluable aids to food production, not only when crops are growing but also after harvesting and during storage.

Many are toxic to animals and humans but are usually applied to plants before the edible part has appeared or sufficiently before harvesting to ensure that the amount remaining on the crop is minimal, making it wholesome when eaten. A permitted maximum residue level (MRL) has been set for each of these chemicals, and the permitted amounts are extremely low.

2. Radioactive Contamination: Radiation and atomic particles emitted from nuclear power stations, nuclear fuel reprocessing plants, or nuclear weapons testing are extremely harmful to living cells. Radioisotopes that enter the atmosphere become widely distributed and may reach the ground thousands of miles away.

When these radioisotopes fall on soil and vegetation, they may be absorbed by plants and, when eaten by humans, become incorporated into body tissues with harmful consequences.

3. Contamination from Packaging Materials: Plastics are increasingly used as packaging materials, and while the polymers themselves are non-toxic, compounds added to improve their properties may not be equally safe. These can contaminate foodstuffs by the migration of chemicals from the packaging material. Paper-based packaging materials, widely used for foodstuffs, could also be a source of contamination.

4. Cross-Contamination: Cross-contamination refers to contamination caused by an already contaminated object. For example, if a knife is used to cut through a cow’s intestine and the same unwashed knife is used to slice bread, cross-contamination occurs because the knife has been contaminated with Escherichia coli from the cow’s intestine and subsequently contaminates the bread. Other examples include polishing a plate with a soiled or dirty napkin to serve food or licking fingers with saliva to pick up paper or foil to wrap snacks.

5. Human Beings and Animals: Humans are the main reservoir of microorganisms. Sick people should not handle foods. People with colds, respiratory disturbances, boils, pimples, acne, or infected cuts are good sources of Staphylococcus aureus. The nails or mouth of a food service worker could be a source of contamination.

6. Dirty Environment/Utensils: If the food preparation area is not hygienically maintained, it could lead to contamination of food. Insanitary equipment or utensils, such as dirty plates, cups, glasses, towels, or tables, are also good sources of contamination.

7. Water: Water, which is essential in food production, could be a source of contamination.

8. Air: Microorganisms found in air could contaminate unprotected foods. Disease-causing organisms may be present in the food preparation area, perhaps on the body or clothing of a food service worker, in floor dust, in the air, on the bodies of rodents or insects, or in meats or poultry.

Primary Sources of Microorganisms in Food

Bacteria can be found virtually everywhere, including on humans, and can enter food products through various routes. The following list outlines some of the most common ways microorganisms enter food products:

1. Soil, Water, and In-Plant Environment: Many bacteria are carried in soil and water, which may contaminate food. The in-plant environment is an important source of contamination due to daily activities and pest infestation. Listeria, Clostridium, Salmonella, and Escherichia are good examples.

2. Animal Feeds: This is a source of salmonellae to poultry and other farm animals and a known source of Listeria monocytogenes to dairy and meat animals when fed silage. Organisms in dry animal feed spread throughout the animal environment and may occur on animal hides, hair, or feathers.

3. Animal Hides: The hide is a source of bacterial contamination of the general environment, hands of establishment employees, and skinned carcasses. Studies show this may be a primary source for E. coli, Salmonella, and Listeria in cattle.

4. Gastrointestinal Tract: The intestinal biota consists of many organisms, including pathogens such as Salmonella, Campylobacter, E. coli, and other microorganisms. Any or all of the Enterobacteriaceae may be expected in the feces of livestock and poultry.

5. Food Handlers: The microbiota on the hands and outer garments of handlers generally reflect the environment and habits of individuals (hygiene), with organisms possibly originating from hides, gastrointestinal tracts, soil, water, dust, or other environmental sources.

6. Food Utensils: Saws, cutting boards, knives, grinders, mixers, etc., may become contaminated during slaughter and processing operations, ensuring a fairly constant level of contamination of meat-borne organisms.

7. Air and Dust: A variety of bacteria may be found in air and dust in food-processing operations at any time. Listeria is an example of a Gram-positive organism that survives in the environment.

8. Vegetables and Vegetable Products: These may be a significant concern in the processing of meat, poultry, and egg products, such as frozen entrees or salads containing meat and poultry components. Many soil and water organisms contaminate vegetables and fruits.

9. Globalization of Food Supply: This is a major factor in contamination, resulting in the transfer of pathogenic agents between countries (import/export), such as the Bovine Spongiform Encephalopathy (BSE) infective agent and Salmonella Typhimurium. Increased international travel also poses a risk of introducing pathogens like foot-and-mouth disease.

10. Terrorist Attacks: There is growing concern in the food industry that terrorists could use pathogens to contaminate food and water supplies in an attempt to disrupt the economy, health, and lifestyle.

Read Also: The Complete Classification of Crops

Preventing Food Contamination

1. Wash and scrub all vegetables carefully, paying particular attention to those intended for use in salads and those to be eaten raw, as residuals from chemical sprays may still be present.
2. Correct washing and sanitizing of dishes, eating utensils, and any equipment or surfaces that will be in contact with food is absolutely essential.
3. Do not keep chemicals used to destroy pests in food preparation areas.
4. Keep fingers away from the mouth, lips, face, and soiled surfaces.
5. Handle no food that can be transferred or picked up with tongs, scoops, forks, spoons, etc. Avoid mixing salads, ground meat, etc., with hands.
6. Avoid the common use of cups, glasses, towels, etc.

Frequently Asked Questions About Food Microbiology and Contamination

1. What is food microbiology, and why is it important?
   Food microbiology studies the microorganisms in foods, including their growth, identification, and pathogenesis. It is critical because it addresses food poisoning, spoilage, preservation, and legislation, helping prevent public health issues caused by harmful microorganisms.
2. What are the main factors affecting microbial growth in food?
   Microbial growth is influenced by intrinsic parameters (e.g., pH, water activity, nutrient content) inherent to the food and extrinsic parameters (e.g., storage temperature, humidity, gases) related to the environment.
3. How does pH affect microbial growth in food?
   Most microorganisms grow best at a pH around 7.0 (6.6–7.5), with few growing below 4.0. Meats with a pH of 5.6 or higher are more susceptible to bacterial, mold, and yeast spoilage.
4. What role does water activity play in food preservation?
   Water activity (a_w) measures the availability of water for microbial growth. Lowering a_w through drying or adding solutes like salt prevents microbial growth, as most foodborne pathogens require a_w greater than 0.9.
5. What are common sources of food contamination?
   Common sources include agricultural chemicals, radioactive isotopes, packaging materials, cross-contamination, humans, animals, dirty environments or utensils, water, air, and globalized food supply chains.
6. How can cross-contamination be prevented in food preparation?
   Cross-contamination can be prevented by thoroughly washing and sanitizing utensils, avoiding contact with contaminated surfaces, and using tools like tongs or spoons instead of hands to handle food.
7. Why is CO₂ used in food preservation?
   Carbon dioxide (CO₂) is effective against various microorganisms and is used in modified-atmosphere packaging to control microbial growth and extend food shelf life.
8. How does globalization contribute to food contamination risks?
   Globalization increases the risk of transferring pathogens like Salmonella Typhimurium or Bovine Spongiform Encephalopathy (BSE) between countries through food imports/exports and international travel.

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