19 Chemistry and Biochemistry of Milk Constituents 429der has poor wettability because the small particles
swell on contact with water, thereby blocking the
channels between the particles (see Kelly et al. 2003
for review). The wettability (often incorrectly
referred to as “solubility”) of spray-dried milk pow-
der may be improved by controlling the drying
process to produce milk powder with coarser, more
easily wetted particles; such powders are said to be
“instantized” and are produced by agglomerating
the fine powder particles, in effect by controlling the
caking process. In the case of whole milk powder,
instantization processes must also overcome the
intrinsic hydrophobic nature of milk fat; this is nor-
mally achieved by adding the amphiphilc agent
lecithin.
The crystallization of lactose in frozen milk prod-
ucts results in destabilization of the casein, which
aggregates when the product is thawed. In this case,
the effect of lactose is indirect; when milk is frozen,
pure water freezes, and the concentration of solutes
in the unfrozen water is increased. Since milk is
supersaturated with calcium phosphate (66 and
57%, respectively, of the Ca and PO 4 are insoluble
and occur in the casein micelles as colloidal calcium
phosphate; see Milk Salts section). When the amount
of water becomes limiting, soluble Ca (H 2 PO 4 ) 2 and
CaHPO 4 crystallize as Ca 3 (PO 4 )2,with the concomi-
tant release of Hand a decrease in pH to approxi-
mately 5.8. During frozen storage, lactose crystal-
lizes as lactose monohydrate, thus reducing the
amount of solvent water and aggravating the prob-
lems of calcium phosphate solubility and pH de-
cline. Thorough crystallization of lactose before
freezing alleviates, but does not eliminate, this prob-
lem. Preheating milk prior to freezing also alleviates
the problem, but prehydrolysis of lactose to the
more soluble sugars glucose and galactose, using-
galactosidase, appears to be the best solution.
Although lactose is hygroscopic when it crystal-
lizes, properly crystallized lactose has very low hy-
groscopicity; consequently, it is a very effective
component of icing sugar.
Lactose has low sweetness (16% as sweet as
sucrose in a 1% solution). This limits its usefulness
as a sweetener (the principal function of sugars in
foods) but makes it is a very useful diluent, for
example, for food colors, flavors, enzymes, and so
on, when a high level of sweetness is undesirable.
Being a reducing sugar, lactose can participate in
the Maillard reaction, with very undesirable conse-
quences in all dairy products: for example, brown
color, off-flavors, reduced solubility, and reduced
nutritional value.FOODAPPLICATIONS OFLACTOSETotal milk production ( 600 106 mt/yr) contains
about 30 106 mt of lactose. Most of this lactose is
consumed as a constituent of milk but whey, a by-
product of the manufacture of cheese and, to a lesser
extent, of casein, contains 8–9 106 mt of lactose.
About 400,000 mt of lactose are isolated/prepared
per year. A number of high-lactose food products
are also produced, for example, approximately
2,000,000 mt of whey permeate powder, regular
whey powder, and electrodialyzed whey powder;
these serve as crude sources of lactose for several
food products, including infant formulas. Thus,
most available lactose is now utilized in some form
or another, and little is wasted.
Although some of its properties, especially its low
sweetness and low solubility, limit the usefulness of
lactose as a sugar, other properties, that is, very low
hygroscopicity if properly crystallized, low sweet-
ness, and reducing properties, make it a valuable
ingredient for the food and pharmaceutical indus-
tries. In the pharmaceutical industry, lactose is wide-
ly used as a diluent in pelleting operations. The prin-
cipal application of lactose in the food industry is in
the humanization of infant formulas; human milk
contains approximately 7% lactose, compared with
about 4.8% in bovine milk. For this application,
demineralized whey is widely used—it is cheaper
and more suitable than purified lactose because it
also supplies whey proteins, which help bring the
casein:whey protein ratio of the formula closer to the
value, 40:60, found in human milk, compared with
80:20 in bovine milk. It is necessary to demineralize
the whey because bovine milk contains about four
times as much inorganic salts as human milk.
Lactose is also used as an agglomerating/free
flowing agent in foods (e.g., butter powders) in the
confectionery industry to improve the functionality
of shortenings, as an anticaking agent in icing mix-
tures at high humidity, or as a reducing sugar if
Maillard browning is desired. The low sweetness of
lactose limits its widespread use as a sugar, but is
advantageous in many applications. Lactose absorbs
compounds and may be used as a diluent for food
flavors and colors or to trap flavors.