Lecture BIOD 12: Food Microbiology

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Lecture BIOD 12 Food Microbiology
- This chapter discusses the microorganisms
associated with foods.
- Some of these microorganisms are used in the
production of certain foods.
- Other microorganisms are disease-causing
organisms that are transmitted via foods.
- Still other microorganisms are associated with
food spoilage.
- The entire sequence of food handling from the
producer to the final consumer must be monitored
carefully for the presence and activity of
microorganisms.

• Role of Microorganisms in Food Production

- Sugars in bread dough are fermented by yeast to
ethanol and C02 C02 causes the bread to rise.
Other microorganisms make special breads, such as
sourdough.
- Pickles, olives, and soy sauce are the products
of microbial fermentation.
- Dairy products are made using a variety of
lactic acid bacteria. Acid produced from
microbial activity causes protein denaturation
organism growth often is sequential frequently
involves Lactococcus and Lactobacillus species,
among others.

• Cheese

- The milk protein casein curdles because of the
action by lactic acid bacteria or the enzyme
rennin or chymosin.
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- Cheese is the curd separated from the liquid
portion of milk, called whey.
- The growth of microorganisms in cheeses is
called ripening.
- Hard cheeses are produced by lactic acid
bacteria growing in the interior of the curd.
- Semisoft cheeses are ripened by bacteria
growing on the surface
- Soft cheeses are ripened by Penicillium growing
on the surface.

• Other Dairy Products

- Old-fashioned buttermilk was produced by lactic
acid bacteria growing during the butter-making
process.
- Commerical buttermilk is made by letting lactic
add bacteria grow in skim milk for 12 hours.
- Sour cream yogurt are produced by
lactobacilli, streptococci, or yeasts growing in
low-fat milk.

• Meat and Fish

- Meat products sausages, country-cured hams,
bologna, salami, etc. 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)
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• Alcoholic Beverages and Vinegar

- Carbohydrates obtained from grains, potatoes,
or molasses are fermented by yeasts to produce
ethanol in the production of beer, ale, and
distilled spirits.
- The sugars in fruits such as grapes are
fermented by yeasts to produce wines.
- In wine-making, lactic acid bacteria convert
malic acid into lactic acid in malolactic
fermentation in fruits with high acidity.
- Acetobacter and Gluconobacter oxidize ethanol
in wine to acetic acid (vinegar).

• Diseases and Foods

• Food-borne infections

- Food infection is the ingestion of
microorganisms, followed by growth, tissue,
invasion and/or release of toxins.
- Includes infections caused by Salmonella,
Campylobacter, Listeria, and Escherichia coli,
particularly E. coli 0157H7 which causes
hemorrhagic colitis.

• Food intoxications

- Ingestion of toxins in foods in which
microorganisms have grown
- The organism need no longer be viable and need
not grow after ingestion
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- It includes staphylococcal food poisoning,
botulism, perfringens food poisoning, and
Bacillus cereus food poisoning.

• Detection of Disease-Causing Microorganisms

- Methods need to be rapid therefore,
traditional culture methods that might take days
to weeks to complete are too slow.
- 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.
- Molecular methods overcome these limitations
and are currently being used wherever feasible
includes DNARNA probe technology and
immunochemical procedures.
- 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.
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- Polymerase chain reaction (PCR) can be used to
detect low level contaminants.
- e.g. PCR can detect as few as 10
toxin-producing E. coli cells in a population of
100,000 cells isolated from soft cheese samples.

• Microorganisms and Food Spoilage

- There are intrinsic and extrinsic factors that
contribute to food spoilage by microorganisms.
- The interaction between the intrinsic
(food-related) and extrinsic (environmental)
factors contribute to the growth of the microbial
communities associated with foods.

• Intrinsic Factors

1. Food composition
- Carbohydrates does not result in major odors.
- Proteins and/or fats result in a variety of
foul odors.
2. pH
- Low pH allows yeasts and molds to become
dominant.
- Higher pH allows bacteria to become dominant.
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- Higher pH favors putrefaction (the anaerobic
breakdown of proteins that releases foul-smelling
amine compounds).
3. Water presence and water availability
- 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).
4. Oxidation-reduction
- Oxidation-reduction potential can be affected
(lowered) by cooking, making foods more
susceptible to anaerobic spoilage.
5. Physical structure
- Grinding 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.
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• Extrinsic factors

1. Temperature lower temperatures reduce
microbial growth and prolong storage life.
2. Relative humidity lower moisture content
helps prolong storage life.
3. 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.

• Food Spoilage

- Food spoilage by microorganisms alters food,
which then has undesirable appearance,
undesirable taste or may contain carcinogens.
- Proteolysis (aerobic) and putrefaction
(anaerobic) are processes that decompose proteins
in meats and dairy products.
- 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.
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- Fungal-derived carcinogens include the
aflatoxins and fumonisins that are produced on
moist corn, grain, and nut products.
- Aflatoxin has also been observed in milk, beer,
cocoa, raisins, and soybean meal.
- 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.
- Canned foods can undergo spoilage can happen
either prior to canning, or as a result of
underprocessing during canning, or leakage of
contaminated water through can seams during
cooling.

• Industrial Food Canning

- Commercial sterilization of food is
accomplished by steam under pressure in a retort.
- Commercial sterilization heats canned foods to
the minimum temperature necessary to destroy
Clostridium botulinum endospores while minimizing
alteration of the food.
- The commercial sterilization process uses
sufficient heat to reduce a population of C.
botulinum by 12 logarithmic cycles (12D
treatment).
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- Endospores of thermophiles can survive
commercial sterilization.
- Canned foods stored above 45C can be spoiled
by thermophilic anaerobes.
- Thermophilic anaerobic spoilage is sometimes
accompanied by gas production if no gas is
formed, the spoilage is called flat sour spoilage.
- Spoilage by mesophilic bacteria is usually from
improper heating procedures or leakage.
- Acidic foods can be preserved by heat of 100C
because microorganisms that survive are not
capable of growth in a low pH.
- However, Byssochlamys, Aspergillus, and
Bacillus coagulans are acid-tolerant and
heat-resistant microbes that can spoil acidic
foods.

• Food Preservation Alternatives

- The earliest methods of preserving foods were
drying, the addition of salt or sugar, and
fermentation.

• Filtration

- Filtration of water, wine, beer juices, soft
drinks and other liquids can keep bacterial
populations low or eliminate them entirely.
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• Low or High Temperature

- Refrigeration and/or freezing retards microbial
growth but does not prevent spoilage.
- Canned food is heated in special containers
called retorts to 115 C for 25-100 minutes to
kill spoilage microorganisms.
- Pasteurization kills disease-causing
organisms substantially reduces the number of
spoilage organisms
a. Low-temperature holding (LTH) 62.8 C for 30
minutes. b. High-temperature short-time (HTST)
71 C for 15 seconds. c. Ultra-high temperature
(UHT) 141 C for 2 seconds.
- Shorter times result in improved flavor and
extended product shelf life.

• Dehydration

- Dehydration procedures (e.g., freeze-drying)
remove water and increase solute concentration.

• Chemical preservatives

- Include simple organic acids, sulfite, ethylene
oxide as a gaseous sterilant, sodium nitrite, and
ethyl formate.
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- Affect microorganisms by disrupting a critical
factor.
- Effectiveness depends on pH.
- 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.
- Nisin is a polypeptide agent that inhibits
peptidoglycan synthesis and can be used to
preserve low-acid foods during canning.

• Radiation

- Nonionizing (ultraviolet or UV) radiation used
for surfaces of food-handling utensils does not
penetrate foods.
- Ionizing (gamma radiation) penetrates 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.