No abstract

Takuan‐zuke is a traditional Japanese fermented pickle, prepared by dehydration of radish root (daikon) by salt‐pressing or sun‐drying followed by aging with salt. We previously reported that alanine, proline, and γ‐aminobutyric acid (GABA) accumulate during daikon dehydration, whereas the level of glutamic acid, their precursor, decreases. We have also reported that dehydration and salt‐aging markedly influence the dynamics of free amino acids. In this study, we quantitatively analyzed free amino acid levels, enzyme activity, and gene expression to characterize takuan‐zuke amino acid metabolism. Enzyme activities related to alanine, proline, GABA, and glutamic acid metabolism were sustained during dehydration. Moreover, genes encoding alanine, proline, and GABA synthases (ALT1, P5CS1, and GAD4) were significantly upregulated during dehydration. These effects may represent cellular stress responses to the dehydration process. The biological response of daikon contributes to the healthy functional aspects that characterize takuan‐zuke. These findings could guide the selection of suitable vegetable varieties to produce pickled vegetables with health‐promoting properties.Practical ApplicationThe fermented pickle takuan‐zuke, prepared by dehydration of radish root (daikon), accumulates amino acids, such as alanine, proline, and GABA, during preparation that provide taste and health benefits. In this study, the aforementioned amino acids were found to accumulate because of the stress response of daikon during the dehydration process and not because of the action of microorganisms during fermentation. Takuan‐zuke processing is a method for improving the nutrition of daikon.

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±0.06 mg/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.

Recently, we reported that the antihypertensive compound, γ-aminobutyrate (GABA), increases over time during the dehydration of salted radish, known as takuan-zuke, a popular pickle in Japan. The objective of this study was to clarify the antihypertensive effects of takuan-zuke. We prepared two types of takuan-zuke by sun-drying (hoshi takuan-zuke) and salt-pressing (shio-oshi takuan-zuke) using dehydration processes. The oral administration of takuan-zuke lowered systolic blood pressure in spontaneously hypertensive rats (SHRs). Shio-oshi takuan-zuke (SR) demonstrated a clear antihypertension effect compared with hoshi takuan-zuke (DR), despite equal GABA concentrations in the feed. Furthermore, takuan-zuke demonstrated angiotensin converting enzyme (ACE) inhibition in vitro. These findings indicated that takuan-zuke contains unknown substances that have hypotensive actions independent of GABA. Further evaluation revealed that takuan-zuke contains polyphenols, arginine, and α-linolenic acid as possible antihypertensive factors. Collectively, our results suggest that the salty Japanese food takuan-zuke has antihypertensive effects in vivo, likely involving complex mechanisms.

This study was performed to clarify the enhancement of the 4-methylthio-3-butenyl isothiocyanate induced yellowing of salted radish root (takuan-zuke) by low pH during short-term salt-aging at low temperature and low salinity. We used two different methods to prepare the dehydrated daikon prior to salt-aging: air-drying outdoors (hoshi takuan-zuke) or salting with a stone press (shio-oshi takuan-zuke). Low salt-aging at low temperature was carried out under pH control with citrate-phosphate buffer. The yellowing of both types of takuan-zuke was accelerated below pH 5, and the color of air-dried takuan-zuke was deeper than that of salt-pressed takuan-zuke. To elucidate this phenomenon, several previously reported yellowing-related compounds were analyzed by high-performance liquid chromatography. The result showed that the production of the primary pigment, 2-[3-(2-thioxopyrrolidin-3-ylidene)methyl]-tryptophan, was low compared with that in previous reports. Therefore, we suggest that an unknown pigment was generated through a previously unreported pathway.