Dairy Chemistry And Biochemistry

LACTOSE 33

MILK,WHEY, PERMEATE Rapid drying Concentrated lactose syrup

* “LACTOSE GLASS”

(Non-crystalline)

a-HYDRATE

Cryslallizatioir

4 MOLECULAR MOBILITY

CAKING OF MILK AND

WHEY POWDERS

AGGREGATES OF

CRYSTALS

Figure 2.9 Formation and crystallization of lactose glass.

Crystallization of lactose in dried milk particles causes ‘caking’ of the
powder into a hard mass. If a considerable portion of lactose in the freshly
dried product is in the crystalline state, caking of the powder on contact
with water is prevented, thereby improving the dispersibility of the powder.
Lactose crystallization is achieved by rehydrating freshly dried powder to
c. 10% water and redrying it, or by removing partly dried powder from the
drier and completing drying in a fluidized bed dryer. This process is used
commercially for the production of ‘instantized’ milk powders. Clustering of
the particles into loose, spongy aggregates occurs; these agglomerates are
readily wettable and dispersible. They exhibit good capillary action and
water readily penetrates the particles, allowing them to sink and disperse,
whereas the particles in non-instantized powder float due to their low
density which contributes to their inability to overcome surface tension.
Also, because of the small size of the particles in conventional spray-dried
powders, close packing results in the formation of inadequate space for
capillary action between the particles, thereby preventing uniform wetting.
As a result, large masses of material are wetted on the outside, forming a
barrier of highly concentrated product which prevents internal wetting and
results in large undispersed lumps. This problem is overcome by agglomer-
ation and, in this respect, lactose crystallization is important since it
facilitates the formation of large, sponge-like aggregates.
The state of lactose has a major effect on the properties of spray-dried
whey powder manufactured by conventional methods, i.e. preheating, con-
densing to about 50% total solids and drying to less than 4% water. The
powder is dusty and very hygroscopic, and when exposed to ambient air it