• Contamination of milk occurs at two levels:
• On farm:
  • Freshly drawn milk contains relatively few bacteria however Micrococcus and Streptococcus are usually found in aseptically drawn fresh milk.
  • During normal milking process, milk is subjected to contamination from udder of animal and adjacent areas.
  • Bacteria found in manure, soil and water contaminate are udder of animal from where they enter into the milk.
  • Other possible source of contamination is hand and finger of milker or other dairy workers.
  • Contamination also occurs from dairy utensils.
• During transport and at processing plant:
  • During transport and manufacturing, contamination occur through tanker, transfer pipes, sampling utensils and other equipment.
  • Sometimes, pathogen may contaminate the milk from hand and finger of milk handler.

Microbial Spoilage of milk and mik products:

• Milk is an excellent culture media for growth of many microorganisms.
• Therefore, different types of microorganisms grow in it and cause spoilage.

i. Spoilage of Milk and cream:

• Souring:
  • Evidence of souring of milk are sour flavor and then coagulation of milk to form solid like curd.
  • Many lactic acid bacteria, coliform and other bacteria ferment sugar of milk and produce acid.
  • At temperature of 10-37^oC, Streptococcus lactis is most likely to cause souring with possible growth of Coliform, Enterococci, Lactobacillus and Micrococcus.
  • At higher temperature, 37-50^oC, Streptococcus thermophilus and Streptococcus faecalis may produce 1% acid and it may be followed by Lactobacillus which produces more acid.
  • Little souring occurs in milk held at refrigeration temperature.
  • Pasteurization of milk kills more active acid forming bacteria but permit survival of thermoduric lactic acid bacteria such as Enterococcus, Streptococcus thermophilus, Lactobacillus, etc.
  • Bacteria other than lactic acid bacteria produce acid specially if conditions are unfavorable for lactic acid bacteria.
  • For example: coliform produce acetic acid, formic acid, ethanol, CO₂, H₂ etc.
  • Similarly, Clostridium produce butyric acid.
• Gas production (Strong fermentation of milk):
  • Sugar fermenting organism produce gas together with acid.
  • Main gas formers, Coliform, Clostridium, Heterofermentative lactic, Propianics bacillus, etc.
  • Coliform, Clostridium, and Bacillus produce both H₂ and CO₂, while others produce only CO₂.
  • Gas production in milk is evidenced by foam at top of liquid milk by gas bubble trapped in curd, by formation of curd.
  • Excessive gas production causes cracking or breakdown of curd causing so called stormy fermentation of milk.
  • Clostridium perfringens mainly causes stormy fermentation.
• Proteolysis:
  • Proteolysis is facilitated by storage at lower temperature by destruction of lactic acid bacteria or by distribution of already produced acid by mold and yeast.
  • Changed cause by proteolytic organism include:
  • Acid proteolysis in which acid production and proteolysis occur simultaneously.
  • Proteolysis with little acidity or even alkalinity.
  • Sweet curdling which is caused by renin like enzyme of microorganisms.
  • Slow proteolysis by intracellular enzyme of bacteria after their autolysis.
  • Residual proteolytic activity of some heat stable proteinase.
  • Acid proteolysis is caused by Micrococcus , Streptococcus faecalis var liquefaciens and some lactose fermenting proteolytic Bacillus species.
  • Sweet curdling is caused by Bacillus cereus.
• Ropiness/ sliminess:
  • Ropiness of milk occur both by bacterial and non-bacterial causes non-bacterial ropiness occurs due to thickness of cream or due to film of cousin or Lactalbumin during cooling.
  • Bacterial ropiness is caused by slimy capsular material of bacteria which usually develop at low storage temperature.
  • Bacteria producing ropiness in milk are Alcaligenes viscolactis, micrococcus freudenreichii, Enterobacter aerogenes, Klebsiella oxytoca, E. coli.
• Change in milk fat:
  • Various bacteria, yeast and mold hydrolyses fat of milk and cause rancidity.
  • Species of Proteus, Pseudomonas fragi, Staphylococcus, Bacillus, Micrococcus, Clostridium, etc. are lipolytic.
  • Pseudomonas fragi and Staphylococcus aureus produce fairly heat resistant lipase.
• Alkali production:
  • Pseudomonas fluorescence and Alcaligene viscolactis produce alkali.
  • Alkali production is due to formation of ammonia from urea and formation of carbonate from organic acid.
• Flavor defect:
  • Acid flavor: Acid flavor may be aromatic or sharp. Sharp flavor is caused by production of acetic acid formic acid, butyric acid etc. by Coliform and Clostridium. It is undesirable. Aromatic flavor is caused by Streptococcus lactic and Leuconostoc when they grow together. It is desirable.
  • Caramel or burnt flavor: It is caused by Streptococcus lactic var. maltigens.
  • Bitter flavor: It is caused by proteolytic organism.
  • Other flavor: They include earthy flavor by Actinomycetes, fruity flavor by Pseudomonas fragi, soapiness by Pseudomonas sapolactic etc.
• Color defect: Growth of pigmented bacteria and other organism give undesirable color. Some examples include:
  • Blue milk: It is caused by Pseudomonas syncyaneum
  • Yellow milk: caused by Pseudomonas synxantha and also by flavobacterium.
  • Red milk: caused by Serratia marcescencs and Micrococcus roseus.
  • Brown milk: caused by Pseudomonas putrefaciens and by enzymatic oxidation of tyrosin by Pseudomonas fluorescence.

ii. Spoilage of Butter:

• Many spoilage microorganisms come in butter from cream or milk from which it is prepared.
• Color defect:
  • Some color defect of butter is non-microbial. They include pink color caused by sulphur-dioxide refrigerant, surface darkening caused by evaporation of water from surface.
  • Discoloration caused by microorganisms depend on type of organism. For example, Stemphylium give black spots, Penicillium give green spot, Alternaria or Phoma give brown spots, Pseudomonas nigrificans give reddish brown spot etc.
• Flavor defect:
  • Cream and butter have capacity to absorb moisture from surrounding.
  • Butter may gain such flavor from absorption of flavor is developed in butter during microbial growth.
  • Some odors in butter caused by growth of organisms include:
  • Fishiness caused by Aeromonas hydrophila.
  • Ester like flavor caused by Pseudomonas fragi.
  • Rancid odor caused by lipase producing organism.
  • Yeasty flavor caused by yeast etc.

iii. Spoilage of Cheese:

• Spoilage of cheese occurs either by mechanical damage or by microorganisms.
• Microbial spoilage of cheese occurs during following three stages:
• Spoilage during manufacturing:
  • During manufacture of most cheese lactic starter culture is added to carry out lactic acid fermentation.
  • If these lactic starters are not effective or when contamination is heavy, many contaminating organisms grow in it and bring undesirable changes in cheese. For.eg. if starter culture is not effective, Clostridium and Bacillus grow and produce holes and other changes.
  • Acid proteolytic bacteria may produce bitter flavor.
  • Leuconostoc may produce holes in cheese.
  • Various organism cause proteolysis, gas production, sliminess and off flavor that damage the quality of cheese.
  • Cheese with too low acidity because of failure of starter culture or because of addition of cream is often made slimy by alcaligenes, melalcaligenes and Pseudomonas fragi.
• Spoilage during ripening:
  • During ripening, spoilage occurs by enzyme released from autolyzed bacteria or by growth of microorganisms during ripening.
  • Main type of spoilage differs with type of cheese.
  • In most of cases, like gas production by Clostridium, Heterofermentative lactis, Propionibacterium, Yeast etc. cause eye formation or cracking of cheese.
  • Clostridium also produce undesirable flavor by production of butyric acid.
  • Certain lactic streptococci give bitter flavor.
  • Some bacteria and yeast give sweet, fruity and yeasty flavor.
  • In cheese with insufficient acidity, putrefaction is caused by anaerobic Clostridium.
  • Microorganisms also caused discoloration on surface of cheese.
  • Bluegreen or black discoloration are produced by reaction of H₂S produced by Microorganisms with metal or metallic salt.
  • Oxidation of tyrosine by bacteria give reddish brown to greyish brown color.
  • Propionibacterium grow as yellow, pink or brown colored complex.
• Spoilage of finished cheese:
  • Soft cheese is most perishable and hard cheese such as cheddar and swiss cheese are most stable.
  • Most common spoilage organism of finished cheese are molds.
  • They grow on surface or into holes or cracks and cause discoloration.
  • Sometimes off flavor is also produced.
• Some molds causing spoilage of cheese are:
  • Cladosporium: It grows on surface and gives black discoloration.
  • Oospora (Geotrichum): Oospora lactis (called dairy mold) grow on surface of soft cheese. In this case, curd gradually becomes liquified under the growth. Oospora crustacea give red spots.
  • Penicillium: Penicillium puberulum and other green spored species grow on surface or into holes and give green coloration.
  • Monilia: Monilia nigra grow on surface of hard cheese and give black discs.

Preservation of milk and milk products from microbial spoilage

1) Reducing contamination:

• Keeping quality of milk is improved when contamination of milk is reduced.
• Udder and adjacent areas should be washed with water and if possible, by germicidal solution before milking.
• Hand and finger of milker should be clean.
• Dairy equipment and other milk processing equipment should be sanitized properly.
• Packaging of milk and milk products in can or packages also avoids contamination.

2) Removal of microorganisms:

• Microorganisms from milk are difficult to remove.
• High speed centrifugation (called bactofugation at 10,000 rpm removes about 99% spores and more than 50% of vegetative cell plus some protein.
• However, bactofugation is not done commercially for removing bacteria from milk.
• Mold can be removed physically from surface of cheese by scrapping or periodic washing.

3) Use of heat:

• For milk, pasteurization is used for preservation.
• Cream can be heated by injecting steam or by combination of steam injection and evacuation in a process called vacreation.
• Heat is also applied for cheese.
• Cooking at 65.6^oC or higher in melting of cheese during manufacture reduces many spoilage microorganisms.
• Evaporated milk is canned and then heated by steam under pressure.

4) Use of low temperature:

• Refrigeration temperature is recommended for milk or similar product during storage in plant during transportation and during storage in home until consumption.
• Fermenter milk and cheese are chilled after their manufacture and kept chilled until they reach the consumption.
• Chilling storage is most commonly applied technique of preservation of milk and milk product in home for short term storage.

5) Drying:

• Many types of milk product are prepared by drying or removing moisture from them.
• There are two types of such product, one is condensed product and the other is dry product.
• One condensed product is evaporated milk, in which 60% moisture is removed, so that remaining high lactose concentration in solution is inhibitory to micro-organisms.
• Bulk condensed milk, sweetened condensed milk, semisolid butter are other examples of condensed product.
• Milk product prepared in dry form include dry milk, ice-cream mix etc.
• Milk can be dried by drum drying or spray drying.

6) Use of chemical preservative:

• There are two types of chemical preservatives i.e. developed and added preservative important in milk product.
• Developed acidity in many fermented milk product retards growth of many microorganisms.
• Propionic acid develop in swiss cheese is inhibitory to mold.
• Many chemical preservatives are added from outside in many milk products.
• Sorbic and propionic acid added in cheese and yoghurt prevent surface spoilage by mold.
• Sugar is added in sweeten condensed milk to reduce A_w.
• NaCl used in cheese during manufacturing gives flavor and also acts as preservative.
• The addition of H₂O₂ combine with mild heat treatment has been used for pasteurization of milk for certain kind of cheese. Some cheese is also smoked.

Microbial spoilage of milk and milk products

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