Unit 5.1 (Difficulty: Hard) Microorganisms in Food

Introduction to Microorganisms in Food

• Microbiological analysis involves detecting and identifying microorganisms as a crucial part of food analysis.
• Addressing whether the correct microbes are present is important during food preparation, for example:
  • Streptobacillus lactis in cheese production.
  • Acetobacter in vinegar production.
• Assessing whether microbes are at acceptable levels is important for food spoilage, for example:
• Lactobacillus in milk at 2 x 10^5 CFUs/mL.
• E. coli in bean sprouts at 5 x 10^3 CFUs/g, where CFU stands for colony-forming unit.
• Microbes impact a food's shelf-life.
• Controlling microbial growth helps prolong a food's shelf-life.
• Food-borne illnesses can occur through:
  • Infection, when bacteria colonize the gut.
  • Intoxication, when bacteria produce a toxin.
  • Intoxification, when bacteria colonize and produce a toxin.

Types of Microorganisms

• Bacteria: Single-celled prokaryotes, such as Lactobacillus or certain E. coli strains found in foodborne illnesses.
• Yeast: Single-celled eukaryotes like Saccharomyces species, used to make beer or wine.
• Mold: A structure formed by some fungi; Penicillum is an example.
• Virus: An infectious agent replicating in other organisms, like Norovirus spreading through foods/surfaces.
• Parasites: Animals or protozoa such as Giardia.

Growth Requirements of Microorganisms

• Most microorganisms are neutrophiles, thriving in neutral pH conditions.
• Acidity has an inhibitory effect on microbial growth.
• Bacteria grow at pH > 4.5 (at aw of 0.85).
• Yeasts grow at pH > 4.0 – 4.5.
• Molds grow at pH < 4.0.
• Acidophiles include molds, yeasts, and lactic acid bacteria.
• Organic acids, indicated by low pH, help minimize food spoilage.
• Water activity influences microbial growth.
• Salinity and water activity are inter-related.

Temperature and Microorganism Growth

• Psychrophiles: "Cold-loving" organisms that grow from -20 °C to 20 °C.
• Psychrotrophs: Cold-tolerant organisms that grow at 0 °C but prefer 20 °C – 40 °C.
• Mesophiles: Thrive in an optimum growth range of 20 °C – 45 °C.
  • Used in making fermentation products.
  • Most human pathogens are mesophiles.
• Thermophiles: "Heat-loving" organisms that thrive between 50 °C and 64 °C.
• Extreme Thermophiles: Thrive between 65 °C and 79 °C.

Atmosphere

Bacteria Identification by Oxygen Requirements

• Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth.
• Obligate aerobes: Require oxygen.
  • Gather at the top of the tube where oxygen concentration is highest.
• Obligate anaerobes: Poisoned by oxygen.
  • Gather at the bottom of the tube where the oxygen concentration is lowest.
• Facultative anaerobes: Grow with or without oxygen.
  • They gather mostly at the top because aerobic respiration generates more ATP.
• Microaerophiles: Need oxygen but are poisoned by high concentrations.
  • They gather in the upper part of the test tube.
• Aerotolerant organisms: Do not require oxygen.
  • Not poisoned by oxygen and are evenly spread throughout the test tube.

Nutrients

• Microorganisms need specific nutrients.
• These include sources of carbon, nitrogen, sulfur, and phosphorous.

Growth Factors and Food Preservation

• pH: Acidifying foods with organic acids or acid-producing microorganisms.
  • Lowers the pH of canned goods.
• Water activity: Storing food with water activity < 80 prevents microorganism growth.
• Salinity: Salting foods prevents the growth of microorganisms.
• Temperature: Heating foods removes bacteria via pasteurization and cooking.
  • Storing food in cold conditions and following maximum holding times at room temperature also control growth.
• Atmosphere: Reduce growth of food spoilage organisms in canned goods.
  • Achieved by limiting oxygen.
• Nutritional requirements: Foods with good nutritional value need to be controlled using the above growth factors.

Spoilage of Acid Foods

• Must be detected by culturing with acidified media (pH 5.0 – 5.5).
• pH isn't the best indicator for predicting microbial response for preservation, as it doesn't indicate:
  • Changes in total acidity (buffering effects).
  • The type of organic acid produced.
• Problematic microorganisms in food lead to food spoilage and illness (food pathogens).
• Some microorganisms, like bacteria or fungi in fermented food, are helpful.
• Controlling microorganism growth requires understanding growth requirements.

Detecting, Identifying, and Counting Microorganisms

• Food microbiologists need to understand what microorganisms are present in a system.
• Food requires storage conditions that prevent growth.
• Preservation methods may remove multiple growth factors for different organisms.
• Approaches to identifying microorganisms in food:
  • Culturing or growing.
  • Molecular biology testing for genetic markers.
  • Testing for specific immunological markers.
  • Specific purpose tests.

Culture Methods

• Grow organisms on media in a petri dish using the traditional approach.
• Different organisms have different growth requirements.
• Pathogenic organisms tend to be more fastidious than nonpathogens.
• Different medias detect different organisms.
• Defined media: all components are known.
• Complex media: involves "undefined" components such as yeast extract.
• Selective media: only selective bacteria can grow.
• Differential media: most bacteria will grow, but it can distinguish certain characteristics.

Biochemical Testing

• Organisms have biochemical reactions.
• These assist in categorization and identification.
• Examples: Gelatinase, IMViC, and lactose fermentation.

Enumerating

• Grow organisms in a suitable medium at different dilutions.
• Get separate colonies and a countable number on the plate.
• Number is expressed as cfu (colony-forming units).
• There is an assumption of each colony arising from a single bacteria.

Molecular Biology

• Utilizes microorganism DNA.
• Probes for a specific and unique part of the genome.
• Can be done in a few hours.
• Can provide very good specific identification.

Immunological

• Utilizes a unique molecular marker of the microorganism.
• Requires an antibody to that marker.

Other Methods

• Quick measure of a characteristic related to microbial activity.
• Titratable acidity (TA) in milk.
  • Lactobacillus in milk produces lactic acid.
  • Low bacteria levels result in low acidity, mostly due to lactic acid.
  • As population increases, acidity increases.