Thermal Food Processing

320 Thermal Food Processing: New Technologies and Quality Issues
milk fat concentration. 107 The HMF level in commercial UHT milk (stored below
50°C) is also related to temperature and time of storage, and increases with higher
temperature. 112
10.3.3 FLUORESCENT PRODUCTS: ADVANCED MAILLARD
REACTION INDICES
According to Van Boekel, 91 several Maillard products are fluorescent. Fluorescence
is typical of intermediate products; they show an initial phase of fluorescence
development followed by a decrease. On the basis of this concept, an
alternative method to evaluate the severity of milk heat treatment is based on
the fluorimetric measurement of denaturated proteins (290-nm excitation and
340-nm emission) and of the fluorescent advanced Maillard reaction products
(330-nm excitation and 420-nm emission). It has been suggested that fluorimetric
measurement of tryptophan can be considered a marker of protein
denaturation during heat treatment, because it is well correlated with betalactoglobulin,
while fluorescent Maillard products can reflect the loss of nutritional
quality during processing. 113–115 The fluorimetric measure of tryptophan
is more helpful for milk subjected to mild heat treatment (pasteurization), while
the fluorescent Maillard products (pyrrole and imidazole derivates) are more
significant for indirectly heated UHT milk (Figure 10.2). Using the data represented
in Figure 10.2, we find a correlation of r = 0.94 between furosine
content and fluorescence of Maillard advanced products in the direct and indirect
UHT milk samples analyzed.
TRP - Fast AMP (fluorescence unit)
60
40
20
0
TRP
Fast AMP
Furosine
Pasteurized High pasteurized Direct UHT Indirect UHT
FIGURE 10.2 Fluorescence of advanced Maillard products (fast AMP) and tryptophan
(TRP) and furosine content in different heat-treated milk samples. (Modified from Birlouez-
Aragon, I. et al., Int. Dairy J., 8, 771–777, 1998.)
200
150
100
50
0
Furosine (mg/100g protein)