Thermally Generated Volatiles in Roselle Tea

Abstract

Roselle (Hibiscus sabdariffa L.) tea with its brilliant red color (Esselen et al., 1975), is one of the most popular drinks in Taiwan (Tsai, 1995). However, its unique flavor is very delicate and barely discernable to detect. There are few reports on the aroma of roselle, ex­cept one referring to the volatiles in seed oil of roselle (Jirovetz et al., 1992). In this study, two methods of extraction (Likens-Nikersen, L-N and thermal desorption, TD-3) were used to mimic the extraction of thermally generated volatiles of roselle prior to their identifica­tion by GC and GC-MS, in order to understand the effect of drying temperature on flavor. Further analysis of these volatiles by principle component analysis (PCA) intended to find the responsible aroma for roselle. Discriminant analysis was used to classify the samples into groups of different drying temperatures according to these volatiles.

Likens-Nickerson steam distillation (L-N) and thermal desorption (TD-3) were used to mimic the preparation of roselle tea. Thermally generated volatiles from roselle were collected and analyzed by GC and GC-MS. Samples were frozen, cold-air dried at 25°C or hot-air dried at 50, 75 or 85°C and used to elucidate the effect of heat treatment on thermal generation of volatiles in roselle tea. Volatile of roselle tea were classified into four groups: fatty acid derivatives, sugar derivatives, phenolic derivatives and terpenes. As compared with L-N extraction, volatiles extracted by TD-3 apparently had a higher content of aliphatic C6 components and terpenes, but thermally generated volatiles, such as furfural, linalool oxide, and eugenol were present in much smaller amounts. Drying process reduced the content of aliphatic C-6 lipid derivatives and terpenes dramatically, but markedly increased the amount of furfural. A combination of the terpene derivative and sugar derivative were found responsible for the roselle aroma as evidenced by Principle component analysis. Discriminant analysis of the volatiles extracted by TD-3 revealed that principle component 2 is most important in contribution of the model and thermally generated furfural is responsible for its correct classification.