456 Part IV: Milk
Pasteur, generally heating milk to 62–65°C for at
least 30 minutes, followed by rapid cooling to less
than 10°C [now referred to as low temperature long
time (LTLT) pasteurization]. However, high tem-
perature short time (HTST) pasteurization, in which
milk is treated at 72–74°C for 15 seconds in a
continuous-flow plate heat exchanger, gradually be-
came the standard industrial procedure for the heat
treatment of liquid milk and cream; compared with
the LTLT process, the HTST process has the advan-
tages of reduced heat damage and flavor changes,
and increased throughput (Kelly and O’Shea 2002).
Later developments in the thermal processing of
milk led to the introduction in the 1940s of ultra-
high-temperature (UHT) processes, in which milk is
heated to a temperature in the range 135–140°C for
2–5 seconds (Lewis and Heppell 2000). UHT treat-
ments can be applied using a range of heat exchang-
er technologies (e.g., indirect and direct processes),
and essentially result in a sterile product that is typi-
cally shelf stable for at least 6 months at room tem-
perature; eventual deterioration of product quality
generally results from physicochemical rather than
microbiological or enzymatic processes.
Sterilized milk products may also be produced
using in-container retort systems; in fact, such pro-
cesses were developed before the work of Pasteur. In
1809, Nicholas Appert developed an in-container
sterilization process that he applied to the preserva-
tion of a range of food products, including milk. In-
container sterilization is generally used for concen-
trated (condensed) milks; typical conditions involve
heating at 115°C for 10–15 minutes. Although not
sterile, a related class of product is preserved by
adding a high level of sugar to concentrated milk;
the sugar preserves the product through osmotic
action (sweetened condensed milk was patented by
Gail Borden in 1856).
The primary function of thermal processing of
milk is to kill undesirable microorganisms; modern
pasteurization is a very effective means for ensuring
that liquid milk is free of the most heat resistant
pathogenic bacteria likely to be present in raw milk.
The original target species was Mycobacterium tu-
berculosis,but a little later, Coxiella burnettibe-
came the target. In recent years, evidence for the
survival of Listeria monocytogenesand, in particu-
lar, Mycobacterium aviumssp. paratuberculosisin
pasteurized milk is of concern and is the subject of
ongoing research (Grant et al. 2001, Ryser 2002).
Most health risks linked to the consumption of pas-
teurized milk are probably due to postpasteurization
contamination of the product.HEAT-INDUCEDCHANGES INMILKPROTEINSThe caseins are very resistant to temperatures nor-
mally used for processing milk, and in general, un-
concentrated milk is stable to the thermal processes
to which it is exposed, for example, HTST pasteur-
ization and UHT or in-container sterilization. How-
ever, very severe heating (e.g., at 140°C for a pro-
longed period) causes simultaneous dissociation and
aggregation of the micelles and eventually coagula-
tion.
However, the whey proteins are typical globular
proteins and are relatively susceptible to changes on
heating milk. -lactoglobulin (-lg) is susceptible
to thermal denaturation, to an extent dependent on
factors such as pH, protein concentration, and ionic
strength (Sawyer 2003); denaturation exposes a high-
ly reactive sulphydryl (-SH) group. Denatured -lg
can react, via sulphydryl-disulphide interchange
reactions, with other proteins, including -casein,
the micelle-stabilizing protein that is concentrated
at the surface of the casein micelles. Thus, heating
milk at a temperature higher than that used for mini-
mal HTST pasteurization (which itself causes little
denaturation of -lg, although even slightly higher
temperatures will cause progressively increased lev-
els of denaturation) results in the formation of
casein-whey protein complexes in milk, with major
effects on many technologically important proper-
ties of milk.
Such complexes are found either at the surface of
the micelles or in the serum phase, depending on the
pH, which determines the likelihood of heat-induced
dissociation of the -casein–-lg complex from the
casein micelle. Largely as a result of this, the heat
stability of milk (expressed as the number of min-
utes at a particular temperature, for example, 140°C,
before visible coagulation of the milk occurs—the
heat coagulation time, HCT) is highly dependent on
pH, and exhibits large differences over a narrow
range of pH values (O’Connell and Fox 2003).
Most milk samples show a type A heat stability–
pH (heat coagulation time, HCT-pH) profile, with a
pronounced minimum and maximum, typically
around pH 6.7 and 7.0, respectively, with decreasing
stability on the acidic side of the maximum and in-