A Review of Non-Chemical Alternatives to SO₂ Fumigation to Prevent Pericarp Browning of Litchi

Abstract

Litchi pericarp browning is a widespread problem caused by a wide range of factors, such as polyphenolsoxidase and peroxidase, disease, desiccation, chilling, senescence, decay, micro-cracks, and heat injury. To overcome these problems, the litchi industry commercially uses SO₂, but it leaves undesirable residues. Furthermore, chemical controls of browning have been threatened by consumer preferences and regulatory issues concerning pesticide residues in fruits. Thus, there is a necessity to develop alternatives to maintain the overall quality. Several alternatives to SO₂ fumigation, such as hot water brushing, modified/controlled atmosphere storage, bio-control agents, surface coating, cold storage, and irradiation, have been proposed.

Keywords:

• Litchi chinensis
• pericarp browning
• sulfur fumigation
• non-chemical methods

INTRODUCTION

Litchi (Litchi chinensis Sonn.) is a tropical, non-climacteric fruit with high commercial value in the international market; it belongs to the family Sapindaceae and is known to have originated from Southeast Asia (Rivera-López et al., 1999; Nakasone and Paull, 1998; Tindall, 1994; Huang et al., 2005). The litchi fruit is highly perishable in nature due to microbial and physiological spoilage, and lasts for only 2–3 days at ambient temperature (26 ± 2°C). Postharvest losses of litchi are estimated to be 20–30% of the harvested fruit and can even be as high as 50% prior to consumption (Jiang et al., 2001, 2002; Wu et al., 2011).

Enzymatic browning is a widespread problem in the litchi and other fruit industries, as it leads to undesirable characteristics of fruits, thereby decreasing fruit quality and value. Browning can be caused by a wide range of different stresses, such as climatic conditions prior to fruit maturation, polyphenols oxidase (PPO) (Huang et al., 1990; Jiang, 2000), peroxidase (POD) (Zhang et al., 2005), fruit disease (Huang and Scott, 1985; Jiang et al., 2002), desiccation (Scott et al., 1982; Underhill and Simons, 1993; Lin et al., 2002a, 2002b), chilling (Tongdee et al., 1982), fruit senescence (Huang and Wang, 1990), decay (Huang and Scott, 1985; Johnson and Sangchote, 1994; Underhill et al., 1997), micro-cracks (Underhill and Critchley, 1993; Underhill and Simons, 1993), and heat injury (Wong et al., 1991; Jiang et al., 2002). The mesocarp cells turn brown first, followed by the epicarp and endocarp (Joubert and Van Lelyveld, 1975). It is thought to be a rapid degradation of the red pigments by polyphenol oxidase, forming brown-colored byproducts (Akamine, 1960; Huang et al., 1990; Jiang et al., 2004; Underhill, 1992; Jaiswal et al., 1987).

The litchi industry commercially uses SO₂ fumigation to overcome these problems (Swarts, 1985). Sulfur dioxide inhibits non-enzymatic formation of colorless quinone-sulfite complexes and enzymatic browning by inactivation of PPO (Kremer-Kohne and Lonsdale, 1991; Oosthuizen 1993; Ray et al., 2005; Swarts and Anderson, 1980), but Botha et al. (1988) found an increase in decay of the fruit Tai So cultivar treated with SO₂. Quinones are otherwise rapidly polymerized to form brown pigments. One of the main concerns with SO₂ fumigation is that it leaves undesirable residues (Kremer-Köhne, 1993). Furthermore, chemical control of postharvest diseases of fruits has been threatened by consumer preferences and regulatory issues concerning pesticide residues in fruits and detrimental effects on the environment (Droby et al., 1991; Wilson et al., 1991; Wisniewski and Wilson, 1992).

Recently, there have been growing concerns regarding SO₂ residue levels present in the fruit, especially by importing countries, such as those in Europe, the United States, and Japan, because some consumers are sensitive to sulfites. A maximum residue limit of 10 mg sulfur/g aril (fresh weight) has been set in Europe, Australia, and Japan (Ducamp-Collin, 2004), whereas in the United States, sulfur is only registered as a postharvest treatment on grapes (Paull et al., 1995). Fumigated fruit absorbs about 20%–30% of the SO₂ applied (Peng and Cheng 1999). Sulfur residues are much higher in the pericarp than in the aril, and decrease rapidly after fumigation (Kremer-Kohne, 1993; Paull et al., 1998; Swarts, 1985).

All of these undesirable effects of SO₂ fumigation have necessitated the development of alternative post-harvest treatments to maintain overall quality during storage and transportation.

Several non-chemical alternative treatments to SO₂ fumigation have been proposed for maintaining the pericarp browning of litchi and have been discussed in this article, for example, hot water brushing (Lichter et al., 2000), modified atmosphere storage (Chen et al., 2001; Ketsa and Leelawatana 1990), bio-control agents (Sivakumar et al., 2007), skin/ surface coating (Datta et al., 1963; Sandhu and Randhawa 1992), cold storage (Liu et al., 2011; Khan et al., 2012), and irradiation (Ilangantileke et al., 1993).

COLD STORAGE

Litchi fruit are best stored at refrigerated temperatures (Tongdee et al., 1982; Paull and Chen, 1987; Huang and Wang, 1990; Schutte et al., 1990; Fontes et al., 1999; Johnson et al., 2002). ‘Haak Yip’ (syn. ‘Fay Zee Sui’) stored for 40 days at 7°C had superior pericarp color to fruit stored at 10, 5, or 0°C (Tongdee et al., 1982). ‘Hei Ye’ (syn. ‘Fay Zee Sui’) stored at 5°C had superior color to fruit stored at 2 or 0°C and superior rot control at 2°C than at other temperatures (Huang and Wang, 1990); and ‘Bombai’ kept better at 4°C than 0°C (Mitra et al., 1996).

Cold storage duration is also very important to litchi shelf life. The temperature increment from 3–5°C to 25°C induced marked increases in activities of lipase, phospholipase D (PLD), and lipoxygenase (LOX). Litchi fruit stored for 10 days at 3–5°C followed by the shelf time at 25°C had lower activities of lipase, PLD, and LOX, and also lower levels of membrane permeability, than fruit stored for 20 and 30 days (Liu et al., 2011). Energy level of the pericarp tissue of cold-stored litchi fruit was similarly dependent on storage time at 3–5°C plus shelf time at 25°C. Jiang et al. (2003) also suggested that the litchi fruit can have a storage life of around 30 days at 3–5°C. Postharvest pericarp browning and fruit quality deterioration can be effectively delayed by cold storage (Khan et al., 2012).

Loosely synthesizing these results, the optimum storage temperature for the retention of pericarp color appears to be between 5 and 10°C, while that for the control of rot development is between 2 and 5°C. This being said, however, Kremer-Kohne and Lonsdale (1991) found that ‘Mauritius’ (syn. ‘Tai So’) browned more slowly after storage at 2°C than at 6°C, and (Zhang and Quantick, 2000) that ‘Huaizhi’ (syn. ‘Wai Chee’) browned more slowly at 1°C than at 5°C. Archibald and Bower (2008) observed that fruit stored at 5.5°C showed a statistically superior color than those at 1°C and remained good for up to 40 days of storage. This higher storage temperature resulted in better color retention but greater incidence of disease.

Paull and Chen (1987) found that temperatures of 2°C resulted in slower pericarp browning and decay than 22°C. The minimum temperature recommendations vary from 0–7°C, depending on the length of storage. Swarts and Anderson (1980) recommended 0–1°C for up to 30 days, and these temperatures are used for the export of litchi fruit from South Africa. Kadam and Deshpande (1995) recommended temperatures of 5°C for 2 weeks of storage and temperatures of 7°C for 3–4 weeks of storage.

PRE-COOLING

Litchi fruits are harvested in May–June when the ambient temperature is high. The harvested fruits, which are soft and juicy, require immediate temperature protection. This can be immediately done by keeping the fruits under shade until they are transported elsewhere. It is essential to remove the field heat of fruits for extended shelf life. Hence, the fruits are pre-cooled. Different types of pre-cooling like air cooling, hydro-cooling, and vacuum cooling have been adopted for pre-cooling the litchi fruits.

‘Hong Huay’ litchi fruits were pre-cooled immediately after harvest by hydro-cooling and room cooling at the production site before packing in corrugated boxes with and without polyethylene (PE) film liners and shipped to the wholesale market by truck within 12 h and stored at 5°C [80% relative humidity (RH)]. The results showed that the internal temperature of litchi fruits was reduced to 5°C within 18 min by hydro-cooling, while room cooling took 71 min. Hydro-cooled litchi fruits packed in corrugated boxes with PE film liners had the best appearance at day 6 during storage. Regardless of pre-cooling, litchi fruits packed in corrugated boxes with PE film liners had longer storage life than those without PE film liners (Ketsa and Leelawatana, 1992). Pornchaloempong et al. (1997) found that the internal temperature of litchi fruit was reduced from 25–27°C to 3°C in 12–15 min in 0–1°C water by hydro-cooled ‘Mauritius’ (syn. ‘Tai So’).

Litchi can be in excellent condition for 12 months when freshly harvested and rapidly pre-cooled at −25°C. In sea transportation at 2°C, fruits, which were hydro-cooled immediately after harvest and packed in plastic bags, remained sound for over a month (Moreuil, 1973).

Fruit can be cooled more quickly by water cooling than by air cooling but the benefits are ambiguous. Ketsa and Leelawatana (1992)found that hydro-cooled ‘Hong Huay’ (syn. ‘Tai So’) fruit were less susceptible to browning than control fruit or forced air cooled fruit; Pornchaloempong et al. (1997) also found a similar effect with ‘Mauritius’ (syn. ‘Tai So’), but Coates (1995) reported significantly higher decay in hydro-cooled than control fruit for ‘Kwai May Pink’.

MODIFIED ATMOSPHERE STORAGE

Modified atmosphere packaging (MAP) has the advantage of low cost and easy implementation at the commercial level (Flores et al., 2004; Mangaraj and Goswami, 2011a, 2011b). Although MAP has been reported to prolong postharvest quality of litchi fruit, the detailed effects of MAP on physiological and biochemical alterations during storage have not been entirely defined or explained (Somboonkaew and Terry, 2010). Modified atmosphere packaging provides three advantages: (i) it helps to reduce browning; (ii) it controls postharvest diseases and maintains a high humidity environment for fruit inside the sealed plastic film, and (iii) it prevents cross-contamination during transportation and storage (Kader, 1994; Pesis et al., 2002; Sivakumar et al., 2007).

The MAP system increased the shelf-life of litchi fruit by 100–150% that of unpacked fruit at various storage temperatures with a quality comparable with freshly harvested fruit. MAP of EDTA-treated fruit resulted in a reduction of weight loss, prevention of browning, and retention of good color and sweet taste during extended storage (Mangaraj et al., 2012).

Modified atmosphere storage in plastic bags and sealed containers has been reported to reduce pericarp browning in litchis (Chen et al., 2001; Ketsa and Leelawatana, 1990). However, Pesis et al. (2002) found that litchis packed in micro-perforated polyethylene bags had less decay, but poorer taste, than fruit in non-microperforated bags.

Somboonkaew and Terry (2010) reported that packaging films were suitable to retain better quality of imported litchi cv. Mauritius fruit during 9 days of storage as compared to unwrapped fruit. PropaFresh^TM PFAM packages significantly reduced sugar transformation and retained anthocyanin contents resulting in brighter redness in the pericarp as well as limiting fruit weight loss, maintaining anthocyanins, sugar, and organic acids in both aril and pericarp.

Combination treatments EDTA, 4-HR, or B. subtilis in BOPP-1 inhibited polyphenol oxidase (PPO) and significantly reduced pericarp browning and severity. Although the combination treatments EDTA, 4-HR, or B. subtilis in BOPP were equally effective in controlling decay and browning, the EDTA and 4-HR affected the natural pinkish-red color of the pericarp by showing higher h1 values (orange–pink). Among the combination treatments, B. subtilis + BOPP-1 had the best potential to control decay, retain the color, and the overall litchi fruit quality during a marketing chain of 20 days (Sivakumar et al., 2008).

CONTROLLED ATMOSPHERE STORAGE

Litchi cv. Huaizhi stored in pure O₂ (100% O₂ and 0% CO₂) for 6 days at 28°C showed significantly reduced pericarp browning (Duan et al., 2004). It is evident from their investigations that pure O₂ inhibited the activities of PPO and anthocyanase involved in the enzymatic browning mechanism. Therefore, pure O₂ atmosphere helps to prevent the degradation of anthocyanin by preventing the hydrolysis of sugar moieties from anthocyanin to anthocyanidin and the degradation of anthocyanidin by PPO to brown polymers.

Higher O₂ concentration in controlled atmosphere (CA) storage limit the PPO and POD activities, maintain higher anthocyanin levels, prevent decay, and retain good fruit quality (Tian et al., 2005). According to Tian et al. (2005), the anthocyanidin content in the pericarp decreased slowly when compared to the control, and the ethanol content responsible for the off-flavors were reduced when the fruit were exposed to 70% O₂ for 1 week, followed by 5% O₂ + 5% CO₂ at 5°C. Superatmosphere O₂ at 50% showed a significant effect on inhibition of browning in cv. Hong Huay for 8 days longer than the ambient temperature, but increased concentrations up to 70% did not show additional control of browning (Techavuthiporn et al., 2006).

Controlled atmospheres of 3.0–5.0% O₂ and 3.0–7.5% CO₂ prolonged litchi fruit storage life; maintained quality; and reduced browning, decay, and decreases in ascorbic acid, TSS, and TA, as compared with air-stored controls (Jiang and Fu, 1999b; Mahajan and Goswami, 2004; Pornchaloempong et al., 1998; Vilasachandran et al., 1997). Controlled atmospheres resulted in less water loss than air controls, despite the fact that both were maintained at 90% RH (Jiang and Fu, 1999). High O₂ treatment in CA storage can limit ethanol production in the aril (Tian et al., 2005). Pure O₂ treatment inhibited activities of PPO and anthocyanase, which may account for slower browning (Duan et al., 2004).

It has previously been reported that CA can prolong the impact of 1-methylcyclopropene (1-MCP) on both physical and sensory responses of apple and these two technologies generally are most effective when used in combination (Bai et al., 2005; Rupersinghe et al., 2000; Watkins et al., 2000). The 1- MCP has been reported to reduce internal browning of pineapple stored at 10°C for 4 weeks (Selvarajah et al., 2001).

Sivakumar and Korsten (2010) investigated that the 1-MCP pre-treatment with CA-1 condition (3% O₂ and7% CO₂) at 2°C for 21 days were more effective in preventing pericarp browning along with delaying senescence, limiting oxidation enzymes activity, maintaining anthocyanin content and acceptable taste, flavor, and overall acceptability. However, the effectiveness of this treatment has to be tested with different cultivars, late seasonal fruit, and the time delay between harvesting and packing operations.

CA reduced litchi decay and maintained peel color better than air during 3 weeks of storage at 5°C and 90–95% RH and after 1 day at 20°C. CA 4:7.5 was found to best maintain red color and minimize peel browning (Pornchaloempong et al., 1998).

HEAT TREATMENT

Hot water brushing has been established as an effective method for improving the quality of various fruits and vegetables, such as bell pepper (Fallik et al., 1999) and mango (Fallik et al., 1999; Prusky et al., 1999). Hot water brushing (HWB) was shown to remove dirt, dust, and fungal spores from the fruit skin as well as to seal microscopic cracks, thus maintaining better keeping quality of the fruit (Fallik et al., 1999), and it not only retards pathogen development but also affects the susceptibility of fruit to infection (Shirra et al., 2000).

Physiological studies demonstrate that polyphenol oxidase (PPO) activity is reduced by the HWB procedure as compared with controls but not to the same extent as inhibition by SO₂ treatments. In addition to its effect on PPO activity, HWB may lead to reduced pH of the pericarp, or more uniform distribution of the acid in it. This result suggests that HWB may act by bruising the external layer of the pericarp allowing the peel to be uniformly exposed to the acid, which may inhibit PPO activity and maintain the anthocyanins in their red-pigmented form (Lichter et al., 2000).

Litchi fruit exposed to steam treatment at 98°C for 30 s followed by hydro-cooling in distilled water at pH 0 for 5 min preserved the red color of the pericarp during storage. However, this technology failed to reach commercial acceptance since the steaming process affected the edible portion (aril) of the fruit (Kaiser et al., 1995).

Jacobi et al. (1993) reported that vapor heat treatment at 45°C core temperature for 42 min maintained the quality of Tai So and Wai Chee litchi cultivars at 5°C for 4 weeks, retaining the appearance and increasing disease control. They also reported that Taiwanese litchi cultivars respond well to vapor heat treatment suggesting that the cultivars were more heat tolerant.

Olesen et al. (2004) reported that the hot water spray was equally effective as the hot water dip. Fruit treated to 52°C for 2 min had significantly slower color decline (slower initial decline in L, slower decrease in chroma, slower increase in hue angle) than the control fruit and the fruit treated to 48°C for 2 min.

The effect of pH is also reflected in the assays for PPO: the phosphate buffer was supposed to normalize the assay conditions but was affected by the acidification of the pericarp. For instance, Jiang et al. (1997) reported that a pH of 4.2 or less in the pericarp may abolish PPO activity and stability and HWB may bring it below this threshold. Hot water treatments of litchi fruits have been reported in combination with the fungicide benomyl that is very effective against pericarp browning (Scott et al., 1982).

The degree of quality loss increased with increasing hot-water immersion temperature (Follett and Sanxter, 2003). Dipping in hot water (98°C) for 30 s, followed by treatment with oxalic acid, enhanced the effectiveness of oxalic acid and resulted in the retention of pericarp redness and gave the best browning inhibition during the storage time by reducing the activities of polyphenol oxidase and peroxidase and maintaining a high level of total anthocyanins in cv. Hong Huay (Saengnil et al., 2006).

Oxalic acid is the most effective anti-browning agent on apple slices (Son et al., 2001), while HWT alone or HWT followed by an HCl dip are also effective in reducing browning and maintaining a distinct red color. These methods seem to be more attractive than the SO₂ treatment of litchi fruit, but after these treatments the aril becomes brown (Litcher et al., 2000; Kaiser, 1994).

SURFACE COATING

The use of coating formulations is another alternative that helps the fruit to retain moisture in the pericarp (Kester and Fennema, 1986). Skin coatings are generally effective in extending fruit storage life, but little benefit has been found for litchi fruit, due to either a continuous dehydration or a pericarp discoloration (Jiang et al., 2003). This result may partially explain the inability of current commercial coatings to restrict water loss and, therefore, inhibit resultant pericarp browning. Thus, different types of coatings based on the characteristics of litchi fruit require further investigation. Acidification of the peel surface and application of coatings can retard dehydration and discoloration (Jaos et al., 2005; Kaewchana et al., 2006; Plotto et al., 2006) by decreasing PPO activity and maintaining anthocyanin in their red-colored form.

Several coatings have shown some degree of success in controlling desiccation browning. ProLong coatings (sucrose esters at 1.5% or 2.5%) also slightly delayed the peak in PPO activity and showed some success in reducing browning in litchi fruit stored at 4°C (Zhang et al., 1997). Polysaccharide coatings were shown to delay browning to some extent, but were not considered commercially viable (York, 1995). Despite some success, coatings are not commonly used in commercial operations.

The loss of pigmentation from the litchi cv. Brewster pericarp was slowed even at room temperature by reformulating a coating of hydroxypropyl cellulose with increase concentration of citric acid (McGuire and Baldwin, 1996). McGuire and Baldwin (1998) reported that the litchi coated with carrageenan and sucrose ester formulations developed less anthracnose decay in cold and ambient storage, and these coatings maintained the red hue of the fruit pericarp and the color intensity.

Chitosan (a high molecular weight cationic polysaccharide) is soluble in dilute organic acids and could theoretically be used as a preservative coating material for fruits (Jiang et al., 2005). The coating is also safe (Hirano et al., 1990; Joas et al., 2005) and shows antifungal activity against several fungi (El Ghaouth et al., 1992; Li and Yu, 2001; Romanazzi et al., 2002).

In a recent study, Zhang and Quantick, (1997) and Jiang and Li (2000) reported that the application of 2% chitosan extended the storage life of ‘Huai zhi’ fruit at 5°C. Since chitosan is soluble in 2% acetic acid, dipping in acidic chitosan solution inhibited PPO activity, and thus, may assist in delaying the pericarp browning of the fruit.

The application of chitosan along with ascorbic acid is very effective to lowered PPO and POD activities during the whole storage time along with increasing the shelf life (Sun et al., 2010). By developing novel strategies of combining ascorbic acid with chitosan coating, pericarp browning and its injury degree were reduced. A similar result was observed by Dong et al. (2004) in peeled litchi fruit.

Jiang et al. (2005) also suggested that the application of chitosan coating delayed the decrease in anthocyanin content, the increase in PPO activity and the changes in color index and eating quality, reduced the decrease in concentrations of total soluble solids and titratable acidity, and partially inhibited decay. The results suggested that treatment with chitosan coating exhibited a potential for shelf life extension at ambient temperature when litchi fruit were removed from cold storage.

Another combination of chitosan and citric acid treatment increased the shelf-life of the fruit by at least 3 weeks compared to untreated control fruit that brown rapidly (Ducamp-Collin et al., 2008). In fact, the coloration does not change during several months, so that shelf-life becomes limited by internal quality rather than external appearance.

Use of organic acids in association with chitosan, instead of sulphur compounds or strong mineral acids, seems to provide an interesting technological alternative for the prevention of browning of litchi fruit (Joas et al., 2005; Caro and Joas, 2005). This treatment has been demonstrated to be more effective when pericarp pH is close to 3, i.e., the pericarp PPO and POD pKs (Le Roux, 1999), by minimizing the impact of postharvest dehydration on the rate of browning.

Plotto et al. (2006) found that the sucrose fatty acid ester formulation Semperfresh preceded by an acid dip, had a positive effect on hue, chroma, and browning control of ‘Mauritius’. Carnauba, a carnauba-based coating, also had a beneficial effect on decay control on ‘Mauritius’, with or without dipping litchi in acid prior to coating.

Further, 1.0% sucrose fatty acid ester (SFE) treatment delays the browning development by approximately 8 days when stored at 5°C with 90–95% RH as compared to the control. It was most effective in reducing weight loss and maintaining higher water content. The effect was also partly due to the inhibition of polyphenol oxidase [catechol oxidase] (PPO) and phenylalanine ammonia-lyase (PAL) activity. All SFE treatments reduced PPO and PAL activity but did not affect total phenolics content (Kaewchana et al., 2006).

Treatment of litchi fruit with acidified coatings, including Semperfresh, acidified Semperfresh (with 2% citric acid), Semperfresh litchi treatment power (LTP) ± citric acid, and chitosan + HCL sometimes resulted in brighter red color (Rattanapanone et al., 2007). Acidified coatings can both lower the fruit surface pH, and thereby reduce the dehydration that contributes to the browning of the peel. Carnauba, a carnauba-based edible coating formulation, had a beneficial effect on decay control on ‘Mauritius’, with or without dipping litchi in acid prior to coating (Plotto et al., 2006).

BIOLOGICAL CONTROL

A number of antagonists have been identified and used for the biological control of the pathogens of litchi (Korsten et al., 1993), strawberry (Tronsmo and Dennis, 1977), mango (Koomen and Jeffries, 1993; Chuang and Ann, 1997), nectarine (Pusey and Wilson, 1984; Smilanick et al., 1993), apple (Piano et al., 1997), apricot (Pusey and Wilson, 1984), avocado (Korsten et al., 1988, 1995), citrus (Wilson and Chalutz, 1989; Huang et al., 1992; Arras, 1996; Arras et al., 1998), longan (Jiang, 1977), pear (Janisiewicz and Roitman, 1988), plum (Pusey and Wilson, 1984), and tomato (Mari et al.,1996).

The biocontrol agent Bacillus subtilis was found to be effective in controlling postharvest decay in litchi cultivars Madras (Korsten et al., 1993) and Huaizhi (Jiang et al., 2001) when kept at cold storage (5°C). Jiang et al. (2001) reported that the culture supernatant of Bacillus subtilis controlled the major litchi pathogen, Peronophythora litchi. Treated fruit could be stored for about 30 days at 5°C with good out-turn quality, whereas about 65% of the water-treated (control) fruit had begun to rot. However, application of the microbial metabolites was less effective in controlling fruit decay at room temperature.

B. subtilis + BOPP-1 had the best potential to control decay, retain the color, and retain the overall litchi fruit quality during a marketing chain of 20 days (Sivakumar et al., 2008).

IRRADIATION

Irradiation in combination with low temperature storage may be recommended as an alternative to SO₂ fumigation during short-term storage (less than 10 days; Ilangantileke et al., 1993). It also helped in maintaining the organoleptic, physical, and biochemical attributes, including antioxidant and radioprotective properties of the fruit (Hajare et al., 2010; Saxena et al., 2011).

A combination treatment of dip and radiation processing was found to be effective in almost completely eliminating the microbial load, leading to extension in shelf life of litchi cultivars Shahi up to 45 days and China up to 30 days of storage at 4°C (Kumar et al., 2012). The processing drastically reduced the activity of PPO inhibiting fruit pericarp browning, and allowing significant retention of anthocyanins cyanidin-3-O-rutinoside and cyanidin-3-O-glucoside and hence the attractive reddish color of the fruit. The processing did not adversely affect any quality attributes of litchi fruits. This can help in expanding the market access for the fruit in non-producing regions.

Irradiation treatment showed differential responses with respect to cultivar and dosage. According to Ilangantileke et al. (1993) irradiation up to 1 kGy dose, in combination with low temperature storage, maintained the market quality of ‘Thai litchi’ by reducing the loss of red pericarp color and decay. However, it failed to retain the overall fruit quality during prolonged cold storage. Further, irradiation is not commercially practiced in many countries for fresh commodities due to the psychological perception of consumers, regarding the safety of irradiated food for human consumption (Jiang et al., 2003).

A low dose of gamma radiation (0.5 kGy) and low temperature storage (4°C) helped in extending its shelf life up to 28 days, by reducing the microbial load and inhibiting the postharvest physiological spoilage (Hajare et al., 2010).

NOVEL TECHNIQUES

Air currents can greatly exacerbate postharvest moisture loss in fresh produce, but no quantitative research has been available on the effect of air currents on moisture loss in litchi fruit. The relationship between air current speed and moisture loss was explored by exposing fresh litchi fruit to various wind speeds and measuring loss of mass. The air currents model is very effective for maintaining the fresh, red color so characteristic of the litchi fruit (Bryant, 2012).

Yang et al. (2009) suggested that the exogenous adenosine triphosphate (ATP) supply reduced reactive oxygen species production, maintained relatively membrane integrity and, thus, inhibited the pericarp browning of harvested litchi fruit. A similar result was obtained by Yi et al. (2008) who reported that the ATP treatment inhibited disease development and pericarp browning of P. litchii inoculated litchi fruit. Treatment with 1 mM ATP effectively reduced skin browning, disease development, and membrane permeability and maintained eating quality of litchi fruit (Song et al., 2006).

CONCLUSIONS

Browning is the most important factor affecting the shelf life and quality of litchi. With the potential harmful effects of chemical treatments like SO₂there is interest in the application of non-chemical treatments to control the pericarp browning of litchi. Since low temperatures reduce both water loss and disease incidence, it remains the most promising means of limiting browning. The effects of modified and controlled atmospheres, used on their own or in combination with postharvest treatments, such as heat, surface coatings, or biocontrol agents, are promising and should be further investigated. Processing is also another way to maintain the quality of litchi. However, there is still a need to concentrate on finding alternatives to fungicides and sulfur dioxide. Future research will be required to achieve better understanding of all of the non-chemical methods.

Acknowledgments

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