文档介绍：1 Principles Which Influence Microbial Growth, Survival, and Death in Foods August 28, 2003 Textbook: Chapter 2 Objectives Define pH, water activity, and O/R potential Describe how pH, water ac tivity, O/R potential and other intrinsic parameters affect microbial growth Describe how various extrinsic parameters affect the ability of anisms to grow in foods Describe the concept of microbial interference and antogonism between anisms Explain the hurdle concept and describe examples of how it can be applied to inhibit and/or eliminate anisms in foods Microbial parameters ?Growth phase ?Forms of cells– vegetative cells, spores ?Types of cells– bacteria, fungi ?Genus, species, strain 2 0 2 4 6 8 10 0 2 4 6 8 10 12 14 16 18 20 22 24 HOURS LOG CFU/ML Bacterial growth curve LAG LOG STATIONARY Spores ?Resistance to chemicals ?Resistance to extreme temperatures ?Resistance to dry conditions ?Resistance to irradiation Intrinsic (food related) parameters ? pH ?Moisture content (water activity or a w ) ?Oxidation-reduction potential (E h ) ?Nutrient content ?Antimicrobial constituents ?Biological structures 3 pH Optimum Maximum Minimum Bacteria - Molds - - - Yeasts - - - Optimum pH ranges for microbial growth pH Yeasts Molds Bacteria Relevant figures Approximate pH growth ranges for some foodborne bacteria Relationship of pH, NaCl, an d Na citrate on the rate of growth of Alcaligenes feacalis in 1% peptone Approximate pH values of some fresh fruits and vegetables Reported minimum pH values for the growth of some foodborne bacteria Approximate pH values of dairy, meat, poultry, and fish products 4 Water activity (a w ) ?Definition: Equilibrium relative humidity/100 ?Range: 0 - (pure water) Water activity ranges for microbial growth Bacteria - S. aureus - Halophilic bacteria – Yeasts - Osmotolerant yeasts - Molds - Xeromyces - Water activity of foo