Identification of phenolic compounds and appraisal of antioxidant and antityrosinase activities from litchi (Litchi sinensis Sonn.) seeds
Introduction
Artificial antioxidants, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and tertiary butylhydroquinone (TBHQ), have been widely used in foods for preventing lipid peroxidation, but the usage of these artificial antioxidants is being gradually restricted in the food industry as they are suspected to be toxic and carcinogenic (Namiki, 1990). There has been an increasing demand for antioxidants from plant origins. Epidemiological studies have shown that the consumption of fruits and vegetables with high phenolic content can reduce the risk of cardio- and cerebro-vascular diseases and cancer mortality (Amin & Yazdanparast, 2007). The protective effect of fruits and vegetables against diseases has been attributed to various phenolic compounds possessing antioxidant activity (Jayaprakasha et al., 2008, Prasad et al., 2005).
Tyrosinase (EC1.14.18.1) is involved in melanin production and the melanin production might be responsible for some of the histopathological features exclusive to malignant cancer. Therefore, tyrosinase inhibitors may be clinically helpful in dealing with skin cancer. In recent years, more attention has been paid to use the natural plant extracts such as tyrosinase inhibitors in the cosmetic industry (Momtaza et al., 2008).
The exploration of green technology and of low cost raw materials is important features for the food industry in making improved use of plant resources. As population increases, food production is more intense and a great quantity of waste is generated.
Litchi (Litchi chinensis Sonn.) is a subtropical fruit originating from south-east Asia. The fruit is accepted by consumers because of its delicious taste and attractive colour. Litchi by-products consist mainly of litchi pericarp and litchi seeds which are discarded as a waste. The previous studies on biochemical activities of litchi fruit are mainly focussed on its pericarp, because it has been found to be a rich source of a multitude of potential antioxidants (Duan et al., 2007, Zhang et al., 2000, Zhao et al., 2006). However, there is no information on litchi seeds, which are commonly used in Chinese Traditional Medicine to relieve neuralgic pain. In the present study, the possibility of litchi seeds being used as a natural antioxidant and tyrosinase inhibitor was investigated, while the major phenolic compounds are identified for the first time. This study could help better utilise litchi seeds, which is not only economical but also environmentally friendly, because they could be recycled in the food industry in the form of value-added products.
Section snippets
Plant materials
Fresh fruits of litchi (Litchi sinensis Sonn.) at the mature stage were collected from an orchard in Guangzhou, China. Fruits were chosen for uniformity in shape and colour. The fruits were carefully washed in potable water. Litchi seeds were manually separated, then dried for 24 h, using a hot air oven at 60 °C, and powdered, using a blender. The moisture content was determined to be 62%.
Chemicals and reagents
1,1-diphenyl-2-picryl hydrazyl (DPPH), ascorbic acid, L-tyrosine, tyrosinase solution (1000 units/ml), gallic
Extraction yield and total phenolic content
Extraction yield and total phenolic content of each extract of litchi seed are given in Table 1. Fifty percent EE gave the highest percentage of extraction yield (26.8 ± 0.24), whereas WE gave the lowest (23.5 ± 0.07). For the total phenolics, ME exhibited the highest value, and WE the lowest. However, there were no significant differences (P < 0.05) in the total phenolic content among all the extracts, except for WE. In our previous work, application of methanol or ethanol gave a higher extraction
Conclusions
In the present study, application of polar solvents, to extract antioxidant compounds from litchi seeds, was investigated. The study indicated that litchi seeds had high phenolic content, exhibited good antioxidant and antityrosinase activities, compared with that of BHT. Furthermore, five compounds were identified from litchi seed, namely gallic acid, procyanidin B2, (−)-gallocatechin, (−)-epicatechin, and (−)-epicatechin-3-gallate. The use of litchi seeds for obtaining natural antioxidant
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos. 30425040 and 30700557) and International Foundation of Science (No. F/4451-1) and Science Foundation of South China Botanical Garden (No. 200807).
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