Microbial growth, Food spoilage and Food borne diseases

 

I.    Microorganism Growth in Foods: Intrinsic Factors, Extrinsic factors

II.   Microbial Growth and Food Spoilage

III.  Controlling Food Spoilage

IV.  Food-borne Diseases

       

I. Microorganism Growth in Foods

  1. Intrinsic Factors

    1. Food composition
      1. Carbohydrates-do not result in major odors
      2. Proteins and/or fats result in a variety of foul odors (e.g., putrefactions)
    2. pH-low pH allows yeasts and molds to become dominant; higher pH allows bacteria to become dominant; higher pH favors putrefaction (the anaerobic breakdown of proteins that releases foul-smelling amine compounds)
    3. Physical structure affects the course and extent of spoilage
      1. Grinding and mixing (e.g., sausage and hamburger) increases surface area, alters cellular structure, and distributes microorganisms throughout the food
      2. Vegetables and fruits have outer skins that protect against spoilage; spoilage microorganisms have enzymes that weaken and penetrate such protective coverings
    4. Presence and availability of water
      1. Drying (removal of water) controls or eliminates food spoilage
      2. Addition of salt or sugar decreases water availability and thereby helps reduce microbial spoilage
      3. Even under these conditions spoilage can occur by certain kinds of microorganisms
        1. Osmophilic-prefer high osmotic pressure
        2. Xerophilic-prefer low water availability
        3. Oxidation-reduction potential can be affected (lowered) by cooking, making foods more susceptible to anaerobic spoilage
        4. Many foods contain natural antimicrobial substances (e.g., fruits and vegetables, milk and eggs, hot sauces, herbs and spices, and unfermented green and black teas)
  2.     Extrinsic factors

     

    1. Temperature and relative humidity-at higher relative humidities, microbial growth is initiated more rapidly, even at lower temperatures
    2. Atmosphere-oxygen usually promotes growth and spoilage even in shrink-wrapped foods since oxygen can diffuse through the plastic; high CO2 tends to decrease pH and reduces spoilage; modified atmosphere packaging (MAP) involves the use of modern shrink wrap materials and vacuum technology to package foods in a desired atmosphere (e.g., high CO2).

    II.    Microbial Growth and Food Spoilage

     

    1. 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; in spoilage of unpasteurized milk a four-step succession of microorganisms occurs
    2. 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
    3. Frozen citrus products are minimally processed and can be spoiled by lactobacilli and yeasts
    4. Grains, corn, and nuts can spoil when held under moist conditions; this can lead to production of toxic substances, including aflatoxins and fumonisins
      1. Ergotism is caused by hallucinogenic alkaloids produced by fungi in corn and grains
      2. Aflatoxins-planar molecules that intercalate into DNA and act as frameshift mutagens and carcinogens; if consumed by dairy cows, aflatoxins can appear in milk; have also been observed in beer, cocoa, raisins, and soybean meal; aflatoxin sensitivity can be influenced by prior disease exposure (e.g., hepatitis B infection increases sensitivity)
      3. Fumonisins-fungal contaminants of corn; cause disease in animals and esophageal cancer in humans; disrupt synthesis and metabolism of sphingolipids
    5. Shellfish and finfish can be contaminated by algal toxins, which cause of variety of illnesses in humans

    III.    Controlling Food Spoilage

     

    1. Removal of microorganisms-filtration of water, wine, beer juices, soft drinks and other liquids can keep bacterial populations low or eliminate them entirely
    2. Low temperature-refrigeration and/or freezing retards microbial growth but does not prevent spoilage
    3. High temperature
      1. Canning
        1. Canned food is heated in special containers called retorts to 115°C for 25-100 minutes to kill spoilage microorganisms
        2. Canned foods can undergo spoilage despite safety precautions; spoilage can be due to spoilage prior to canning, underprocessing during canning, or leakage of contaminated water through can seams during cooling
      2. Pasteurization-kills disease-causing organisms; substantially reduces the number of spoilage organisms
        1. Low-temperature holding (LTH)-6
        2. 8°C for 30 minutes
        3. High-temperature short-time (HTST)-71°C for 15 seconds
        4. Ultra-high temperature (UHT)-141°C for 2 seconds
        5. Shorter times result in improved flavor and extended shelf life
      3. Heat treatments are based on a statistical process involving the probability that the number of remaining viable microorganisms will be below a certain level after a specified time at a specified temperature
    4. Water availability-dehydration procedures (e.g., freeze-drying) remove water and increase solute concentration
    5. Chemical-based preservation
      1. Regulated by the U.S. Food and Drug Administration (FDA); preservatives are listed as "generally recognized as safe" or GRAS; include simple organic acids, sulfite, ethylene oxide as a gaseous sterilant, sodium nitrite, and ethyl formate; affect microorganisms by disrupting a critical factor
      2. Effectiveness depends on pH; nitrites protect against Clostridium botulinum, but are of some concern because of their potential to form carcinogenic nitrosamines when meats preserved with them are cooked
    6. Radiation-nonionizing (ultraviolet or UV) radiation is used for surfaces of food-handling utensils, but does not penetrate foods; ionizing (gamma radiation) penetrates well but must be used with moist foods to produce peroxides, which oxidize sensitive cellular constituents (radappertization); ionizing radiation is used for seafoods, fruits, vegetables, and meats
    7. Microbial product-based inhibition
      1. Bacteriocins-bacteriocidal proteins produced by bacteria; active against only closely related bacteria (e.g., nisin)
      2. Bacteriocins disrupt proton motive force either as a result of inhibition of murein synthesis or detergent-like effects on cytoplasmic membrane

IV.    Food-borne Diseases

  1. Food-borne illnesses impact the entire world; are either infections or intoxications; are associated with poor hygiene practices

Food-borne infections

  1. Due to ingestion of microorganisms, followed by growth, tissue invasion and/or release of toxins
  2. Salmonellosis-caused by a variety of Salmonella serovars; commonly transmitted by meats, poultry, and eggs; can arise from contamination of food by workers in food-proccessing plants and restaurants and in canning process
  3. Campylobacter jejuni-transmitted by uncooked or poorly cooked poultry products, raw milk and red meats; thorough cooking prevents transmission
  4. Listeriosis-transmitted by dairy products
  5. Enteropathogenic, enteroinvasive, and enterotoxigenic Escherichia coli
    1. Spread by fecal-oral route; found in meat products, in unpasteurized fruit drinks, and on fruits and vegetables
    2. Prevention requires prevention of food contamination throughout all stages of production, handling, and cooking; gamma irradiation may be used in the future as a prevention and control measure
  6. Variant Creutzfeld-Jakob disease-transmitted by ingestion of beef from infected cattle; transmission between animals is due to the use of mammalian tissue in ruminant animal feeds; prevention and control is difficult
  7. Foods transported and consumed in uncooked state are increasingly important sources of food-borne infection, especially as there is increasingly rapid movement of people and products around the world

    1. Sprouts can be a problem if germinated in contaminated water; furthermore, as seeds germinate, they release molecules that promote microbial growth.

           b.   Shellfish and finfish can be contaminated by pathogens (e.g., Vibrio and viruses)found in raw sewage

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