Dairy Ingredients for Food Processing

Chemical, Physical, and Functional Characteristics of Dairy Ingredients 47

heat or refrigeration required to change the

temperature of milk.

Thermal Conductivity

Thermal conductivity is the rate of heat trans-

fer through a material. It determines how fast

milk is cooled or heated and it increases with

temperature. However, it decreases with an

increase in fat or total solids. The thermal

conductivity of milk at 37 ° C is 193 J/msK

and 223 J/msK at 80 ° C. Thermal conductiv-

ity is frequently exploited to extend the shelf

life of milk.

Optical Properties

Optical properties include appearance, fl avor,

and refractivity. They mostly infl uence a con-

sumer ’ s perception of a product.

Appearance

The appearance of milk is perceived as a

measure of quality, purity, and richness by

consumers. The opaque, white color of milk

is due to the scattering of light by the fat

globules, casein micelles, and colloidal

calcium phosphate. The intensity of the color

is proportional to the size and number of

these particles, which is increased by homog-

enization. Therefore, homogenized products

appear whiter than unhomogenized products.

Furthermore, the yellowish color depends on

the concentration of carotene, and the green-

ish color of whey and milk serum depends on

the concentration of ribofl avin.

Refractivity

The refraction of light is infl uenced by the

concentrations of solutes in solution. Each

solute has its own refractivity index. The

refractive index of the solute and solvent

determines a solution ’ s refractivity. The

refractive index of whole milk is 1.3440 to

1.3485. Water has the highest refractive

Curd Tension

Curd tension is an important factor in cheese
making and milk digestibility. The curd
tension of milk is 28 to 54 g. Heating of milk
and the addition of salts (i.e., calcium) causes
a reduction in curd tension.

Thermal Properties

Thermal properties infl uence the quality,
fl ow rate, and storage considerations for
milk. Thermal properties include heat stabil-
ity and transfer, expansion of milk (coeffi -
cient of thermal expansion), heat capacity,
specifi c heat, and thermal conductivity.
Generally, the rate of heat transfer is max-
imized in dairy processing operations, which
results in a better quality product. Heat stabil-
ity is mainly determined by the protein com-
position, pH, and various salt concentrations.
Heat expansion affects the fl ow rate and
storage conditions during processing treat-
ments (i.e., pasteurization). Milk expands
approximately 0.335cm^3 /Kg/ ° C between 5 ° C
and 40 ° C.

Heat Capacity

Heat capacity is the capacity of a substance
to store heat. It is measured in J/kg/K. The
heat capacity of skim milk is 3,906 J/kg/K at
50 ° C and increases linearly to 4,218 J/kg/K
at 140 ° C. The heat capacity of milk and
cream depends on the fat content, because
high fat content results in a lower heat
capacity.

Specifi c Heat

Specifi c heat is the amount of heat required
to raise a unit temperature of a substance by
a certain temperature interval. The specifi c
heat of whole milk is 3.931 kJ/kg/K at 80 ° C.
Specifi c heat is infl uenced by the fat and
water content of a product and is used in the
dairy industry to determine the amount of