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are outside the scope of this chapter and are covered in the reviews of Fox and
Morrissey (1977), Singh and Creamer (1992), O'Connell and Fox (2003) and
Singh (2004). However, milks showing Type A characteristics can be con-
verted into Type B profiles and vice versa. For a list of methods and a
discussion of these observations, see Horne and Muir (1990). Interestingly,
several of these methods involve manipulating the levels of milk salts, parti-
cularly Ca and phosphate. For many years, it was considered that differences
in the heat stability of milk were due to variations in the composition of milk
salts and this led Sommer and Hart (1919) to propose the salt balance theory
referred to above in our discussion of ethanol stability. O'Connell and Fox
(2003) have suggested that subsequent attempts to correlate heat stability
with natural variations in the composition of milk salts failed because the
original studies were based on deliberate additions of salts to milk at levels
outside natural variability. This overlooks the fact that the experiments of
Sommer and co-workers employed a different protocol for the heat stability
assay, namely a measurement of the heat coagulation temperature, the tem-
perature at which milk instantaneously coagulates (i.e. effectively coagulates
within a short time,
<
2 min). Because it is a measure of instantaneous
coagulation, it is unaffected by changes that occur on prolonged heating.
Instead, the response to changing pH, as observed by Miller and Sommer
(1940), is remarkably similar to the sigmoidal ethanol stability/pH profile.
Moreover, the addition of Ca shifts this profile to more alkaline values while
the addition of phosphate has the opposite effect of producing an acidic shift,
just like the response of ethanol stability profiles. Horne and Muir (1990)
suggested that such behaviour indicated that heat-induced coagulation as
measured by this assay might follow a similar, if not identical, pathway to
alcohol-induced coagulation as described above, involving the precipitation
of Ca phosphate and a decrease in Ca activity with increasing pH. Such a
scenario also ties in with the observation that the amount of free Ca
2+
has
been associated by various authors with the heat stability of milk, powdered
milk and recombined milk (Augustin and Clarke, 1990; Singh and Creamer,
1992; Williams et al., 2005). Addition of Ca to milk results in a decrease in
heat stability due to the increase in free [Ca
2+
] (Philippe et al., 2004). Seasonal
changes in milk salts (soluble Ca) have been correlated with the heat stability
of milk (Kelly et al., 1982). Salts, such as orthophosphates, are often added to
milk concentrates (or ultra-high-temperature sterilized milks) during proces-
sing to improve heat stability. Orthophosphates reduce the Ca
2+
activity,
which is mainly responsible for the improved heat stability (Augustin and
Clarke, 1990). O'Connell and Fox (2001) suggested that heat-induced pre-
cipitation of CCP is involved in the thermal coagulation of milk and that the
specific effect of
-lactoglobulin at the pH of maximum stability may be
related to its ability to chelate Ca.
