EFFECT OF MILK PROCESSING ON THE CONCENTRATION OF FOLATE-BINDING PROTEIN (FBP), FOLATE-BINDING CAPACITY AND RETENTION OF 5-METHYLTETRAHYDROFOLATE

The main objective of this study was to investigate the effects of pas teurisation, UHT processing and fermentation on the concentration of f olate-binding proteins (FBP) and their folate binding capacity in comp arison with the retention of the most predominant folate from, 5-CH3TH F. The amount of folate-binding protein (FBP) was analysed using enzym e-linked immunosorbent assay (ELISA). Unprocessed milk and pasteurised milk were found to contain similar amounts, 211 and 168 nmol/l, of FB P, respectively, UHT-processed milk and Yoghurt naturelle(R), both pro cessed at temperatures above 90 degrees C, contained only 5.2 and 0.2 nmol/l FBP, respectively. As an indication of the protein-binding capa city, free and protein-bound folates were analysed after charcoal trea tment using the radio-protein binding assay method (RPBA). These resul ts indicated that all folates in unprocessed milk and pasteurised milk were protein-bound, while folates in UHT-processed milk and Yoghurt n aturelle(R) occurred freely which is supported by our findings on FBP. High-performance Liquid chromatography analysis indicated that unproc essed milk, pasteurised milk, UHT-processed milk and Yoghurt naturelle (R) contained 44.8 +/- 2.1 (n = 10), 41.1 +/- 0.9 (n = 10), 36.1 +/- 1 .8 (n = 10) and 35.6 +/- 9.1 mu g/l (n = 10) 5-methyltetrahydrofolates (5-CH3THF), respectively, after deconjugation. Corresponding values f or total milk folates analysed using radio-protein binding assay were 80.4 +/- 0.9 (n = 10), 64.2 +/- 2.7 (n = 10), 48.2 +/- 1.8 (n = 10) an d 54.0 +/- 8.2 mu g/l (n = 10), respectively. Hence, both methods indi cated significant (P < 0.05) losses of 5-CH,THF as a result of pasteur isation, UHT processing and fermentation, compared with unprocessed mi lk. In spite of apparent discrepancies in folate concentrations obtain ed using the two different methods, these results support the equimola r ratio of FBP and folates in unprocessed and pasteurised milk when da ta on 5-CH3THF, obtained using HPLC were corrected for differences in recovery. Thus, heat processing of milk not only reduced the amount of 5-CH3 THF significantly, but also changed the concentration of FBP an d the folate-binding capacity of FBP, which may have implications on t he bioavailability of milk folates.