9/18/2016 Typical aw of various foods Microbes have different aw requirements for growth Foods a Decreasing aw Fresh,raw fruits,vegetables,meat,and fish >0.98 increases lag phase Cooked meat,bread 0.95-0.98 decreases growth rate decreases the number of cells at stationary Cured meat products,cheeses 0.91-0.95 phase Sausages,syrups 0.87-0.91 Grouped by their minimal aw requirement Rice,beans,peas 0.80-0.87 Reflects the mechanisms used by each bacterium Jams,marmalades 0.75-0.80 to protect against osmotic stress Candies 0.65-0.75 Compatible solutes are synthesized Dried fruits 0.60-0.65 Accumulate intracellularly Equilibrate the intracellular water activity Dehydrated vermicelli,spices,mild powder 0.20-0.60 Do not interfere with cellular metabolism Minimal aw required for growth of Extrinsic Factors that Influence foodborne microbes at 25C Microbial Growth Group of Extrinsic factors are external to the food Minimal a required microorganisms ·Temperature Most bacteria 0.91-0.88 -Growth rate slower at 4oC Most yeasts 0.88 -Physiology differs at 4C Regular molds 0.80 Genes expressed differ at 4C Halophilic bacteria 0.75 ·Gas composition Xerotolerant molds 0.71 Xerophilic molds and 0.62-0.60 osmophilic yeasts 16 9/18/2016 16 Typical aw of various foods Foods aw Fresh, raw fruits vegetables meat and fish raw fruits, vegetables, meat, and fish >0 98 . Cooked meat, bread 0.95-0.98 Cured meat products, cheeses 0.91-0.95 Sausages, syrups 0.87-0.91 Rice, beans, peas 0.80-0.87 Jams, marmalades 0 75 . -0 80 . Candies 0.65-0.75 Dried fruits 0.60-0.65 Dehydrated vermicelli, spices, mild powder 0.20-0.60 61 Microbes have different aw requirements for growth • Decreasing aw - increases lag phase - decreases growth rate - decreases the number of cells at stationary phase • Grouped by their minimal aw requirement • Reflects the mechanisms used by each bacterium to p g rotect against osmotic stress • Compatible solutes are synthesized - Accumulate intracellularly - Equilibrate the intracellular water activity - Do not interfere with cellular metabolism 62 Minimal aw required for growth of foodborne microbes at 25oC Group of Minimal aw required microorganisms Minimal aw required Most bacteria 0.91-0.88 Most yeasts 0.88 Regular molds 0.80 H l hili b t i Halophilic bacteria 0 75 . Xerotolerant molds 0.71 Xerophilic molds and osmophilic yeasts 0.62-0.60 63 Extrinsic Factors that Influence Microbial Growth Extrinsic factors are external to the food • Temperature - Growth rate slower at 4oC - Physiology differs at 4oC - Genes expressed differ at 4oC • Gas composition 64