Black rice is a food crop that contains a variety of useful substances, especially anthocyanins (ACNs) widely known as a representative natural antioxidant. However, application of black rice containing ACN has been limited as ACN belongs to one of the most unstable compounds among natural products, and its crystallization is difficult. Therefore, in this study, the improvement of ACN stability was attempted by using cyclodextrin (CD), a versatile host molecule capable of forming a complex with various guest molecules to improve their thermal and photo- stability. The optimization of CD production from various rice starch was also conducted and treated with isoamylase, CGTase from Thermoanaerobacter sp., and glucoamylase. The physicochemical properties of five different cultivars of rice starch, Dodammi (DD), Ilpummi (IP), Saegoami (SG), Sintoheukmi (SH), and Geonganghongmi (GH) were investigated and the yields of CD production were compared. Due to the highest amylose content of 66%, DD showed significant differences from the other cultivars in the results of various physicochemical analyses such as HPAEC, RVA, DSC, XRD and SEM. From the preliminary experiments to determine the optimum conditions for CD production using SG starch, the maximum yield of 76.64% was obtained when 5 U/g isoamylase was treated for 8 h, 25 U/g CGTase was treated for 6 h, and 50 U/g glucoamylase was treated for 6h. When CD was produced from the five cultivars at the optimal conditions, DD starch showed the highest yield of 82.91%. Therefore, DD was selected as the final substrate for CD production.After extraction, ACN was pulverized by lyophilization. To investigate the effect of CD on ACN stability, 0-2% of β-CD was mixed with 0.1% ACN at pH 2, 4, 6, and 8 for 24 h, and a decrease of antioxidant activity during heat treatment (96°C) and UVB irradiation for 24 h was monitored using ABTS method. In the heat treatment condition, the addition of β-CD at pH 2 resulted in a significant improvement in antioxidative stability of ACN. A similar improvement of ACN antioxidative stability with β-CD was observed for UVB irradiation tests at pH 2. The degradation constant (kd) value and half-life (t1/2) of antioxidant activity suggested that ACN existed in a structure susceptible to heat at pH 2 and vulnerable to UVB at pH 4, and CD worked to protect ACN at these pHs. As a result of comparing the antioxidative stability of ACN with the addition of CD produced from DD (DD-CD) and maltodextrin (MD) at pH 2 and 4 under the same conditions, the addition of DD-CD was superior to the addition of MD and β-CD. This result indicated that CD was superior to MD in complex formation with ACN, and α-, β-, and γ-CD contained in DD-CD were more effective to protect ACN than β-CD only.In conclusion, this study confirmed that highly efficient CDs could be produced from rice starch by sequential treatment of isoamylase, CGTase, and glucoamylase. Also, CD was found to form a complex with ACN to alleviate pH-dependent thermal and photo- instability of ACN and suggested the possibility of industrial application. However, in the production of DD-CD, the activity of CGTase was inhibited by sugar and CD. Therefore, a process of separating and adsorbing CD was required during the process. In addition, since ACN is known to be unstable in environments other than light or heat, further studies about the effect of DD-CD on the storage stability and oxidative stability of ACN will be needed. It is highly feasible that DD-CD contained stable antioxidant materials can be developed and also, the utilization of black rice can be expanded in the future.
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Stability enhancement of black rice anthocyanin using cyclodextrin produced from rice starch
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