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This is module 7, dairy microbiology.
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The first part of this module will review fluid milk microbiology.
Bovine milk is 87% water, 5% carbohydrates, 4% proteins, and 4% lipids. The pH is
normally neutral at around pH 6.6. Lactose is the principle carbohydrate with small
amounts of glucose and citrate. Casein makes up approximately 80% of the protein
with the remaining in whey proteins such as albumin and lactalbumin. Milk lipids
are mostly triglycerides with small amounts of diglycerides. Milk also contains
traces of many vitamins and minerals.
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With the exception of mastitic cows, milk is essentially sterile inside the cow. Once
it hits the external environment it is immediately contaminated. Contaminates come
from the cow and the environment the cow is in. Approximately 1000 cells/ml are
expected in good quality milk. Lactose is the majority carbohydrate the biota are
almost exclusively able to grow using lactose. Since regulations require that milk be
chilled within 1 hour suggests that the majority of milk biota will be psychrotrophs.
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The lactic acid bacteria are a group of Gram positive bacteria, non-respiring (no
ETS), non-spore forming, cocci or rods. The lactic acid bacteria associated with
milk differ from those that colonize vegetables. There are two main differences.
First they possess a plasmid that encodes the genes necessary for lactose
degradation and membrane transport. Secondly, over time they have had mutations
that inhibited their ability to synthesize many needed nutrients on their own. Since
milk is such a complete substrate, they simply have to obtain these nutrients from
the milk. When a bacteria must obtain lots of nutrients from the environment, they
are termed fastidious.
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Coliforms, mostly non-pathogenic E. coli, can also grow psychrotrophically, utilizing
lactose. It is interesting to note that E. coli O157:H7 cannot utilize lactose and
therefore is not a major concern in milk. The coliforms are fecal in origin and will
come from fecal contamination of the milk environment.
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Spores make their way into milk from the environment (soils) and can survive
pasteurization temperatures. Luckily, Clostridium does not grow due to the high Eh
(oxidative) environment of milk. Bacillus cannot utilize lactose, however, many can
grow using protease degradation of milk proteins. Many Bacillus species are
psychrotrophic and are one of the main groups to spoil fluid milk after pasteurization
in home refrigerators.
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Naturally, Pseudomonas will find its way into the biota of milk, since it is such as
common resident in the processing environment and a psychrotroph.
Psuedomonas does not ferment lactose, but like Bacillus, will use proteases to grow
from proteins.
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The result of milk biota left to their own devises is spoilage. The most common
defect is lactic acid production causing an acidic flavor. When enough acid
accumulates the milk will curdle. Secondly, due to protein degradation, bitter
byproducts are formed that affect both the odor and the flavor of milk.
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Milk pathogens come from three sources: bovine diseases, fecal contamination, and
environmental contamination.
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The bovine diseases brucellosis, anthrax, Q fever, tuberculosis and mastitis can all
contaminate milk. Each of these bovine etiologic agents can cause illness in
humans. Careful herd management is used to ensure healthy dairy cows to
minimize these diseases. Careful pasteurization will also destroy all of these
pathogens as well.
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Fecal contamination can contaminate milk with Salmonella, E. coli O157:H7, or
Campylobacter. Good sanitation should minimize the potential and pasteurization
would eliminate these pathogens from milk.
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Lastly, Listeria monocytogenes can contaminate milk due directly from the
processing environment. Here too, pasteurization will eliminate these pathogens.
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Milk pasteurization is a vital process to eliminate the pathogens that may be present
in milk. Pa
steurization is based on destroying 5 logs of the most heat resistant
pathogen, Coxiella burnetii. Using any method that achieves this log kill will kill all
known pathogens at the same time. For many years the most common
pasteurization process was the low temperature long time or LTLT. This is heating a
vat of milk to 145°F for 30 minutes. Since milk may develop a scorched flavor
during this process, two alternatives have been developed. The first is high
temperature short time or HTST. This is 161°F for 15 seconds. This is a common
method used in a tube pasteurizer where milk flows past a heat exchanger for a
dwell of 15 seconds at 161°F. It is then immediately cooled to preserve its flavor. If
the heat exchanger is set to approximatley 275°F for a dwell of only a few seconds,
then the process is called ultra high temperature pasteurization or UHT. This milk
can be stored at room temperature.
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The second part of this module will review dairy fermentations such as buttermilk,
yogurt and cheese.
The majority of fermented dairy products are accomplished by the fermentation of
lactose by the lactic acid bacteria. From the bacteria’s perspective, they are simply
using fermentation to generate energy. In most cases the majority byproduct is
lactic acid. However there are some lactic acid bacteria that produce ethanol,
propionic acid, acetic acid, and carbon dioxide. When the lactic acid bacteria can
only produce lactic acid, these are termed homofermentative. When the lactic acid
bacteria can also produce the other byproducts, then these are termed
heterofermentative.
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The lactic acid bacteria group include Lactobacillus, Lactococcus, Leuconostoc,
Enterococus, Pediococcus, and Streptococcus.
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The main (starter) cultures in yogurt are Lactobacillus bulgaricus and Streptococcus
thermophilus. Their function is to ferment lactose into lactic acid. The increase in
lactic acid decreases pH and causes the milk to clot, or form the soft gel that is
characteristic of yogurt. Yogurt fermentation is homofermentative.
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Kefir is a very interesting heterofermentative dairy fermentation. It uses a complex
mix of homofermentative lactic acid bacteria, heterofermentative lactic acid bacteria
and yeasts. The resulting milk is sour from lactic acid, has flavors from propionic
and acetic acids and has carbonation from CO2.
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