- Watermelon (Citrullus lanatus) is a member of the Cucurbitaceae family, along with squash, cucumbers, pumpkins, cantaloupes, and other melons
- This annual fruit crop has a smooth green rind; the flesh is most commonly red but may be yellow or orange
- Watermelon can be seeded or seedless, hybrid or open-pollinated
- Watermelon has been grown in the United States since the 16th century after being introduced in Florida and quickly spreading across the country
- Total U.S. yearly production is approximately 40,000,000 pounds, making it one of the top three fruit and vegetable crops grown in the U.S.
- Almost one fourth of watermelons consumed in the U.S. are imported from Mexico and Central America
- Outbreaks have most often been associated with pre-cut watermelons that are prepackaged alone or are included in fruit mixes.
Watermelons are a member of the Cucurbitaceae family, along with many other kinds of squash, melons, pumpkins, and cucumbers. The plants grow as vines and produce separate male and female flowers. Fruit weights range from 5-50 pounds depending on the variety and growing conditions. Open-pollinated and hybrid types are available, as well as a variety of flesh colors ranging from red to yellow. Rinds are smooth and green but can be striped, spotted, or solid and range from light green to almost black.
Watermelons were first cultivated in ancient Egypt and have since spread throughout the warm regions of the world. The fruit reached both India and China hundreds of years before the Americas, thought to have arrived in the 7th and 10th century, respectively. They were first brought to North America in 1673 by European colonists and the slave trade from Africa, leading to their introduction in the southern United States. The first U.S. watermelon were grown in Florida, which soon spread to Massachusetts and into Central America as colonists continued expanding their territory. Watermelons are still widely grown in Africa and used as a water source during times of drought. These sweet summer fruits are now produced in over 90 countries around the world.
Currently, the seedless watermelon is the most commonly produced and consumed type of watermelon. The first seedless watermelons were developed in the 1930s, however commercial production began in the 1990s. Until commercial production began, plant breeders sought to improve the sugar and flavor levels of the seedless watermelon that was once a blander variety. Seedless watermelons are self-sterile hybrids that are produced when crossing a diploid watermelon with a tetraploid watermelon, resulting in a genetic triploid watermelon that is unable to produce seeds. As result, these watermelons only produce the white, tasteless seed coats in the flesh that can go undetected while eating. Production costs of seedless watermelons are greater than their seeded counterparts due to high seed prices, germination demands, and requirement to interplant with seeded varieties; these higher production costs are reflected in the higher market prices of seedless watermelon. Despite these higher prices, consumer demand for seedless watermelon is high since this type is more convenient to eat.
While cantaloupes are the leading source of melon-associated outbreaks (58%), watermelon have been linked to 13 outbreaks between 1973 and 2011. Salmonella spp. were the causative agent for a majority (7/13) of the outbreaks, followed by norovirus (2/13). The point of contamination for most outbreaks are unknown, however production and point of service have been implicated as the primarily areas of concern.
In 2017, watermelons were associated with an outbreak of Salmonella Newport in Washington and Oregon. Pre-cut watermelon, pre-cut cantaloupe, and fruit mixes were linked to these Salmonella infections, leading to 24 total illnesses wherein 6 individuals were hospitalized, and one death was reported. Most cases reported pre-cut fruit at Fred Meyer and QFC grocery stores, leading the traceback investigation to the supplier Mary’s Harvest Fresh Foods.
Watermelons were included in a 2018 Salmonella outbreak and recall of pre-cut melons and mixed fruit salads containing melons. This outbreak, which spanned from April to July of 2018 involved 9 states. resulting in 77 illnesses and 36 hospitalizations. Investigation into the outbreak linked the Salmonella Adelaide outbreak strain to pre-cut melon supplied by Caito Foods, which included pre-cut with cantaloupe, pre-cut watermelon, and pre-cut fruit mixes. Caito Food initiated a recall on June of 2018 as a result of illnesses reported in the area and reports made by state departments of public health.
Caito Foods was linked to additional outbreak of Salmonella Carrau the following year that occurred between March 2019 and May 2019. A total of 137 people were infected during the outbreak, leading to 38 hospitalization and no reported deaths. Traceback evidence revealed that pre-cut melon from Caito Foods was the source of the outbreak; Caito foods subsequently issued a recall on pre-cut watermelon, pre-cut cantaloupe, and fruit mixes containing melon as they had the previous year. In February 2020, Caito Foods announced it would be closing their fresh-cut food manufacturing operations due to loss of a major customer.
According to the USDA, the U.S. produced 40.1 million pounds of watermelons in 2017. Recently seedless watermelons have surpassed seeded watermelons in production as seedless varieties are in higher demand by consumers. Most U.S. watermelons are produced in Texas, California, Georgia, or California. While most watermelon consumed in the U.S. are produced domestically, approximately one-quarter in the U.S. market are imported from Mexico or Central American countries. Worldwide, the U.S. ranks seventh in total watermelon production.
The ideal soil conditions for growing watermelons are sandy loam soils with good drainage and a slightly acidic pH. Loose and minimally compacted soil allow for optimal root growth. When planted, watermelon seeds will typically germinate between 68 and 95ºF. Often, watermelons will be grown as transplants in a greenhouse and planted in the field after three weeks.
Watermelon are a warm-season crop and are injured by temperature dropping out the ideal range. To maintain ideal soil conditions, winter cover crops help protect the soil by reducing compaction and adding organic matter. Since sandy soils require more frequent and light irrigation compared to heavy soils, drip irrigation is the common method for watering watermelon plants to ensure uniform watering and even moisture. With field irrigation, soil moisture should not drop below 50%.
Seedless watermelons have sterile pollen, necessitating seeded watermelon to be planted to provide pollen. Most strategies for pollinating involve planting a pollinizer row every third row to provide sufficient pollen to the seedless watermelons. Interplanting these rows of pollinizers will subsequently stimulate fruit growth of the watermelon. Both the seedless and the pollinizing seeded watermelons will be harvested and sold, so pollenizer rows are marked to separate them from the triploid varieties.
Time to reach maturity will vary based on the variety of watermelon, usually falling within the range of 70 to 90 days. Generally, smaller varieties will take less time to reach maturity than larger varieties. Seedless varieties usually fall at the end of this range, taking 80 to 85 days to reach maturity.
Watermelon are considered optimal for eating when flesh matures to a sweet flavor, crisp texture, and deep red color. Sugar content is an important factor for consumer, so melons can be selected from the field and tested for sugar content using a hand refractometer to assess ripeness of the field. Signs of a mature watermelon include wilting vines, yellowing of the ground spot, a soft hollow thumping sound, and the green band of striped varieties breaking up towards the ends of the watermelon.
Watermelon that are destined for distant markets are usually picked prior to peak ripeness to minimize flesh breakdown. Watermelons are hand-harvested, however should be minimally handled to reduce damage to the rind. The watermelons are typically harvested into bin, trucks, and buses for packing. Following harvest, cooling watermelons to 45-50ºF and maintaining a high relative humidity can help improve shelf life and taste by removing field heat that can continue ripening and lead to decay.
To maintain optimal quality, watermelon should be stored at 60ºF including when in transit, though most are shipped in trucks and trailers without refrigeration. During shipping and storage watermelons should not be enclosed with ethylene-emitting fruits or vegetables, which can lead to internal breakdown of the melon. Too low of temperature while cooling the watermelon can lead to chilling injury as characterized by decreased redness, leakage of juices, and decay. Once cut, watermelon should be wrapped and stored at 32ºF. Watermelons should be consumed within two to three weeks of harvesting.
Outbreaks associated with watermelon and other melons have been mainly attributed to contamination from the rind of melon to the edible flesh during cutting. The main pathogens associated with melon include Escherichia coli, Salmonella, and rotavirus. Whole melons with signs of decay or damaged rinds are at increased risk of being contaminated with pathogens and should not be sold to or purchased by consumers.
Many consumers do not wash watermelons before cutting or peeling, which increases risk of pathogens found on the rind being transferred the flesh when cutting with the knife blade. The outer surface of watermelon should be thoroughly washed with cold water before being cut and scrubbed clean of dirt and contaminants. In addition, it is recommended that consumers and food handlers wash their hands thoroughly before cutting melons. After being cut, watermelon should be kept at a temperature of 41ºF or below within two hours of being cut. If cut watermelon is not refrigerated within two hours and is left in the temperature Danger Zone (41ºF to 135ºF), it should be discarded.
Watermelon consumption in the United States has been steadily rising over the past 20 years. Per capita watermelon consumption was 13.8 lbs in 2000, which has gradually risen to 15.7 lbs. in 2018. Globally, China is reported to be both the leading producer and consumer of watermelon.
Given that watermelon is a seasonal summer fruit, it is most highly consumed during summer months when domestic production is highest. Most watermelon is sold as fresh produce. It is often consumed in slices, diced, in fruit mixes or salads, or as watermelon juice. Seedless watermelons are the most popular and produced types due to their texture, color, and convenience without seeds. Other types of watermelons consumed throughout the U.S. included seeded, mini or petite, and yellow and orange varieties. The common varieties include Crimson Sweet, Charleston Gray, Blacktail Mountain, Georgia Rattlesnake, Orangeglo, Moon and Stars, and Ali Baba.
As referenced in its name, watermelon is composed primarily of water with a 92% average water content. With this high-water content, watermelon is naturally low in calories with 80 calories per two-cup serving. Watermelon is high in natural sugar and total carbohydrates, while being low in both protein and fat. Additionally, watermelon is a good source of vitamin C, pantothenic acid, magnesium, copper, biotin, and B vitamins. Aside from these micronutrients, watermelon is one of the highest sources of the phytonutrient lycopene which is associated antioxidant, anti-inflammatory, and antihypertensive properties. One serving of watermelon has about 12.7 mg of lycopene per two-cup serving, which is higher than any other fruit or vegetable including red tomatoes. Some studies into watermelon consumption suggests that adding watermelon to an atherogenic diet can improve cardiovascular risk factors including inflammation and abnormal lipid levels, however no large-scale human trials have yet investigated these effects. Additionally, watermelon juice has been proposed as a functional food, wherein its effects on inflammation and health is an ongoing area of research.
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