Sliminess / Ropiness
Changes in milk fat
Changes in milk flavor
Changes in milk color
Milk contains lactose which produces acids (such as propionic,
lactic, and butyric) and CO2 on fermentation. When this
fermentation occurs at faster rate, then raw milk present a foamy
layer on the upper surface. If it continues then air bubbles becomes
entrapped and gas becomes saturated throughout the body of the milk.
It is also termed as “Stormy Fermentation”. Such milk is not fit for
human consumption. Gas production may be due to gas producing
bacteria such as: Colliforms, clostridium and bacillus species.
It is the destruction of
milk proteins especially of casein. There are different types of
Milk whey separated and pH >5.2. Milk taste will be soured.
Milk whey separated and pH >6.9 (towards neutral and alkaline). Milk
taste will be bitter.
It is due to bacterial
contamination i.e. bacillus cerus – it release enzymes “protease”
which targets the casein. Bacteria use lactose and convert into acids
and aldehyde components. Milk is sweet in taste.
If raw milk is heated to
certain pasteurized temperature or boiling temperature, it will
cause destruction of bacteria which results into release of
endogenous proteases in the milk. These proteases cause slow
proteolysis of the milk.
Proteolysis due to Anaerobic Bacteria:
Bacillus and clostridial
species are heat resistant. Their presence is not eliminated easily
stage. That’s why result into specific kind of smell names as
butyrine smell. Clostridium species are mostly involved in the
alkaline proteolysis of milk.
Roppiness or Sliminess:
In case of roppiness or sliminess milk viscosity is increased. If
you touch the surface of milk, a cord/rope like structure is formed.
Roppiness or sliminess may be bacterial or non-bacterial in origin:
It may be due to normal thickness of cream itself. Such milk is not
regarded as spoiled milk.
may be due thin film of casein and lactalbumin. These when mix
together at refrigeration temperature, sliminess may be seen in the
bottom part of the milk
there is non-infectious mastitis to the animal, then leukocytes and
fibrin contents enhanced. These two make the milk thicker and
produce roppiness or sliminess in the milk.
Conditions are irreversible.
Certain microorganisms have properties to produce sliminess at the
top and other produce throughout the body of the milk. Mostly
capsulated bacteria produce sliminess.
Changes in Milk Fat:
There are three particular conditions which cause changes in the
Oxidation of Unsaturated Fatty Acids:
Upon oxidation, milk fat yields a lot of aldehydes, ketones and
variety of acids. Oxidation imparts tellowy odour.
Hydrolysis of Overall Milk Fat:
Hydrolysis occurs due to the acidity of enzyme “lipase” which
separates the molecules of glycerols from fatty acids and results
into release of free fatty acids in the milk which brought obnoxious
change in the milk. Hydrolysis gives rise to putrefied odour (rotten
egg like smell).
Combined Condition of Above:
Change in milk fat may be due to certain bacteria which gives a
combination of both odours discussed before:
Change in the Color of Milk:
Normal color of milk is light or off-white (cow) and shining white
(buffalo). Variety of bacteria responsible for bringing change in
the color of the milk:
Blue color -------------------------- Pseudomonas synciani
Red color --------------------------- Brubibacterium erythrogenes,
Yellow color ----------------------- Pseudomonas synxantha
Brown color ------------------------ Pseudomonas putrificians
Green color ------------------------- Pseudomonas aurogenosa
These above are all bacteria except brubibacterium which is yeast.
Milk with any abnormal discoloration must be declared as unwholesome
Change in the Flavour of Milk:
The change in the flavour of the milk is discussed as follows:
is due to acidic changes in the milk. Sourness in the milk is
described in 3 different forms:
Clean: Low contents of acids due to environment of Streptococcus
lactis. There will be sour acidic flavour.
Aromatic: It is due to bacteria which are responsible for moderated
type of acidic components.
Sharp: It is due to the environment of clostridial species; high
is due to alkaline changes in the milk.
Pseudomonas mucidolense (a bacterium) produces potato like flavour
in the milk.
typical flavour of fishiness is produced by Acromian hydrophila. It
is due to formation of tri-methyl amine. Trimethyl amine gives fishiness flavour to the milk.