Chemistry Reference
In-Depth Information
infant formulae. Added iron and ascorbic acid appear to have a particularly
destructive effect on tryptophan in dairy systems. It is hypothesized that the
main pathway for tryptophan destruction under mild heating conditions is
not direct glycation of the indole N but a consequence of oxidation due either
to the activity of the Fe-ascorbate system or the pro-oxidant activity of
Maillard reaction products following ascorbylation of lysine
"
-NH
2
groups
(Puscasu and Birlouez-Aragon, 2002; Lecl`re et al., 2002). The presence of
oxygen is particularly detrimental under such conditions (Gliguem and
Birlouez-Aragon, 2005). The high levels of blocked lysine in some fortified
milks clearly limit their usefulness as dietary supplements (Evangelisti et al.,
1999; Rada-Mendoza et al., 2005).
Similarly, the higher extent of the Maillard reaction in liquid-sterilized
infant formulae, compared with powdered and UHT counterparts, is well
described (Guerra-Hernandez et al., 2002; Puig et al., 2003). The considerably
higher lysine losses and furosine levels in follow-on formulae may be related
to the processing method but are also related likely to the impact of added
nutrients, such as maltodextrin or iron, on the Maillard reaction (Ferrer et al.,
2003; Pereyra Gonzales et al., 2003).
Maillard reactions may limit the bioavailability of undamaged amino
acids by inhibiting digestive enzymes and/or by inhibiting amino acid trans-
port at the intestinal level. In addition, there is evidence that severely browned
proteins are less digestible than native proteins (Dalsgaard et al., 2007).
Gothwal and Bhavadasan (1991) reported that although slight browning of
milk was accompanied by an increase in the rate of trypsin proteolysis,
severely browned milk samples are less readily hydrolysed by trypsin. A
stored dried casein-glucose mixture showed a loss of digestibility of
14-88%, depending on conditions (Culver and Swaisgood, 1989). The avail-
able chemical evidence on the effect of Maillard reactions on the nutritional
value of food proteins is supported by clinical evidence that diets rich in
Maillard reaction products have a negative impact on protein digestibility
(Seiquer et al., 2006). Moreover, protein-bound lactose is resistant to hydro-
lysis by
-galactosidase (Morgan et al., 1999a)
Maillard reactions may also contribute to the destruction of vitamins in
food systems. Vitamin C can participate directly in the reaction and it has
been proposed that vitamins B1, B6, B12 and pantothenic acid may react with
pre-melanoidins (Hurrell, 1990). Although vitamin B1 contains an amino
group and, therefore, could participate in Maillard reactions, it is unclear to
what extent this mechanism contributes to its destruction in food systems.
Folic acid is now added in the free form to many manufactured foods and
through its free amino group is believed to participate in Maillard reactions.
There is evidence that dietary Maillard reaction products may disrupt
mineral homoeostasis in vivo, reducing the bioavailability of some minerals.
