Microbial Spoilage of Milk and Milk Products (Cream, Butter, Cheese, Yoghurt, Ice-cream)

• Milk is known to be balanced food as well as highly nutritious food as it is rich in proteins, fats, carbohydrates, vitamins, minerals, essential amino acids, etc.
• Milk and its products consist of numerous nutrient content, it serves as an excellent growth medium for all of the microorganisms (i.e., bacteria, viruses, fungi, and protozoans).
• The microbial content of raw milk is important for the production of hygienic milk foods.
• Milk and its products such as milk, butter, cream, and cheese are all susceptible to microbial spoilage.

Contamination source of milk

The milk source is the mammals such as cows, buffalo, sheep, and goats. Milk is not sterile when obtained from animals. The source of milk contamination are:

1. the contamination from milking animals
2. the interior and exterior of the udder
3. the coat of the milking animals
4. the surrounding air of the farm and storage
5. the animal feed 
6. the quality of water used
7. the quality of milk handling equipment and storage tanks used
8. biofilms formation on  diary plant pipes

Spoilage of milk

• The main components of milk are water, fat, protein, and lactose.
• Its high water activity, moderate pH (6.4–6.6), and high nutrient content make milk an excellent medium for microbial growth.
• Both raw milk and pasteurized milk contain many types of microorganisms, they are refrigerated, yet they have limited shelf life. 
• During refrigerated storage (at dairy farms and processing plants) before pasteurization, only psychrotrophs can grow in refrigerated milk storage such as Pseudomonas, Flavobacterium, Alcaligenes, Listeria monocytogenes, Yersinia enterocolitica, some coliforms, and Bacillus spp. 
• Those that spoil milk after heating are the thermoduric microorganisms surviving pasteurization such as Micrococcus, some Enterococcus, Streptococcus, some Lactobacillus, and spores of Bacillus and Clostridium.
• Molds and yeasts are usually eliminated during the pasteurization process therefore they cause milk spoilage after the heat treatment such as Aspergillus, Byssochlamys, Cladosporium, Candida spp.
• To a lesser extent, the protozoan pathogens such as Cryptosporidium and Giardia have been found to contaminate milk.

The defects that occur in milk due to microorganism are:

1. Off flavors
2. Discoloration
3. Gas production
4. Lactic acid production/ Souring 
5. Proteolysis 
6. Lipolysis  with development of rancidity
7. Sweet curdling

Spoilage of Milk products

The microbial quality of milk product mainly depends upon:

• The type of milk and milk product used (raw milk, condensed milk, dried milk, cream, butter, etc.)
• The product used for its enhancement such as gelatin, nuts, fruits, sugar, chocolate, coloring agent, etc., 
• Sanitary level of types of equipment, the efficiency of pasteurization, and hygienic level during production and packaging.

1. Spoilage of Cream

• The cream is a milk product made from a butterfat layer deposited on the top of milk before homogenization.
• Cooled milk is used for the production of cream so psychrotrophic bacteria are the main causes of spoilage are caused by Pseudomonas, Alcaligenes, Acinetobacter, Aeromonas, and Achromobacter.
• At room temperature, the main spoilage-causing organisms are Corynebacterium, Bacillus, Micrococcus, Lactobacillus, and Staphylococcus.
• The cream is highly susceptible to pathogenic microorganisms like E. coli, Salmonella Typhimurium, and Listeria monocytogenes.

2. Spoilage of Butter

• Butter is a milk product made by the separation of milk and subsequent churning of the cream.
• The main source of microorganisms found in butter is cream which is starting material of butter. Therefore, the main spoilage is caused by Pseudomonas, Alcaligenes, Acinetobacter, Aeromonas, and Achromobacter.
• The primary spoilage organisms in butter are molds such as Thamnidium, Cladosporium, and Aspergillus.
• The pathogenic microorganisms like Listeria monocytogenes, Brucella, Mycobacterium, Campylobacter jejuni, Yersinia enterocolitica, and Salmonella Typhimurium.

3. Spoilage of Cheese

• Cheese is a fermented milk product that is made by coagulating the casein present in milk by using the enzyme rennet.
• The ripening in cheese is achieved due to the proteolytic and lipolytic activities of various microorganisms.
• The low moisture content of hard and semi-hard ripened cheeses makes them susceptible to fungi compared to bacteria.
• Soft and fresh cheeses are spoiled easily due to their higher pH, moisture content, and lower salinity. 
• The bacterial cheese spoilage is caused by Clostridium spp., (especially C. pasteurianum, C. butyricum, C. sporogenes, and C. tyrobutyricum), Bacillus polymyxa, Flavobacterium, Pseudomonas spp., Alcaligenes, and Achromobacter.
• Yeasts are common cheese spoilage organisms e.g. Candida spp.,  Debaryomyces hansenii, Geotrichum candidum, and Pichia spp.
• The mold spoilage in cheese is caused mainly by Penicillium spp. and Cladosporium spp.
• Major pathogenic bacteria found in cheese are Listeria monocytogenes, Salmonella spp., and Escherichia coli.

4. Spoilage of Yoghurt

• Yogurt is a lactic acid fermented milk product that is produced by using symbiotic cultures of two Lactic acid bacteria (Streptococcus thermophilus and Lb. delbrueckii subsp. Bulgaricus).
• Fruits and nuts added to yogurt for enhancement of flavor are the main sources of contamination.
• Molds and yeasts are the primary contaminants in yogurt. 
• The most common mold causing yogurt spoilage are Aspergillus, Penicillium, Rhizopus, Fusarium, and Trichoderma.
• The most common yeasts causing yogurt spoilage are Candida spp., Debaryomyces, Kluyveromyces, Torulopsis, and Saccharomyces spp.
• They are responsible for off-flavor, gas production, discoloration, etc.
• Molds and yeasts that cause yogurt spoilage leads to a decrease in acidity, leading to proteolysis and putrefaction by bacteria
• The common food-borne pathogens reported are Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, and Yersinia enterocolitica.

5. Spoilage of Ice-cream

• Ice cream is a frozen milk product produced by freezing a pasteurized mixture of milk, cream, and milk solids other than fat, sugars, emulsifier, and stabilizers.
• It includes various flavor-enhancing ingredients like nuts, fruit pulp, confectionary products, eggs, and egg products.
• Being a nutritious food, ice-cream serves as a good medium for microbial growth due to the high nutritive value, almost neutral pH, and long storage duration.
•  It is a frozen milk product hence ice cream spoilage is mainly caused by psychrotrophs such as Pseudomonas, Flavobacterium, Alcaligenes, Listeria monocytogenes.
• Certain molds such as Aspergillus, Fusarium, Geotrichum, Mucor, Penicillium, and yeasts such as Zygosaccharomyces, Saccharomyces, and Cryptococcus also cause ice-cream spoilage.
• Major pathogenic bacteria found in ice-cream are Listeria monocytogenes, Salmonella spp., and Escherichia coli.

The defect found in ice cream due to this spoilage causing microorganisms are:

1. bitterness and off-flavor
2. rancidity
3. greenish pigments
4. discoloration
5. surface taints

 References

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