Unit 5.1 (Difficulty: Medium) Microorganisms in Food

Introduction to Microorganisms in Food

• Microbiological analysis involves detecting and identifying microorganisms, which is critical in food analysis.
• Ensuring the correct microbes are present is important during food preparation ;examples include Streptobacillus lactis in cheese and Acetobacter in vinegar.
• The presence of microbes at acceptable levels is important to prevent food spoilage.
  • For example, Lactobacillus in milk (2 x 10^5 CFUs/mL) and E. coli in bean sprouts (5 x 10^3 CFUs/g).
• CFU stands for colony-forming unit.
• Microbes impact the shelf-life of food.
• Controlling microbial growth and extending shelf-life involves various methods.
• Foodborne diseases can result from:
  • Infection, where bacteria colonize the gut.
  • Intoxication, where bacteria produce a toxin.
  • Intoxification, where bacteria colonize and produce a toxin.

Types of Microorganisms

• Bacteria are single-celled prokaryotes:
  • Lactobacillus and some strains of E. coli are types of bacteria.
• Yeast are single-celled eukaryotes.
  • Saccharomyces species are used to make beer and wine.
• Mold is a structure formed by some fungi.
  • Penicillum species are a type of mold.
• Virus: infectious agent
  • Norovirus can spread through foods/surfaces.
• Parasites: animals pr protozoa
  • Giardia is a parasite.

Microbial Growth Requirements

• Most microorganisms are neutrophiles and prefer a neutral pH.
• Acidity inhibits the growth of most microorganisms.
• Growth ranges for microorganisms vary:
  • Bacteria: pH > 4.5 (at aw of 0.85)
  • Yeasts: pH > 4.0 – 4.5
  • Molds: pH < 4.0
• Important acidophiles (organisms that thrive in acidic environments) include:
  • Mold
  • Yeasts
  • Lactic acid bacteria

Controlling Food Spoilage

• Organic acids in foods like fruits, soft drinks, fermented milk products, sauerkraut, and pickles contribute to their preservation by lowering the pH.
• Organic acids are an effective way to minimize food spoilage.

Environmental Factors Affecting Microbial Growth

• Salinity is inter-related with water activity.
• Psychrophiles ("cold-loving") can grow from -20 °C to 20 °C.
• Psychrotrophs (cold-tolerant) can grow at 0 °C but prefer 20 °C – 40 °C.
• Mesophiles have an optimum growth range of 20 °C – 45 °C; they can ferment products like yogurt, cheese, wine, and beer; most human pathogens are mesophiles.
• Thermophiles ("heat-loving") tend to thrive between 50 °C and 64 °C.
• Extreme Thermophiles tend to thrive between 65 °C and 79 °C.

Impact of Atmosphere on Microbial Growth

• Bacteria (aerobic and anaerobic) can be identified by growing them in test tubes of thioglycollate broth.
• Obligate aerobes:
  • Require oxygen
  • Gather at the top of the tube .
• Obligate anaerobes:
  • are poisoned by oxygen
  • Gather at the bottom of the tube
• Facultative anaerobes:
  • Grow with or without oxygen
  • Gather mostly at the top
• Microaerophiles:
  • Need oxygen
  • Poisoned by high oxygen concentrations
  • Gather in the upper part of the test tube
• Aerotolerant organisms:
  • Do not require oxygen
  • Not poisoned by oxygen
  • Spread throughout the test tube.

Nutrients and Growth Factors

• Microorganisms require specific nutrients:
  • Carbon
  • Nitrogen
  • Sulfur
  • Phosphorous
• Methods of food preservation related to growth factors:
  • pH: Acidifying foods to lower pH (e.g., canned goods below 4.6)
  • Water activity: Storing food with water activity < 80.
  • Salinity: Salting foods
  • Temperature: Heating (pasteurization, cooking to 165 °F) or storing food in cold conditions.
  • Atmosphere: Limiting oxygen for canned goods.
  • Nutritional requirements:Food microorganisms can be controlled using a growth factor.

Additional Factors in Food Microbiology

• Spoilage of acid foods (pH 4.6 or lower) requires detection by cultural methods using acidified media (pH 5.0 – 5.5).
• pH is not necessarily the best indicator for predicting microbial response for preservation because it does not indicate:
  • Changes in total acidity (buffering effects)
  • The type of organic acid produced
• Microorganisms can lead to food spoilage or illness
• Microorganisms that cause illness are food pathogens.
• Some microorganisms are helpful in food, such as bacteria/fungi in fermented food.
• Controlling their growth and food storage is important.
• Multiple preservation methods may be needed to remove multiple growth factors for different organisms.

Detecting, Identifying, and Counting Microorganisms

• Methods to identify microorganisms in food include:
  • Culturing or growing the organism
  • Molecular biology testing for genetic markers
  • Testing for specific immunological markers
  • Specific purpose tests
• Culture methods involve a traditional approach of growing organisms on suitable media in a petri dish.
• Different organisms have different growth requirements and pathogenic organisms tend to be more fastidious than nonpathogens.
  • Different medias may be required to detect different organisms
• Defined media: When all components are known
• Complex media: Use "undefined" components such as yeast extract or tryptone.
• Selective media: Only selective bacteria can grow.
• Differential: Most bacteria will grow to distinguish characteristics.
  • E.g., blood agar to breakdown red blood cells
• Bacteria can take 1 to 2 days for bacteria and longer for mold and yeast to grow.
• A more recent method involves paper backed thin films of growth media (Petrifilm).
• Biochemical testing: Each organism has biochemical reactions to categorize and identify it.
  • E.g., gelatinase, IMViC, fermentation of lactose
• Enumerating (counting) involves:
  • Growing the organisms in a suitable medium at different dilutions.
  • It is useful to get separate colonies and an easily countable, or accurate number on the plate
• The number of colonies is expressed as cfu (colony forming units).
• There is an assumption of each colony arising from a single bacteria.

Molecular and Immunological Identification

• Molecular biology identification makes use of the DNA of the microorganism and probes a specific and unique part of the genome, it can be done in a few hours.
• Immunological identification:
  • Uses a unique molecular marker of the microorganism. -Requires an antibody to that marker

Other Identification Methods

• Examples of other measures related to microbial activity:
  • Titratable acidity (TA) in milk
    • Lactobacillus in milk produces lactic acid.
    • Low levels of bacteria mean low acidity
    • Acidity increases as the population increases.