Principles of Food Science Unit 7 Review: Microbes in Foods

Key Concepts and Definitions

• Microorganisms are tiny living organisms that can only be seen under a microscope and include bacteria, viruses, fungi, and protozoa
• Pathogenic microorganisms cause disease or illness in humans while beneficial microbes are used to produce fermented foods (yogurt, cheese, sauerkraut)
• Foodborne illness results from consuming food contaminated with pathogenic microorganisms or their toxins
• Microbial growth refers to the increase in the number of microbial cells and is affected by factors such as temperature, pH, water activity, and nutrient availability
• Food spoilage occurs when microorganisms grow and metabolize food components, leading to undesirable changes in texture, flavor, and appearance
• Colony-forming units (CFUs) represent the number of viable microbial cells in a sample and are used to quantify microbial growth
• Microbial contamination introduces microorganisms into food from various sources (raw materials, equipment, personnel)
  • Can occur at any stage of food production, processing, or handling

Types of Microorganisms in Food

• Bacteria are single-celled prokaryotic organisms that can be found in a wide range of food products
  • Examples include Escherichia coli, Salmonella, Listeria monocytogenes, and lactic acid bacteria
• Fungi, including yeasts and molds, are eukaryotic organisms that can grow on various food substrates
  • Yeasts are used in the production of bread, beer, and wine
  • Molds are responsible for the ripening of certain cheeses (Camembert, Roquefort)
• Viruses are non-living entities that require host cells to replicate and can cause foodborne illnesses (Norovirus, Hepatitis A)
• Protozoa are single-celled eukaryotic organisms that can contaminate food and water sources (Cryptosporidium, Giardia)
• Algae, such as spirulina and chlorella, are used as food additives or supplements due to their nutritional value
• Archaea are prokaryotic organisms that can survive in extreme environments and are not typically associated with food

Factors Affecting Microbial Growth in Foods

• Temperature plays a crucial role in microbial growth, with most microorganisms growing best at temperatures between 20°C and 45°C (danger zone)
• pH affects microbial growth, with most bacteria preferring a neutral pH range (6.5-7.5), while yeasts and molds can tolerate more acidic conditions
• Water activity (aw) is a measure of available water in food and influences microbial growth
  • Most bacteria require an aw of 0.90 or higher for growth
• Nutrient availability, including carbohydrates, proteins, and vitamins, is essential for microbial growth
• Oxygen levels determine the growth of aerobic (require oxygen) or anaerobic (grow in the absence of oxygen) microorganisms
• Antimicrobial compounds, such as salt, sugar, and preservatives, can inhibit or slow down microbial growth
• Biological structures, like plant cell walls or animal tissues, can provide a physical barrier against microbial growth

Beneficial Microbes in Food Production

• Lactic acid bacteria (LAB) are used in the production of fermented dairy products (yogurt, cheese, kefir) and vegetables (pickles, sauerkraut, kimchi)
  • LAB convert lactose into lactic acid, lowering the pH and inhibiting the growth of spoilage microorganisms
• Yeasts, such as Saccharomyces cerevisiae, are used in the production of bread, beer, and wine
  • Yeasts ferment sugars into ethanol and carbon dioxide, providing flavor and texture to the final product
• Molds, like Penicillium roqueforti and Penicillium camemberti, are used in the ripening of blue-veined and soft-ripened cheeses, respectively
• Probiotics are live microorganisms that, when consumed in adequate amounts, confer health benefits to the host
  • Examples include Lactobacillus and Bifidobacterium species
• Starter cultures are carefully selected microorganisms added to food to initiate fermentation and ensure consistent product quality
• Microbial enzymes, such as rennet (chymosin) from Rhizomucor miehei, are used in cheese production to coagulate milk proteins

Foodborne Pathogens and Diseases

• Salmonella is a gram-negative bacterium that causes salmonellosis, characterized by diarrhea, fever, and abdominal cramps
  • Often associated with contaminated poultry, eggs, and produce
• Listeria monocytogenes is a gram-positive bacterium that causes listeriosis, a severe infection in pregnant women, newborns, and immunocompromised individuals
  • Found in ready-to-eat foods, such as deli meats and soft cheeses
• Escherichia coli O157:H7 is a pathogenic strain that produces Shiga toxins, causing hemorrhagic colitis and hemolytic uremic syndrome (HUS)
  • Associated with undercooked ground beef and contaminated produce
• Clostridium botulinum is an anaerobic bacterium that produces a potent neurotoxin, causing botulism, a rare but severe paralytic illness
  • Can grow in improperly canned low-acid foods
• Staphylococcus aureus is a gram-positive bacterium that produces heat-stable enterotoxins, causing staphylococcal food poisoning
  • Often associated with foods handled by infected individuals
• Norovirus is a highly contagious virus that causes acute gastroenteritis, with symptoms including vomiting and diarrhea
  • Transmitted through contaminated food, water, or person-to-person contact

Microbial Spoilage of Foods

• Spoilage bacteria, such as Pseudomonas and Acinetobacter species, cause off-flavors, off-odors, and slime formation in refrigerated foods (meat, poultry, fish)
• Yeasts and molds can cause visible growth, off-flavors, and mycotoxin production in foods with low water activity (bread, fruits, cheese)
• Enzymatic spoilage occurs when microbial enzymes break down food components, leading to undesirable changes in texture and flavor
  • Examples include lipases (rancidity in fats and oils) and proteases (softening of meat)
• Fermentative spoilage is caused by microorganisms that ferment sugars, producing acids, gas, and off-flavors
  • Can occur in vacuum-packaged meats and canned foods
• Microbial metabolites, such as biogenic amines (histamine) and sulfur compounds, can accumulate in spoiled foods and cause adverse health effects
• Spoilage indicators, like pH changes or gas production, can be used to detect microbial spoilage in foods
• Shelf life is the time period during which a food remains safe and retains its desired quality attributes
  • Determined by factors such as initial microbial load, storage conditions, and packaging

Detection and Enumeration Methods

• Plate count methods involve spreading a diluted food sample onto an agar plate and counting the number of colonies that grow after incubation
  • Examples include aerobic plate count (APC) and yeast and mold count (YMC)
• Most probable number (MPN) is a statistical method used to estimate the number of viable microorganisms in a sample through serial dilutions and multiple tube fermentation
• Membrane filtration is used to concentrate microorganisms from liquid samples by passing them through a fine filter and then transferring the filter to an agar plate for incubation
• Rapid detection methods, such as ATP bioluminescence and immunoassays, provide quick results for assessing microbial contamination in food
  • ATP bioluminescence measures the amount of ATP present, which correlates with the number of viable cells
  • Immunoassays use antibodies to detect specific microbial antigens or toxins
• Molecular methods, like polymerase chain reaction (PCR) and whole genome sequencing (WGS), are used for the specific detection and identification of microorganisms
• Microscopy techniques, such as light microscopy and electron microscopy, allow for the visualization and morphological characterization of microorganisms in food samples

Food Preservation Techniques

• Thermal processing, including pasteurization and sterilization, uses heat to inactivate microorganisms and enzymes in food
  • Pasteurization involves heating food to a specific temperature for a set time to reduce the number of spoilage and pathogenic microorganisms
  • Sterilization, such as canning, involves heating food to a high temperature (121°C) to eliminate all viable microorganisms and spores
• Refrigeration and freezing slow down or stop microbial growth by lowering the temperature below the optimal range for most microorganisms
• Drying and dehydration remove water from food, reducing the water activity to levels that inhibit microbial growth
  • Examples include dried fruits, jerky, and powdered milk
• Chemical preservation uses additives, such as salt, sugar, and organic acids, to create an unfavorable environment for microbial growth
  • Antimicrobial agents, like nitrites and sorbates, are used to inhibit specific microorganisms
• Modified atmosphere packaging (MAP) involves altering the gas composition within the package to extend shelf life and inhibit microbial growth
  • Commonly used gases include carbon dioxide, nitrogen, and oxygen
• Fermentation is a process in which microorganisms convert sugars into acids, alcohol, or other compounds, creating an environment that inhibits the growth of spoilage and pathogenic microorganisms
• High-pressure processing (HPP) uses high hydrostatic pressure to inactivate microorganisms and enzymes while maintaining the sensory and nutritional qualities of the food