Investigation into the cooked flavour in heat-treated milk
The effect of heat treatment of milk on whey proteins,
sulphydryl (reactive and total), disulphide groups, half
cystine, volatile sulphur compounds and sensory properties
was studied in order to relate changes in the flavour of
milks to changes in chemical composition.
The degree of heat denaturation of whey proteins increased
generally in the following order: HTST treatment, direct
UHT treatment, indirect UHT treatment, and the severest
was laboratory heat treatment where milks were held at
temperatures below 1000 C for several minutes. At the same
heating temperature and holding time indirect UHT treatment
was found to result in more whey protein denaturation
compared with direct UHT treatment.
The sensitivity of whey proteins to denaturation was
confirmed to be In decreasing order: serum albumin,
S-lactoglobulin B, S-lactoglobulin A and a-lactalbumin.
Laboratory heat treatment resulted in a peak of reactive
sulphydryl groups, followed by a decrease, as the heating
time is prolonged. HTST treatment and UHT treatments
(direct and indirect) increased reactive sulphydryl groups
from nearly zero in raw milk to values dependent on the
heating temperature. Generally, heat treatment caused a
decrease in concentration of total sulphydryl groups,
disulphide groups and half cystine.
Hydrogen sulphide and dimethyl sulphide were identified in
heat-treated milks. Dimethyl sulphide was found to be
present in raw milk as well as in heat treated milks. The
increase in concentration of each of the volatile sulphur
compounds was found to parallel the intensity of heating
to the extent that they appeared to be linearly related.
Volatile sulphur compounds decreased rapidly during storage
at 22°C compared with~slower decrease during storage at 4oC.
The panel was unable to detect differences between direct
UHT milk and pasteurised milk. On the other hand, most
panelists could detect differences between indirect UHT
milk and pasteurised milk. The panelists were not able to
distinguish between milks processed, respectively, at 77oc,
830 C and 890 C for 20 seconds using HTST treatment. A
significant number of correct judgments w~s observed Wl+~
940 C milk. The statements describing the difference were:
heated taste, strong flavour, sulphurous, burnt flavour
and very pronounced cooked flavour. All these statements
are related to the cooked flavour.
A good linear correlation was clearly observed between
reactive sulphydryl groups and the whey proteins very
sensitive to heat treatment, eg S-lactoglobulin A,
S-lactoglobulin B and serum albumin. There was also a
good linear correlation between reactive sulphydryl groups
and mean whey protein denaturation.
It was found that hydrogen sulphide concentration increased
with increasing concentration of reactive sulphydryl
groups, and with decreasing concentration of total
sulphydryl groups, disulphide groups and half cystine respectively.
There was also a good correlation between hydrogen
sulphide and S-lactoglobulin A, S-lactoglobulin B and serum
The degree of cooked flavour, the degree of whey protein
denaturation, the concentration of reactive sulphydryl groups,
the concentration of half cystine and the concentration of
hydrogen sulphide show good correlation and similar
dependence upon the severity of heat treatment.
The data also suggest that the cooked flavour threshold
a) 59 per cent denaturation of S-lactoglobulin (A + B);
b) reactive sulphydryl group concentration of 0.037 mmole
per litre of milk;
c) half-cystine concentration of 1.082 mmole per litre
d) hydrogen sulphide concentration of 3.4~g per litre