Chemistry Reference
In-Depth Information
lactose-free milk, the decrease in pH on heating is dramatically less. pH also
decreases during the storage of UHT milk; the extent of the change is
proportional to lactose content and to the extent of browning and is higher
at elevated temperatures (Venkatachalam et al., 1993). Although formic acid
is derived from degradation of sugar moieties, its formation was increased in
heated milk at higher casein concentrations, suggesting the involvement of a
Maillard reaction (Berg, 1993). Berg (1993) concluded that from a quantita-
tive point of view, direct degradation of lactose is a more important source of
heat-induced acidity in milk than the Maillard reaction.
The degradation of lactulose also results in the formation of several C5
compounds. In addition to the formation of furfural and furfuryl alcohol, as
discussed earlier, the formation of 3-deoxypentosulose (Troyano et al.,
1992a) and deoxyribose (Berg, 1993) has been reported.
The concentration of lactulose in UHT milks has been reported to range
from 2 to 25 mg dl
-1
while the concentration may exceed 75 mg dl
-1
in
sterilized milks (Olano and Calvo, 1989). Interestingly, Klostermeyer and
Geier (1983) reported that the lactulose content of UHT milk increased
during a production cycle, which they attributed to the build-up of alkaline
scale on the heating surfaces. Their observation may offer a partial explana-
tion for the higher lactulose concentration in indirectly heated UHT milks
than in those subjected to direct heating. In addition, it is likely that the
greater severity of heating during indirect UHT processing contributes to a
higher lactulose content (Elliott et al., 2005). Based on the differences between
directly and indirectly heat-treated UHT milks, Andrews (1984) suggested
that lactulose concentrations could be used to classify such milks according to
the processing method.
In UHT milks, the concentration of galactose has been reported to
range from 9 to 12 mg dl
-1
and the concentration exceeds 16 mg dl
-1
in
in-container-sterilized milks, although it is unclear to what extent non-
enzymatic degradation of lactose may contribute to such levels. Berg (1993)
reported that the amount of free galactose formed in milk at 1408C was more
than 60% that of the lactulose formed. Epilactose concentrations up to 5 mg
dl
-1
have been reported for in-container-sterilized milk with values of 1-3 mg
dl
-1
for UHT milks. Tagatose formation appears to require relatively severe
heating conditions, such as in-container sterilization (Troyano et al., 1992b,
1994). However, its formation in skim milk powder during storage has been
reported; no tagatose was detectable in fresh spray-dried milk powder
(Troyano et al., 1994).
Calvo and Olano (1989) reported that the presence of proteins in milk
systems reduced the amount of lactulose formed whereas it increased the
formation of epilactose and galactose. Furthermore, these authors showed
that the galactose did not originate from the degradation of lactose-protein
