Behavior analysis of γ-aminobutyrate and glutamate decarboxylase activity in salted radish roots (Takuan-zuke)

Abstract

We prepared salted radish roots and analyzed the changes in γ-aminobutyrate (GABA) and glutamate decarboxylase (GAD) activity during dehydration and long-term salting. The amounts of glutamate (Glu) and GABA in the fresh roots were 5.72±0.23 and 0.28±0.01 mg/g dry weight (DW), respectively. The amount of GABA by sundrying and salt-pressing process increased to 7.30±1.57 and 4.98±0.06mg/g DW, respectively. In contrast, the Glu content decreased to 0.71±0.13 mg/g DW. The GABA content in salted radish was further increased by 1.2-3.0-fold by the addition of monosodium glutamate (MSG, 0.1-0.5% weight of the radish) at the same time of salting. Fresh radish showed the highest GAD activity, which decreased during the dehydration and salting process. On the other hand, in the liquid leached from the radishes during the salting process (agari-mizu in Japanese) with MSG, a timedependent decrease of Glu and increase of GABA were observed. Thus, it was suggested that Glu was converted to GABA by microorganisms in the agari-mizu and transferred to the salted radish. The color of the salted radish without MSG was pale yellow, while that with MSG turned grayish in color after 2 months of salting.Thus, the addition of MSG during the salting process decreased the shelf-life of the product.