C. tora contains many phenolic compounds, such as emodin, rhein, chry-sophanol, and obtusin, some of which belong to anthraquinones (56). Some anthraquinones have been reported to have antioxidant activity (57). The total polyphenols in extracts of C. tora decreased significantly (p > 0.05) during roasting. The amount of polyphenols in extracts of C. tora decreased from 180.64 mg/g (unroasted) to 103.55 mg of gallic acid/g extracts of C. tora after roasting at 250°C. This means that polyphenols were degraded during the roasting process. The decrease of polyphenol contents in C. tora after roasting is correlated to the decrease in antioxidant activity. Therefore, it can be suggested that the decrease in antioxidant activity of extracts of roasted C. tora was related to the decrease in polyphenols.
B. Anthraquinones: Characteristic and Content in Cassia Extracts
Anthraquinones have been reported to be the main active components in C. tora, including aloe-emodin, anthrone, aurantiobtusin, chrysophanol, emo-din, obtusifolin, physcion, rhein, etc. (17,56). The basic chemical structures of anthraquinones are shown in Fig. 1. The anthraquinones, with two ketone groups added at position C9 and position C10, occur naturally in plants. Total anthraquinones, including free anthraqinones (anthraquinones agly-con) and bound anthraquinones (anthraquinones O-glycosides), were deter-
Rs O Ri
Rs O Ri
Aloe-emodin Chrysophanol Emodin
Ri=R6=R8=OH, R2=H, R3-CH3 Ri=Rg=OH, R2=Ré=H, R3=COOH
Figure 1 Basic chemical structures of anthraquinones.
mined by Yen and Chung (32). Their results showed that the total content of anthraquinones in extracts of C. tora were degraded by thermal treatment, in the order of unroasted (88.2mg/g) > 150°C-roasted(70.7mg/g) > 200°C-roasted (26.9 mg/g) > 250°C-roasted (14.9 mg/g). The data also indicate that most of the anthraquinones in extracts of C. tora are in a bound form and contain glycosides. This is in agreement with the studies of Fairbirn and Moss (58), who reported that the anthraquinones in Cassia plants are partly free but mostly are present as glycosides (with aglycon occurring usually as a reduced form, e.g., anthrones).
The individual anthraquinone content in extracts of C. tora was also measured by Wu (31) using HPLC. Three anthraquinones, chrysophenol, emodin and rhein, have been detected in extracts of C. tora under experimental conditions. The unroasted sample contains the highest anthraqui-nones content; the content of rhein, chrysophanol, and emodin was 10.4, 0.6, and 0.3 mg/g Cassia extract, respectively. The anthraquinone content decreased with increased roasting temperature. It is noted that the extracts of C. tora prepared by roasting at 250° C did not show any detectable anthraqui-nones. Zhang et al. (27) indicated that anthraquinones in C. tora were degraded to a free form (aglycon) by roasting. The content of these three anthraquinones has only one-eighth of the total content of anthraquinones compared with the results reported by the Yen and Chung (32). Most individual anthraquinones or anthrones have shown antioxidant activity in linoleic acid peroxidation systems (57). Thus, the decrease in antioxidant activity of roasted C. tora was related to the decrease in anthraquinones.
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