Food Microbiology Overview

Food Microbiology

• Food microbiology examines food for safety ensuring standards are met by processing companies
• Objectives include confirming food safety standards, ensuring food meets internal standards set by the processing company, verifying materials entering the food meet standards, determining presence of specific microorganisms, and performing quality control.

Methods for Food Microbiological Examination

• Culture-Based Methods: Classical techniques to isolate, enumerate, and identify microorganisms.

  • Plate Count Methods:
    • Standard Plate Count (SPC) measures viable microorganisms in a sample, using spread plate and pour plate techniques.
    • Selective and Differential Media isolate specific types of microorganisms (e.g., MacConkey agar for coliforms).
  • Enrichment Culture Techniques: Enhance the growth of specific microorganisms in low concentrations.
    • Most Probable Number (MPN) Method provides a statistical estimation of microbial populations in samples.

• Microscopic Methods:

  • Direct Microscopic Count (DMC) observes bacteria directly under a microscope (e.g., using a Petroff-Hausser counting chamber).
  • Fluorescence Microscopy uses fluorescent dyes to identify live/dead cells (e.g., acridine orange or DAPI stains).

• Biochemical Methods:

  • Catalase and Oxidase Tests identify specific metabolic properties of microbes.
  • Fermentation Tests and API (Analytical Profile Index) Systems also determine metabolic properties.

• Rapid Detection and Enumeration Techniques:

  • Immunological Methods: Enzyme-Linked Immunosorbent Assay (ELISA) and Latex Agglutination Tests.
  • Molecular Methods:
    • Polymerase Chain Reaction (PCR) detects specific DNA/RNA of microorganisms.
    • Quantitative PCR (qPCR) quantifies microbial DNA.
    • Loop-Mediated Isothermal Amplification (LAMP).
  • Biosensors detect microbial metabolites or specific microbial activity in food samples.

• Advanced and Emerging Technologies:

  • Next-Generation Sequencing (NGS) provides a detailed microbial profile of food samples.
  • Microarrays detect multiple genes simultaneously.
  • Whole Genome Sequencing (WGS).

• Physicochemical Methods:

  • ATP Bioluminescence Assay measures ATP to estimate microbial contamination.
  • Mass Spectrometry (e.g., MALDI-TOF).

• Rapid Automation-Based Systems:

  • Automated colony counters (e.g., Petrifilm).
  • Real-time PCR-based systems.

Sampling

• Sampling Plan: One of the biggest problems in food microbial examination is the sampling plan; samples should be representative.
• Number of Samples: 5-10 samples per batch or the square root of the number of packs per batch.
• Representative Samples: Samples must be representative of the entire batch.

Two-Class Sampling Plans

• Two-class plans specify n (number of units to examine), c (maximum acceptable number of unsatisfactorily units), and m (maximum acceptable bacterial count per gram) where values above m indicate unacceptability.

Choosing a Sampling Plan

• Choosing a sampling plan depends on the related health hazard and condition of use.
• The International Commission on Microbiological Specifications for Foods (ICMSF) suggests sampling plans related to hazard conditions for various food categories, including raw meat, processed meat, poultry products, dried milk, cheese, seafood, vegetables, fruit, cereal products, infant food, and dietetic foods.

Microbiological Criteria for Ice Cream

• Specific microbiological criteria for ice cream: Listeria monocytogenes, Salmonella, Clostridium, Staphylococcus aureus, and Aerobic colony count (30°C).

Dilution factor for food samples

• Dilution factor is calculated by dividing the volume of sample added to the tube by the final volume in the tube.

Total Viable Count (TVC)

• TVC, also known as Standard Plate Count (SPC) or Aerobic Plate Count (APC), is a common method to determine the number of viable bacteria in a food sample.

Examination Methods details

• Agar Media: Nutrient agar or plate count agar media (pH 7.0 to 7.4) are used for enumerating bacterial growth.
• Incubation Temperatures:
  • Thermophiles: 55°C
  • Mesophiles: 35-37°C
  • Spoilage/Psychrophiles: 20-30°C

Indicator Organisms

• Indicator organisms in food suggest the probability of pathogen presence (e.g., coliforms, E. coli, Enterococci, Staphylococcus aureus, Listeria monocytogenes, Salmonella).
• Coliforms are common in the gastrointestinal tracts of humans and animals; E.coli indicators of fecal contamination.
• Other indicator organisms, like Enterococci, Staphylococcus aureus, Salmonella, and Listeria monocytogenes, also aid in evaluating food safety with regards to contamination, sanitation, and potentially extensive contamination.

Additional Food Poisoning Bacteria

• Enterococci: Indicators of fecal pollution but less conclusive evidence of fecal contamination. Composed of E. faecalis and E. faecium.
• Staphylococcus aureus: Produces thermostable deoxyribonuclease (TNase), which helps rapidly enumerate staphylococcus in food. Growth ≥ 10⁶ indicates possible toxin production. It's an indicator of improper handling and heat processing.
• Salmonella: Used as an indicator for cross-contamination, environmental contamination, and dust movement into food.
• Listeria monocytogenes: Used to assess low-temperature food storage, especially within containers, drains, and sanitation. Presence suggests inadequate sanitation.
• Compylobacter: Used to recover low numbers of the organism.
• Bacillus cereus: Used to assess Bacillus food poisoning, especially in starchy foods like rice.
• Escherichia coli: Used as an indicator of hygiene level in raw and processed plant products. Presence of the organism in food suggests potential contamination with fecal matter.
• Clostridium (botulinum, perfringenes): Evaluate thermal effects on processing, especially in canned food products.
• Yeast and Molds: Used to assess spoilage in fruit, vegetables, and other food items.