Variations in Water-Soluble Saccharides and Phenols in Longan Fruit Pericarp After Drying
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VARIATIONS IN WATER-SOLUBLE SACCHARIDES AND PHENOLS IN LONGAN FRUIT PERICARP AFTER DRYING BAO YANG1, MOUMING ZHAO2, JOHN SHI3, GUIPING CHENG1, NEUNGNAPA RUENROENGKLIN1 and YUEMING JIANG1,4 1South China Botanical Garden The Chinese Academy of Sciences Guangzhou Leyiju 510650, China 2College of Light Industry and Food Sciences South China University of Technology Guangzhou 510640, China 3Food Research Center Agriculture and Agri-Food Canada Guelph ON N1G 5C9, Canada Accepted for Publication April 27, 2007 ABSTRACT Variations in the contents of water-soluble polysaccharides (WSP), water-soluble total saccharides (WSTS), gallic acid and total phenols in longan pericarp tissues of three major cultivars (“Shixia,” “Chuliang” and “Fuyan”) planted in Guangzhou, Gaozhou and Haikou, respectively, were evaluated. Microwave radiation, solar radiation and lyophilization were employed to dry the longan pericarps. A great variation in the contents of WSTS, WSP, total phenols and gallic acid existed in pericarp tissues of longan fruit among the three cultivars, locations and drying methods. The highest content (248.7-mg glucose equivalents [GE]/g dry weight [DW]) of WSTS was obtained in “Shixia” fruit from Haikou, while the lowest content (58.9-mg GE/ g DW) was detected in “Fuyan” fruit from Guangzhou using lyophilization. WSP content was in a decreasing order except for cv. “Shixia” from Haikou, using the three various drying methods: microwave radiation > solar radiation > lyophilization. The longan pericarp tissues contained a large amount of phenolic compounds. “Shixia” fruit from Haikou had the highest content (49.4-mg gallic acid equivalents/g DW) of total phenols. The contents of total phenols followed a decreasing order for the three drying methods: lyophilization > microwave radiation > solar radiation. No gallic acid was 4 Corresponding author. TEL: +86-20-37252525; FAX: +86-20-37252831; EMAIL: ymjiang@ scbg.ac.cn Journal of Food Process Engineering 31 (2008) 66–77. All Rights Reserved. 66 © 2008, The Author(s) Journal compilation © 2008, Blackwell Publishing SACCHARIDES AND PHENOLS IN LONGAN FRUIT PERICARP 67 detected in either fresh or lyophilized pericarp tissues, but it was found in the dried pericarp tissues with a higher content in fruit pericarp tissues dried by microwave radiation than by solar radiation. PRACTICAL APPLICATIONS There is increasing interest of utilizing saccharides and phenols from plant sources for their uses in medicine and cosmetics. Microwave radiation, solar radiation and lyophilization were employed to dry longan fruit pericarp tissues of three major cultivars in relation to their various production locations, while contents of water-soluble total saccharides (WSTS), water-soluble polysaccharides (WSP), total phenols and gallic acid in the pericarp were evaluated after these dryings. Significant variations in the contents of WSP, WSTS, gallic acid and total phenols of longan pericarp tissues among the three major cultivars and various locations and drying methods were found. This study can help utilize better the longan fruit pericarp in practice. INTRODUCTION Polysaccharides and phenols widely exist in plant tissues. Phenols are secondary metabolites, which play a role in growth and development of plants. In recent years, there has been an increased interest of utilizing polysaccha- rides and phenols from plant sources for their uses in medicine and cosmetics. The health effects of plant polysaccharides and phenols in the human diet involve anticancer, immune modulation, antibacteria and anticardiovascular disease (Sonoda et al. 1998; Randhir and Shetty 2004; Deters et al. 2005). Longan (Dimocarpus longan Lour.) is an exotic fruit popular in Southeast Asia, especially in China (Jiang et al. 2002). It is being accepted by consumers for its special taste and flavor, and has become an important fruit for export. Rangkadilok et al. (2005) reported that a significant amount of phenols was found in longan fruit pericarp tissues. Unfortunately, little information on polysaccharides in longan fruit pericarp is available. Thus, it is interesting to investigate the contents of phenols and saccharides in longan fruit pericarp tissues of different cultivars and locations, and then utilize better the fruit. As the longan pericarp deteriorates rapidly at ambient temperature, appropriate drying measures are requested to maintain the quality of the fruit pericarp. However, the classical drying procedure using solar radiation requires a long time. Recently, microwave radiation has been used to dry plant tissues. Application of microwave is based on the direct effect of molecules by ionic conduction and dipole rotation. Polar molecules strongly absorb 68 B. YANG ET AL. microwave energy because they have a permanent dipole moment, which results in rapid temperature rise and fast drying (Varith et al. 2007). The objective of this study was to evaluate the contents of water-soluble total saccharides (WSTS), water-soluble saccharides (WSP), total phenols and gallic acid of longan fruit pericarp using lyophilization, microwave radiation and solar radiation, and then compare their differences using the three drying methods. Longan fruits of three major cultivars in relation to various produc- tion locations were used. This study could help utilize better the longan fruit pericarp. MATERIALS AND METHODS Plant Materials Fresh fruit of three major longan (Dimocarpus longan Lour.) cultivars “Shixia,” “Chuliang” and “Fuyan” at a commercially mature stage harvested from Guangzhou (Guangdong province), Gaozhou (Guangdong province) and Haikou (Hainan province), respectively, were purchased from a commercial market in August 2006. The fruits were selected for uniformity of shape and color. Chemicals Gallic acid, glucose, Folin-Ciocalteu reagent, acetic acid and acetonitrile were from Sigma Chemical Co. (St Louis, MO). Ethanol, phenol and sulfuric acid were obtained from Guangzhou Reagent Co. (Guangzhou, China). All other chemicals used were of analytical grade. Treatments Fresh Pericarp without Treatment. The fresh longan fruit pericarp tissues were cut into small pieces for extraction of WSP and phenols, subjected to further analysis to establish baseline quality. Lyophilization. Small fresh pieces were rapidly frozen in liquid nitrogen, then stored at -40C for 24 h using a GLZY-0.5Bfreeze dryer (PDFD, Shanghai, China) equipped with a compressor (Bitzer, Stuttgart, Germany) and a vacuum system (Edwards, Delaware, USA), and were finally ground to powder. Drying by Microwave Radiation. A domestic digital microwave oven (Galanz P7023TP-K7, Guangdong Galanz Enterprise Group Co. Ltd., SACCHARIDES AND PHENOLS IN LONGAN FRUIT PERICARP 69 Guangdong, China) was used to dry the fresh longan pericarp tissues. The small fresh pieces were dried for 20 min in the microwave oven at 400 W power, cooled to 25C and then ground to powder. Drying by Solar Radiation. Small fresh pericarp tissues were exposed directly to sunshine for 8 h (9:00 a.m.–5:00 p.m.) in August at an average daily temperature of 35C and a clear weather. The dried pericarp tissues were then ground to powder for quality analysis. Moisture Content Determination. Moisture content in the pericarp tissues was determined by drying the pericarp in a hot air oven at 105C for 6 h. The moisture contents of fresh and dried pericarp tissues are shown in Table 1. The dried pericarp tissues by microwave radiation had lower moisture contents compared with those by solar radiation or lyophilization. Extraction and Quantification of WSTS and WSP Preparations for WSTS and WSP were conducted by the method of Yang et al. (2006) with some modifications. The dried pericarp tissues (4 g) were extracted for 3 h with 100 mL of distilled water at 50C for three times, and were then filtered through Whatman No. 1 paper (Whatman plc, Middlesex, UK). The filtrates were combined and concentrated to 25 mL using a rotary evaporator at 50C under low pressure. The proteins in the extract were removed using the Sevag reagent (Navarini et al. 1999). After removal of the Sevag reagent, one aliquot of the extract was used to determine the content of WSTS by phenol-sulfuric acid method (Dubois et al. 1956). Other aliquots were added to 100 mL of anhydrate ethanol before the mixture was maintained overnight at 4C to precipitate WSP. The precipitated WSP was dissolved in 100 mL of distilled water after centrifugation at 5,000 rpm for 15 min. The content of WSP was also determined by the phenol-sulfuric acid method. Glucose was used to make a standard curve. The contents of WSTS and WSP in pericarp tissues were expressed as milligram of glucose equivalents (GE) on dry weight (DW) basis. DW was calculated as the weight of pericarp tissues free of its original water content. Extraction and Quantification of Total Phenols and Gallic Acid The dried pericarp tissues (4.0 g) were extracted for 3 h at 40C in 100 mL of 95% ethanol by the methods of Lee and Wicker (1991) and Zhao et al. (2006) with a minor modification. After filtering the extract through Whatman No. 1 paper, the residue was re-extracted and filtered for three times. The filtrates were combined and dried using a rotary evaporator at 40C under low pressure. Total phenolic content was determined by the Folin-Ciocalteu 70 B. YANG TABLE 1. MOISTURE CONTENT OF FRESH AND DRIED LONGAN FRUIT PERICARP* Samples 1 (%) 2 (%) 3 (%) 4 (%) 5 (%) 6 (%) 7 (%) 8 (%) 9 (%) Fresh tissues 51.6 Ϯ 1.5 59.8 Ϯ 0.9 63.2 Ϯ 1.2 60.4 Ϯ 1.2 64.1 Ϯ 1.0 52.9 Ϯ 0.8 58.6 Ϯ 0.8 54.7 Ϯ 0.5 53.6 Ϯ 0.7 Drying methods: Lyophilization 10.7 Ϯ 0.6 11.7 Ϯ 0.6 10.9 Ϯ 0.9 11.4 Ϯ 0.7 13.2 Ϯ 0.9 12.2 Ϯ 1.1 11.9 Ϯ 1.0 10.9 Ϯ 0.5 12.4 Ϯ 0.8 ET AL. Microwave radiation 7.3 Ϯ 0.8 6.5 Ϯ 0.5 7.0 Ϯ 0.9 6.3 Ϯ 0.9 8.6 Ϯ 0.2 7.9 Ϯ 0.3 9.1 Ϯ 0.4 8.2 Ϯ 0.5 6.7 Ϯ 0.4 Solar radiation 11.7 Ϯ 0.8 11.3 Ϯ 0.7 10.8 Ϯ 0.5 12.9 Ϯ 0.7 13.4 Ϯ 0.7 12.5 Ϯ 0.3 13.3 Ϯ 0.9 12.9 Ϯ 0.7 11.7 Ϯ 0.4 Each data was presented as mean Ϯ SD of three replicated determinations.