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Copyright © The McGraw-Hill Companies. Permission required ...

Copyright © The McGraw-Hill Companies. Permission required ...






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  • Left: fresh milk Right: curdled mild. The curdled milk has undergone a natural 4-step sequence of spoilage organisms activity, resulting in separated curds and whey.
  • The letter designations refer to the color of the compounds under ultraviolet light after extraction from the grain and separation by chromatography. The B 1 and B 2 compounds fluoresce with a blue color and the G 1 and G 2 appear green. The two type of M aflatoxins are found in the milk of lactating animals that have ingested type B aflatoxins.
  • Beer production requires malting, and the use of hops and boiling for clarification, flavor development, and inactivation of malting enzymes, to produce the wort. Only after completion of these steps can the actual fermentation be carried out.

Copyright © The McGraw-Hill Companies. Permission required ... Copyright © The McGraw-Hill Companies. Permission required ... Presentation Transcript

  • Chapter 41 Microbiology of Food
  • Microorganism growth in foods
  • Intrinsic factors
    • Composition
    • pH
    • Presence and availability of water
    • Oxidation-reduction potential
      • altered by cooking
    • Physical structure
    • Presence of antimicrobial substances
  • 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
  • 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
  • Physical structure
    • Grinding and mixing increase surface area and distribute microbes
      • promotes microbial growth
    • Outer skin of vegetables and fruits slows microbial growth
  • 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
  • Extrinsic factors
    • Temperature
      • lower temperatures retard microbial growth
    • Relative humidity
      • higher levels promote microbial growth
    • Atmosphere
      • oxygen promotes growth
      • modified atmosphere packaging
        • use of shrink wrap and vacuum technologies to package food in controlled atmospheres
  • 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
        • fungal toxins contaminate grains
  • Toxins
    • Ergotism
      • toxic condition caused by growth of a fungus, Claviceps purpura , in grains
      • (← production of hallucinogenic alkaloids )
    • Aflatoxins
      • carcinogens produced in fungus-infected grains and nut products ( Aspergillus flavus )
    • Fumonisins
      • carcinogens produced in fungus-infected corn
      • ( Fusarium moniliforme )
  • Intercalate into DNA, causing frameshift mutations aflatoxins
  • Disrupt synthesis and metabolism of sphingolipids Fumonisins FB1: R = OH FB2: R = H
  • Algal toxins
  • Controlling food spoilage
  • Removal of microorganisms
    • Usually achieved by filtration
    • Commonly used for water, beer, wine, juices, soft drinks, and other liquids
  • Low temperature
    • Refrigeration at 5°C retards but does not stop microbial growth
      • microorganisms can still cause spoilage with extended spoilage
      • growth at temperatures below -10°C has been observed
  • High temperature
    • Canning
    • Pasteurization
  • Canning
    • Food heated in special containers to 115° C for 25 to 100 minutes
    • Kills spoilage microbes , but not necessarily all microbes in food
  • Spoilage of canned goods
    • Spoilage prior to canning
    • Underprocessing
    • Leakage of contaminated water into cans during cooling process
  • 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
  • Chemical-based preservation
    • GRAS
      • chemical agents “generally recognized as safe”
    • pH of food impacts effectiveness of chemical preservative
  • Radiation
    • Ultraviolet (UV) radiation
      • used for surfaces of food-handling equipment
      • does not penetrate foods
    • Radappertization
      • use of ionizing radiation (e.g. gamma ray) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables
      • kills microbes in moist foods by producing peroxides from water
        • peroxides oxidize cellular constituents
  • Microbial product-based inhibition
    • Bacteriocins
      • bactericidal proteins active against related species
      • some dissipate proton motive force of susceptible bacteria
      • some form pores in plasma membranes
      • some inhibit protein or RNA synthesis
    • * e.g., nisin
      • - produced by some strains of Streptococcus lactis
      • - used in low-acid foods to inactivate Clostridium botulinum during canning process
  • Food-borne diseases
    • Two primary types
      • food-borne infections
      • food intoxications
  • Food-borne infection
    • Ingestion of microbes , followed by growth, tissue invasion, and/or release of toxins
    • Raw foods (e.g., sprouts, raspberries, and seafood) are important sources
  • Nucleic acid can be detected even when plaque-forming ability is lost
  • 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
  • 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
  • Comparison of PCR sensitivity and growth for detection of Salmonella
  • Surveillance for food-borne disease
    • 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
  • 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
  • Microbiology of fermented foods
    • Major fermentations used are lactic , propionic , and ethanolic fermentations
  • Fermented milks
    • Mesophilic – Lactobacillus and Lactococcus
    • Thermophilic – Lactobacillus and Streptococcus
    • Therapeutic – Lactobacillus and Bifidobacterium
    • Yeast lactic – yeasts, lactic acid bacteria, and acetic acid bacteria
    • Mold lactic – filamentous fungi and lactic acid bacteria
  • Cheese production
    • Milk
    • lactic acid bacteria and rennin 
    • Curd
    • removal of whey 
    • ripening by microbial action 
    • Cheese
  • Meat and fish
    • Sausages
    • Hams
    • Bologna
    • Salami
    • Izushi (fermentation of fish, rice, and vegetables by Lactobacillus spp.)
    • Katsuobushi (fermentation of tuna by Aspergillus glaucus)
  • Production of alcoholic beverages
    • Begins with formation of liquid containing carbohydrates in readily fermentable form
      • must
        • juice from crushed grapes
      • mashing
        • hydrolysis of complex carbohydrates in cereals by addition of water and heating mixture
        • yields wort – clear liquid containing fermentable carbohydrates
  • racking – removes sediments or Microbial oxidation of ethanol to acetic acid yields wine vinegar
  • Beers and ales
    • Malt
      • germinated barley grains having activated enzymes
    • Mash
      • the malt after being mixed with water in order to hydrolyze starch to usable carbohydrates
    • Bottom yeasts
      • used in production of beers
    • Top yeasts
      • used in production of ales
  • pitched
  • Distilled spirits
    • Similar to beer-making process
      • usually begins with ‘ sour mash ’
        • mash inoculated with homolactic bacterium : lower the mash pH
      • following fermentation, is distilled to concentrate alcohol
  • Production of breads
    • Involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions
      • maximizes CO 2 production , which leavens bread
    • Other microbes used to make special breads (e.g., sourdough bread)
    • Can be spoiled by Bacillus species that causes ropiness
  • Other fermented foods
    • Silages
      • fermented grass, corn, and other fresh animal feeds
  • Making sauerkraut
  • Microorganisms as foods and food amendments
    • Variety of bacteria, yeasts, and other fungi are used as animal and human food sources
    • - mushrooms, Spirulina , etc.
    • Probiotics
      • microbial dietary adjuvants
      • microbes added to diet in order to provide health benefits beyond basic nutritive value
  • Benefits of probiotics
    • Immunodilation
    • Control of diarrhea
    • Anticancer effects
    • Possible modulation of Crohn’s Disease
    • In beef cattle,
      • Lactobacillus acidophilus decrease E. coli
    • In poultry,
      • Bacillus subtilis limits Salmonella colonization of gut by the process of competitive exclusion
  • Prebiotics
    • Oligosaccharide polymers that are not processed until they enter large intestine
    • Synbiotic system
      • combination of prebiotics and probiotics
      • results in an increase in production of butyric acid and propionic acids that may be responsible for possible beneficial effects of probiotics