Abstract:
Litchi fruit pericarp tissue is considered an important source of dietary phenolics. This study consisted of two experiments. The first was conducted to examine the effects of various extraction temperatures (30, 40, 50, 60, 70 and 80 degrees C) and pH values (2, 3, 4, 5 and 6) on the extraction yield of phenolics from litchi fruit pericarp. Extraction was most efficient at pH 4.0, while an extraction temperature of 60 degrees C was the best in terms of the combined extraction yield of phenolics and the stability of the extracted litchi anthocyanins. The second experiment was carried out to further evaluate the effects of various temperatures (25, 35, 45, 55 and 65 degrees C) and pH values (1, 3, 5 and 7) on the total antioxidant ability and scavenging activities of DPPH radicals, hydroxyl radical and superoxide anion of the extracted anthocyanins. The results indicated that use of 45-60 degrees C or pH 3-4 exhibited a relatively high antioxidant activity. The study will help improve extraction yield of phenolics from litchi fruit pericarp and promote better utilization of the extracted litchi anthocyanins as antioxidants.
Introduction
Litchi (Litchi chinensis Sonn.) is a subtropical fruit that originated in South-East Asia [1]. In recent years, litchi production has increased steadily around the world. Litchi fruit pericarp (LFP) accounts for approximately 15% by weight of the whole fresh fruit [2] and contains significant amounts of phenolics, among which anthocyanins are the major polyphenols. Anthocyanins play an important pharmacological role against various human diseases, such as cardiovascular disease, cancer, inflammation and allergies [3–6]. Furthermore, some studies have indicated that LFP is a powerful free radical scavenger and exhibits strong antioxidant activity [7, 8], which suggest its use as a readily accessible source of natural antioxidants and/or a possible supplement in the food or pharmaceutical industry [2, 6].
Extraction yields of phenolics from plant tissues depend on the various extraction conditions [2]. Most phenolics present in plant tissues are soluble in polar solvents and can be extracted using methanol containing small amount of hydrochloric or formic acid [3, 9, 10]. The low pH value of the extraction solution can prevent the oxidation of phenolics, while the use of low temperatures may preserve anthocyanin stability [2, 11]. Thus, an investigation of the efficient extraction of phenolics from LFP requires evaluation under various temperatures and solvent pH values.
Anthocyanins are the major phenolics present in LFP. The major anthocyanins from LFP tissues were identified as epicatechin, proanthocyanidin B4 and proanthocyanidin B2 [2, 10]. Anthocyanins show good antioxidant ability [7], but they are relatively unstable [9]. As the stability of litchi anthocyanins is dependent on various factors, such as pH value and temperature [12], the antioxidant ability of the litchi anthocyanins under the conditions of various temperatures and pH values also needs further evaluation.
The objective of this study was to examine the effects of various extraction temperatures and pH values on the extraction yield of phenolics from LFP and then evaluate total antioxidant ability and scavenging activities towards α,α-diphenyl-β-picrylhydrazy (DPPH) and hydroxyl radicals and superoxide anion of the extracted litchi anthocyanins under various temperatures and pH conditions. The study will help improve extraction of phenolics from litchi fruit pericarp and promote better utilization of the extracted litchi anthocyanins as antioxidants.
