اثرات تیمار موم بر روی کیفیت و فیزیولوژی پس از برداشت میوه اناناس در سردخانه Effects of wax treatment on quality and postharvest physiology of pineapple fruit in cold storage

INTRODUCTION Pineapple (Ananas comosus (L.) Merr.) is an important fruit crop grown in many tropical and subtropical countries. Fresh pineapple fruit is perishable (Chen and Paull, 2001; Avallone et al., 2003; Soares et al., 2005; Wilsonwijeratnam et al., 2005; Ko et al., 2006) and cold storage is the main method to slow the product deterioration in terms of consumer perception and nutritional value (Zhang et al., 2009; Cantín et al., 2010). However, low temperature results in chilling injury symptoms in pineapple fruits during or after cold storage (Selvarajah et al., 2001; Zhou et al., 2003). Chilling injury hygrophanous flesh in pineapple fruits and is the major is manifested as internal browning (black heart) and postharvest limitation for the pineapple industry, as fruits rapidly develop blackheart following low temperature, which severely restricts refrigerated seafreight export (Selvarajah et al., 2001). Thus, postharvest treatments that can alleviate chilling injury and extend shelf-life of pineapple fruits are urgently needed. The development of effective methods to alleviate chilling injury in pineapple fruits has been widely reported and these include heat treatment and controlled atmosphere (Wilsonwijeratnam et al., 2005; Nimitkeatkai et al., 2006). However, these methods do not eliminate significantly chilling injury. At present, wax (edible coatings) have been used as an effective technology to increase the quality of postharvest fruits and vegetables (Qiuping and Wenshui, 2007; Fan et al., 2009; Tzoumaki et al., 2009; Tietel et al., 2010). Coatings could effectively retard the loss of the water, titratable acidity and ascorbic acid of sweet cherries (Dang et al., 2010). Waxing could improve firmness, titratable acidity, ascorbic acidity and the water content for Murcott tangor stored at 15°C for 56 days (Chien et al., 2007). Waxing, acting as semipermeable barriers, may be an effective method to alleviate chilling injury (Meng et al., 2008; Ahmed et al., 2009). However, only two studies reported the application of waxing of pineapple fruits during cold storage, with emphasis on chilling injury symptoms, but less attention to coating-induced quality alterations and their physiological responses in the fruits (Paul and Rohrbach, 1985; Wijeratnam et al., 2006). Meanwhile, the main pineapple cultivar grown in the world is ‘Smooth Cayenne’, and in China, the most popular cultivar is ‘Comte de Paris’, accounting for more than 80% of the total pineapple production (Li et al., 2011). However, few data exist extending storage life of the cultivar ‘Comte de Paris’. The objective of this study was to find a new method for extending storage life of pineapple cultivar ‘Comte de Paris’ and to investigate the method of how to alleviate chilling injury and maintain fruit quality during cold storage. Thus, this study was performed to comprehensively investigate the effects of application of a novel wax formulation on the changes in several parameters related to quality and life extend of pineapple fruits (weight loss, firmness, color saturation, total soluble solids (TSS), soluble sugars, titratable acidity (TA), ascorbic acid (AsA) and soluble proteins) and on their physiological activities including the changes in relative leakage rate, malondialdehyde (MDA) and membrane integrity in response to cold storage of the chilling sensitive pineapple fruits. The results obtained from this study indicated that Sta-Fresh 2952 wax treatment is more effective in alleviating chilling injury and in maintaining the quality of these pineapple fruits. MATERIALS AND METHODS Plant material and experimental design Pineapple fruits (A. comosus cv. Comte de Paris) (at about 3 matured stages) were selected on the basis of the uniformity of the color and size from a commercial grower in Zhanjiang, China. The stage of ripeness was determined by visual assessment of the shell (Selvarajah et al., 2001). The scale ranges from 0 to 5: 0, all eyes are totally green; 1, < 20% of the eyes are predominantly yellow; 2, 20 to 40% of the eyes are tinged with yellow; 3, up to 65% of the eyes are predominantly yellow; 4, 65 to 90% of the eyes are fully yellow; 5, > 90% of the eyes are fully yellow and no more than 20% of the eyes are reddish orange. All fruits were cleaned and soaked in 0.05% (w/v) Iprodione solution (Kuaida, Jiangsu, China) for 2 min to eliminate potential microbes. Afterwards, the treated fruits were divided into seven groups; each group (10 fruits) was placed in a clean plastic box. The fruits of one group were dipped in water (as control). The fruits of six groups were respectively treated with StaFresh 2952 (FMC) wax solution at 30, 60 and 90 g/l and Sta-Fresh 7055 (FMC) wax solutions at 40, 80 and 120 g/l. After being air dried, the samples were placed in polyethylene bags (0.04 mm), stored at 90% relative humidity (RH) and 7°C for 21 day and transferred to 25°C for 3 day to simulate shelf conditions for chilling injury and quality evaluation. Five fruits from each box were randomly sampled to determine quality characteristics of fruits after storage periods and then the optimal treatment condition was chosen. Forty (40) fruits were treated with the optimal treatment and 40 fruits were dipped in water (as control). All fruits were stored at 7°C and some samples were taken at intervals. After 21 day storage at 7°C, the residual samples were transferred to 25°C for three days storage. Five fruits from each box were randomly sampled every 7 days in cold storage and each day in room temperature (25°C) storage to determine physical and biochemical changes during storage periods.