Item

Factors affecting the yield of isothiocyanates, the flavour compounds, in wasabi (Wasabia japonica (Miq.) Matsum)

Sultana, Tamanna
Date
2003
Type
Thesis
Fields of Research
Abstract
Wasabi (Wasabia japonica (Miq.) Matsum) is an aromatic herb that adds a sharp hot taste and pungent smell to food and, therefore, is highly appreciated in Japanese cuisine. Recently wasabi has emerged as a new flavour in western cuisine. The unique flavour of wasabi comes from the isothiocyanate (ITC) content which is the key quality parameter of wasabi. This research was performed to investigate the factors which affect the ITC profile in wasabi, as a way to support the emerging wasabi industry in New Zealand. The present study highlighted the existence of a misconception concerning the flavour difference between wasabi and horseradish. Wasabi is known as Japanese horseradish, but differs in its ITC profile compared to European horseradish. The key indicator is 2-phenylethyl ITC, which is found only in European horseradish and gives it a radish like flavour. Thus, it is very important for processors to understand the differences in ITC profile between wasabi and horseradish to manufacture a product which has a unique wasabi flavour. Seven ITCs have been investigated in New Zealand grown wasabi and, among them, allyl ITC was the most abundant ITC, contributing 86-94% of the total ITC concentration. 3-butenyl ITC and 4-pentenyl ITC made up 3-8% of the total ITC while the other ITCs (iso-propyl ITC, isobutyl ITC, sec-butyl ITC, 5-hexenyl ITC) contribute very small amounts to the total ITC content. The two cultivation methods (soil and water) showed no variation in ITC yields of larger rhizomes (>45 g). In soil cultivated plants, ammonium sulphate fertiliser (containing 21 % N with 24% S) was very effective in significantly increasing ITC yields from 2316 mg/kg to 3366 mg/kg (a 45% increase of allyl ITC in the rhizomes compared to the control level) when applied at a rate of 200 kg N /ha. This treatment also highlighted the importance of N and S nutrients for ITC synthesis in wasabi, as both N and S are components of ITCs and the precursor glucosinolates (GSLs). A 61-77% increase of ITC yield was also observed in the rhizomes when the plants commenced flowering after 18 months of growth. Consequently, this result suggests that flowering is an important event in the crop management process. Among the four plant parts of wasabi, rhizomes yielded the highest level of all ITCs and traditionally this is the most important plant part. Other parts like petioles and leaves gave lower yields of ITCs, but could be used when lower concentration of ITCs are required in the final product. The larger rhizome, the outer layer and distal portion of the rhizome had a higher ITC yield than smaller rhizomes, the inner tissues and proximal portion of the rhizome respectively. Suitable storage conditions for rhizomes are very important to retain the quality of the raw materials and, therefore, products. This research showed that defrosting can potentially lead to large losses of ITCs from frozen rhizomes. For example, a 99.7% loss of allyl ITC was observed after 16 days when snap frozen rhizomes were defrosted slowly at 4°C. Storage at -10°C or lower temperatures (up to -80°C) for two months retained the original ITC levels in the rhizomes. The shelf life analysis of allyl ITC in mayonnaise-type sauces containing wasabi oil (4400 mg of total ITC/kg of oil) gave a life of 8-9 weeks (considering 5% loss of allyl ITC to be the end of satisfactory storage) when stored at between 4 to 30°C. Shelf life was reduced to less than 2 weeks for a tomato based sauce prepared with a lower amount of wasabi oil, higher water content and heat treated by pasteurisation at 90°C for 30 minutes under the same storage conditions. Overall the data obtained from this study will assist farmers and processors to produce high quality products using recently introduced wasabi crop in New Zealand.
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