Effect of iron and lactose supplementation of milk on the Maillard reaction and tryptophan content

New liquid UHT milks supplemented with iron (0.9-4 mg/100 ml), vitamin C (1-7 mg/100 ml), lactose (2-4 g/100 ml) and linoleic acid (200-400 mg/l 100 ml), named growth milks, have recently become available to satisfy the specific nutritional needs of children aged 1-3 years. But the iron-vitamin C mixture could activate the lactose-induced Maillard reaction and tryptophan (Trp) oxidation in proteins. We have therefore examined the Amadori product and Trp concentrations of these milks. Forty-two commercial growth milks from five firms were analysed for the Maillard reaction and the soluble protein Trp content and compared with 64 UHT milks. The furosine concentration of total proteins was two to four times higher in 'growth' milks than in standard UHT milks, indicating a proportional loss of available lysine. The Trp fluorescence of undenatured proteins soluble at pH 4.6 was almost three times lower in 'growth' than in standard milks and Trp concentration 36% lower suggesting destruction of this oxidation-sensitive amino-acid. The mechanism of Trp destruction remains to be elucidated, and the roles of iron and Amadori products determined.