extension of bitter gourd (momordica charantia l.) storage life through the use of reduced...

EXTENSION OF BITTER GOURD (MOMORDICA CHARANTZA L.)

STORAGE LIFE THROUGH THE USE OF REDUCED TEMPERATURE AND POLYETHYLENE WRAPS

MAJEED MOHAMMED' and LYNDA D. WICKHAM

Department of Crop Science Faculty of Agriculture

The University of the West Indies St. Augustine, Trinidad and Tobago, W.I.

Accepted for Publication May 14, 1993

ABSTRACT

Bitter gourd (Momordica charantia L.) fruits were stored individually wrapped in low density polyethylene (LDPE) film or unwrapped for up to 21 days at 5-7C, 20-22C and 28-30C, respectively. Assessment was done on several quality parameters including marketable quality. Storage offilm-wrapped fruit at 5-7C resulted in extension of shelf-life in excess of two weeks and delayed appearance of chilling injury symptoms. Additionally, film-wrapped fruits stored at 5- 7C were still marketable after 21 days, had lowest fresh weight losses, less softening, reduced incidence ofpostharvest rots and minimal changes in vitamin C content and pH. Storage of individually wrapped fruits at reduced temperatures therefore offers an effective method of prolonging the shelf-life of bitter gourd.

INTRODUCTION

Bitter gourd (Momordica charantia, L.) is a member of the Cucurbitaceae family and is also called, depending on the country where it is cultivated, bitter-melon, African cucumber, karela, carille, art pumpkin, balsam pear, maiden apple or koe. Perhaps better known for the use of leaf, root and fruit for medicinal purposes (Okabe et al. 1982; Awasthi and Jaiswai 1986), the commodity has great potential as a food source in both developing and industrialized countries and is rich in iron, phosphorus and ascorbic acid. In the Caribbean,as in the Far East and India, the young fruits are normally cooked

'Mailing address: Dr. M. Mohammed, Department of Crop Science, Faculty of Agriculture, University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies.

Journal of Food Quality 16 (1993) 371-382. All Rights Reserved. Copyright 1993 by Food & Nutrition Press, Inc., Trumbull, Connecticut. 37 1

3 72 M. MOHAMMED AND L.D. WICKHAM

and eaten as a vegetable. To a lesser extent, bitter gourds are also canned in the form of pickles and the seed used as a condiment.

The fruit is intensely bitter, long, green and warty in appearance, and is highly fragile and perishable (Snowdon 1991). It is produced and consumed mainly in tropical and subtropical areas of the world with high levels of consumption and popularity among ethnic minority groups in the more developed countries (Hallam and Molina 1988). Within recent years, the demand for bitter gourd for use as a fresh vegetable has grown in Europe and North America among Chinese, African and Indian ethnic populations. Quantities exported from Trinidad and Tobago to North America and Europe have trebled from 1989 to 1991 (Trinidad and Tobago Export Development Corp. 1992). Export potential could be expanded if a larger proportion of high quality bitter gourds could reach overseas markets. However, increase in quantities exported is constrained by the rapid rate of postharvest deterioration and the lack of documented information on postharvest handling and storage characteristics of the fruit.

The bitter gourd fruit has a high moisture content, a large surface area:volume ratio and a relatively thin cuticle, which make it very susceptible to moisture loss and physical injury. Senescence of the fruit is rapid under tropical ambient conditions and is seen as early signs of yellowing. This is followed by changes normally associated with ripening of the fruit: excessive softening, development of a bright yellow color, and intense red pigmentation of the arils. This is undesirable as the fruit is generally eaten only in the green, turgid state. Fungal and bacterial rot also contribute to postharvest deterioration of bitter gourds. Thus, the rate of moisture loss, senescent changes and microbial invasion all have to be reduced for successful postharvest maintenance of quality. Reduced temperature storage is the obvious alternative but, as for so many other commodities of tropical origin, the incidence of chilling injury limits its use.

Several studies have demonstrated that individual wrapping of various fruits and vegetables with polymeric films can reduce spoilage, extend shelf-life, retard ripening and reduce chilling injury (Ben-Yehoshua 1985; Purvis 1985; Rij and Ross 1988; Mayberry and Hartz 1992). The objective of this study was to test the efficacy of polyethylene wrapping in extending storage life of bitter gourds and its effect on incidence of chilling injury during reduced temperature storage.

MATERIALS AND METHODS

Two separate experiments were conducted on bitter gourd fruits approximately 1 h following harvest from a farmer’s plot in Central Trinidad.

EXTENDING STORAGE LIFE OF BITTER GOURD 373

Fruits free from physical injuries, insect damage and pesticide residues were sorted according to weight (1 65-200 g each) and color and washed in a 500 ppm sodium hypochlorite solution in two large plastic containers (20-L capacity). Fruits were spread in a single layer on absorbent paper and air-dried for 45 min with an oscillating fan to ensure complete evaporation of water droplets from the fruit surface.

In experiment 1, fruits were .either individually hand-wrapped and sealed in low density polyethylene (LDPE), 12.7 pm thick (gas transmission rates at 22C, 1 atm.:- 630ml m-*h-' CO,; 109ml m-*h-' 0,, or left unwrapped (air control) and stored in ventilated cardboard cartons in single layers in separate storage rooms at 5-7C and 60-70% r.h.; 20-22C and 50-60% r.h. and 28-30C and 55-60% r.h. up to 18 days. At three-day intervals fruit samples were assessed for percentage weight loss, firmness, pH, vitamin C content, chilling injury, decay and general marketable quality.

The weight of each fruit was taken before and after each storage interval for calculation of percentage weight losses. Firmness was determined on the opposite sides of each fruit at 4 locations with a Chatillon (Model DDP-5) penetrometer, equipped with a 0.64 cm V-shaped chisel hammerhead and a gauge to record the force (g cm-2) required for penetration (Miller et al. 1983). The pH of expressed juice was determined with an Orion digital pH meter. Marketable quality was evaluated according to the scoring method used by Sherman and Allen (1983) as 1-9: 1, unusable; 3, unsaleable; 5, fair; 7, good; and 9, excellent. Vitamin C was determined by titrating a 25 ml sample of the fruit extract plus 1 ml of 10% potassium iodide and 2 ml 2N sulphuric acid with 0.0025 N potassium iodute. The Vitamin C equivalent of the iodate was calculated and expressed as mg 1OOg-' fresh weight (Kefford 1957). Fruits were classified into one of five chilling injury categories where 1 = no injury; 2 = slight; 3 = moderate (limit to marketability); 4 = slightly severe with moderate decay and 5 = extremely severe with extensive secondary infections. The chilling injury index was determined for each fruit by summing the products of the number of fruits in each category and then dividing this sum by the total number of fruits assessed (Wild and Hood 1989; Hatton and Cubbedge 1982).

The experiment consisted of six replicates with each replicate containing 10 fruits. Data were analyzed as a completely randomized design with a factorial arrangement of variables, and significance tested by the F- test and Duncan's multiple range test where applicable, after transformation for ranking (Steel and Torrie 1960).

Koch's postulate (Brathwaite 1981) was carried out on decayed fruits to determine the fungi that were responsible for the decay. Samples from the tip of the advancing edge of the lesion were removed and cultured in a sterile environment on neo-peptone glucose (NPG) agar plates until pure colonies were retained. Each genus was identified and used to inoculate mature-green fruits

374 M. MOHAMMED AND L.D. WICKHAM

to distinguish fungi that were naturally associated with bitter gourd from those that were pathogenic on it. Identification was done by matching the spores or fruiting bodies of the fungi under the light microscope. Prior to visual identification, fungal spores were placed on (a) the unbroken surface of the host fruit and (b) open wound on the surface of the host fruit.

In experiment 2, bitter gourds were obtained from the same field and subjected to similar selection and prestorage procedures as described in Experiment 1. Treatments consisted of individually wrapped and sealed, and unwrapped fruits being stored at 5-7C up to 21 days. Data were taken on percentage weight losses, chilling injury index, bioelectrical resistance and electrolyte leakage at 7 day intervals. At each interval, three replicates of 3 fruits per replicate were transferred to 20-22C and 28-30C for 1 and 2 days, respectively, to simulate market conditions and examined for the parameters mentioned above. Storage regimes (SR1 to SR5) used were as follows:

SR1 = 7, 14 or 21 days, respectively, at 5-7C; SR2 = 7, 14 or 21 days, respectively, at 5-7C + 1 day at 20-22C; SR3 = 7, 14 or 21 days, respectively, at 5-7C + 2 days at 20-22C; SR4 = 7, 14 or 21 days, respectively, at 5-7C + 1 day at 28-3OC; SR5 = 7, 14 or 21 days, respectively, at 5-7C + 2 days at 28-30C.

Bioelectrical resistance was determined with a resistance meter (02-86 Shigometer) using the techniques of Weaver and Jackson (1966) as modified by Lougheed et al. (1981). The two fine nickel-plated electrodes were inserted into two opposite sectors around the equatorial region of each fruit and the average of six readings for each fruit was recorded. The current pulse was 0.5pA for 0.5 ms, and the interval between pulses was 10 ms. Measurement range was 0-500 Kilohm.

The changes in permeability of mesocarp tissue of bitter gourds were determined using a method modified from that of Bangert (1974). Tissue cylinders were extracted with a metal cork borer from the equatorial zone of each fruit and divided into 10 discs each 1.50 mm thick and 6.0 mm in diameter. The discs were rinsed three times in 0.4 M mannitol and incubated in 40 ml of 0.4 M mannitol in test tubes sealed with rubber stoppers at 26 k 0.5C in a shaking water bath for 3 h, After incubation, relative electrolyte leakage from the tissue discs was estimated by measuring conductivity of the mannitol solution with a Fisher conductivity meter (Model 152), equipped with a immersion type temperature-compensated 1 cm conductivity probe. Total electrolyte leakage was taken following freezing of the tissue discs together in the incubation solution for 24 h then thawing in a shaking water bath at 26 -+ 0.5C. Electrolyte leakage after 24 h was calculated as a percentage of total electrolyte leakage. Both experiments were conducted twice.


TABLE 1 . EFFECT OF POLYETHYLENE WRAP ON MARKETABLE QUALITY, FIRMNESS,

PERCENTAGE FRESH WEIGHT LOSSES, PH AND VITAMIN C OF BITTER GOURDS STORED AT 20-22C AND 28-30C AFTER 3 AND 6 DAYS

Storage Temperature ( C )

Parameter Wrapping Treatment 20-22 c 28-30 C

3 days 6 days 3 days 6 days

Marketable Wrapped 1 . 4 0 dy 2.40 b 4.20 c 2.10 b QualityZ Unwrapped 5.80 c 1.70 a 6.20 d 1.50 a

Firmness Wrapped 421.00 d 300.00 b 349.20 c 136.50 a (g c c 2 I Unwrapped 342.00 c 161.10 a 200.10 b 130.00 a

Weight Wrapped 0.86 a 1 . 2 1 a 0.68 a 17.70 c Losses(%) Unwrapped 11.63 b 12.69 b 7 . 4 1 b 29.95 d

PA Wrapped 5 . 4 1 a 5.76 bc 5 . 8 1 a 5.98 b Unwrapped 5.61 b 5.96 c 5.73 a 6.69 c

Vitamin C Wrapped 5.28 a 3.83 a 6.85 b 2.98 d

( mg/ 1009-1) Unwrapped 5.45 a 3.88 a 7 . 0 3 b 3.88 a

'Rated on a scale of 1-9, with 9 = excellent and 1 = unusable YMean separation by Duncan's multiple range test, p = 0.05

RESULTS AND DISCUSSION

Bitter gourds stored at 5-7C gave an increased storage life that was 15 days longer than those stored at 20-22C and 28-30C (Fig. 1 , Table 1). At the two higher temperatures, storage life was shortened due to rampant growth of stem scar and surface decay organisms, which rendered all fruits unmarketable after 6 days (data not shown).

Refrigerated storage of individually wrapped bitter gourds conferred additional benefits by delaying ripening and senescence when compared with unwrapped fruits. The reduced breakdown of respiratory substrates at 5-7C vs. 20-22C and 28-30(3, in conjunction with the polyethylene wrap, which provided an effective moisture vapor barrier, accounted for lower fresh weight losses, better maintenance of fruit firmness and superior marketable quality ratings in wrapped compared with unwrapped fruits (Fig. 1, Table 1). Unwrapped fruits at 5-7C maintained a green color and were decay-free throughout. However, marketable quality declined to 5.6 after 18 days due to


I S.E. = 20.8

chyho ~ l u r y 3.6 1 1

I -.-+ ,./- .A,

1.61

= S.E.= ? .I2 6.6

I

3 6 O l 2 1 6 % 0.Yr

FIG. 1. EFFECT OF POLYETHYLENE WRAPPING ON MARKETABLE QUALITY, WEIGHT LOSSES, PH AND CHILLING INJURY OF BITTER GOURDS

STORED AT 5-7C FOR UP TO 18 DAYS


extensive shrivelling as a result of the 37.9% reduction in weight losses, reduction in firmness and the appearance of visible symptoms of chilling injury (Fig. l), as indicated by pitting on ribbed regions that frequently coalesced longitudinally. Unwrapped fruits stored at the two higher temperatures and particularly those stored at 28-30C, had a shorter shelf-life (6 days) than fruit that were stored wrapped and sealed at lower temperatures (18 days). Fruits stored at the higher temperatures appeared desiccated and ripe. A distinct color change from green to orange-yellow was accompanied by a marked splitting of fruit and a color change of the arillated seeds from creamy-white to an intense red.

Wrapped fruits were still of exceptional quality after 18 days of refrigerated storage (Fig. 1). Because nonperforated wraps were used, the atmosphere surrounding the fruit was very likely modified with respect to CO, and O2 concentration, which may have also influenced the delayed evidence of ripening and senescence. In other studies, Lester and Bruton (1986) reported that a reduction in water loss accomplished by individually wrapping cantaloupe delayed ripening. Rij and Ross (1988) made similar claims from their investigations on shrink-wrapped honeydew melons, although the mechanism by which shrink-wrapping produce delays ripening remains to be determined.

The extent of changes in pH of the fruits during storage (Fig. 1, Table 1) may have been related to the water-saturated atmosphere within the wrap. Thus, the lower pH for wrapped versus unwrapped bitter gourds may have resulted from the concentrating effect of water loss in unwrapped fruits on the intracellular contents (Hulbert and Bhowmik 1987; Anthony et al. 1989). The increased levels of CO,, which resulted from the modified atmosphere, may also have resulted in the lower pH of wrapped fruits (Kader et al. 1973).

The absence of a significant reduction in vitamin C of fruits stored for 3 and 6 days at 5-7C compared with fruits at 20-22C and 28-30C3, respectively, (Table 1) was in agreement with similar deductions by Watada (1987) with respect to retention of vitamin C in vegetable crops stored under reduced temperatures.

Visible symptoms of chilling injury apparent in unwrapped samples after 12 days at 5-7C (Fig.1) appeared to be alleviated by the polyethylene wrap. Further investigations were necessary to verify this, since serious chilling injury problems can be encountered when fruits are removed from low temperature storage and kept at room temperature, as is normally done at the retail market. Thus fruits were stored at 5-7C for a longer period (21 days) and examined at 7 day intervals and upon transfer for 1 and 2 days at 20-22C and 28-30C respectively (Experiment 2). Data indicated (Table 2) that chilling injury symptoms were not evident until the third seven-day interval (SRI) and were again significantly higher in unwrapped than wrapped samples. Electrolyte leakage, which is an objective measurement of chilling injury (King and Ludford


1983; Lougheed et al. 1981), corresponded to the development of visible chilling injury symptoms that ranged from moderate to severe with respect to time and polyethylene wrap treatments (Tables 2,3). However, differences in bioelectrical resistance measurements shown in Table 3 suggested that wrapped fruits had significantly (P< 0.05) higher readings than unwrapped fruits as early as 7 days at 5-7C, that is, even when visible symptoms were not apparent. Also, fruits transferred to higher temperatures gave significant reductions in bioelectrical resistance much earlier than electrolyte leakage and visible symptom expressions (Table 3). These findings suggest that membrane integrity was impaired due to chilling injury prior to the appearance of visible symptom expression and determined much earlier with bioelectrical resistance measurements than electrolyte leakage. Accordingly, bioelectrical resistance is the more sensitive indicator of chilling injury and has the potential for use as a predictor of early symptom development.

TABLE 2. EFFECT OF POLYETHYLENE WRAP ON PERCENTAGE WEIGHT LOSSES AND

CHILLING INJURY INDEX OF BITTER GOURDS AFTER 7, 14 AND 21 DAYS AT 5-7C PLUS 1 AND 2 DAYS, RESPECTIVELY, AT 20-22C AND 28-30C

Storage period (days)

Parameter Storage 7 days 14 days 2 1 days Regime

W vw W uw W vw

Weight SR1 0.4 aY 11.9 c 0.5 a 2 6 . 1 c 0 . 9 a 38.4 e losses ( 2 ) SR2 0 . 6 ab 16.3 d 0 .8 ab 2 8 . 1 d 1 . 8 ab 42.2 f

SR3 0.7 ab 21.5 e 0 . 9 a 32 .0 e 3 . 1 bc 45.7 g SR4 0 . 8 ab 25 .5 t 1.4 b 33.4 f 3 . 8 cd 54.1 h SR5 0 . 9 b 27.8 g 1 . 5 b 35 .5 g 4.4 d 60 .6 i

Chi l l ing

indexX SR2 1.0 a 1.6 ab 1 . 0 a 2 . 9 d 3.2 ab 4 .3 O e SR3 1 . 6 ab 2 . 8 de 1 .9 bc 3 . 6 e f 3.5 bc 4 .6 eg SR4 1 .9 bc 3 . 1 e 2 .3 c 3.9 f g 4.2 de 5 . 0 f SR5 2 . 3 cd 3.7 f 2 . 8 d 4 . 2 g 4 . 6 eg 5 . 0 f

S R 1 1 . 0 a 1.0 a 1 . 0 a 1 . 9 bc 2.8 a 3 . 8 cd injury

W = Wrapped fruit

Y = Mean separation by Duncan’s multiple range test, P = 0.05.

UW = Unwrapped fruit = Rated on a scale of 1-5, with 5 = extremely severe and 1 = none

Both experiments confirmed previous studies conducted by Wright (1974) that water stress or moisture loss is an essential prerequisite for the chilling response. Data in Fig. 1 and Tables 1 and 2 indicate that the higher weight losses experienced for unwrapped versus wrapped fruits corresponded to a greater severity of chilling injury as indicated by secondary infection, surface discoloration consisting of brown spots, russetting, and internal breakdown.


TABLE 3. EFFECT OF POLYETHYLENE WRAP ON BIOELECTRICAL RESISTANCE AND

ELECTROLYTE LEAKAGE OF BITTER GOURDS AFTER 7 , 1 4 AND 21 DAYS AT 5-7C PLUS 1 AND 2 DAYS, RESPECTIVELY, AT 20-22C AND 28-30C

Storage period (days)

Parameter Storage 7 days 14 days 2 1 days Regime

W uw W uw W UW

B i o e l e c t r i c a l p SR1 77 .4 gY 6 6 . 2 f 6 8 . 5 h 4 5 . 1 f 5 5 . 6 g 30 .9 e r e s i s t a n c e SR2 6 2 . 1 e f 5 1 . 1 cd 53 .0 g 46 .6 f 40.2 f 2 0 . 1 bc (kilohma ) SR3 5 6 . 1 de 4 2 . 0 b 4 0 . 6 e 37 .2 de 2 8 . 6 de 15 .9 c

SR4 5 0 . 3 cd 36.6 ab 35 .2 c d 3 0 . 0 b 2 2 . 6 cd 1 0 . 2 a SR5 4 0 . 1 b 3 1 . 0 a 2 8 . 5 b 2 3 . 6 a 1 5 . 0 b 6 . 6 a

E l e c t r o l y t e q SR1 42 .2 ab 4 2 . 0 a 40 .6 ab 38.5 a 3 8 . 0 ab 3 4 . 2 a leakage ( % ) SR2 43.6 abc 45.9 abcd 4 4 . 2 bc 4 6 . 1 cd 41.0 bc 50 .2 c

SR3 46 .6 abcd 47 .6 c d 48 .4 cde 49 .2 de 5 1 . 6 cd 52 .9 c d SR4 4 1 . 0 bcd 4 9 . 1 d e 4 9 . 9 def 52.6 ef 5 5 . 6 cde 60 .2 d e f SR5 49 .6 de 5 2 . 6 e 5 4 . 6 f g 5 8 . 1 g 6 3 . 3 e f 6 8 . 9 f

W = Wrapped fruit UW = Unwrapped fruit

Prestorage bioelectrical resistance = 88.6 kilohms Prestorage electrolyte leakage = 20.2% Mean separation by Duncan’s multiple range test, P = 0.05

Unwrapped fruits with a chilling injury rating of 4.0 and above (Table 2) had evidence of secondary fungal invasion on fruit surfaces and appeared very desiccated. Chilling injury occurred earlier in unwrapped than wrapped fruits; for example, unwrapped fruits after 14 days at SR5 had a chilling injury rating of 4.2. A similar rating was not obtained for wrapped fruits until 21 days at SR4. The severity of chilling injury and overall fruit senescence differed between wrapped and unwrapped treatments. Wrapped fruits with a chilling injury rating above 4.0 (SR4 and SR5 after 21 days) had large sunken pits that were water-soaked and dark brownish-green in color. These areas were well- macerated and invaded by bacterial soft rot (Erwinia Sp.) organisms. Fruits were mushy throughout with the pericarp, changing from a green to uniform yellow-orange color and the arils surrounding the seeds changing to scarlet. Unwrapped fruits from SR5 after 14 days and SR2 to SR5 after 21 days had intense color changes for pericarp and arils described above. Pits were enlarged, but remained dark green on the ribbed regions. Causal agents of surface rot were identified to be Fusarium sp., Curvularia sp., Gloesporium sp. and Chaetomella sp. After 6 days at the higher temperature, fruits exhibited a


longitudinal splitting upon color development similar to that observed during senescence on the vine and in Experiment 1.

With decay being a major limiting factor in wrapped and unwrapped samples, although at different times and temperatures, the need to explore the potential of prestorage treatments, such as hot water dips, to enhance bitter gourd quality is warranted. Further research is also underway to investigate the physiological significance of ripening and senescence of bitter gourds in relation to water stress and chilling injury.

ACKNOWLEDGMENTS

The technical assistance of Mr. Keshwar John is gratefully acknowledged.

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extension of bitter gourd (momordica charantia l.) storage life through the use of reduced...

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