BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come
468 Part 4: Milkeffective 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 wasMycobacterium tuberculo-
sisbut, a little later,Coxiella burnettibecame the target. In recent
years, evidence for the survival ofListeria monocytogenesand,
in particular,Mycobacterium aviumsubsp.paratuberculosisin
pasteurised milk is of concern and the subject of ongoing re-
search (Grant et al. 2001, Ryser 2002). Most health risks linked
to the consumption of pasteurised milk are probably due to post-
pasteurisation contamination of the product.Heat-Induced Changes in Milk ProteinsThe caseins are very resistant to temperatures normally used for
processing milk and, in general, unconcentrated milk is stable to
the thermal processes to which it is exposed, for example HTST
pasteurisation, UHT or in-container sterilisation. However, very
severe heating (e.g., at 140◦C for a prolonged period) causes
simultaneous dissociation and aggregation of the micelles and
eventually coagulation.
However, the whey proteins are typical globular proteins
and are relatively susceptible to changes on heating milk.
β-Lactoglobulin (β-lg) is susceptible to thermal denaturation,
to an extent dependant on factors such as pH, protein concentra-
tion and ionic strength (Sawyer 2003); denaturation exposes a
highly reactive sulphydryl (-SH) group. Denaturedβ-lg can re-
act,viasulphydryl–disulphide interchange reactions, with other
proteins includingκ-casein, the micelle-stabilising protein that
is concentrated at the surface of the casein micelles. Thus, heat-
ing milk at a temperature higher than that used for minimal
HTST pasteurisation (which itself causes only low levels of de-
naturation ofβ-lg, although even slightly higher temperatures
will cause progressively increased levels of denaturation) re-
sults in the formation of casein/whey protein complexes in milk,
with major effects on many technologically important properties
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 theκ-casein/β-lg
complex from the casein micelles. Largely as a result of this,
the heat stability of milk (expressed as the number of minutes
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, with large differences over a narrow
range of pH values (O’Connell and Fox 2003).
Most milk samples show a type A heat stability–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 increasing stability on the alkaline
side of the minimum. The rare type B HCT–pH profile has no
minimum, and heat stability increases progressively throughout
the pH range 6.4–7.4.
In addition to pH, a number of factors affect the heat stability
of milk (O’Connell and Fox 2002), including: Reduction in the concentration of Ca 2 +or Mg 2 +increases
stability in pH range 6.5–7.5 Lactose hydrolysis increases heat stability throughout the
pH range
Addition ofκ-casein eliminates the minimum in the type A
profile
Addition ofβ-lg to a type B milk converts it to a type A
profile
Addition of phosphates increases heat stability
Reducing agents reduce heat stability and convert a type A
to a type B profile
Alcohols and sulphydryl-blocking agents reduce the heat
stability of milkConcentrated (e.g., evaporated) milk is much less thermally
stable than unconcentrated milk and its HCT–pH profile, nor-
mally assayed at 120◦C, is quite different (O’Connell and Fox
2003). It shows a maximum at approximately pH 6.4, with de-
creasing stability at higher and lower pH values (Fig. 25.1).
Severe heating has a number of effects on the casein
molecules, including dephosphorylation, deamidation of glu-
tamine and asparagine residues, cleavage and formation of co-
valent cross-links; these changes may result in protein–protein
interactions that contribute to thermal instability.
The heat stability of milk of different species differ, primarily
due to differences in protein and mineral content; for example
equine, camel, buffalo and caprine milk are less heat stable than
bovine milk (O’Connell and Fox 2003).Stability of UHT Milk on StorageUHT milk is stable to long-term storage at ambient temperatures
if microbiological sterility has been achieved by the thermal
process and maintained by aseptic packaging in hermetically
sealed containers. The shelf life of UHT milk is often limited bypHHeat coagulation timeFigure 25.1.Heat coagulation time (HCT)–pH profiles of typical
type A bovine milk (____), type B or serum protein-free milk
(____), as determined at 140◦C, or concentrated milk (......), as
determined at 120◦C.