Spoilage of Milk

Definition: Any undesirable change or deterioration in the quality of milk refers as Spoilage of milk. These changes can be like change in appearance, colour, odour, taste etc. Milk contains complex biochemical composition and high water activity, and this provides an excellent culture medium for the growth and multiplication of microorganisms which leads to serious health issues.

Spoilage of milk can be due to:
A primary factor: It includes microbes or enzymes.
A secondary factor: Includes the addition of unwanted chemical or toxin and conditions that are naturally present in the food like moisture, oxygen content that supports the growth of microorganisms.

Content: Spoilage of Milk

  1. Sources
  2. Factors Affecting Milk Spoilage
  3. Types of Microorganisms in milk
  4. Microbial examination of milk

Sources of spoilage of milk

There are two factors involves in spoilage of milk: Internal and external factors.


Factors Affecting Milk Spoilage

It can categorise into:
factors affecting milk spoilageIntrinsic factors: These are innate to the food composition. It includes many elements like moisture content, pH, nutrient content, antimicrobial constituents of food.

Extrinsic factors: These are innate to environmental factors. It includes temperature, relative humidity, oxygen availability, microbial interaction.

Types of microorganisms in milk

It groups into three categories
microorganisms type

Biochemical Types

It consists of organisms that bring biochemical changes in the milk. Biochemical change is a process where microbes produce some intracellular and extracellular enzymes that bring about the change in biochemical constituents of milk like protein, carbohydrates etc.

Souring of milk: It includes microorganisms that embitter the taste of milk.
Examples: Lactobacillus sp., Streptococcus sp.
Signs of spoilage: sour milk, curd formation etc.

Sweet curdling: It includes microorganisms that coagulate the milk.
Examples: Bacillus sp., Proteus sp., Micrococcus sp. etc.
Signs of spoilage: pH becomes alkaline leads to curd formation.

Gas production: It includes microorganisms that produce acid and gas mainly coliforms, cause “stormy fermentation of milk”. It evidences at the top of the milk.
Examples: Clostridium sp., Bacillus sp., coliform
Signs of spoilage: Explosion of curds.

Ropiness: It includes microorganisms that make the milk highly viscous, sticky and characteristic silk-like thread appears. It is due to the slimy capsular material of an organism. It is of two types: surface ropiness and ropiness throughout the milk.
Examples: Alcaligenes sp., Klebsiella sp., Enterobacter sp. Etc.

Colour changes in milk: It results from prolonged storage of milk. Wet, humid climate favours the growth of microorganisms to bring this kind of change includes:
Blue colour: E.g. Pseudomonas syncyanea
Yellow colour: E.g. Pseudomonas synxantha
Red colour: E.g. Micrococcus roseus
Brown colour: E.g. Pseudomonas putreficans
Grey colour: E.g. Clostridium sp.

Dairy mould: It includes the growth of moulds in milk that can produce toxins that cause serious health issues.
Examples: Penicillium sp., Geotrichum sp. Etc.
Signs of spoilage: Mouldy appearance

Temperature Characteristic Types

Based on temperature, the microorganisms can classify into the following four types:

Psychrophilic bacteriaIt defines as cold-loving bacteria”, which grows below 10֯ C.
Examples: Flavobacterium sp., Pseudomonas sp., Coliforms.
Changes in milk: Off flavour and odour.

Mesophilic bacteria: These grow at a temperature between 25-40֯ C.
Lactic streptococci, coliforms: Off flavour
Streptococcus lactins: Malty and caramel taint
Pseudomonas sp.: Fishy flavour

Thermophilic bacteria: It defines as “heat-loving bacteria”, which grows at a temperature between 55-65 ֯C.
Examples: Bacillus sp.
Changes in milk: Off flavour and odour.

Besides this, there is one more type that is “Thermoduric bacteria“, which indirectly contaminate the food by contaminating the container. It can result from faulty cleaning and improper handling and survive at pasteurisation temperature but cannot grow.
Examples: Micrococcus luteus, Bacillus subtilis etc.

Pathogenic Type

Some pathogenic kinds of microorganisms found in milk that causes serious diseases. Diseases can be transmitted either through raw milk, cow and others.
Examples: Tuberculosis is caused by Mycobacterium tuberculosis in both cow and man. Bacillus anthracis causes anthrax in both cow and man. Streptococcus pyogenes causes scarlet fever in man.

Microbial examination of milk

It can be done by following methods:

Standard Plate Count

It is also called “Viable Count Method“, examines the viable count of bacteria present in the milk. It gives a rough and direct assessment of a viable number of bacteria that is a very simple method to carry out.

Procedure: Take 1ml of milk sample. Then prepare serial dilutions by transferring 1ml of a sample from each tube, i.e. 10, 100 etc. Then, transfer solid agar media on sterile plates. Spread 0.5ml of a sample onto the plate. At last, incubated for 24-48 hours at 35-37֯ C and count the number of colonies.

Interpretation of result: More than 300 colonies indicates the milk is unpotable.
standard plate count

If the solution is more diluted, then it will produce the highest number of colonies, i.e. milk is impotable, whereas a less diluted solution will produce less number of colonies.

Coliform Count

It is used to examine the presence of coliform bacteria that cause the fermentation of milk by the production of acid and gas. This becomes necessary to detect the presence of coliform because it makes the milk unpotable for human consumption.

Procedure: Firstly, take MacConkey fluid medium. Then add milk of different concentrations in a fermentation tube. After that Durham tube is dipped and incubated for 24-48 hours at 35-37֯ C. In this test result are made based on fermentation property of coliforms.

Interpretation of result:
Positive: If the colour changes from purple to yellow and forms gas bubbles in Durham tube.
Negative: If there is no acid and gas formation.
coliform test

Methylene Blue Reductase Test

It is the quickest method to determine the microbial load. It identifies the quality of milk, based on the colour-retaining property. A speed of reduction of methylene blue colour is directly proportional to the volume of bacteria present in the milk sample.
In simple words, an increase in the number of bacterial flora will reduce the colour of methylene blue more rapidly due to the consumption of oxygen.

Procedure: Add a definite quantity of methylene blue to 10ml of milk. After that, hold the sample at 37֯ C until the colour disappears.

Interpretation of result:
Decolouration time —— Result
30min-2hrs —— poor quality
2-6hrs —— Fair quality
6-8hrs —— Good quality
Over 8 hours —— Best quality
Shorter the decolouration time, higher is the volume of the bacterial flora present in the milk and poor is the quality of milk and vice versa.

Resazurin Test

It is very similar to the methylene blue reductase test

Procedure: Firstly add resazurin to the milk sample. Then incubated it for 10min and observe the shades of colour.

Interpretation of result:
Positive: Formation of pink colour indicates the presence of bacteria that reduce resazurin.
Negative: Colour remains unchanged, i.e. bacteria are not present in the milk indicates that milk is of good quality.
resazurin test

Phosphatase Test

It is used to check the pasteurisation process, verifies whether milk is pasteurised or not. Phosphatase is an enzyme that is usually present in the milk. This enzyme gets inactivated if pasteurisation performed properly.

Procedure: Firstly, the 5ml of milk is taken. Then a few drops of sodium biphenyl phosphate is added. After that, incubated for 10-15min

Interpretation of result:
Positive: Blue colour appears, indicates the presence of a phosphatase, i.e. milk pasteurises appropriately.
Negative: No changes in colour, indicates the absence of phosphatase, i.e. milk pasteurises improperly.
phosphatase test

Turbidity Test

It checks the sterilisation process of milk, whether the milk boiled correctly or not, to the temperature prescribed for sterilisation. If milk sterilizes properly, then all coagulable heat proteins get precipitated.

Procedure: Firstly take 5ml of sterilised milk. Then add a few drops of ammonium sulphate and boil in a water bath for 5min.

Interpretation of result:
: If turbidity appears, i.e. milk sterilizes improperly.
Negative: No turbidity indicates milk sterilizes properly.
turbidity test

Direct Microscopic Count

It is a rapid method for microbial examination, which determines cell, morphology.

Procedure: Firstly, take 0.01ml of raw milk in glass slide (hemocytometer) and air dry. Then add one drop of methylene blue. At last, count bacterial clumps in a colony counter.

direct microscopic count
It counts both viable and non-viable cells.

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