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Food microbiology

Food microbiology. Preparing food Preserving food When microbes go wrong. The fine line between fermenting and spoiling. What kind of food does the microbe grow on? Which microbe grows on it (what is its end product)? What is “agreeable” and what is “disagreeable”?.

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Food microbiology

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  1. Food microbiology Preparing food Preserving food When microbes go wrong

  2. The fine line between fermenting and spoiling • What kind of food does the microbe grow on? • Which microbe grows on it (what is its end product)? • What is “agreeable” and what is “disagreeable”?

  3. Which microbes grow in food, and why? • Intrinsic factors • Nutrient content • Water availability (aw) • pH (acid conditions may inhibit growth of harmful microbes) • Biological barriers • Antimicrobial substances • Think about bacteria and fungi

  4. How do intrinsic factors favor the growth of some organisms over others? • “Water activity” is affected (reduced) by salt and sugar content • Pathogens tend to prefer high aw • Food microbiologists quantify this • Toxin production and pH: “high-acid” foods are protected • Some foods have proteases and acids that inhibit microbes

  5. Extrinsic factors • Environmental features • Ambient temperature • Cold temperatures slow growth (some organisms more than others) • Atmosphere: presence or absence of oxygen • If conditions change, different microbes can grow • Implications for vacuum packing?

  6. How do you detect microbes in food? • Plate count • Can be performed on differential media • Measures organisms that can form colonies on plates • MPN • Enrichment • Both methods useful for detecting very low levels of microbes

  7. Some pathogens can’t be cultured! How do we measure those? • Viable but nonculturable (VNC) state- like dormancy • Cells become VNC by environmental stressors • VNC organisms can be “reactivated” by changes in temperature, nutrients, etc. • Salmonella, Campylobacter, Escherichia, Shigella, and Vibrio can all become VNC

  8. Microbes and food production • Obligate fermenters (can only ferment): • Streptococcus • Lactococcus • Lactobacillus • Leuconostoc, etc • Starter cultures must be preserved • Other microbes added for flavor and aesthetics

  9. Commercial cheese production • Fermentation products cause curdling • If ripened, mixture is dehydrated and salted • Shaped and ripened (timing is key)

  10. More fermented foods • Pickled foods • Salting and packing • Nature (fermentation) does the rest • Fermented meats • Sugar needed to “feed” fermenting bacteria • Starter cultures (not abundant in meat) • Nitrates inhibit spoilage • Heating, then drying

  11. Yeasts produce different fermentation products • Ethanol and carbon dioxide • Alcoholic beverages • Bread

  12. Distinguishing features of alcoholic fermentation • What is fermented? • Fruit (wine) • Rice (sake) • Grains (beer, spirits) • How is the product finished? • Spirits are distilled • Beer and wine are filtered

  13. Fermentation has enhanced and preserved food for centuries • By bacteria, yeasts and molds • In absence of oxygen • Acids and alcohols are produced • Vinegar (oxidized alcohol!) • Specialized, commercial strains used for each

  14. Molds are used in food production, too

  15. Food spoilage • End products are obnoxious • Sometimes harmful • Bacteria tend to spoil moist foods • Pseudomonas and other psychrotrophic species damage refrigerated foods • Fungi dry or acidic foods • Penicillium, Rhizopus, Aspergillus, among others

  16. Diseases transmitted in food • Toxins (“foodborne intoxication”) • Staph toxin is heat-stable; botulism toxin is not • C. perfringens, B. cereus also produce toxins • Spore formers are adapted to many environments

  17. Foodborne infection • Organisms actually cause illness • Symptoms develop more slowly than with intoxication • Prevention • Proper cooking • Rapid cooling • Avoiding cross-contamination

  18. Food preservation • Pre-industrial • Drying (lowering the aw) • Pickling (salting) • Canning (low-acid vs high-acid) • Cooling • Mass production • Pasteurization • Preservatives • Freezing (freeze-drying) • Irradiation

  19. Summary • Microbes eat the same foods we do! • Controlled microbial growth can enhance our diets and help preserve foods • Metabolism of some microbes leads to spoilage of foods • Some microbes in food are pathogens • Minimizing microbial contamination of food is a priority

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