460 Part IV: Milk
1988, Oldfield 1998). The extent of transfer of phos-
phate to the colloidal phase, which is greater than
that of calcium, depends on the temperature of pre-
heating. The concentrations of soluble calcium and
phosphorus in reconstituted milk powder are gener-
ally lower than those in the original milk, due to
irreversible shifts induced during drying (Le Graet
and Brule 1982).
Holding milk concentrate at 60°C for an
extended period before spray-drying can increase
the viscosity of the concentrate, making it more dif-
ficult to atomize and thereby affecting the properties
of the final powder.
Alternative technologies for concentrating milk
are available, the most significant of which is prob-
ably membrane separation using reverse osmosis
(RO). Reverse osmosis can achieve only a relatively
low level of total solids (20%) and has a relative-
ly low throughput, but is far less thermally severe
than evaporation (Písecky 1997). Other membrane
techniques used to concentrate or fractionate milk
include nanofiltration (NF), which essentially re-
moves only water, and ultrafiltration (UF), which
allows standardization of the composition (e.g., pro-
tein and lactose contents) of the final powder
(Mistry and Pulgar 1996, Horton 1997).
SPRAY-DRYING OFMILK
Today, milk powders are produced in large, highly
efficient spray-dryers; the choice of dryer design
(e.g., single versus multistage, with integrated or
external fluidized bed drying steps, choice of atom-
izer, air inlet, and outlet temperatures) depends on
the final product characteristics required. Due to a
wide range of applications of milk powders, from
simple reconstitution to use in cheese manufacture
or incorporation as an ingredient into complex food
products, the precise functionality required for a
powder can vary widely.
Skim milk powders (SMP) are usually classified
on the basis of heat treatment (largely meaning the
intensity of preheating during manufacture), which
influences their solubility and flavor (Caríc and
Kalab 1987, Kyle 1993, Pellegrino et al. 1995).
Instant powders (readily soluble in cold or warm
water) are desirable for applications requiring
reconstitution in the home, and are produced by
careful production of agglomerated powders con-
taining an extensive network of air spaces that can
fill rapidly with water on contact (Písecky 1997). A
number of techniques are used to produce agglomer-
ated powders; these include (1) feeding fines (small
powder particles) back to the atomization zone dur-
ing drying (in straight-through agglomeration pro-
cesses) and (2) wetting dry powder in chambers that
promote sticking together of moistened particles,
which when subsequently dried, produce porous
agglomerated powder particles (rewet processes).
Milk powder contains occluded air (in vacuoles
within individual powder particles) and interstitial
air (entrapped between neighboring powder parti-
cles). The amount of occluded air depends on the
heat treatment applied to the feed (whey protein
denaturation determines foaming properties), the
method of atomization, and outlet air temperature,
while the interstitial air content of a powder depends
on the size, shape, and surface geometry of individ-
ual powder particles. The drying process can be
manipulated (e.g., by using multiple stages or by
returning fines to the atomization zone) to increase
the levels of interstitial and occluded air and instan-
tize the powder (Kelly et al. 2003).
In the case of whole milk powders (WMP), a key
characteristic is the state of the milk fat, whether ful-
ly emulsified or partially in free form (the latter is
desirable for WMP used in chocolate manufacture).
The production of instant WMP requires the use of
an amphiphilic additive such as lecithin, as well as
agglomeration, to overcome the intrinsic hydropho-
bicity of milk fat (Jensen 1975, Sanderson 1978).
Lecithin may be added either during multistage dry-
ing of milk (between the main drying chamber and
the fluidized bed dryer) or separately, in a rewet
process.
During spray-drying of milk, there is relatively
little change in milk proteins, apart from that caused
by preheating and evaporation. The constituent most
affected by the drying stage, per se, is probably lac-
tose, which on the rapid removal of moisture from
the matrix assumes an amorphous glassy state,
which is hygroscopic and can readily absorb mois-
ture if the powder is exposed to a high relative
humidity. This can result in crystallization of lac-
tose, with a concomitant uptake of water, which can
cause caking and plasticization of the powder (Kelly
et al. 2003).
Amorphous lactose is the principal constituent of
both SMP and WMP and forms the continuous
matrix in which proteins, fat globules, and air vac-