Download What are somatic cells

What are somatic cells?
Somatic cells normally comprise mainly of white blood cells (leucocytes) and tissues
from the udder (epithelial cells). The white blood cells particularly increase as a result of
udder infection (mastitis). The somatic cell count of a cow’s milk thus gives us a good
indication of degree that the udder is infected with organisms that cause mastitis.
Significance of high SCC (somatic cell count) to the farmer
A high SCC, which is an indicator of mastitis, causes a serious financial loss for the dairy
farmers. Mastitis results in the reduction of milk production, cost of preventing and
treating the disease and shortening of productive life of a dairy cow. Loss of money is
also incurred due to fines from being penalized for having milk, which does not fall
within the SCC standard range, which is usually less than 400 000cells/ml.
Significance of high SCC (somatic cell count) to Dairy Processor
Mastitis or infection of the cow’s udder causes a rise in the somatic cell counts with
consequent changes in the milk composition and the functional characteristic of milk. As
SCC increases to 500 000 cells/ml and above, the processing properties of milk such as
protein, fat, lactose, anions and enzymes change. This leads to a decline in quality of
manufactured dairy products in terms of enzyme activity, product yield and raw milk
composition. Enzymes like lipase and plasmin occur naturally in milk but increase during
udder infection and these enzymes are responsible for the proteolysis and lipolytic
activity, which affect product quality.
The table below summarizes the effect of high SCC on specific Dairy products
Product
Change
Raw milk
Development of rancid flavour
Lower heat stability of whey proteins
Pasteurized milk
Cheese
Butter
Deterioration in shelf life, flavour and
quality
Reduced starter activity, changed clotting
time, reduction of curd firmness, losses of
fat and casein with whey and a lower
cheese yield
Less flavour, oxidative taste and a longer
churning time
Resazurin test
The test is done to assess the reducing activities of microbes in milk thus its shelf life.
Resazurin is a blue dye, which becomes colourless when it is chemically reduced by
removal of oxygen. When it is added to the milk sample, the metabolic activity of
bacteria present in the sample has the effect of changing the colour of the dye at a rate,
which bears a direct relationship to the number of bacteria in the sample. On receiving
milk the resazurin test is used as a quick screening test, which may form basis of a bad
churn supply. If the sample starts to shade immediately, the consignment is considered
unfit for human consumption. The resazurin dye changes through a range of three
colours, the blue colour changes to pink resorufin then to colourless dihydroresorufin.
The first colour change is brought about easily, bacteria and leucocytes reduce the dye.
Procedure involves shaking the milk sample vigorously to mix and aerate. Pipette 10 ml
of sample milk into a sterile test tube. Add aseptically, 1 ml of freshly prepared 0.005%
resazurin reagent. Close tube with rubber stopper and invert it twice, incubate at 37.5°C
in a covered water bath for 10 minutes. Compare the colour of the dye with the original in
a lovibond comparator disc. Significant decrease in colour intensity indicates poor quality
milk. A disc reading less than 0.5 indicates poor quality milk and above 4 indicates good
quality milk. Anaerobic and psychrotrophic microorganisms are not accounted for in this
test. The test is used as a platform rejection test. Milk, which fails this test, is rejected.
Titratable acidity
They are two types of acidity - initial acidity due to acid salts and dissolved carbon
dioxide and secondary acidity, which is formed as a result of bacterial action on lactose,
converting it to lactic acid. The two are combined to form the percentage of lactic acid in
milk. The development and control of acidity in milk, cream and whey is of the greatest
importance in milk processing and the manufacturing of cheese, butter and other dairy
products. High acidity results in precipitation of milk proteins when milk is heated
resulting in the clogging of pasteurisation heat plates.
The determination of acidity in milk or other dairy products is based on fact that an alkali
is added in sufficient quantity and strength to an acid, to neutralise the acid. An indicator
that displays a marked colour change in acid and alkaline solution is used to determine
whether all the acidity has been neutralised.
CH3CH (OH) COOH + NaOH
(Lactic acid)
(Sodium hydroxide)
H2O + CH3CH (OH) COONa
(Water)
(Salt)
Total bacterial
This is a method used to estimate amount of colony forming units in a sample and it
makes use of a pour plate method. The samples are plated at different dilutions and
incubated for 24 hours. This makes use of special type of media called plate count. A
comparison of bacterial load before pasteurisation and after pasteurisation gives an
indication of the efficiency of the pasteuriser. The TBC is also used as a quality check for
milk suppliers. If a producer’s milk is out of specifications a warning letter is sent to the
dairy farmer, warning him on his milk being liable to rejection if the TBC remains high.
Spores
Bacterial spores are the most resistant microorganisms to steam sterilisation. Bacterial
spores are not all equally resistant to steam but some bacteria are able to survive
pasteurisation and sterilization by the formation of spores. The most resistant spores are
usually named thermo resistant spores and they are all thermophiles, that is they can grow
at rather high temperatures, ranging from 40ºC to 50ºC. Counts of mesophilic spores are
often referred to as total spore counts due to the fact that mesophilic spores represent
more than 90% of the total number of spore formers in most environments. Spores are the
reason for most spoilage in pasteurised and sterilised milks, an example of bacteria
mainly able to survive by the formation of spores are of genera Bacillus, Bacillus subtilus
and Bacillus cereus.
Coliform bacterium
Coliform bacteria are short rods defined as aerobic and facultative anaerobic, gram
negative, non-spore forming, which ferment lactose with the production of acid and gas
formation. Leading species are E.Coli and Enterobacter aerogenes. E.Coli causes gastro
enteritis, food poisoning in man, it produces anterotoxins that induce vomiting,
abdominal pains and nausea therefore must not be present in the final product. Coliform
bacteria can be grouped into two faecal and presumptive. Presumptive coliforms are
incubated at 37ºC and faecal Coliform bacteria group include coliform bacteria capable
of growth at an elevated temperature (44.5 or 45ºC). The original purpose of elevated
incubation tests is to differentiate Coliform of faecal and those of non-faecal origin. The
presence of Coliform in milk and milk products is evidence of poor hygiene. The test for
Coliform on pasteurised milk is used to detect post pasteurisation contamination since the
bacteria are destroyed during pasteurisation.
Test for Faecal Coliforms
Green bile selective media is used to test for faecal coliforms. E.Coli produces the
enzymes formic hydrogenylase, which splits formic acid, producing carbon dioxide
brilliant and hydrogen in the ratio 1:1.The Brilliant green bile (BGB) broth contains
lactose and brilliant bile. The test involves introducing the milk sample into a test tube
containing 5 ml of sterilised BGB media and inverted Durham tube. The positive result is
shown by a gas production as seen trapped in the Durham tube. The same procedure is
used for testing for presumptive coliforms by replacing BGB with MacConkey broth.
Results are interpreted in the same manner with gas production showing presence of
coliform bacteria.