UF/IFAS - Vegetarian Newsletter

A study was conducted to determine the effect of the ethylene antagonist 1-methylcyclopropene (SmartFresh^®, AgroFresh, Inc. Philadelphia, PA) on harvested, commercially ripe watermelon (Citrullus lanatus Thunb. Matsum and Nakai) fruit (cvs. Mardi Gras and Abbott and Cobb 5244, a seedless type) stored in the presence of ethylene or under commercial holding conditions. Watermelon exposed to ethylene, a possible occurrence for watermelon fruit held in mixed-commodity facilities or shipments, rapidly deteriorated, exhibiting flesh and rind water-soaking and otherwise appearing as over-ripe or chill-injured fruit. Prior treatment of watermelon fruit with SmartFresh^® afforded complete protection against the adverse effects of added ethylene. Furthermore, watermelon fruit maintained under commercial holding conditions (air storage at 55 ^oF) also benefited significantly from prior treatment with SmartFresh, indicating that the low levels of ethylene accumulating in storage facilities (below 50 ppb) can potentially damage watermelon fruit. The results show that SmartFresh^® has great potential to benefit the quality of watermelon fruit stored under a range of postharvest conditions.

Harvested watermelon fruit are very sensitive to ethylene gas, with injury occurring at concentrations of 1 ppm and lower. Symptoms of ethylene exposure in watermelon fruit include flesh deterioration and rind softening, rendering the fruit unfit for marketing and consumption. Since the primary injury occurs internally, the effects of ethylene are not easily determined from visual inspection of intact fruit. Interestingly, the symptoms of ethylene exposure bear visual similarities to over-ripeness or low-temperature injury. Accordingly, inadvertent exposure to ethylene, such as could occur during mixed-load shipments or during holding in distribution centers containing ethylene-producing commodities (eg., tomatoes, avocados) might result in injury to watermelon mistakenly attributed to causes other than ethylene.

The recent introduction to the market of SmartFresh^® (AgroFresh, Inc. Philadelphia, PA), an antiethylene product used commercially with apple and other ethylene-producing commodities, has proven effective in prolonging the shelf-life of many fruits. SmartFresh works by interfering with the ability of commodities to ‘see’ ethylene produced naturally, reducing the rate of ripening and permitting extended shelf life. In these cases, the objective is to reduce the rate of ripening and softening of the targeted fruits, while eventually allowing recovery from the effects of SmartFresh and resumption of normal ripening and quality characteristics.

In contrast to the climacteric fruits apples, tomatoes, and avocados that are harvested prior to full ripeness, watermelon fruit are ideally harvested at a ready-to-eat, fully ripe stage of development. It is at this time that the fruit possess optimum sweetness, aroma, texture, and other quality parameters. For this reason, the goal with watermelon fruit is to reduce or arrest continued development during storage. Since harvested watermelon fruit do not produce high levels of ethylene (Elkashif et al., 1989), yet respond negatively when exposed to ethylene of exogenous (external) sources (Elkashif and Huber, 1988), studies were conducted to determine the benefits of SmartFresh treatment on watermelon fruit prior to storage in the presence of added ethylene.

Since it is not uncommon for watermelon fruit to be held at 55 ^oF for up to 2 weeks and longer prior to marketing http://www.ba.ars.usda.gov/hb66/contents.html, other studies were performed to determine if SmartFresh would afford protection against the low levels of ethylene that can accumulate naturally in storage facilities.

Watermelon (Citrullus lanatus cv. Mardi Gras (Rogers brand from Syngenta) or Abbott and Cobb 5244, a seedless cultivar) fruit were harvested when commercially ripe in the Spring of 2001 and 2002 from plants grown at the Suwannee Valley Research Station, Live Oak, FL. A number of fruit were sliced at the time of harvest to confirm uniformity of ripeness, and the fruit were transported directly to the Postharvest Research facilities in Gainesville. Fruit were selected for uniformity of size and washed with sodium hypochlorite (150 µl l^-1 free chlorine) for postharvest decay control.

Watermelon fruit (Abbott and Cobb 5244) were placed in 174 liter (about 46 US gallons) steel chambers, sealed, and the containers provided with SmartFresh (5 ppm) generated from a commercial formulation (AgroFresh Inc., a division of Rohm and Haas Co.). Total exposure time to SmartFresh was 18 h at 20 ^oC (68^oF). Afterward, the containers were opened, vented to release excess SmartFresh, and fruit subsequently transferred to 174 liter containers and exposed to either air or ethylene for 8 days at 68 ^oF. Fruit were analyzed at intervals during storage.

In a separate experiment, harvested ripe watermelon fruit (cv. Mardi Gras) were exposed to either air or 1 ppm SmartFresh (24 h, 13 ^oC/55^oF ) and then stored in air for up to 21 d at 55 ^oF. This experiment involved no application of external ethylene and was designed to study the effects of SmartFresh application to watermelon fruit held under simulated commercial storage/shipping conditions (2 to 3 weeks at 55^oF).

The firmness of Abbott and Cobb 5244 watermelon fruit pretreated with air or SmartFresh [18 h at 20 ^oC (68^oF)] followed by storage in air or ethylene at 68 ^oF for 8 days is shown in Figure 1. Firmness was determined directly on the flesh of sliced fruit. This experiment used a relatively high (68 ^oF) temperature compared to recommended commercial storage recommendations in order to accelerate the effects of ethylene exposure over a relatively short storage period. As illustrated, watermelon fruit exposed to ethylene (G) deteriorated (softened) rapidly and extensively, being unsuitable for consumption after as few as 2 days exposure to ethylene gas. External symptoms were negligible after only 2 days exposure. In contrast, SmartFresh-treated fruit (#) were effectively protected from the effects of external ethylene, softening at rates comparable to fruit exposed only to air ("). Figure 2 illustrates the appearance of Abbott and Cobb 5244 watermelon fruit following 3 days exposure to ethylene, either without (left) or with prior exposure to SmartFresh.

Figure 1. Firmness of Abbott and Cobb 5244 watermelon fruit treated with SmartFresh or air followed by storage in air or ethylene at 68 ^oF. Fruit treated only with air ("); fruit treated only with ethylene (G); fruit treated with 5 ppm SmartFresh for 18 h before storage in air (!); fruit treated with 5 ppm SmartFresh for 18 h and then stored in continuous ethylene (#). Values are means  SE from 5 fruit, with 12 measurements performed per fruit.
1 Newton = 0.225 pounds

Figure 2. Cross section of Abbott and Cobb 5244 seedless watermelon fruit following exposure to ethylene for 3 days at 68 ^oF. Control fruit on the left, which did not receive SmartFresh, show clear evidence of watersoaking of both flesh and rind. Fruit on the right, pretreated with SmartFresh, was protected from the effects of ethylene exposure.

The benefits of SmartFresh treatment were quite evident for fruit exposed to added ethylene; however, close scrutiny of the data for day 8 of storage (see Figure 1) show that the flesh of fruit treated with SmartFresh followed by storage in air only (!) was significantly firmer than fruit stored in air without prior exposure to SmartFresh ("). These results prompted a study to examine the benefits of SmartFresh application for fruit destined for holding under simulated commercial conditions (55 ^oF, 85 to 90% RH) in the absence of exogenously provided ethylene.

Flesh firmness values of watermelon fruit, cv. Mardi Gras, during storage for up to 21 days at 55 ^oF are shown in Table 1. Firmness of control fruit (no SmartFresh) declined rapidly and continuously during storage, reaching after 13 days values 24% (no SmartFresh) lower than values at the start of the experiment. In contrast, fruit pretreated with SmartFresh had lost only 4% of their original firmness. After 21 days, control and SmartFresh-treated fruit had softened 39% and 11%, respectively. These firmness differences were also readily evident upon applying slight finger pressure to intact fruit (data not shown).

Table 1. Firmness of of control and SmartFresh-treated ‘Mardi Gras’ watermelon fruit during storage at 55 ^o F.
Days at 13 ^OC	Firmness (N)
	Control	SmartFresh-treated
	Mean ± SE	Mean ± SE
0 d (prior to 1-MCP treatment)	5.61 ± 0.28	5.61 ± 0.28
7 days	4.98 ± 0.33	5.51 ± 0.23
13 days	4.27 ± 0.33	5.37 ± 0.38
21 days	3.43 ± 0.29	4.97 ± 0.31

Fruit utilized in the second experiment, even though not provided with exogenous ethylene, clearly benefited from prior exposure to SmartFresh (see Table 1 and Figure 3). Storage facility ethylene in the second experiment demonstrated the presence of the gas at concentrations of 40 to 50 ppb (part per billion). These levels of ethylene are nearly 1000 times lower than those applied to the ‘Mardi Gras’ fruit in the first experiment, indicating that watermelon fruit subjected to storage for as few as 2 weeks can respond negatively to extremely low levels of ethylene.. Since these fruit were stored in the absence of other commodities, the ethylene accumulating in the storage facilities likely originated from the low levels of ethylene produced by the watermelon fruit (Elkashif et al., 1989). Evidently, the levels accumulating (40 to 50 ppb) are sufficient to inflict injury on unprotected (no SmartFresh) fruit, particularly during longer holding periods. The adverse influence of ultra-low levels (ppb range) of ethylene on a number of fruits and vegetables at distribution and retail facilities was reported by Wills et al. (2000).

Figure 3. Cross section of ‘Mardi Gras’ watermelon fruit (2 replicate fruits) after storage in air for
13 days at 68 ^o F. The arrows and the bordered enclosure illustrate regions of damage
in control fruit (no SmartFresh). Similar damage is not evident in the SmartFresh –treated fruit.

In summary, a single, short-term exposure of watermelon fruit to SmartFresh proved beneficial for fruit stored in the presence and absence of added ethylene. Consequently, watermelons so treated have potential for extended, high quality shelf life. Future experiments will address the use of SmartFresh-treated watermelon in fresh-cut operations. Watermelon fruit in fresh-cut form is extremely fragile compared with other fruits. The successful application of SmartFresh technology to fresh-cut operations with watermelon would be of immense benefit to the fresh-cut industry.

Elkashif, M. E. and Huber, D. J. 1988. Electrolyte leakage, firmness, and scanning electron microscopic studies of watermelon fruit treated with ethylene. J. Amer. Soc. Hort. Sci. 113:378-381.

Elkashif, M. E., D. J. Huber and J. K. Brecht. 1989. Respiration and ethylene production in harvested watermelon fruit: evidence for nonclimacteric respiratory behavior. J. Amer. Soc. Hort. Sci. 114:81-85.

Karakurt, Y., and Huber, D.J., 2002. Cell wall-degrading enzymes and pectin solubility and depolymerization in immature and ripe watermelon (Citrullus lanatus) fruit in response to exogenous ethylene. Physiol. Plant. 116, 398-405.

Wills, R. B. H., Ku, V. V. V., Shohet, D., and Kim, G. H. 2000. Importance of low ethylene levels to delay senescence of non-climacteric fruit and vegetables. Aust J Exp Agric 39, 221-224.

For further information on the commercial applications and availability of SmartFresh, contact AgroFresh, Inc., Philadelphia, PA or online at http://www.rohmhaas.com/agrofresh/ .