Testing of folate conjugase from chicken pancreas vs. commercial enzyme and studying the effect of cooking on folate retention in Thai foods

Abstract

Crude enzyme from chicken pancreas as a source of folate conjugase was prepared in a lyophilised form. Homogeneity, stability and activities were checked against a commercial enzyme. Subsequently, the prepared crude enzyme was used to investigate the process of folate extraction in various food matrices and study the effect of cooking on folate retention in several Thai foods. The lyophilised enzyme was homogeneous and contained 4 μg endogenous folate per g of prepared lyophilised crude enzyme. The stability of the lyophilised enzyme and the diluted enzyme, kept at 4 °C, was at least 12 months and 3 days, respectively. The activity of the prepared lyophilised folate conjugase, used at the level of 20 mg/g sample, was evaluated by a microbiological assay of total folate in brown rice, egg, whole milk powder, soybeans and asparagus. It showed comparable results to that of the commercial enzyme. Single-enzyme treatment (folate conjugase alone) was sufficient for the determination of folate content in brown rice, soybeans and asparagus. However, for egg and whole milk powder, it was necessary to apply tri-enzyme treatment as this showed a significantly higher level (at P < 0.05) of total folate than that obtained using only single- or di-enzyme (protease + folate conjugase) treatments. Among the representative foods studied, the levels of total folate ranked in order of concentration, were soybeans > egg > asparagus > brown rice > steamed-mackerel (common form sold) (305, 117, 95, 41 and 24 μg/100 g wet weight basis, respectively). After cooking, the true retention of total folate in the cooked foods ranged from 60% in brown rice (cooked in an electric rice cooker) up to 82% in boiled asparagus. On a wet weight basis, the total folate content in boiled soybeans, boiled asparagus, boiled egg, fried mackerel and cooked brown rice was 100, 84, 72, 22 and 12 μg/100 g, respectively. One boiled egg (50 g) contributes 36 μg folate, which is equal to 18% of the Thai Recommended Daily Intake (RDI), and thus it can be considered as a good source of folate based on one serving size. A serving of boiled soybeans (70 g) or boiled asparagus (80 g) is an excellent source of folate, each contributing about 35% of the Thai RDI.