Effect of milk processing on the concentration of folate-binding protein (FBP), folate-binding capacity and retention of 5-methyltetrahydrofolate

Abstract

The main objective of this study was to investigate the effects of pasteurisation, UHT processing and fermentation on the concentration of folate-binding proteins (FBP) and their folate binding capacity in comparison with the retention of the most predominant folate from, 5-CH3THF. The amount of folate-binding protein (FBP) was analysed using enzyme-linked immunosorbent assay (ELISA). Unprocessed milk and pasteurised milk were found to contain similar amounts, 211 and 168 nmol/l, of FBP, respectively. UHT-processed milk and Yoghurt naturelle®, both processed at temperatures above 90°C, contained only 5.2 and 0.2 nmol/l FBP, respectively. As an indication of the protein-binding capacity, free and protein-bound folates were analysed after charcoal treatment using the radio-protein binding assay method (RPBA). These results indicated that all folates in unprocessed milk and pasteurised milk were protein-bound, while folates in UHT-processed milk and Yoghurt naturelle® occurred freely which is supported by our findings on FBP. High-performance liquid chromatography analysis indicated that unprocessed milk, pasteurised milk, UHT-processed milk and Yoghurt naturelle® contained 44.8 ± 2.1 (n = 10), 41.1 ± 0.9 (n = 10), 36.1 ± 1.8 (n = 10) and 35.6 ± 9.1 μg/l (n = 10) 5-methyltetrahydrofolates (5-CH3THF), respectively, after deconjugation. Corresponding values for 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) and 54.0 ± 8.2 μg/l (n = 10), respectively. Hence, both methods indicated significant (P < 0.05) losses of 5-CH3THF as a result of pasteurisation, UHT processing and fermentation, compared with unprocessed milk. In spite of apparent discrepancies in folate concentrations obtained using the two different methods, these results support the equimolar ratio of FBP and folates in unprocessed and pasteurised milk when data 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 and the folate-binding capacity of FBP, which may have implications on the bioavailability of milk folates.