Milk-and-Dairy-Products-in-Human-Nutrition-FAO

Chapter 3 – Milk and dairy product composition 73
Hedegaard et al., 2006; Smet et al., 2009; Hiller and Lorenzen, 2010), where mainly
lysine residues in casein react with lactose and other sugars (Gliguem and Birlouez-
Aragon, 2005). Maillard reactions lead to browning of milk, associated with the formation
of brown melanoidins. No Maillard reactions are expected to occur during
pasteurization (Andersson and Öste, 1994). Even UHT treatment causes only very
small losses of lysine, ranging from 0–5 percent (Andersson and Öste, 1994). One
study involved powdered infant formulas sterilized by autoclaving for 5 minutes at
105 °C and 5 600 kg/m 2 of pressure after being reconstituted with hot water (80 °C)
(Yeung et al., 2006). The authors reported a 20 percent reduction in total protein
after autoclaving compared with conventional preparation. Concentrations of total
free amino acids were significantly lower (P = 0.01) and individual amino acids
were lower in autoclaved infant formulas. In particular, losses in valine (72 percent),
glutamine (60 percent) and lysine (40 percent) were noted. The concentration of
ammonia was significantly higher (P = 0.0003) after autoclaving, and may reflect
degradation of protein and amino acids (Yeung et al., 2006).
Reactions of lactose
Heat treatment of milk results in isomerization of part of the lactose to lactulose
(Zhang et al., 2010). Lactulose is reported to stimulate the growth of bifidobacteria
(Zhang et al., 2010). The amount of lactulose formed depends on the extent of
heat-treatment, with the lactulose content sometimes being used as a measure of
the extent of heat-treatment (Montilla, Moreno and Olano, 2005). Lactulose contents
in commercial samples purchased from local stores ranged between 1.3 and
3.2 mg/100 g of pasteurized milk, 9.5 and 43.7 mg/100 g of UHT milk and 62 and
71 mg/100 g of sterilized milk (Montilla, Moreno and Olano, 2005). Reconstituted
milk powder was found to contain only 2.4–4.9 mg of lactulose /100 g, the low values
reflecting the milder processing conditions to which milk powder is subjected,
and the slower isomerization in the solid state.
Other effects
Heat treatment would also be expected to cause isomerization of certain FAs.
Herzallah, Humeid and Al-Ismail (2005) found that pasteurization (63 °C for
30 minutes) increased the trans-isomer content of milk by 31 percent but that the
higher temperature (but shorter duration) involved in UHT treatment did not result
in significant (P < 0.05) increases in trans-isomer content. Siddique et al. (2010)
found that different UHT processing temperatures and storage temperatures had
no influence on ash content.
Conclusions
It is clear that although heat treatment is essential to ensure total microbiological
safety, it also reduces various nutrient contents, and some of these losses are compounded
by storage. A study of semi-skimmed cow milk and fortified milk subjected
to UHT (135 °C, 3–4 seconds) treatment or in-bottle sterilization (110 °C,
20 minutes), stored in different packaging for various storage periods (3 days, 1,
2 and 4 months) concluded that “a radical modification of the milk composition
occurs during storage, which aggravates the changes firstly induced by the sterilization
heat treatment. Optimal quality would require UHT (treatment), packaging in