BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come
470 Part 4: MilkM. tuberculosis. A negative result (residual activity below a set
maximum value) in a phosphatase test (assay) is regarded as
an indication that milk has been pasteurised correctly (Wilbey
1996). Detailed kinetic studies on the thermal inactivation of
alkaline phosphatase under conditions similar to pasteurisation
have been published (McKellar et al. 1994, Lu et al. 2001).
To test for over-pasteurisation (excessive heating) of milk,
a more heat-stable enzyme, for example lactoperoxidase, has
been used as the indicator enzyme (Storch test); lactoperoxidase
may also be used as an index of the efficacy of pasteurisation of
cream, which must be heated more severely than milk to ensure
the killing of target bacteria, due to the protective effect of the
fat therein.
Recently, other enzymes have been studied as indicators
(sometimes called Time Temperature Integrators, TTIs) of heat
treatments (particularly at temperatures above 72◦C) that may
be applied to milk; these include catalase, lipoprotein lipase,
acid phosphatase,N-acetyl-β-glucosaminidase andγ-glutamyl-
transferase (McKellar et al. 1996, Wilbey 1996). The latter has
been a focus of particular attention due to its potential role as an
indicator of the efficacy of heat treatments in the range 70–80◦C
for 16 seconds (McKellar et al. 1996).
The alkaline milk proteinase, plasmin, is resistant to pasteuri-
sation; indeed, its activity may increase during storage of pas-
teurised milk, due to inactivation of inhibitors of plasminogen
activators (Richardson 1983). Treatment under UHT conditions
greatly reduces plasmin activity (Enright et al. 1999); there is
some evidence that the low level of plasmin activity in UHT
milk contributes to the destabilisation of the proteins, leading
to defects such as age gelation, although this is not universally
accepted (for review see Datta and Deeth 2001).
Since most indigenous milk enzymes are inactivated in UHT-
sterilised milk products and in all in-container sterilised prod-
ucts, enzymes are not suitable indices of adequate processing
and chemical indices are more usually used (e.g., the concentra-
tion of lactulose or extent of denaturation ofβ-lg). The use of a
range of indicators for different heat treatments applied to milk
was reviewed by Claeys et al. (2002).
The thermal inactivation of bacterial enzymes in milk is also
of considerable significance. For example, psychrotrophic bac-
teria of the genusPseudomonasproduce heat-stable lipases and
proteases during growth in refrigerated milk; while pasteurisa-
tion readily kills the bacteria, the enzymes survive such treat-
ment and may contribute to the deterioration of dairy products
made therefrom (Stepaniak and Sørhaug 1995).Pseudomonas
enzymes partially survive in UHT milk and may be involved in
its age gelation (Datta and Deeth 2001).CHANGES ON EVAPORATION AND
DRYING OF MILKDehydration, either partial (e.g., concentration to∼40–50% total
solids, TS) or (almost) total (e.g., spray-drying to∼96–97% TS)
is a common method for the preservation of milk. Reviews of
the technology of drying of milk and its effects on milk quality
include Car ́ıc and Kalab (1987), Pisecky (1997), Early (1998) ́
and Kelly et al. (2003).Concentration of MilkThe most common technology for concentrating milk is ther-
mal evaporation in a multi-effect (stage) falling-film evaporator,
which is used either as a prelude to spray-drying, to increase
the efficiency of the latter, or to produce a range of concentrated
dairy products (e.g., sterilised concentrated milk, sweetened con-
densed milk).
In many processes, a key stage in milk evaporation is pre-
heating the feed. The minimum requirements of this step are to
pasteurise the milk (e.g., skim milk, whole milk, filled milk) to
ensure food safety and to bring its temperature to the boiling
point in the first effect of the evaporator, for thermal efficiency.
However, the functionality of the final product can be deter-
mined and modified by this processing step; preheating con-
ditions range from conventional pasteurisation to 90◦C for 10
minutes or 120◦C for 2 minutes. The most significant result of
such heating is denaturation ofβ-lg, to a degree depending on
the severity of heating. During subsequent evaporation and, if
applied, spray-drying, relatively little denaturation of the whey
proteins occurs, as the temperature of the milk generally does
not exceed 70◦C (Singh and Creamer 1991). However, further
association of whey proteins with the casein micelles can occur
during evaporation, probably because the decrease in pH reduces
protein charge, facilitating association reactions (Oldfield 1998).
Evaporation also increases the concentrations of lactose and
salts in milk and induces a partially reversible transfer of soluble
calcium phosphate to the colloidal form, with a concomitant
decrease in pH (Le Graet and Brule 1982, Nieuwenhuijse et al.
1988, Oldfield 1998). The extent of the transfer of phosphate
to the colloidal phase, which is greater than that of calcium,
depends on the temperature of preheating. The concentrations of
soluble calcium and phosphate in reconstituted milk powder are
generally 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 difficult to atomise and thereby affecting the
properties of the final powder.
Alternative technologies for concentrating milk are available,
the most significant of which is probably membrane separation
using reverse osmosis (RO). RO can achieve only a relatively
low level of total solids (<20%) and has a relatively low through-
put, but is far less thermally severe than evaporation (Pisecky ́
1997). Other membrane techniques may be used to concentrate
or fractionate milk, such as nanofiltration (NF), which essen-
tially removes only water, and ultrafiltration (UF), which allows
standardisation of the composition (e.g., protein and lactose con-
tents) of the final powder (Mistry and Pulgar 1996, Horton 1997).
The most significant area of application for membrane process-
ing in the dairy industry to date is probably in whey processing,
as will be discussed below.Spray-Drying of MilkToday, milk powders are produced in large, highly efficient
spray-dryers; the choice of dryer design (e.g., single versus