46 Chapter 2
Surface Properties
The surface tension of milk is determined by
the work required to increase the surface area
of it, expressed as dynes/cm or mN/m. The
surface tension of whole milk is approxi-
mately 50 to 52 dynes/cm; skim milk, 55 to
60 dynes/cm; and cream, 46 to 47 dynes/cm
at 20 ° C.
The surface properties affect the absorp-
tion, formation, and stability of emulsions,
which infl uence creaming, fat globule mem-
brane function, foaming, and emulsifi er use
in dairy products. Surface tension is infl u-
enced by the concentration of casein and
temperature. Homogenization, sterilization,
and other processes involving heat have been
shown to increase the surface tension of milk.
Surface tension can be used to follow the
changes in surface - active components during
milk processing, the foaming tendency of
milk, and the release of fatty acids during
lipolysis.Foaming
Foaming properties affect the handling of
milk and its incorporation in dairy ingredi-
ents. Foam is air cells contained within a
protein fi lm matrix. The formation of stable
foam depends on the lowering of surface
tension that allows the spreading of surface -
active components into thin fi lms only if the
fi lms are suffi ciently elastic and stable enough
to prevent the coalescence of the air cells
created.
Stable foam is essential to produce the
correct overrun and texture in frozen dairy
desserts including frozen and whipped
yogurts. Foam control is required during pro-
cessing to reduce the development of foam
in pipelines and during heat treatment because
the presence of foam can lead to ineffective
pasteurization. The minimum amount of
foam is produced at temperatures between
30 ° C and 35 ° C; however, it increases below
20 ° C and above 30 ° C.The oxidation - reduction potential can be
altered by various factors including the con-
centration of dissolved oxygen, ascorbic
acid, ribofl avin, pH, temperature, and cystine -
cysteine contents present in the milk. The
oxidation - reduction potential of milk can be
used to estimate the amount of lactose avail-
able for fermentative bacteria because the
amount of lactose available correlates with
the amount of dissolved oxygen. The
oxidation - reduction potential can be deter-
mined using the methylene blue reduction
test.
Rheological Properties
Rheology is the study of the transition of
materials subjected to applied forces, in
which a distinction is usually made between
fl uids and solids. Fluids fl ow under the infl u-
ence of forces, whereas the solids stretch,
buckle, or break. The rheological properties
of milk include viscosity, surface tension,
and foaming, which are used in the assess-
ment and monitoring of the quality of milk -
derived products including cheese, yogurt,
butter, and cream.
Viscosity
Viscosity is the resistance of fl ow in centi-
poise (cP) units. Several factors infl uence
viscosity including the temperature, concen-
tration, and state of the casein micelles and
fat globules. Casein micelles affect viscosity
the greatest. Protein hydration also has
resulted in an increase in viscosity.
Viscosity can be used to determine the
amount of casein micelle aggregation in milk
as well as the rate of creaming, mass and heat
transfer, and fl ow conditions in dairy pro-
cesses, and in the designing of dairy process-
ing equipment. Viscosity contributes to
organoleptic properties including mouth feel
and fl avor. The viscosity of whole milk is
approximately 1.9 cP; skim milk, 1.5 cP; and
whey, 1.2 cP at 20 ° C.