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Food Microbiology Elok Zubaidah

Food Microbiology Elok Zubaidah. Food Microorganisms. Bacteria Yeast Mold Viruses. How Long do Bacteria Need to Grow?. Under ideal conditions, some bacteria can double in number every 15-30 minutes Example: bacteria with a 20 min doubling time:. TIME. Start. 20 Min. 40 Min. 1 Hour.

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Food Microbiology Elok Zubaidah

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  1. Food Microbiology Elok Zubaidah

  2. Food Microorganisms • Bacteria • Yeast • Mold • Viruses

  3. How Long do Bacteria Need to Grow? • Under ideal conditions, some bacteria can double in number every 15-30 minutes • Example: bacteria with a 20 min doubling time: TIME Start 20 Min 40 Min 1 Hour

  4. Bacterial Multiplication • Time Numbers • 0 1 • 20 2 • 40 4 • 80 16 • 160 256 • 420 2,097,152

  5. Bacterial Growth Curve Stationary Phase Number of Bacteria (log10) Growth Phase Death Phase Lag Phase Time

  6. Changing the Bacterial Growth Curve Number of Bacteria (log10) IDEAL CONDITIONS SUB-OPTIMAL* Much longer Lag Phase Time *Sub-optimal means lowered: pH, AW,Temp., etc.

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

  8. Microorganism transfer • Soil and Water • Plants and animals • Raw to processed food / cross contamination • Person to Food • Person to Person

  9. Microorganism Growth in Foods

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

  11. Factors Affecting Microbial Growth in Foods What are the factors affecting microbial growth in foods? F- A- T- T- O- M- (P)-

  12. Clarification of Acidity (pH)

  13. pH scale: • no matter what extracellular pH a microorganism prefers, intracellular pH is relatively near neutrality

  14. Classification of Bacteria by Temperature Requirements

  15. variations in temperature optima:

  16. oxygen relationships of microorganisms:

  17. Relation to Oxygen • Aerobes: use oxygen in metabolism; obligate. • Microaerophiles: require oxygen (also obligate), but in small amounts. • Anaerobes: grow without oxygen; SEE NEXT A: aerobeB: microaerophile • Capnophiles: require larger amounts of carbon dioxide than are found normally in air.

  18. Anaerobes grow without O2 • Classifications vary, but our definitions: • Obligate (strict) anaerobes: killed or inhibited by oxygen. • Aerotolerant anaerobes: do not use oxygen, but not killed by it. • Facultative anaerobes: can grow with or without oxygen C: could be facultative or aerotolerant.D: strict anaerobe

  19. Protections of bacteria against oxygen • Bacteria possess protective enzymes, catalase and superoxide dismutase. • Catalase breaks down hydrogen peroxide into water and oxygen gas. • Superoxide dismutase breaks superoxide down into peroxide and oxygen gas. • Anaerobes missing one or both; slow or no growth in the presence of oxygen. Fe3+ -SOD + O2- → Fe2+ -SOD + O2 Fe2+ -SOD + O2- + 2H+ → Fe 3+ -SOD + H2O2

  20. enzymes that destroy toxic oxygen species:

  21. toxic forms of oxygen:

  22. enzymes that destroy toxic oxygen species:

  23. testing microbial culture for presence of catalase: negative positive catalase H2O2 + H2O2 2 H2O + O2

  24. Water activity and microbial growth Relative growth or reaction rate 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Water activity • Most bacteria can’t grow below aw = 0.85 • Most yeasts & molds can’t grow below aw = 0.65

  25. Low water activity:halophiles, osmophiles, and xerotolerant • Water is critical for life; remove some, and things can’t grow. (food preservation: jerky, etc.) • Halophiles/halotolerant: relationship to high salt. • Marine bacteria; archaea and really high salt. • Osmophiles: can stand hypertonic environments whether salt, sugar, or other dissolved solutes • Fungi very good at this; grandma’s wax over jelly. • Xerotolerant: dry. Subject to desiccation. Fungi best • Bread, dry rot of wood • Survival of bacterial endospores.

  26. salt tolerance:

  27. Typical water activity of some foods Aw – examples 0.95 – fruits, vegetables, meat, fish, milk 0.91 – some cheeses, ham 0.87 – salami, pepperoni, dry cheeses, margarine, 0.80 – fruit juice concentrates, sweetened condensed milk, syrups, flour, rice, high sugar cakes 0.75 – jam, marmalade 0.65 – oatmeal, fudge, marshmallows, jelly, molasses, sugar, nuts 0.60 – dried fruits, honey 0.50 – dried pasta, dried spices 0.30 – cookies, crackers 0.03 – dry milk, dehydrated soups, corn flakes

  28. Most bacterial pathogens can not grow or produce toxins at AW < 0.91 • Exception: Staph. aureus can grow and produce toxin down to 0.85 • How do you think AW can be decreased? • 1. • 2. • 3. • How do these methods work?

  29. Growth of microbes in food follows a typical microbial growth pattern • Growth rate depends on the nutritional value and temperature of the food • Number of microbes depends on both inoculum size and • growth rate • Food spoilage occurs at high populations density • (at stationary phase) - retarding microbial growth delays • spoilage

  30. Food Spoilage Food is considered spoiled when an undesirable change in the color, flavor, odor or texture has occurred. Foreign substances in food products make foods undesirable. Spoilage is a natural phenomenon; it occurs at varying rates depending on the storage temperature, kind of food involved, kind of microorganisms present, packaging materials used, food additives used and method of preservation.

  31. It is a gradual process occurring because of poor sanitation, enzymatic or chemical reactions, improper temperature controls, microbial growth or physical abuse. • II. Microbial Growth and Food Spoilage • Meats and dairy products are ideal environments for spoilage by microorganisms because of their high nutritional value and the presence of easily utilizable carbohydrates, fats, and proteins; proteolysis (aerobic) and putrefaction (anaerobic) decompose proteins;

  32. Fruits and vegetables have much lower protein and fat content than meats and dairy products and undergo different kind of spoilage; • the presence of readily degradable carbohydrates in vegetables favors spoilage by bacteria; high oxidation-reduction potential favors aerobic and facultative bacteria; molds usually initiate spoilage in whole fruits

  33. 1. Frozen citrus products are minimally processed and can be spoiled by lactobacilli and yeasts • 2. Grains, corn, and nuts can spoil when held under moist conditions; this can lead to production of toxic substances, including aflatoxins and fumonisins • 3. Shellfish and finfish can be contaminated by algal toxins, which cause of variety of illnesses in humans

  34. The microbial agent causing spoilage depends on the source of the food and its nutritional value: • Meats may be contaminated by intestinal pathogens released during slaughter • Dairy products - lactic acid bacteria • Fruit and vegetables - soil and water microbes • Some microbes that cause spoilage may be human pathogens but the majority are not!

  35. Growth of microbes in food follows a typical microbial growth pattern • Growth rate depends on the nutritional value and temperature of the food • Number of microbes depends on both inoculum size and • growth rate • Food spoilage occurs at high populations density • (at stationary phase) - retarding microbial growth delays • spoilage

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