The availability of lysine, methionine and tryptophan in condensed milk and milk powder. In vitro digestion studies

Abstract

1. 1. Destruction and inactivation of α-amino groups, tryptophan, tyrosine, methionine and lysine were determined in fresh and boiled milk and in preserved milk manufactured according to four widely used processes: i.e., spray-dried and roller-dried milk powders, sweetened condensed, and evaporated milks. 2. 2. No destruction of tryptophan, tyrosine, or methionine occurred in any milk. 3. 3. A 6% destruction of α-amino groups was noted in a slightly scorched roller-powder milk (roller-powder milk B). 4. 4. No destruction of lysine was apparent in boiled milk, whereas a slight destruction took place in spray-dried milk (3.6 %) and sweetened condensed milk (4.8%). Destruction was appreciable in evaporated milk (8.4%) and average roller-dried milk A (13%), while in roller-powder B it was considerable (26.6%). 5. 5. Inactivation of the same amino acids was measured by an in vitro digestibility test, which is described in detail. 6. 6. No inactivation of tryptophan and tyrosine was found in any milk. 7. 7. Thirteen per cent of the α-amino groups was inactivated in roller-powder B. 8. 8. Methionine was slightly inactivated in roller-dried milks only. 9. 9. Lysine was inactivated in evaporated milk (11.2%), roller-milk powder A (20%), and B (45.8%). 10. 10. There is a close correlation between destruction and inactivation of lysine ( r = 0.992). Both are attributed to a sugar-amine condensation. 11. 11. The sum of destruction and inactivation gives the total deterioration of lysine. 12. 12. Availability of lysine was computed by deducting total deterioration from 100 (availability in fresh milk taken as 100). 13. 13. Lysine availability ( y) in industrial milks can be estimated from lysine destruction ( x) using the regression equation: y = 109.58 − 3.099 x.