Uses and Nutritional Composition

The principal use of watermelons is consumption of its crisp, succulent, cooling flesh as a dessert or snack. The rind may be used as a basis for a delicious sweet pickle. Seeds are dried and used as a snack food in China, Israel and perhaps elsewhere. In Africa seeds may be ground into a coarse flour or oil may be extracted from them. Imperfect fruit are used as livestock feed and immature fruit may be prepared and used as summer squash. In the drier areas of Africa watermelon fruit provide a source of liquid as a ‘living canteen’. In Japan, perfect specimens are used as gifts for important occasions. Finally, watermelons are the object of competition in the southern United States and elsewhere. Who can grow the largest watermelon? A 293 lb. watermelon was grown by B. Carson of Arrington, Tennessee, USA in 1990. Seed spiting contests are popular at local festivals and the record distance for this event is 75 ft. Folklore relating to watermelon culture and use in the southern United States abounds.

            Watermelon fruit are not outstanding in any of the traditional nutritional components (See the Nutritional Composition of Watermelon table). Seeds, however, are relatively rich in protein, lipids, and carbohydrates. Used mostly as a snack food, their total contribution to nutrition is also relatively small.

Nutritional Composition of Watermelon. Amounts per 100 g (3 ½ oz) edible portion. Source: USDA 2003.

Sweetness   

Sweetness in Diploid and Triploid Watermelon Fruit

1. Donald N. Maynard, Amy M. Dunlap, and Brian J. Sidoti

Gulf Coast Research and Education Center, University of Florida, 5007 60 St. E., Bradenton, FL 34203

Internal watermelon fruit quality is a composite of flesh color and texture, freedom from defects (such as hollowheart), optimum maturity, sweetness, seed size and frequency in diploids, and freedom from seeds in triploids (Maynard, 2001).

According to Maynard (2001), "sweetness, one of the prime quality factors in watermelon fruit is related to total soluble solids (TSS), as measured by ^°Brix with a refractometer." The U.S. Standards for Grades of Watermelons (USDA, 1978) indicates watermelons may be labeled as having good internal quality with 8% TSS as determined in a random sample by an approved refractometer. Likewise, fruit may be labeled as having very good internal quality with TSS of 10% or greater. Having personally sampled thousands of watermelon fruit, it is my contention that fruit with 8% TSS are in fact not very good and those with 10% TSS are barely enjoyable. Most people would thoroughly enjoy fruit with 11%-12% TSS".

TSS is a measure of the concentration of the reducing sugars fructose and glucose and the non reducing sugar sucrose. The relative concentration of these sugars is influenced by cultivar and stage of maturity. Glucose and fructose concentrations generally increase up to 24 days after pollination (DAP) and decline thereafter. Sucrose is first detectable at 20 DAP and increases thereafter. The relative concentration of these sugars is important since they vary in perceived sweetness with sucrose having a value at 1.0, glucose 0.60-0.75, and fructose 1.40-1.75. Accordingly, cultivars or maturity that result in high fructose concentrations is a desirable feature (Elmstrom and Davis, 1981).

TSS are determined routinely by watermelon breeders and cultivar evaluators as one estimator of fruit quality along with a myriad of other characteristics.

Methods. Diploid and triploid watermelon cultivars and advanced experimental hybrids were evaluated each spring season from 1991 through 2001 at the University of Florida's Gulf Coast Research and Education Center at Bradenton. The number of entries in each class was determined by commercial seed producer submissions. TSS were determined on two fruit in each plot at each harvest. Accordingly, determinations were made on 12 fruit with three replications and two harvests or on 16 fruit with four replications and two harvests for each entry. A hand-held refractometer (Atago ATC-1, 32-10 Honcho, Itabashi-ku, Tokyo 173-001, Japan) was used from 1991 to 1998 and a digital refractometer (Atago PR-101) was used from 1999 to 2001 for TSS determinations. Fruit were sampled by cutting from stem to blossom end, removing a section of tissue from the center (heart) of the fruit, and squeezing a few drops of juice on the refractometer prism surface. TSS data were subjected to analysis of variance and Duncan's multiple range test was used for mean separation (SAS, 2001).

Results. TSS for diploid and triploid watermelon cultivars that were evaluated at least four seasons are shown in Table 1. The range in diploid TSS was from 11.4% for 'Festival' to 12.6% for 'Sultan'. For triploid cultivars, TSS varied from 11.7% for 'Jack of Hearts' to 13.4% for 'Tri-X-Carousel'. TSS of 11 triploid cultivars exceeded the highest diploid fruit TSS. Only two triploid cultivars ('Summer Sweet 5032' and 'Jack of Hearts') had TSS that were lower than the second highest TSS that was found in diploid 'Royal Majesty' fruit.

The average TSS of diploid and triploid watermelon fruit by year from 1991 through 2001 averaged over all cultivars and determinations in that year is shown in Table 2. Average TSS in triploid fruit was higher than that in diploid fruit in 9 of 11 years and there was no difference in the other 2 years. TSS in diploid fruit varied from 11.1% in 1991 (Maynard, 1991) to 12.9% in 2000 (Maynard and Dunlap, 2000) while triploid fruit TSS ranged from 12.0 in 1991 and 1997 (Maynard, 1997) to 13.8% in 2000. The 1991 (486 mm) and 1997 (423 mm) spring seasons were characterized by much higher than normal rainfall during the growing season, whereas rainfall was sparse during the spring 2000 (114 mm) season. Accordingly, the highest TSS in watermelon fruit occur in seasons of low rainfall, usually accompanied by high light and low disease incidence; conditions that also favor high TSS. The 11-year average TSS was higher in triploid fruit, 12.7%, than in diploid fruit, 11.8%. What factors may account for or contribute to higher TSS in triploid watermelon fruit than in diploid fruit? Some possibilities are: 1) energy used to produce seeds in diploid fruit is diverted to sugar production in triploid fruit, 2) triploid plants are generally larger than diploid plants and therefore have greater photosynthetic potential, 3) triploid fruit are generally smaller than diploid fruit so that equivalent sugar content is concentrated in triploid fruit. There may be other explanations as well.

Literature cited

1. Elmstrom, G. W. and P. L. Davis. 1981. Sugars in developing and mature fruits of several watermelon cultivars. J. Amer. Soc. Hart. Sci. 106:330-333.

2. Maynard, D. N. 1991. Seedless watermelon variety evaluation, Spring 1991. GCREC (University of Florida) Res. Rept. BRA 1991-21.

3. Maynard, D. N. 1997. Triploid watermelon cultigen evaluation, Spring 1997. GCREC (University of Florida) Res. Rept. BRA 1997-15.

4. Maynard, D. N. 2001. An introduction to the watermelon p. 9-20. In: D.N. Maynard (ed.) Watermelons. Characteristics, production and marketing. ASHS Press, Alexandria, Va.

5. Maynard, D. N. and A. M. Dunlap. 2000. Triploid watermelon cultigen evaluation, Spring 2000. GCREC (University of Florida) Res. Rept. BR-A2000-4.

6. SAS. 2001. SAS Institute Inc., Cary, N.C.

7. U.S. Dept. Agr. 1978. U.S. standards for grades of watermelons. AMS, USDA, Washington, D.C.

Table 1. Total soluble solids of diploid and triploid watermelon cultivars included in at least four trials. Gulf Coast Research and Education Center, University of Florida.

`Mean separation in columns by Duncan's multiple range test, 5% level.

Table 2. Total soluble solids of diploid and triploid watermelons by year. Gulf Coast Research and Education Center, University of Florida.

'Mean separation in rows by Duncan's multiple range test, 5% level.

Lycopene  

           Red-fleshed watermelon fruit contained 6300 to 6800 mg/100 g lycopene whereas orange-fleshed and yellow-fleshed fruit contained 370 to 420 mg/100 g and 10 to 80 mg/100 g fresh weight, respectively. Within the red-fleshed types, fruit of diploid (seeded) hybrid varieties generally had higher lycopene concentrations than fruit of diploid (seeded) open-pollinated varieties. One exception was ‘Dixielee’ that was developed specifically for intense red flesh. Triploid (seedless) variety fruit had lycopene concentrations as high or higher than those in fruit of diploid (seeded) hybrid varieties. Fruit at peak ripeness had higher lycopene concentrations that unripe (-7 days) or overripe (+ 7 days) fruit. Minimally processed (fresh cut) watermelon fruit lost about 10% of its lycopene after 7 or 10 days at 2^oC.

            Until recently, lycopene was thought to be important only as it contributed to flesh color but it is now recognized that it makes an enormous contribution to human health. Watermelon flesh has an average of 4100 mg/100 g (range 2300-7200) lycopene compared to 3100 mg/100 g (range 879-4900) in raw tomato, 3362 mg/100 g in pink grapefruit, and 5400 μg/100 g (range 5340-5500) in raw guava. Processed tomato products have two or three times greater lycopene concentrations than raw tomatoes because of water depletion in those products. Lycopene is one of the major carotenoids in Western diets and accounts for about 50% of the carotenoids in human serum. Among the common dietary carotenoids, lycopene has the highest singlet oxygen quenching capacity in vitro. Other outstanding features are its high concentration in testes, adrenal gland, and prostate. In contrast to other carotenoids its serum values are not regularly reduced by smoking or alcohol consumption, but is reduced by increasing age. Remarkable inverse relationships between lycopene intake or serum values have been observed in particular for cancers of the prostate, pancreas, and to a certain extent of the stomach. In some studies, lycopene was the only carotenoid associated with risk reduction. Accordingly, although watermelon consumption may help reduce risk of certain cancers, tomato has a larger role in risk reduction because of greater per capita consumption, 165 lb, compared to 17 lb for watermelons.