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Lecture #9

Lecture #9. Food Microbiology. Conditions for Spoilage. Water pH Physical structure Oxygen temperature. Microorganism Growth in Foods. Intrinsic Factors. composition pH presence and availability of water oxidation-reduction potential altered by cooking physical structure

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Lecture #9

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  1. Lecture #9 Food Microbiology

  2. Conditions for Spoilage • Water • pH • Physical structure • Oxygen • temperature

  3. Microorganism Growth in Foods

  4. Intrinsic Factors • composition • pH • presence and availability of water • oxidation-reduction potential • altered by cooking • physical structure • presence of antimicrobial substances

  5. Composition and pH • putrefaction • proteolysis and anaerobic breakdown of proteins, yielding foul-smelling amine compounds • pH impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food

  6. Water availability • in general, lower water activity inhibits microbial growth • water activity lowered by: • drying • addition of salt or sugar • osmophilic microorganisms • prefer high osmotic pressure • xerophilic microorganisms • prefer low water activity

  7. Physical structure • grinding and mixing increase surface area and distribute microbes • promotes microbial growth • outer skin of vegetables and fruits slows microbial growth

  8. Antimicrobial substances • coumarins – fruits and vegetables • lysozyme – cow’s milk and eggs • aldehydic and phenolic compounds – herbs and spices • allicin – garlic • polyphenols – green and black teas

  9. Extrinsic Factors • temperature • lower temperatures retard microbial growth • relative humidity • higher levels promote microbial growth • atmosphere • oxygen promotes growth • modified atmosphere packaging (MAP) • use of shrink wrap and vacuum technologies to package food in controlled atmospheres

  10. Microbial Growth and Food Spoilage • food spoilage • results from growth of microbes in food • alters food visibly and in other ways, rendering it unsuitable for consumption • involves predictable succession of microbes • different foods undergo different types of spoilage processes • toxins are sometimes produced • algal toxins may contaminate shellfish and finfish

  11. Food Spoilage • Approximately 1/3rd of all food manufactured in world is lost to spoilage • Microbial content of foods (microbial load): qualitative (which bugs) and quantitative (how many bugs) • Shelf life • Non-perishable foods (pasta) • Semiperishable foods (bread) • Perishable foods (eggs)

  12. General Principles • Minimize contamination by: • Good management processes • Acceptable sanitary practices • Rapid movement of food through processing plant • Well-tested preservation procedures

  13. Spoilage • Meat • Cutting board contamination • Conveyor belts • Temperature • Failure to distribute quickly • Fecal bacteria from intestines • Fish • Polluted waters • Transportation boxes

  14. Spoilage • Poultry and Eggs • Human contact • Penetration by bacteria • Milk and Dairy Products • Lactobacillus and Streptococcus species that survive pasturization (sour milk) • Breads • Spores and fungi that survive baking • Grains • Fungi produce toxins

  15. Food-Borne Diseases • two primary types • food-borne infections • food intoxications

  16. Preventing Foodborne Disease • Food infections (microbes are transferred to consumer) • Food poisoning (results from the toxin consumption)

  17. Food-Borne Intoxications • ingestion of toxins in foods in which microbes have grown • include staphylococcal food poisoning, botulism, Clostridium perfringens food poisoning, and Bacillus cereus food poisoning

  18. Toxins • ergotism • toxic condition caused by growth of a fungus in grains • aflatoxins • carcinogens produced in fungus-infected grains and nut products • fumonisins • carcinogens produced in fungus-infected corn

  19. Controlling Food Spoilage

  20. Removal of Microorganisms • usually achieved by filtration • commonly used for water, beer, wine, juices, soft drinks, and other liquids

  21. Low Temperature • refrigeration at 5°C retards but does not stop microbial growth • psychrophiles and psychrotrophs can still cause spoilage • growth at temperatures below -10°C has been observed

  22. High Temperature • canning • pasteurization

  23. Canning • food heated in special containers (retorts) to 115 °C for 25 to 100 minutes • kills spoilage microbes, but not necessarily all microbes in food

  24. Spoilage of canned goods • spoilage prior to canning • underprocessing • leakage of contaminated water into cans during cooling process

  25. Pasteurization • kills pathogens and substantially reduces number of spoilage organisms • different pasteurization procedures heat for different lengths of time • shorter heating times result in improved flavor

  26. Water Availability

  27. Chemical-Based Preservation • GRAS • chemical agents “generally recognized as safe” • pH of food impacts effectiveness of chemical preservative

  28. Radiation • ultraviolet (UV) radiation • used for surfaces of food-handling equipment • does not penetrate foods • Gamma radiation • use of ionizing radiation (gamma radiation) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables

  29. Detection of Food-Borne Pathogens • must be rapid and sensitive • methods include: • culture techniques – may be too slow • immunological techniques - very sensitive • molecular techniques • probes used to detect specific DNA or RNA • sensitive and specific

  30. comparison of PCR and growth for detection of Salmonella

  31. nucleic acid can be detected even when plaque-forming ability is lost

  32. Surveillance for food-bornedisease • PulseNet • established by Centers for Disease Control • uses pulsed-field gel electrophoresis under carefully controlled and duplicated conditions to determine distinctive DNA pattern of each bacterial pathogen • enables public health officials to link pathogens associated with disease outbreaks in different parts of the world to a specific food source

  33. Surveillance… • FoodNet • active surveillance network used to follow nine major food-borne diseases • enables public health officials to rapidly trace the course and cause of infection in days rather than weeks

  34. Helpful Suggestions • Refrigerate quickly • Wash hands • Clean cutting boards • Leftovers • Avoid home-canned foods

  35. Microbiology of Fermented Foods • major fermentations used are lactic, propionic, and ethanolic fermentations

  36. Fermentation Any partial breakdown of carbohydrates taking place in the absence of oxygen.

  37. Meat and Fish • sausages • hams • bologna • salami • izushi – fish, rice and vegetables • katsuobushi – tuna

  38. Wine White vs. Red: juice or juice and skin Yeasts: Ferment when no oxygen around. Saccharomyces species Dry Sweet Sparkling Fortified

  39. Production of Breads • involves growth of Saccharomycescerevisiae (baker’s yeast) under aerobic conditions • maximizes CO2 production, which leavens bread • other microbes used to make special breads (e.g., sourdough bread) • can be spoiled by Bacillus species that produce ropiness

  40. Other Fermented Foods • silages • fermented grass, corn, and other fresh animal feeds

  41. Microorganisms as Foods and Food Amendments • variety of bacteria, yeasts, and other fungi are used as animal and human food sources • probiotics • microbial dietary adjuvants • microbes added to diet in order to provide health benefits beyond basic nutritive value

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