POSTHARVEST SULPHUR DIOXIDE FUMIGATION AND LOW ACID DIP FOR PERICARP COLOUR RETENTION AND DECAY PREVENTION ON LITCHI Agricultural Research and Extension Unit ABSTRACT Litchi (Litchi chinensis) is a tropical fruit of high commercial value. Its main postharvest problem is the rapid pericarp browning followed by decay and loss of flavour. Consequently, export of high quality litchi fruits depends exclusively on post-harvest treatments to suppress pericarp browning and disease development. The current standard treatment of litchi fruits in Mauritius is sulphur dioxide fumigation to overcome browning. This treatment bleaches the fruits which gradually turn light yellow to pink but the fruits never revert to their initial red attractive colour. To preserve and enhance peel coloration and to reduce post-harvest decay, sulphur-treated litchi fruits were dipped in different concentrations (3.5, 3.0, 2.5, 2.0 %) of hydrochloric acid solutions combined with 0.2 %. Sulphur treated fruits with no acid treatment, fruits treated with 3.5 % HCL alone and untreated fruits were kept as control. All acid treated fruits helped the fruits to recover an attractive bright red pericarp colour. However with dipping in 3.0 % hydrochloric acid being the most promising treatment. The latter also gave an effective control of post-harvest decay thus extending shelf life to 5 weeks under storage at 2 C and 90 95 % R.H as compared to 2 weeks for sulphur treated fruits that were not acid treated. KEYWORDS: pericarp, post-harvest, sulphur dioxide fumigation, browning, acid treatment INTRODUCTION The litchi fruit is in high demand as an exotic commodity because of its appealing natural colour, rich taste and aroma. Its red pericarp colour is an important attribute for high quality fresh fruits. Litchi is classified among the highly perishable tropical fruit. Pericarp browning has long been considered as the main post-harvest problem of litchi (Akamine, 1960). Under ambient conditions, once harvested litchi fruits have a short post-harvest life. Unless desiccation is controlled, the bright red pericarp turns brown and brittle within 2 to 3 days. This process is accelerated by micro-cracking of the pericarp, which leads to leakage of the cytoplasm and consequent fungal infection (Underhill and Simons, 1993). These factors contribute to poor consumer appeal and hence a dramatic reduction in its commercial value. Among the methods that have been devised to retain litchi peel colour (Holcrof & Mitchan, 1996, Ray, 1998) suphur dioxide applied as fumigant is one of the most widely and cost effectively used methods. It helps to prevent skin browning and controls saprophytic surface fungi, two of the major post-harvest problems on harvested fresh litchi (Swarts, 1983; Zauberman et al., 1989). The suphur dioxide treatment causes the red peel colour to be bleached to yellowish green, which is slowly and partially restored to dull yellow to pink colour after 24 to 48 hours under ambient conditions. The fruit does not revert to the original red colour. Furthermore this treatment holds no benefit for a storage period beyond 1 2 weeks as mould begins to develop. Sulphur dioxide interacts with the membranes, making the rind pliable and leaky to solutes. It also directly reacts with the anthocyanin rendering them colourless and stabilizing them against further degradation (Timberlake and Bridle, 1975). In 1989, Zauberman et al reported that dipping of sulphur-treated litchi fruits in dilute acid helped to promote fruit quality and restore the red colour after bleaching. Later studies by Fuchs et al. (1993) also showed that litchi pericarp colour after sulphur dioxide treatment was significantly improved by AMAS 2003. Food and Agricultu ral Research Council, Réduit, Mauritius. 41
acid treatment. Low ph dip helps to restore the red colour to the anthocyanin, rendered colourless after sulphur dioxide treatment. The aim of this study was to develop a postharvest treatment that would restore the red colour of fruits, inhibit browning and suppress disease development. Low concentration of hydrochloric acid dip treatments after sulphur dioxide fumigation were evaluated on the pericarp colour retention, postharvest disease development and quality of litchi fruits stored at 2º C with RH 80 95 %. The hydrochloric acid concentrations used were 3.5 %, 3.0 %, 2.5 % and 2.0 %. MATERIALS AND METHODS For the litchi season 2001 2002, fruits of the variety Taiso, were harvested at full maturity from a commercial orchard and brought to the Laboratory within 2 hours for trial. The fruits with no defect such as cracks, insect punctures and skin tearing were selected. The fruits were then packed in ventilated crates and treated for 25 minutes with SO 2 fumes by burning of sulphur powder under a tarpaulin (@1 kg of pure sulphur powder / 1,600 kg of fruits), similar to the current practice adopted by local litchi exporters. The SO 2 -treated bleached fruits were then aerated for one hour before being stored at 2º C. After an overnight storage at 2º C following SO 2 treatment, the fruits were divided into five different batches and 4 batches were dipped for 2 minutes in the respective concentrations of hydrochloric acid solutions: 3.5%, 3.0 %, 2.5 % and 2.0 % (30 %HCL: water mixture made in the ratio of 1:8, 1:10, 1:12 and 1:15). The last batch was not dipped into acid. An additional batch of fruits not treated with SO 2 and acid was included in the experiment. Preliminary studies conducted earlier showed that S0 2 treatment alone was not effective in controlling fungal growth on litchi peel. Exclusion of a fungicide resulted in development of mould mainly composed of Penicillium spp. which developed during cold storage period and spread rapidly when transferred at ambient conditions (22-25º C). This suggests that the pathogens are deeply embedded in, or strongly adhere to, the rough surface of the peel. All fruits except those untreated received a fungicide, (tradename - Sportax) at the rate of 0.2 % for the control of mould growth. (Sportax) at the rate of 0.2 % was applied after S0 2 treatment or included in the acid dip solution. Each treatment was replicated 4 times with fruit samples constituting of 50 individual fruit units each. After each acid dip treatment, 3 randomly drawn samples of 10 fruits each were analysed for total soluble solid content (TSS) and % acidity (titratable acid content). Similarly 3 equivalent samples of untreated freshly harvested fruits were also analysed. The treated samples were air dried for 10 20 minutes, packed in clear perforated polyethylene bags (thickness 40 µ), tied and then stored at 2º C at 85 95 % relative humidity. After 2 weeks of storage fruit samples were assessed for weight loss, pericarp browning, post-harvest disease incidence and peel colour acceptability at regular weekly interval. Extent of pericarp browning was assessed using a scale of 0 5 as given in Table 1. The average browning index was obtained by taking the mean index of individual fruits. Fruits samples with an average index above 3 were considered commercially unacceptable. Fruits were also examined for mould development and were considered infected when a visible lesion was observed. Results were expressed as percentage of fruits infected on a 0 to 5 scale as shown in Table 1, while fruit peel colour acceptability was based on a arbitrary scale of 1 to 5 where 1= very poor and 5 = excellent (as given in Table 1 ) At each assessment, fruit aril of each sample was also evaluated for sensory attributes like, sweetness, sourness and presence of off-flavour using a group of 8 untrained panelists. The assessment was recorded on an arbritrary scale of 1 9, where 1=lowest (extremely weak) and 9 = highest (extremely strong). 42
Table 1 Scales used for quality assessment Pericarp browning index Post-harvest disease index Peel colour acceptability index 0. no browning (excellent quality) 0. no fruit infected 5 excellent 1. slight browning 1. > 0-5 % infected fruits 4 good 2. < ¼ browning 2. > 5-10 %infected fruits 3 fair (acceptable) 3. ¼ to ½ browning 3. > 10 25 % infected fruits 2 poor (unacceptable for export) 4. > ½ to ¾ browning (poor quality) 4. > 25 50 % of infected fruits 1 very poor (totally unacceptable) 5. >3/4 browning (very poor quality) 5. > 50 % of infected fruits RESULTS AND DISCUSSION Evaluation of fruit quality PERICARP COLOUR In general, fruits treated with SO 2 followed by acid dip resulted in better coloured fruits when compared to the control. The acid dip helped the bleached fruits to restore a bright red attractive and appealing peel colour. Furthermore no browning occurred on fruits from any of the acid treatments. (Figure 1) While, no significant difference in pericarp browning was noted between untreated fruits and fruits treated only with hydrochloric acid (3.0% for 2 minutes). Both had short shelf life as their pericarp browning index significantly increased with increased storage time (P<0.05). The fruits started to show sign of pericarp browning as from the first week of storage at 2 C and were almost of unacceptable peel colour after 3 weeks of storage. SO 2 treatment alone produced fruits with an acceptable appearance with a colour ranging from yellow to pink-red. However, during storage the rate of browning index of SO 2 treated fruits changed significantly more slowly than that of untreated fruits or fruits treated with hydrochloric dip only. Figure 1 Changes in fruit pericarp colour during storage at 2 C, 85 95 % relative humidity 5 4 Mean Pericarp browning index 3 2 1 0 0 2 3 4 5 Storage Period weeks Untreated HCl dip + S02 fumigation + 43
Fruit cracking A problem associated with acid dip was fruit cracking. Micro-cracking of the pericarp may serves as entry for microorganisms and leads to leakage of fruit juice leakage and consequently fungal infection (Underhill and Simons, 1993). Preliminary observation trial showed that acid dip carried out just after SO 2 treatment, when the fruits were at ambient temperatures resulted in high % of cracking (27.3 %) but when the SO 2 treated fruits were cooled to 10 C and 2 C before acid dip, the % incidence of cracking declined from 5.2 % to 0.04% respectively (Table 2). Hence, in later experiments the fruits were cooled overnight at 2 C before acid dip treatment. With this treatment fruit cracking was not apparent visually. This showed that by lowering the fruit temperature to 2 C at the time of acid dip was effective in reducing or eliminating fruit cracking, which is an important attribute of quality. Table 2 Fruit cracking as a function of temperature of fruits at time of acid dip treatment Fruit temperature ( C) % of fruits with cracked peel Approx 25 (ambient room condition) 27.34 10 5.22 2 0.04 Effect of treatments on total soluble solids and acidity of fruit pulp Chemical analysis of samples of litchi fruit aril before and just after acid treatments showed no significant change in total soluble solid content (TSS measured in o Brix) and % acidity (titratable acid content) (Table 3). Result obtained is in line with works carried out by Zauberman et al (1989, 1991) and Underhall and Critchley (1990) where they demonstrated very little evidence of acid penetration into the pulp after acid dip treatment and thus no remarkable effect on the eating quality. Table 3 Mean TSS and % acidity of measured litchi samples prior to storage TSS content ( º Brix ) % Titratable acidity Mean SE ± Mean SE ± No treatment 19.20 0.16 0.48 0.02 SO 2 fumigated 20.44 0.24 0.45 0.05 SO 2 fumigated + 3.5 % HCl acid dip 19.37 0.25 0.46 0.02 SO 2 fumigated + 3.0 % HCl acid dip 18.64 0.27 0.44 0.01 SO 2 fumigated + 2.5 % HCl acid dip 19.59 0.32 0.44 0.00 SO 2 fumigated + 2.0 % HCl acid dip 19.82 0.23 0.44 0.01 ns ns Effect of storage on quality of fruit pulp Result of this experiment showed that both TSS and % acidity showed a general tendency to decrease with storage time. The % acidity decreased significantly (P<0.05) with increase in storage time at 2 o C, while the drop in TSS content is significantly less pronounced. Over the storage period, the differential rate of decrease in brix and % acidity leads to an increase in the sugar to acid ratio (Figure 2). This imbalance in the sugar: acid ratio is confirmed by a reduction in the eating quality as the fruits developed a bland taste, with an increase in storage time. 44
Mean % weight loss Postharvest sulphur dioxide fumigation and low acid dip for pericarp colour retention and decay prevention on litchi. Effect of treatment on weight loss of fruit During storage at 2 C, no significant difference in weight loss was observed among the different treatments (Figure 3). Weight loss increases continuously with storage time. The average % weight loss recorded after 2 and 5 weeks of storage at 2 C, RH 85-95 % were 0.8 and 1.7 % respectively. Figure 2 Changes in mean TSS and % acidity observed in fruit aril during storage period at 2 C, 85-95 % RH. 20 0.5 19 0.4 18 0.3 17 0.2 16 0.1 15 0 1 2 3 4 5 6 Storage Period ( weeks ) Mean TSS Mean % acidity 0 Figure 3 Mean weight loss recorded over storage period at 2 C, RH 85-95 %. 2.5 2.0 1.5 1.0 0.5 0.0 0 1 2 3 4 5 6 Storage Period ( weeks ) 45
Effect of treatments on control of postharvest diseases Results of disease development showed that postharvest disease incidence of fruits treated S0 2 fumigation followed by hydrochloric acid dip with were significantly (p<0.05) lower compared to fruits treated with HCl and or fruits treated with S0 2 and (Figure 4). Untreated fruits showed high level of disease incidence (Table 4). Fruits treated with S0 2 and Prochoraz or HCl and showed significantly less infection than untreated fruits. These results agree with previous findings on litchi cv Mauritius and Red McLean (Duvenhage, 1994). Table 4 Result of quality assessment of fruit stored for 5 weeks at 2 C, RH 85-95 % Mean % weight loss Pericarp browning index Mean disease severity index Peel colour acceptability index Untreated 1.84 ns 5.00 c 4.80 e 1.00 d SO 2 only 1.80 ns 1.82 b 2.20 c 2.46 c HCl dip only 1.67 ns 5.00 c 3.00 d 1.00 d SO 2 + 1: 8 HCl dip 1.53 ns 0.00 a 0.00 a 5.00 a SO 2 + 1:10 HCl dip 1.60 ns 0.00 a 0.00 a 5.00 a SO 2 + 1:12 HCl dip 1.85 ns 0.00 a 1.20 b 4.56 b SO 2 + 1:15 HCl dip 1.66 ns 0.00 a 1.80 c 4.48 b Values in each column not followed by the same letters are significantly different according t o Duncan s Multiple range test (p=0.05) Fruits treated with S0 2 followed by HCl and showed a good control of mould development. Furthermore it was noted that with acid concentration of 3.5 and 3.0 % no contamination was obtained up to 5 weeks of storage at 2 C and RH 85 95 % but as the concentration of HCl decreased, there is indication of increase in disease index. This indicates that HCl itself has a fungistatic effect. Figure 4 Postharvest disease severity index recorded on fruits subject to the different treatments under storage Mean disease severity index 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 1 2 3 4 5 Storage Period ( weeks ) Untreated SO2 + 2.5 % HCl + HCl dip + SO2 + 3.0 % HCl + SO2 fumigation + SO2 + 3.5 % HCl + SO2 + 2.0 % HCl + 46
Fruit sensory evaluation An informal taste panel set up with 8 unexperienced panelists to evaluate quality of fruits following the different treatments showed that there was no significant difference in the sweetness and sourness index assessed after 2 weeks of storage at 2ºC (Figure 5). Compared to other treatments, the mean off flavour index was found to be significantly higher for fruits treated with 3.5 % HCl alone or following S0 2 treatment. Treatment with 3.0% HCL followng S0 2 treatment did not differ sighnificantly from the untreated or S0 2 treatment or lower acid dip concentrations. Figure 5 Result of sensory evaluation after 2 weeks storage at 2ºC 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Untreated S02 fumigation + HCl dip + S02 + 3.5 %HCl dip + S02 + 3.0 % HCl dip + S02 + 2.5 % HCl dip + Sweetness index Sourness index Off Flavour index S02 + 2.0 % HCl dip + CONCLUSION Additional treatment by HCl acid dip (3.0% for 2 minutes) resulted in fruits superior in quality as compared to application of S0 2 alone. It was found that the acid treated sulphured fruits were not prone to pericarp browning up to 5 weeks at 2ºC, RH 85 95 %. This treatment was also useful in suppressing mould development especially at higher concentrations of acid. The addition of gave an even better control of disease development. Moreover, the use of S0 2, HCl and did not alter the eating quality of the fruits. Further experimentation to develop recommendation for commercial application need to be carried out to determine the timing for the treatment after harvest and optimum level of HCl to be used. Possibility for identification of alternatives to fungicidal treatments has also to be undertaken. ACKNOWLEDGEMENTS I wish to express my thanks to Mr. Eshan Gobindram, Assistant Research Scientist, for assisting me in this experiments, Mr R. Ramnauth, the Senior Biometrician for his support in data analysis and guidance, Mrs N.Ramburn for reviewing the manuscript and the taste panelists for the sensory evaluation. 47
REFERENCES AKAMINE EK. 1960. Preventing the darkening of fresh litchi prepared for export. Technol. Progr. Rep. Hawaii Agri. Exp. Station. 127, 1 17. DUVENHAGE JA. 1994. Control of postharvest decay and browning of litchi fruit by sodium metabisulphite and low ph dips an update. South African Litchi Growers Association 6. 36 38. FUCHS Y, ZAUBERMAN G, RONEN R, ROT I, WEKSLER A, and AKERMAN M. 1993. The physiological basis of litchi fruit pericarp colour retention. Acta Horticulture 343, p 29-33. HOLCROFT DM, MITCHAM EJ. 1996. Postharvest physiology and handling of litchi ( Litchi chinensis Sonn. ). Postharvest Biol. Technol. 9, 265-281. RAY PK. 1998. Postharvest handling of litchi fruits in relation to colour retention a critical appraisal. Journal of Food Science and Technology. 35, 103-116. SWARTS DH. 1983. Post-harvest handling of litchis. Tech. Bull. L1.1. Citrus Subtrop. Fruit. Res. Inst. Nelspruit, South Africa. TIMBERLAKE CFand BRIDLE P. 1975. The anthocyanin. In: Harborne JB, Mabry TJ, Mabry H. eds. The Flavonoids. Chapman and Hall. London, pp. 214-266. UNDERHILLSJR and SIMONS DH. 1993. Lychee (Litchi chinensis Sonn.) pericarp desiccation and the importance of postharvest micro-cracking. Scientia. Hort. 54: 287-294. ZAUBERMAN G, RONEN R, AKERMAN and FUCHS Y. 1989. Low ph treatment protects litchi fruit colour. Acta Hort. 269, 309-314. 48