| 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- Microorganism Growth in Foods
- Intrinsic Factors
- Food composition
- Carbohydrates-do not result in major odors
- Proteins and/or fats result in a variety of foul odors (e.g., putrefactions)
- 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)
- Physical structure affects the course and extent of spoilage
- Grinding and mixing (e.g., sausage and hamburger) increases surface area, alters cellular structure, and distributes microorganisms throughout the food
- Vegetables and fruits have outer skins that protect against spoilage; spoilage microorganisms have enzymes that weaken and penetrate such protective coverings
- Presence and availability of water
- Drying (removal of water) controls or eliminates food spoilage
- Addition of salt or sugar decreases water availability and thereby helps reduce microbial spoilage
- Even under these conditions spoilage can occur by certain kinds of microorganisms
- Osmophilic-prefer high osmotic pressure
- Xerophilic-prefer low water availability
- Oxidation-reduction potential can be affected (lowered) by cooking, making foods more susceptible to anaerobic spoilage
- 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)
- Extrinsic factors
- Temperature and relative humidity-at higher relative humidities, microbial growth is initiated more rapidly, even at lower temperatures
- 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)
- 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; in spoilage of unpasteurized milk a four-step succession of microorganisms occurs
- 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
- Frozen citrus products are minimally processed and can be spoiled by lactobacilli and yeasts
- Grains, corn, and nuts can spoil when held under moist conditions; this can lead to production of toxic substances, including aflatoxins and fumonisins
- Ergotism is caused by hallucinogenic alkaloids produced by fungi in corn and grains
- 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)
- Fumonisins-fungal contaminants of corn; cause disease in animals and esophageal cancer in humans; disrupt synthesis and metabolism of sphingolipids
- Shellfish and finfish can be contaminated by algal toxins, which cause of variety of illnesses in humans
- Controlling Food Spoilage
- Removal of microorganisms-filtration of water, wine, beer juices, soft drinks and other liquids can keep bacterial populations low or eliminate them entirely
- Low temperature-refrigeration and/or freezing retards microbial growth but does not prevent spoilage
- High temperature
- Canning
- Canned food is heated in special containers called retorts to 115°C for 25-100 minutes to kill spoilage microorganisms
- 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
- Pasteurization-kills disease-causing organisms; substantially reduces the number of spoilage organisms
- Low-temperature holding (LTH)-6
- 8°C for 30 minutes
- High-temperature short-time (HTST)-71°C for 15 seconds
- Ultra-high temperature (UHT)-141°C for 2 seconds
- Shorter times result in improved flavor and extended shelf life
- 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
- Water availability-dehydration procedures (e.g., freeze-drying) remove water and increase solute concentration
- Chemical-based preservation
- 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
- 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
- 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
- Microbial product-based inhibition
- Bacteriocins-bacteriocidal proteins produced by bacteria; active against only closely related bacteria (e.g., nisin)
- Bacteriocins disrupt proton motive force either as a result of inhibition of murein synthesis or detergent-like effects on cytoplasmic membrane
- Food-borne Diseases
- Food-borne illnesses impact the entire world; are either infections or intoxications; are associated with poor hygiene practices
- Food-borne infections
- Due to ingestion of microorganisms, followed by growth, tissue invasion and/or release of toxins
- 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
- Campylobacter jejuni-transmitted by uncooked or poorly cooked poultry products, raw milk and red meats; thorough cooking prevents transmission
- Listeriosis-transmitted by dairy products
- Enteropathogenic, enteroinvasive, and enterotoxigenic Escherichia coli
- Spread by fecal-oral route; found in meat products, in unpasteurized fruit drinks, and on fruits and vegetables
- 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
- 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
- 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
- Sprouts can be a problem if germinated in contaminated water; furthermore, as seeds germinate, they release molecules that promote microbial growth
- Shellfish and finfish can be contaminated by pathogens (e.g., Vibrio and viruses) found in raw sewage
- Raspberries are often transported by air to far-away markets; if contaminated, outbreak occurs far from source of pathogen
- Food intoxications
- Ingestion of microbial toxins in foods
- 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
- Clostridum botulinum, C. perfringens, and B. subtilis also cause food intoxication
- Botulism, caused by C. botulinum, is discussed in chapter 39
- 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
- Bacillus cereus food poisoning is associated with starchy foods
- Detection of Food-borne Pathogens
- 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
- Molecular methods are valuable for three reasons
- They can detect the presence of a single, specific pathogen
- They can detect viruses that cannot be conveniently cultured
- They can identify slow-growing or non-culturable pathogens
- Some examples
- DNA probes can be linked to enzymatic, isotopic, chromogenic, or luminescent/fluorescent markers; are very rapid
- 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
- 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)
- Microbiology of Fermented Foods
- 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
- Mesophilic-acid produced from microbial activity at temperatures lower than 45°C causes protein denaturation (e.g., cultured buttermilk and sour cream)
- Thermophilic-fermentations carried out at about 45°C (e.g., yogurt)
- Therapeutic-fermented milks may have beneficial therapeutic effects
- Acidophilus milk contains L. acidophilus; improves general health by altering intestinal microflora; may help control colon cancer
- 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
- Yeast lactic-these fermentations include kefir, which is made by the action of yeasts, lactic acid bacteria, and acetic acid bacteria
- Mold lactic-this fermentation is used to make viili, a Finnish beverage; carried out by the mold Geotrichium candidum and lactic acid bacteria
- Cheeses-produced by coagulation of curd, expression of whey, and ripening by microbial fermentation; cheese can be internally inoculated or surface ripened
- Meat and Fish
- Meat products include sausages, country-cured hams, bologna, and salami; frequently involves Pediococcus cerevisiae and Lactobacillus plantarum
- Fish products include izushi (fresh fish, rice, and vegetables incubated with Lactobacillus spp.) and katsuobushi (tuna incubated with Aspergillus glaucus)
- Production of Alcoholic Beverages
- Wines and champagnes
- 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)
- 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
- Racking-removal of sediments accumulated during the fermentation process
- 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
- For champagnes, fermentation is continued in bottles to produce a naturally sparkling wine
- Beers and ales
- 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)
- 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
- 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
- Production of breads
- Aerobic yeast fermentation is used to produce carbon dioxide with minimal alcohol production; other fermentation add flavors
- Other microorganisms make special breads, such as sourdough
- Bread products can be spoiled by Bacillus species that produce ropiness
- Other fermented foods
- Sufu, fermented tofu (a chemically coagulated soybean milk product) and tempeh, made from soybean mash, are made by the action of molds
- Sauerkraut-fermented cabbage; involves a microbial succession mediated by Leuconostoc mesenteroides, Lactobacillus plantarum, and Lactobacillus brevis
- Pickles are cucumbers fermented in brine by a variety of bacteria; process involves a complex microbial succession
- Silages-animal feeds produced by anaerobic, lactic-type mixed fermentation of grass, corn, and other fresh animal feeds
- Microorganisms as Foods and Food Amendments
- Microbes that are eaten include a variety of bacteria, yeasts, and other fungi (e.g., mushrooms, Spirulina)
- Probiotics-the addition of microorganisms to the diet in order to provide health benefits beyond basic nutritive value; also called microbial dietary adjuvants
- Early claims for health benefits were not based on scientific investigation; however, studies are now being done using a simulated human intestinal ecosystem (SHIME)
- Prebiotics-oligosaccharide polymers that are not processed until reaching the large intestine; often combined with probiotics to create a symbiotic system
- 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
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