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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University

Microbiology of Food

Study Outline

  1. 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)
  2. 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
  3. 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
  4. Food-borne Diseases
    1. Food-borne illnesses impact the entire world; are either infections or intoxications; are associated with poor hygiene practices
    2. 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
        2. Shellfish and finfish can be contaminated by pathogens (e.g., Vibrio and viruses) found in raw sewage
        3. Raspberries are often transported by air to far-away markets; if contaminated, outbreak occurs far from source of pathogen
    3. Food intoxications
      1. Ingestion of microbial toxins in foods
      2. Staphylococcal food poisoning is caused by exotoxins released by Staphylococcus aureus, which is frequently transmitted from its normal habitat (nasal cavity) to food by person's hands; improper refrigeration leads to growth of bacterium and toxin production
      3. Clostridum botulinum, C. perfringens, and B. subtilis also cause food intoxication
        1. Botulism, caused by C. botulinum, is discussed in chapter 39
        2. C. perfringens is a common inhabitant of food, soil, water, spices and intestinal tract; upon ingestion, endospores germinate and produce enterotoxins within the intestine; this causes food poisoning; often occurs when meats are cooked slowly
        3. Bacillus cereus food poisoning is associated with starchy foods
  5. Detection of Food-borne Pathogens
    1. Methods need to be rapid; therefore, traditional culture methods that might take days to weeks to complete are too slow; identification is also complicated by low numbers of pathogens compared to normal microflora; chemical and physical properties of food can make isolation of food-borne pathogens difficult
    2. Molecular methods are valuable for three reasons
      1. They can detect the presence of a single, specific pathogen
      2. They can detect viruses that cannot be conveniently cultured
      3. They can identify slow-growing or non-culturable pathogens
    3. Some examples
      1. DNA probes can be linked to enzymatic, isotopic, chromogenic, or luminescent/fluorescent markers; are very rapid
      2. PCR can detect small numbers of pathogens (e.g., as few as 10 toxin-producing E. coli cells in a population of 100,000 cells isolated from soft cheese samples; as few as two colony- forming units of Salmonella); PCR systems are being developed for Campylobacter jejuni and Arcobacter butzleri
      3. Food-borne pathogen fingerprinting is an integral part of an initiative by the Centers for Disease Control (CDC) to control food-borne pathogens; The CDC has established a procedure (PulseNet) in which pulse-field gel electrophoresis is used under carefully controlled and standardized conditions to detect the distinctive DNA pattern of nine major food pathogens; these pathogens are being followed in an surveillance network (FoodNet)
  6. Microbiology of Fermented Foods
    1. Fermented milks-at least 400 different fermented milks are produced throughout the world; fermentations are carried out by mesophilic, thermophilic, and therapeutic lactic acid bacteria, as well as by yeasts and molds
      1. Mesophilic-acid produced from microbial activity at temperatures lower than 45°C causes protein denaturation (e.g., cultured buttermilk and sour cream)
      2. Thermophilic-fermentations carried out at about 45°C (e.g., yogurt)
      3. Therapeutic-fermented milks may have beneficial therapeutic effects
        1. Acidophilus milk contains L. acidophilus; improves general health by altering intestinal microflora; may help control colon cancer
        2. Bifid-amended fermented milk products (containing Bifidobacterium spp.) improve lactose tolerance, possess anticancer activity, help reduce serum cholesterol levels, assist calcium absorption, and promote the synthesis of B-complex vitamins; may also reduce or prevent the excretion of rotaviruses, a cause of diarrhea among children
      4. Yeast lactic-these fermentations include kefir, which is made by the action of yeasts, lactic acid bacteria, and acetic acid bacteria
      5. Mold lactic-this fermentation is used to make viili, a Finnish beverage; carried out by the mold Geotrichium candidum and lactic acid bacteria
    2. Cheeses-produced by coagulation of curd, expression of whey, and ripening by microbial fermentation; cheese can be internally inoculated or surface ripened
    3. Meat and Fish
      1. Meat products include sausages, country-cured hams, bologna, and salami; frequently involves Pediococcus cerevisiae and Lactobacillus plantarum
      2. Fish products include izushi (fresh fish, rice, and vegetables incubated with Lactobacillus spp.) and katsuobushi (tuna incubated with Aspergillus glaucus)
    4. Production of Alcoholic Beverages
      1. Wines and champagnes
        1. Grapes are crushed and liquids that contain fermentable substrates (musts) are separated; musts can be fermented immediately, but the results can be unpredictable; usually must is sterilized by pasteurization or with sulfur dioxide fumigant; to make a red wine, the skins of a red grape are left in contact with the must before the fermentation process; if must was sterilized, the desired strain of Saccharomyces cerevisiae or S. ellipsoideus is added, and the mixture fermented (10 to 18% alcohol)
        2. For dry wine (no free sugar), the amount of sugar is limited so that all sugar is fermented before fermentation stops; for sweet wine (free sugar present), the fermentation is inhibited by alcohol accumulation before all sugar is used up; in the aging process flavoring compounds accumulate
        3. Racking-removal of sediments accumulated during the fermentation process
        4. Brandy (burned wine) is made by distilling wine to increase alcohol concentration; wine vinegar is made by controlled microbial oxidation (by Acetobacter or Gluconobacter) to produce acetic acid from ethanol
        5. For champagnes, fermentation is continued in bottles to produce a naturally sparkling wine
      2. Beers and ales
        1. Malt is produced by germination of the barley grains and the activation of their enzymes to produce a malt; mash is produced from malt by enzymatic starch hydrolysis to accumulate utilizable carbohydrates; mash is heated with hops (dried flowers of the female vine Humulus lupulis) to provide flavor and clarify the wort (hydrolyzed proteins and carbohydrates); hops inactivate hydrolytic enzymes so that wort can be pitched (inoculated with yeast)
        2. Beer is produced with a bottom yeast, such as Saccharomyces carlsbergensis and ale is produced with a top yeast, such as S. cerevisiae; freshly fermented (green) beers are lagered (aged), bottled, and carbonated; beer can be pasteurized or filtered to remove microorganisms and minimize flavor changes
      3. Distilled spirits-beerlike fermented liquid is distilled to concentrate alcohol; type of liquor depends on composition of starting mash; flavorings can also be added; a sour mash involving Lactobacills delbrueckii mediated fermentation is often used
    5. Production of breads
      1. Aerobic yeast fermentation is used to produce carbon dioxide with minimal alcohol production; other fermentation add flavors
      2. Other microorganisms make special breads, such as sourdough
      3. Bread products can be spoiled by Bacillus species that produce ropiness
    6. Other fermented foods
      1. Sufu, fermented tofu (a chemically coagulated soybean milk product) and tempeh, made from soybean mash, are made by the action of molds
      2. Sauerkraut-fermented cabbage; involves a microbial succession mediated by Leuconostoc mesenteroides, Lactobacillus plantarum, and Lactobacillus brevis
      3. Pickles are cucumbers fermented in brine by a variety of bacteria; process involves a complex microbial succession
      4. Silages-animal feeds produced by anaerobic, lactic-type mixed fermentation of grass, corn, and other fresh animal feeds
  7. Microorganisms as Foods and Food Amendments
    1. Microbes that are eaten include a variety of bacteria, yeasts, and other fungi (e.g., mushrooms, Spirulina)
    2. Probiotics-the addition of microorganisms to the diet in order to provide health benefits beyond basic nutritive value; also called microbial dietary adjuvants
      1. Early claims for health benefits were not based on scientific investigation; however, studies are now being done using a simulated human intestinal ecosystem (SHIME)
      2. Prebiotics-oligosaccharide polymers that are not processed until reaching the large intestine; often combined with probiotics to create a symbiotic system
      3. Probiotics are being used with poultry to increase body weight and feed conversion; also reduce colforms and Campylobacter; may be useful in preventing Salmonella from colonizing gut due to competitive exclusion