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Microbiology, 4/e Prescott, Harley, Klein | ||||||
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Microbiology, 4/e Prescott, Harley, Klein | ||||||
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43 Microbiology of Food
CHAPTER OVERVIEW
This chapter discusses the microorganisms associated with foods. Some of these microorganisms are used in the production of certain foods. Others are disease-causing organisms that are transmitted via foods. Still others are associated with food spoilage. Therefore, the entire sequence of food handlingCfrom the producer to the final consumerCmust be monitored carefully for the presence and activity of microorganisms.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:
! discuss the interaction of intrinsic (food-related) and extrinsic (environmental) factors with the microbial communities associated with foods and the successional changes this interaction produces
! describe the various physical, chemical, and biological processes used to preserve foods
! discuss sterilization processes used in the production of foods and how these processes are monitored
! discuss the various diseases that can be transmitted to humans by foods
! differentiate between food infections and food intoxications
! discuss the detection of disease-causing organisms in foods
! describe the fermentation of dairy products, grains, meats, fruits, and vegetables
! discuss the disease-causing chemicals produced by fungi growing in moist corn and grain products
! discuss wine production by alcoholic fermentation of fruit juices or musts
! discuss the direct use of microbial cells as nutrients (single-cell protein) by humans and animals
CHAPTER OUTLINE
I. Microorganisms and Food Spoilage
A. Intrinsic Factors
1. Food composition
a. CarbohydratesCdoes not result in major odors
b. Proteins and/or fats result in a variety of foul odors
2. pHClow 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. Water presence and water availability
a. Drying (removal of water) controls or eliminates food spoilage
b. Addition of salt or sugar decreases water availability and thereby helps reduce microbial spoilage
c. Even under these conditions spoilage can occur by certain kinds of microorganisms
(1) OsmophilicCprefer high osmotic pressure
(2) XerophilicCprefer low water availability
4. Oxidation-reduction potential can be affected (lowered) by cooking, making foods more susceptible to anaerobic spoilage
5. Physical structureCgrinding and mixing increases surface area and distributes microorganisms, resulting in more rapid spoilage; vegetables and fruits have outer skins that protect them from spoilage
6. Some foods, particularly spices, may contain antimicrobial substances
B. Extrinsic factors
1. TemperatureClower temperatures reduce microbial growth and prolong storage life
2. Relative humidityClower moisture content helps prolong storage life
3. AtmosphereCoxygen 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
C. Food Spoilage
1. Food spoilage by microorganisms alters food, which then has undesirable appearance, contains hallucinogens, or contains carcinogens
2. Proteolysis (aerobic) and putrefaction (anaerobic) are processes that decompose proteins in meats and dairy products
3. Spoilage often occurs in a successional relationship: the growth of one type of organism creates conditions conducive to the growth of a different type of organism, which eventually leads to the spoilage of the food; this is seen in the spoilage of unpasteurized milk
4. Fungal-derived carcinogens include the aflatoxins and fumonisins that are produced on moist corn, grain, and nut products
5. Aflatoxin has also been observed in milk, beer, cocoa, raisins, and soybean meal
6. 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
7. Canned foods can undergo spoilage
a. Prior to canning
b. As a result of underprocessing during canning
c. Leakage of contaminated water through can seams during cooling
II. Food Preservation Alternatives
A. FiltrationCFiltration of water, wine, beer juices, soft drinks and other liquids can keep bacterial populations low or eliminate them entirely
B. Low or High Temperature
1. Refrigeration and/or freezing retards microbial growth but does not prevent spoilage
2. Canned food is heated in special containers called retorts to 115EC for 25-100 minutes to kill spoilage microorganisms
3. PasteurizationCkills disease-causing organisms; substantially reduces the number of spoilage organisms
a. Low-temperature holding (LTH)C62.8EC for 30 minutes
b. High-temperature short-time (HTST)C71EC for 15 seconds
c. Ultra-high temperature (UHT)C141EC for 2 seconds
d. Shorter times result in improved flavor and extended product shelf life
4. 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
C. Chemicals and Radiation
1. Chemical preservatives
a. Regulated by the U.S. Food and Drug Administration
b. Include simple organic acids, sulfite, ethylene oxide as a gaseous sterilant, sodium nitrite, and ethyl formate
c. Affect microorganisms by disrupting a critical factor
d. Effectiveness depends on pH
e. Nitrites protect against Clostridium botulinum, but are of some concern; however, because of their potential to form carcinogenic nitrosamines during the cooking of meats preserved with them
f. Nisin is a polypeptide agent that inhibits peptidoglycan synthesis and can be used to preserve low-acid foods during canning
2. Radiation
a. Nonionizing (ultraviolet or UV) radiationCused for surfaces of food-handling utensils; does not penetrate foods
b. Ionizing (gamma radiation)Cpenetrates well, but must be used with moist foods to produce peroxides from water, resulting in oxidation of sensitive cellular constituents (radappertization); used for seafoods, fruits, and vegetables; can be used to sterilize meats
III. Diseases and Foods
A. Diseases transmitted by foods
1. Food-borne infections
a. Ingestion of microorganisms, followed by growth, tissue, invasion and/or release of toxins
b. Includes infections caused by Salmonella, Campylobacter, Listeria, and Escherichia coli, particularly E. coli 0157:H7 which causes hemorrhagic colitis
2. Food intoxications
a. Ingestion of toxins in foods in which microorganisms have grown
b. The organism need no longer be viable and need not grow after ingestion
c. Induces staphylococcal food poisoning, botulism, perfringens food poisoning, and Bacillus cereus food poisoning
B. Detection of Disease-Causing Microorganisms
1. Methods need to be rapid; therefore, traditional culture methods that might take days to weeks to complete are too slow
2. Methods need to be sensitive; therefore, traditional methods that are not sensitive enough to detect a low concentration of pathogens against a high background of normal microflora complicate matters
3. Molecular methods overcome these limitations and are currently being used wherever feasible; includes DNA:RNA probe technology and immunochemical procedures; most recently, the use of immunomagnetic reagents allows for both detection and removal for further study using a magnetic collector
4. Molecular methods are valuable for three reasons
a. They can detect the presence of a single, specific pathogen
b. They can detect viruses that cannot be conveniently cultured
c. They can identify slow-growing or non-culturable pathogens
5. Polymerase chain reaction (PCR) can be used to detect low level contaminants (e.g., as few as 10 toxin-producing E. coli cells in a population of 100,000 cells isolated from soft cheese samples)
6. Recently bioluminescence methods (e.g., monitoring the presence of ATP) have been used to detect potential pathogens on utensils and other surfaces
IV. Microbiology of Fermented Foods
A. Dairy productsCacid produced from microbial activity causes protein denaturation; organism growth often is sequential; frequently involves Lactococcus and Lactobacillus species, among others
1. Cultured buttermilkCfermentation of skim milk
2. Sour creamCsame fermentation but of cream
3. YogurtCtwo microorganisms (S. thermophilus and L. bulgaricus) are used for acid and aroma production, respectively
4. Acidophilus milk contains L. acidophilus; improves general health by altering intestinal microflora; may help control colon cancer
5. 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
6. CheesesCproduced by coagulation of curd, expression of whey, and ripening by microbial fermentation; cheese can be internally inoculated or surface ripened
B. Meat and Fish
1. Meat productsCsausages, country-cured hams, bologna, salami, etc.; 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)
C. Wine, Beer, and Other Fermented Alcoholic Beverages
1. Wines and champagnes
a. Grapes are crushed and liquids that contains fermentable substrates (musts) are separated
b. Must is sterilized with sulfur dioxide fumigant or by pasteurization
c. Mashing involves incubation with water; insoluble material is removed to yield a wort, a clear liquid containing fermentable sugars and other simple molecules
d. All grapes, regardless of color, have white juices; to make a red wine, the skins of a red grape are left in contact with the must before the fermentation process
e. Desired strain of Saccharomyces cerevisiae or S. ellipsoideus is added, and the mixture fermented (10 to 18% alcohol)
f. Dry wine (no free sugar)Camount of sugar is limited so that all sugar is fermented before alcohol level increases to a level that inhibits further yeast activity
g. Sweet wine (free sugar present)Cfermentation is inhibited by alcohol accumulation before all sugar is used up
h. AgingCfinal fermentation to accumulate flavoring compounds
i. RackingCremoval of sediments accumulated during the fermentation process
j. ChampagnesCfermentation is continued in bottles to produce a naturally sparkling wine
k. Brandy (burned wine)Cdistilled to increase alcohol concentration
l. Wine vinegarCcontrolled microbial oxidation (by Acetobacter or Gluconobacter) to produce acetic acid from ethanol
2. Beers and ales
a. Malt is produced by germination of the barley grains and the activation of their enzymes
b. Mash is produced by enzymatic starch hydrolysis to accumulate utilizable carbohydrates
c. Mash is heated with hops (dried flowers of the female vine Humulus lupulis) to provide flavor and clarify the wort (hydrolyzed proteins and carbohydrates); inactivates hydrolytic enzymes so that wort can be pitched (inoculated with yeast)
d. BeerCproduced with a bottom yeast, such as S. carlsbergensis
e. AleCproduced with a top yeast, such as S. cerevisiae
f. Freshly fermented (green) beers are lagered (aged), bottled, and carbonated
g. Pasteurized or filtered to remove microorganisms
3. Distilled spiritsCbeerlike 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
D. Bread and Other Fermented Plant Products
1. Aerobic yeast fermentation is used to produce carbon dioxide with minimal alcohol production
2. Other fermentation products add flavors
3. Other microorganisms make special breads, such as sourdough
4. Other products
a. Susu fermentation of tofu, a chemically coagulated soybean milk product
b. SauerkrautCfermented cabbage; involves a microbial succession mediated by Leuconostoc mesenteroides and Lactobacillus plantarum
c. PicklesCinvolves a complex microbial succession
d. SilagesCanimal feeds produced by anaerobic, lactic-type mixed fermentation of grass, corn and other fresh animal feeds