Factors affecting microbial activity in foods. Bacterial growth curve. Intrinsic Factors Extrinsic Factors

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1 Factors affecting microbial activity in foods Intrinsic Factors Extrinsic Factors Bacterial growth curve Log bacterial No. Incubation time Possible fate of a microorganism after gaining access to a food product Growth Number of cells Survival Storage time Death

2 Intrinsic Factors ph Moisture content Oxidation-reduction potential (Eh) Nutrient content Antimicrobial constituents Biological structures Extrinsic Factors Temperature of storage Time of storage Gaseous environment Relative humidity Intrinsic factors Intrinsic: Pertaining to the food product

3 ph Effects on microorganisms: Affects enzymatic function Affects nutrient transport into the cell ph Cytoplasmic ph is neutral When microorganisms are in a low ph environment they must prevent H + intake, or free H + as the same rate as it is taken. Effect of H + transport: Increase in denaturization of membrane proteins and transport enzymes Affects cell premeability and decreases intracellular ph Optimal ph values for growth of different types of microorganisms Type of microorganism Optimum ph Bacteria Yeasts Molds

4 ph of selected foods Food ph Egg yolk Seafood Meats Liver Vegetables Fruits Effect of food ph on microbial activity For canned foods product ph determines the type of treatment to apply Minimal ph for production of botulin toxin = = 12D treatment ( fold reduction) Effect of food ph on microbial activity Beef carcasses DFD (dry, firm, dark) When cattle are subjected to stress before slaughter a glycogen depletion occurs There are no lactic acid sources during rigor mortis ph of normal meat: , ph of DFD meat: >6.0 DFD meat is more susceptible to bacterial deterioration and is of lower quality

5 Most common causes of food deterioration by product ph Product Fruits Juices, soft drinks Vegetables Meat ph Most common causes of spoilage Yeasts, molds Yeasts, molds Bacteria Bacteria Growth of E. coli in the presence of acetic acid (Diez- Gonzalez and Russell, Microbiol., 143: ) O157:H7 K Internal ph O157:H7 K-12 OD ph Acetate concn. (mm) Acetate concn. (mm) Moisture content Water activity (a w )

6 Water activity (a w ) Definitions Physical chemical: A w = p/p 0 p = vapor pressure of food system p 0 = vapor pressure of solvent (water) Biochemical: Amount of molecules available to serve as vehicle for biochemical reactions Microbial growth Water activity (a w ) Vapor pressure Atmospheric pressure Water activity (a w ) Na + Na + H - O 2 + H Cl - Cl -

7 Minimum Aw requirements for growth of different types of microorganisms Type of microorganism Minimum required Aw Most bacteria 0.90 Yeasts 0.80 Molds 0.70 Aw values for selected foods Food Aw Meat Milk Fruits Vegetables Bread Evaporated milk Tomato paste Cheese Condensed milk Jellies Syrup Crackers <0.60 Pasta Water activity (a w ) Mostly used for preservation purposes Limited effect on food safety Good control measure for pathogens that need to grow in order to become a hazard Staphylococcus aureus Clostridium botulinum

8 Minimum a w for growth of S. aureus Minimum for growth 0.86 Minimum for enterotoxin B production 0.90 Survival of Salmonella in meringue powder as affected by a w 250 Days of storage a w REDOX Potential (Eh) Definitions Oxidation Release of electrons from a compound Reduction Incorporation of electrons into a compound Always occur in pairs Oxidation - Reduction

9 REDOX Potential (Eh) When an oxidation-reduction reaction occurs, an electric potential is created Oxidated = positive Reduced = negative Depending on the concentration of oxidizing and reducing compounds: Greater reduction or oxidation capacity Eh value Eh and microbial growth Anaerobiosis -200 mv Aerobiosis 200 mv -200 a 200 mv includes growth of microaerobic, aerobic and anaerobic organisms Eh values of selected foods Food Fruits, vegetables, juices Milk Meat Ground meat Cheeses Eh (mv) 300 to a -200 Adapted from Jay, J. M., 5th. Ed., 1998

10 Eh of meat Immediately after slaughter 250 mv 30 h post mortem Without microbial growth With microbial growth -130 mv -250 mv Barnes and Ingram. J. Sci. Food Agr. 6: Nutrient content Availability Ex. Ground beef Breaking tissues promotes fluid leakage More nutrient availability Content Most foods can support microbial growth Antimicrobial constituents Spices Essential oils Eugenol in clove and cinnamon Alicin in garlic Cinnamic aldehyde in cinnamon Carvacrol and thymol in oregano Allyl isothiocianyde in mustard

11 Antimicrobial constituents Raw cow s milk Lactoferrin Lactoperoxidase Lysozyme Immunoglobulines Egg white Lisozyme Conalbumen Fruits Organic acids Biological structures Natural covers. Prevent microbial access and the resulting food spoilage Hull of fruits and vegetables Nut shell Egg shell After harvests wounds Stem scar in fruits. Promotes bacterial penetration