Microbiology of Food
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;
unpasteurized milk a four-step succession of microorganisms occurs
Fruits and vegetables have much lower protein and fat content
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- carcinogens
aflatoxins can appear in milk; have
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
High-temperature short-time (HTST)-71°C for 15 seconds
Ultra-high temperature (UHT)-141°C for 2 seconds
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
Shorter times result in improved flavor and extended shelf life
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; “Mad Cows
Disease”
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 personís 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