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Abstract:

The name of folates comprises the water soluble vitamins, which are structurally related and which have similar biological activity to folic acid. The important factors contributing to the folate stability during processing are temperature, pH, light, oxygen, etc. In food, endogenous folates could be destroyed during storage and processing. In literature, combined high pressure-temperature processing of foods has been introduced to allow microorganism inactivation while better maintaining food quality characteristics. Hitherto, study on possible effects of high pressure on folates is limited and qualitative in nature; especially on kinetics of folate degradation. Vitamin C plays an important role in the protection of folate oxidation due to its act as an O2-scavenger, or a reducer of the quinonoid dihydrofolate, a primary product of folate oxidation. Hereto, the main purpose of this investigation was to study the effect of intrinsic (pH, oxygen, presence of ascorbic acid) and extrinsic (temperature, pressure) factors on folate stability on a kinetic basic. In the first phase of the study, effects of pH on thermo and pressure stability of folates were studied in different buffer solution (acetate buffer pH 3.4, 5.0; phosphate buffer pH 7.0 and borate buffer pH 9.2). Thermal degradation of folates followed pseudo first order kinetics. At all pH’s studied, it was found that (i) folic acid and 5-formyltetrahydrofolic acid were more thermo and pressure stable compared to tetrahydrofolic acid and 5-methyltetrahydrofolic acid; (ii) all folates studied had the highest stability at neutral pH (pH 7.0) or alkaline pH (pH 9.2) and (iii) at all pH values, only slightly or no reduction in folic acid was found e.g. after pressure treatment at 800 MPa and 60°C for 7 h. In the second phase of the study, thermal and combined high pressure temperature degradation kinetics of 5-methyltetrahydrofolic acid and 5-formyltetrahydrofolic acid were studied, respectively in phosphate buffer (0.1 M, pH 7.0) and acetate buffer (0.2 M, pH 5.0). In the pressure/temperature combinations studied, the degradation rate constants of 5-methyltetrahydrofolic acid and 5-formyltetrahydrofolic acid were enhanced by increasing pressure at constant temperature. A remarkable synergistic effect of pressure and temperature on 5-methyltetrahydrofolic acid and 5-formyltetrahydrofolic acid degradation occurred at temperatures above 40°C. A kinetic model describing combined pressure and temperature effect on the 5-methyltetrahydrofolic acid and 5-formyltetrahydrofolic degradation rate constants was constructed. In the third phase of the study, thermal and combined high pressure-thermal degradation kinetics of ascorbic acid in acetate buffer (0.2 M, pH 5.0) and phosphate buffer (0.1 M, pH 7.0) were studied at different molar ratio between oxygen and ascorbic acid. Thermal degradation of ascorbic acid could be described by a biphasic model, indicating degradation under aerobic and anaerobic conditions. It was observed that the ascorbic acid degradation had already occurred during pressure build up probably due to oxidation (aerobic degradation). When the oxygen was totally used up, the anaerobic degradation dominated and took place much more slowly than aerobic degradation. Thus, it seemed that ascorbic acid was stable at pressures up to 700 MPa combined with temperatures up to 70°C (up to 100 min of treatment after pressure build up). However, ascorbic acid was degraded at extreme pressure temperature combinations such as temperatures above 70°C combined with pressures equal to or above 600 MPa. The molar ratio between ascorbic acid and oxygen is an important parameter to estimate the proportion of ascorbic acid aerobic degradation and to determine the concentration of ascorbic acid needed to protect folates during processing. In the fourth phase of the study, the effect of antioxidants (e.g. b-mercaptoethanol, ascorbic acid) on 5-formyltetrahydrofolic acid stability was studied during thermal and high pressure thermal processing. The stability of 5-formyltetrahydrofolic acid increased in presence of antioxidants. Based on this study, it was observed that the degradation of 5-formyltetrahydrofolic acid during treatments was not only due to the oxidation but also the conversion to 5,10- methenyltetrahydrofolic acid. Moreover, ascorbic acid did not only increase the stability of 5-formyltetrahydrofolic acid but also enhanced the conversion of 5-formyltetrahydrofolic acid to 5,10-methenyltetrahydrofolic acid.