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Bacteria are Responsible for Spoilage
Other than the lactic acid bacteria (LABs), many other bacteria will produce acid to
ferment milk if the conditions are not favorable for the LABs.

Coliforms will produce a mixture of acids, gases, and alcohols

Some species of micrococci, micro-bacterium, and bacilli can produce acid in milk as
well
Clostridium spp. inhibits the growth of LABs and may produce butyric acid

The defects/bacterial spoilage in milk:
Gas Production
Gas production is usually accompanied by acid formation, which is undesirable in milk
and milk products.
Some of the most notorious gas formers include coliforms, yeasts, clostridium species,
and gas forming bacillus that produce a mixture of carbon (IV) oxide and hydrogen
gases.
To identify milk that has gas, check for foam at the top layer of the milk, ripping of the
curd, and rapid frothy/stormy fermentation of the milk.
Proteolysis
This is the hydrolysis of milk proteins by microorganisms to produce peptides that gives
the milk a bitter taste/flavor.
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Storage of milk at low temperatures and destruction of lactic acid producers in the milk
through heat treatment favor this process. Moulds and yeasts may also destroy any
lactic acid formed in the milk and accelerate the process.
Proteolytic bacteria produce the following forms of bacterial spoilage in milk:

Acid proteolysis, which involves production of acid and proteolysis

Proteolysis that produces both acidity and alkalinity
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Sweet curdling, which results from the activity of the rennin-like enzymes that these
bacteria produce at the early stages of proteolysis

Slow proteolysis, which result from the activity of the bacterial endo-enzymes after
autolysis
Acid proteolysis leads to production of a lot of whey and formation of shrunken curd. The
bacteria further digests the curd, which changes the color of the curd from opaque to a
little translucent. Some bacteria may completely dissolve the curd.
There are three major causes of acid proteolysis, namely:



Micrococcus spp.: – these are very notorious and may even cause the proteolysis of
freshly drawn milk as some of them inhabit the cow’s udder
Streptococcus faecalis and Streptococcus liquifasciens: – very active proteolytic
bacteria that may cause proteolysis of pasteurized milk
Spores of some strains of Bacillus spp.: – especially the lactose fermenting and
proteolytic strains such as Bacillus cereus, which can survive high temperature
pasteurization and cause acid proteolysis.
Some bacteria that may not be able to ferment lactic acid may still cause proteolysis and
this varies with different species of bacteria. Some strains may act on the casein directly
and produce little proteolysis.
These three scenarios are very likely to occur following the action of these bacteria:
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
They may produce very little or non-existent acidity making the milk alkaline

Most of these bacteria cause sweet curdling of the milk before they digest the casein

Others hydrolyze the protein extremely fast that no curdling is observed. This results into
a clear liquid that lacks curd.
The most active proteolytic bacteria are found among the following species of bacteria.
Non-spore forming bacteria include:

Micrococcus

Alcaligenes

Achromobacter

Pseudomonas

Proteus

Flavobacterium
Spore forming bacteria include:


Bacillus spp.
Clostridium spp.
Most of these bacteria can grow and cause proteolysis and bitterness of milk that is held
at chilling temperature. However, most of them are thermoduric (except some species of
micrococci) and should not be present in pasteurized milk.
Proteolysis by the endo-enzymes after bacterial autolysis is slow and insignificant in
milk. However, when long time is allowed for the process, it can be significant, especially
in the curing of cheese.
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Ropiness
Ropiness is a form of bacterial spoilage in milk that makes the milk highly viscous or
sticky. Ropy milk has characteristic silk-like threads that may vary in length from a few
inches to several feet.
You would test for this by dipping a pointed device (like a needle) on the surface of the
milk (after incubating for 12-48 hours) and raising the needle to see if there is presence
of a “rope.”
Testing for #ropiness in milk: dip a pointed device on the surface & raise to check for a
“rope.”
This defect affects milk, cream and whey. The effect is very significant in milk and cream
meant for the market.
Ropiness can be classified as either bacterial or non-bacterial in nature
Bacterial Ropiness
Caused by a slimy capsular material produced by the bacterial cells (which is usually
either gums or mucus).
Further classify bacterial ropiness under these two categories:
Surface Ropiness: – observed at the top of the milk and is caused byAlcalegenes
viscolactis, which is majorly found in the soil and water.
 Ropiness observed throughout the milk: – caused by:
1. Some coliforms, (e.g. Enterobacter aerogenes, Enterobacter cloacae, and in rare
curcumstances Escherichia coli.)
2. Some species of LABs, (e.g. Streptococcus lactis, Lactobacillus bulgaricus, Lactobacillus
cereus, Streptococcus cremoris, and Lactobacillus plantarum). Most of these
microorganisms grow in chains, which leads to the formation of the ropes in milk.
3. Other microbes like Micrococci and Bacilli.

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Non-bacterial Ropiness
This form of bacterial spoilage in milk may occur as a result of:
1. Presence of mastitis in the milk, especially fibrin and leucocytes from the cow’s blood
present in the milk.
2. Thick cream at the top of the milk
3. Casein film or lactalbumin that occur in milk during cooling
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