Thermal Food Processing

314 Thermal Food Processing: New Technologies and Quality Issues
Several folate fractions have been identified in milk samples with HPLC
analysis, including tetrahydrofolic acid, 5-methyltetrahydrofolic acid (5-MTHFA),
the main form, and 5-formyltetrahydrofolic acid. 67 There is significant seasonal
variation in the total folate concentration of milk, with a peak occurring in summer.
Oxygen is known to induce degradation of 5-methyltetrahydrofolic acid.
Effects of the presence of different oxygen levels on the thermal degradation of
5-MTHFA in the UHT region at 110, 120, 140, and 150°C were investigated in
a buffer model food system. 68 Results showed that in the presence of oxygen, the
overall folate degradation is a second-order reaction; Arrhenius activation energy
values for aerobic and anaerobic degradation were 106 and 62 kJ/mol, respectively.
Results confirmed the importance of degassing milk before heat treatment
to maximize recovery of folate. 68
The concentration of folate-binding protein (FBP), which might have an
impact on folate absorption, is significantly lower in UHT milk and fermented
milk, both of which are processed at temperatures of >90°C, than in pasteurized
milk. 69 Wigertz and coworkers 70 investigated the relation between retention of 5methyltetrahydrofolate
and folate-binding protein (FBP) concentration. Raw and
pasteurized milk samples contained similar amounts of FBP: 211 and 168 nmol/l,
respectively, while UHT-processed milk and yogurt, both processed at high temperatures,
contained only 5.2 and 0.2 nmol/l FBP, respectively. All folates in raw
and pasteurized milk were found to be protein bound, while folates in UHTprocessed
milk and yogurt occurred freely. Raw milk, pasteurized milk, UHT milk,
and yogurt contained 44.8, 41.1, 36.1, and 35.6 µg/l 5-methyltetrahydrofolates,
respectively, after deconjugation. It was concluded that significant losses of 5methyltetrahydrofolates
take place as a result of processing. Results supported the
equimolar ratio of FBP and folates in raw and pasteurized milk.
Heat processing of milk reduces the amount of 5-methyltetrahydrofolate and
the concentration and folate-binding capacity of FBP. 70 What can be the implications
of FBP denaturation on folate bioavailability in milk and to what extent
this denaturation can affect folate retention during processing and storage are
questions that need further investigation. 71
10.2.4.3 Fat-Soluble Vitamin: Beta-Carotene,
Retinol, and Vitamin E
The beta-carotene content of milk varies markedly, depending on the animal
species, feed, and season (e.g., fresh pasture is richer in carotenoids than hay or
silage). A naturally yellowish color of cow’s milk is associated with the presence
of carotenoids. Milk of goat and buffalo does not contain carotenoids, probably
because of a different animal metabolism, and usually dairy products from these
animals are whiter than cow products. The beta-carotene content of milk is
unaffected by UHT processing.
The main form of vitamin A in milk, all-trans retinol, comprises a beta-ionone
ring with a side chain composed of two isoprene units and four double bonds in
the trans configuration. Among retinol cis isomers, 13-cis retinol has the largest