EFFECT OF DRYING METHODS ON Γ-PGA, ISOFLAVONE CONTENTS AND ACE INHIBITORY ACTIVITY OF NATTO (A FERMENTED SOYBEAN FOOD)

This study investigates the effects of drying methods on angiotensin-converting enzyme (ACE) inhibitory activity, ?-polyglutamic acid (?-PGA) and isoflavone contents of steamed soybeans fermented by Bacillus subtilis (natto). In this study, the fermented steamed soybeans (natto) are dried in different methods, namely conventional oven-drying at 50C, vacuum-drying at 40C and freeze-drying at -50C, and ground into powder for analysis after drying for 2 days. Results indicate that vacuum-drying, freeze-drying or conventional oven-drying does not affect the stability of ?-PGA and isoflavone contents but does affect ACE inhibitory activity in all dried natto samples. PRACTICAL APPLICATIONS Natto is a traditional fermented soybean food product containing functional compounds, but it is not acceptable to most consumers because of its strong unpleasant smell. Its odor sometimes causes problems with natto product quality control, and a fresh product has a short shelf life. To avoid these problems, some biotechnical and functional food companies have tried drying it into powder form or into capsulized form and sell it as a functional food. The results obtained from this research demonstrate that natto dried by vacuum-drying, freeze-drying or oven-drying does not affect the stability of ?-PGA or isoflavone but has an effect on the ACE inhibitory activity in natto. The drying process is useful to commercial production of natto and product quality, minimizing the risk of secondary fermentation releasing ammonia. The greatest advantage of the drying process is that it can maintain the functional compounds of the product.