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:
- the contamination from milking animals
- the interior and exterior of the udder
- the coat of the milking animals
- the surrounding air of the farm and storage
- the animal feed
- the quality of water used
- the quality of milk handling equipment and storage
tanks used
- 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.
- Off flavors
- Discoloration
- Gas production
- Lactic acid production/ Souring
- Proteolysis
- Lipolysis with development of rancidity
- Sweet curdling
Kind of
defect |
Related
microorganisms |
Shorter shelf
life, rancidity, and bitterness |
Bacillus spp. |
Gelation |
Psychrotrophic
bacteria (Gram-negative and Gram-positive) |
Increase of
free fatty acids and casein hydrolyses, destabilizing the casein micelles
(acid coagulation of milk) |
Bacillus spp |
Undesirable
flavor: unclean, fruity, bitter, rancid, yeasty |
Pseudomonas
fragi, P. fluorescens Flavobacterium,
Acinetobacter, Alcaligens |
Sour (acid,
gas) |
Lactic acid
bacteria |
Ropy or slimy |
Coliforms,
Pseudomonas spp, Alcaligenes, Micrococcus, Bacillus subtilis |
Flavor
|
Streptococcus
lactis, leuconostoc, clostridium spp |
|
Streptococcus
lactis |
Color changes -Blue milk |
Pseudomonas
syncyanea, Streptococcus Lactis |
-Yellow milk |
Pseudomonas
Synxantha, Flavobacterium spp |
-Red milk |
Serratia
spp, Brevibacterium erthrogenes, 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.
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.
Kinds of
defect |
Related
Microorganisms |
Bitty cream,
sweet curdling |
Bacillus
cereus |
Bitterness,
thinning in sterilized cream |
Bacillus
licheniformis, B. coagulans, Bacillus subtilis |
Yeast or
fruity flavors, gas formation |
Candida
lipolyticum, Geotrichum candidum. |
Surface
taints |
Penicillium spp |
Foamy |
Candida,
Torulopsis |
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 |
Lipolytic
spoilage |
Rhodotorula,
Cryptococcus, Torulopsis, Candida lipolytica |
Yeasty smell |
Geotrichum
candidum |
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 |
Moldy |
Penicillium,
Scopulariopsis, Mucor, other molds |
Bitterness,
putrefaction and rancid odor, liquefaction, gelatinization of curd, and slime
and mucous formation |
Pseudomonas
fluorescens, P. fragi |
Undesirables
flavor: rancid taste in hard cheeses |
Bacillus
spp |
off-white,
tan, or yellow surface discolorations |
Geotrichum |
2. Soft
cheese |
|
Black mold |
Mucor |
3. Cottage
cheese |
|
Slimy curd,
putrid odor |
Pseudomonas |
Discoloration |
Flavobacterium, yeasts,
molds |
Slimy,
gelatinous |
Pseudomonas,
Alcaligenes, Flavobacterium, coliforms |
Fruity |
Yeasts |
4. Cheddar
cheese |
|
Sweet,
yeasty, fruity |
Yeasts |
5. Swiss
cheese |
|
Gassy, sweet
Off odor |
Yeasts
(Torulopsis) |
Off·odor |
C.
sporogenes |
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.
Kinds of
defect |
Related
Microorganisms |
Yeasty,
fruity, cheesy, or bitter |
Yeast |
Surface
taints |
Molds |
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.
- bitterness and off-flavor
- rancidity
- greenish pigments
- discoloration
- surface taints