Microbial Spoilage of Milk and Milk Products (Milk, Butter,

Cheese, Yoghurt)
Contamination of milk:
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 sources 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

 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:

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

 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 10-37oC, Streptococcus lactis is most likely to cause souring with possible
growth of Coliform, Enterococci, Lactobacillus and Micrococcus.
 At higher temperature, 37-50oC, 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.

Kind of defect Related microorganisms

Shorter shelf life, rancidity, and
Bacillus spp.
bitterness
 Psychrotrophic bacteria (Gram-negative and
Gelation
Gram-positive)
Increase of free fatty acids and casein
hydrolyses, destabilizing the casein Bacillus spp
micelles (acid coagulation of milk)
Undesirable flavor: unclean, fruity, Pseudomonas fragi, P. fluorescens
bitter, rancid, yeasty Flavobacterium, Acinetobacter, Alcaligens
Sour (acid, gas) Lactic acid bacteria
Coliforms, Pseudomonas spp, Alcaligenes,
Ropy or slimy
Micrococcus, Bacillus subtilis
Flavor Streptococcus lactis, leuconostoc,
Sour or acid flavor clostridium spp
Burnt or caramel flavor Streptococcus lactis
Color changes Pseudomonas syncyanea, Streptococcus
-Blue milk Lactis
Pseudomonas Synxantha, Flavobacterium
-Yellow milk
spp
Serratia spp, Brevibacterium erthrogenes,
-Red milk
micrococcus
– Brown milk Pseudomonas putrefaciens, P. fluorescens

Kind of defect Related microorganisms

Shorter shelf life, rancidity, and
Bacillus spp.
bitterness
Psychrotrophic bacteria (Gram-negative and
Gelation
Gram-positive)
Increase of free fatty acids and casein
hydrolyses, destabilizing the casein Bacillus spp
micelles (acid coagulation of milk)
Undesirable flavor: unclean, fruity, Pseudomonas fragi, P. fluorescens
bitter, rancid, yeasty Flavobacterium, Acinetobacter, Alcaligens
Sour (acid, gas) Lactic acid bacteria
Coliforms, Pseudomonas spp, Alcaligenes,
Ropy or slimy
Micrococcus, Bacillus subtilis
Flavor Streptococcus lactis, leuconostoc,
Sour or acid flavor clostridium spp
Burnt or caramel flavor Streptococcus lactis
Color changes Pseudomonas syncyanea, Streptococcus
-Blue milk Lactis
Pseudomonas Synxantha, Flavobacterium
-Yellow milk
spp
Serratia spp, Brevibacterium erthrogenes,
-Red milk
micrococcus
– Brown milk Pseudomonas putrefaciens, P. fluorescens
 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.
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.
Kinds of defect Related Microorganisms
Surface discoloration and taints Bacteria : Pseudomonas putrefaciens,
Flavobacterium spp, Alteromonas
 Molds: Penicillium, Aspergillus, Mucor,
Cladosporium, Rhizopus spp
Black discoloration Pseudomonas nigrificans
Off flavor Pseudomonas spp, Lactococcus lactis
Rhodotorula, Cryptococcus, Torulopsis,
Lipolytic spoilage
Candida lipolytica
Yeasty smell Geotrichum candidum

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.
 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.
Kinds of defect Related Microorganisms
 1. Cheese
Gassy, Butyric acid Clostridium tyrobutyricum
Gassy, floating, or split curd Leuconostoc, S. lactis subsp. diacetylactis
Penicillium, Scopulariopsis, Mucor, other
Moldy
molds
Bitterness, putrefaction and rancid odor,
liquefaction, gelatinization of curd, and Pseudomonas fluorescens, P. fragi
slime and mucous formation
Undesirables flavor: rancid taste in hard
Bacillus spp
cheeses
off-white, tan, or yellow surface
Geotrichum
discolorations
2. Soft cheese
Black mold Mucor
3. Cottage cheese
Slimy curd, putrid odor Pseudomonas
Discoloration Flavobacterium, yeasts, molds
Pseudomonas, Alcaligenes,
Slimy, gelatinous
Flavobacterium, coliforms
Fruity Yeasts
4. Cheddar cheese
Sweet, yeasty, fruity Yeasts
5. Swiss cheese
Gassy, sweet Off odor Yeasts (Torulopsis)
Off·odor C. sporogenes
Kinds of defect Related Microorganisms

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.
 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.
Kinds of defect Related Microorganisms
Yeasty, fruity, cheesy,
Yeast
or bitter
Surface taints Molds
reservation 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 65.6oC 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 Aw.
 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
Contamination of milk:

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 10-37oC, Streptococcus lactis is most likely to cause souring with possible growth
of Coliform, Enterococci, Lactobacillus and Micrococcus.

At higher temperature, 37-50oC, 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.