food microbiology 08/11/11. ideal environment for microbial growth: moist nutrient-rich ph neutral...
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Ideal environment for microbial growth:•Moist•Nutrient-rich•pH neutral
Water activity (aw): designate amount of water available in foods Pure water aw = 1.0 Most bacteria aw > 0.90
Most fungi aw > 0.80
Low pH: inhibits most bacterial growth &production of some toxins (Exceptions: lactic acid bacteria and fungi)
Growth of Microorganisms in Food
• Warm• Oxygen-rich
Food SpoilageGenerally not harmfulMay be presence of • Food-borne bacterial pathogen• Fungal toxin
Common spoilage bacteria• Pseudomonas (meat, fish, eggs)• Psychrophilic organisms• Endospore forming organisms (Clostridium & Bacillus
species)
Common spoilage fungi (lower aw & acidic environments)
Rhizopus Penicillium Aspergillus
Pepe, O. et al. 2003. Appl. Environ. Microbiol. 69(4):2321-2329
Foodborne Illness“food poisoning”
Infection : colonization by a pathogen Most food-borne pathogens: obligate anaerobes
•Salmonella •Campylobacter
•Escherichia coil O157:H7
Intoxication: ingestion of microbial toxin•Staphylococcus aureus•Clostridium botulinum
Salmonellosis (Infection)1.4 million cases (100s of deaths) Gastroenteritis/typhoid fever
Organism: Salmonella enterica Subspecies: entericaSerovars: Enteritidis, Typhi, Typhimurium
Virulence factors:Type III secretion system •Cell invasion•Toxin that inhibits the immune system
Associated with poultry products• Inadequate cooking• Cross-contamination common
Sensitive to stomach acid- must ingest high numbers
Staphylococcus aureus (Intoxication)S. aureus: not good at competing with other organisms
Superantigen enterotoxin:Heat stable
Mechanism of toxin:• Poorly understood• Affect sub-epithelial
macrophages (inflammation)• Change in local vascular
permeability
Food PreservationPreventing growth and metabolic activities of organisms that cause
spoilage and foodborne illness
Canning– Steam under pressure – Destroys spoilage and pathogenic organisms (including endospores)– High acid environment
Pasteurization– High temperatures for short periods– Reduces number of spoilage organisms