Strawberries

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Photos by: Jared Guerrero and Dr. William E. Keene

Externally-reviewed

Key Facts

  • Strawberries have been associated with foodborne illnesses including E. coli O157:H7, Norovirus, and Hepatitis A. The susceptibility of strawberries to foodborne illness highlights the importance of following recommended food safety practices from the farm to the table.
  • The United States is the world’s largest producer of strawberries. The top three strawberry-producing states are California, Florida, and Oregon. California produced approximately 2.7 billion pounds of strawberries in 2013, far more than any other state and more than 50,000 acres of land are dedicated to producing strawberries.
  • Strawberries are grown in every state in the United States and almost every country in the world. They are the most widely grown fruit crop. Today, there are over 100 varieties of strawberries.
  • Americans consume about eight pounds of strawberries every year; approximately 75% of that amount is fresh strawberries and the remainder is frozen. 94% of U.S. households consume strawberries.
  • Strawberries are a great addition to a healthy diet. They are an excellent source of vitamin C and high in flavonoids (bioactive compounds), fiber, potassium, and several antioxidants. One cup of strawberries contains only 55 calories.

IntroductionPPOD_Strawberry_2

The cultivated strawberry belongs to the genus Fragaria in the Rosaceae family and is a hybrid of two native species, F. chiloensis and F. virginiana. Commercial strawberries are grown in a broad range of climates including temperate, grassland, Mediterranean, taiga, and subtropical. The fruit’s peak season occurs between April and June in the Northern Hemisphere. Typically, strawberry plants are short-day sensitive and fruit in spring. New varieties may be day-neutral and are capable of fruiting under all day lengths, so long as temperatures are cool. This has enabled strawberries to be produced nearly year-around in the coastal regions of California and in summer and fall in colder climates. From 1970 to 2013, the production of strawberries in the U.S. increased more than six-fold, from 496 million pounds to nearly three billion pounds, making the United States the largest producer of strawberries in the world.

The cultivated strawberry is an herbaceous, perennial plant, as leaves remain alive and green during the winter in mild climates and survive under mulch in more severe climates. The strawberry plant produces leaves, flowers, and runners off of very short woody stems or crowns. Axillary buds at the base of each crown can stay dormant or differentiate into runners, flowers, or leaves, depending on the temperature and day length. Leaf development and root growth are strongly regulated by temperature.  The edible portion of the plant, or “strawberry”, is not a true berry; rather, it is a swollen receptacle covered with about 200 seeds.

Foodborne OutbreaksStrawberryCollage_Edit2png

Strawberries have been the culprit in foodborne outbreaks of Hepatitis A, Norovirus, and E. coli O157:H7. Strawberries are a delicate, highly perishable fruit and, therefore, are harvested by hand and not washed as often or as thoroughly as harder-skinned fruit. In order to maintain maximum shelf life, fresh strawberries are typically field-harvested and placed directly into retail containers without being washed. Frozen strawberries are typically picked and hulled in the field and washed before freezing. The strawberry’s rough surface provides many places for viruses to hide, and once a berry has been contaminated, the virus is unlikely to be washed off. Freezing may only preserve pathogens, and since frozen berries are sold “ready-to-eat”, they can pose an even higher risk of causing illness.

Several outbreaks of Hepatitis A during the last two decades have been attributed to strawberries. Outbreaks in 1990, 1997, 2007, and 2016 resulted in a total of 475 illnesses. The recent 2016 multistate outbreak resulted in 143 reported illnesses across seven states, with 56 requiring hospitalization. The outbreak was linked to imported frozen strawberries from Egypt, and nearly all those who were ill confirmed they had consumed a fruit smoothie containing strawberries from a Tropical Smoothie Café. Hepatitis A outbreaks are normally associated with farm worker health and hygiene. Because Hepatitis A is spread through the “fecal-to-oral route”, and the fruit is harvested by hand, it is essential that good food safety practices are utilized in the field. Workers who pick berries in the field can transmit the virus from hands to berries if proper hand-washing technique is not followed. Another cause of Hepatitis A transmission to strawberry fruit is the use of sewage-contaminated water to irrigate strawberry crops. It was determined that this was likely the cause of the 2016 outbreak.

Norovirus is a serious health threat associated with strawberries. Three Norovirus outbreaks in the U.S., one in 2005 and two in 2007, resulted in total of 67 individuals falling ill, and in 2012, the largest foodborne outbreak ever recorded in Germany was linked to frozen, imported strawberries from China. The outbreak affected close to 11,000 people. Norovirus outbreaks are typically associated with contamination by food handlers; however, as with the 2016 Hepatitis A outbreak in the U.S., the detection of different genotypes in the German outbreak suggested contamination by sewage.

Deer feces in strawberry fields from 2011 Oregon outbreak. Photo by: Dr. William E. Keene
Deer feces in strawberry fields from 2011 Oregon outbreak. Photo by: Dr. William E. Keene

In 2011, an E. coli O157:H7 outbreak in Oregon became the first known instance in which strawberries were associated with a shiga toxin-producing E. coli (STEC) outbreak. E. coli O157:H7 foodborne outbreaks are associated with ruminant animals’ feces and/or contaminated water. In this outbreak, deer feces were implicated as the source of contamination after investigators discovered fecal material in the fields of the locally owned grower, Jaquith Strawberry Farm. Samples of deer feces with the outbreak strain were recovered from multiple locations on the farm during the investigation. Those affected became ill after consuming the contaminated strawberries, which had mostly been purchased at local roadside fruit stands. Fifteen people were reported ill, seven were hospitalized, and one died in the outbreak.

In 2005, the California Strawberry Commission published food safety recommendations specific for strawberry growers based on good agricultural practices (GAPs). This report recommends that producers avoid field contamination by not picking strawberries in areas with obvious fecal contamination. Fruits and vegetables can become contaminated through soils and water (e.g. contaminated irrigation water) or through cross-contamination from workers or food handlers. Soil and water can become contaminated through the feces of various animal species, particularly birds.

Production

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Matted-row system diagram showing original and runner plants.

Soil & Planting

Strawberry production requires attention to cultural practices such as variety, selection, weed control, frost control, and winter protection. There are two major production systems utilized in the world. The first system is the annual hill, which uses raised plastic beds (also referred to as “strawberry plasticulture”); the second is the perennial matted row. The matted row system employs runners as the primary yield component where plants are allowed to shoot runners freely into narrow rows. The hill system relies on crowns as the primary yield component. In this system, any runners that form are removed. Perennial masted rows use short day cultivars in climates with short summers and cold winters. The annual hill system is primarily used in areas having mild winters and either hot or moderate summers.

Strawberries have been developed by both private and public breeding programs. Private programs do not release varieties for growers outside of the private group, and they may not be given a name. The popularity of public varieties changes frequently, so one variety may be popular for just a short time before it is replaced by another. Also, varieties tend not to do well across broad geographic areas. Typically, varieties are adapted to the area in which they were developed. Strawberries prefer a slightly acidic pH (pH 5.5 to 6.5) but are well-adapted to a variety of different soil types, as long as air and water drainage are adequate. Planting strawberries is not advised in soils previously planted with solanaceous crops (tomato, potato, pepper, eggplant). Without proper fumigation or planting of verticillium-resistant varieties, these soils may contain Verticillium wilt, which is a persistent fungal organism. Sites recently planted to sod may contain white grub, which can damage the strawberry roots. Strawberries require nitrogen fertilizer application every year beginning the year of planting. Nitrogen is important for plant growth, runner production, and fruit bud formation. Information on various pests of strawberries can be found in the American Phytopathological Society’s Diseases and Insects of Strawberry book. Information on soil management for strawberries can be found in the Cornell University, Berry Soil and Nutrient Management – A Guide for Educators and Growers handbook.

Strawberry production in Florida utilizes the raised plastic bed method, with about two rows of plants per bed. Many varieties are cultivated in Florida, and the specific varieties grown can change from year to year. They can include Camarosa, Carmine, Camino Real, Gaviota, Strawberry Festival, Sweet Charlie, Treasure, Ventana, and Winter Dawn. Pests that affect cultivation include the twospotted spider mite, thrips, lepidopterous larvae (fall and southern armyworms), corn earworms, and sap beetles. Occasionally, insects are seen on the fruit, but are managed on an as-needed basis; they include fruit flies, whiteflies, lygus bugs, saltmarsh caterpillars, aphids, leafrollers, tobacco budworms, mole crickets, and ants.

Irrigation

Good irrigation plays an important role in growing and maintaining healthy strawberry plants by allowing for optimal plant growth and crop production. Determining the proper amount of irrigation is necessary because both excessive and minimal watering of plants can cause problems. Excessive irrigation can lead to nutrient deficiencies in the root zone because it can cause leaching of sulfur, boron, and nitrogen. In addition, over-watering can lead to either slower root growth or the rotting of the roots altogether. On the other hand, inadequate watering can lead to reduced strawberry size, decreased yield, and diminished fruit quality. The recommended ways for irrigating this crop are limited. Ideal methods include overhead irrigation, sprinklers, or drip lines. Overhead or sprinkler irrigation can also serve other purposes, such as frost protection in areas where frost is an issue or as a way to provide cooling to the crop in hotter weather. Irrigation can also be used to apply fertilizers and pesticides to the crop. Methods such as flood or furrow irrigation should be avoided.

Harvesting, Packaging, & Storage

Photo by: Dr. William E. Keene
Photo by: Dr. William E. Keene

Florida produces between 10 and 15% of the total amount of strawberries in the United States and 100% of domestic strawberries in the winter. During the 2014 crop year, 207 million pounds of strawberries valued at $306.5 million were produced on 10,900 acres. Florida strawberries are harvested by hand every three days during harvest season. Generally, there is one picker per acre who picks for four hours during off-peak parts of the production season. One and a half pickers per acre are used during peak parts of the harvesting season and picking lasts for eight hours a day. Typically, plastic shipping containers are filled in the field to reduce the amount of handling.

A high-quality strawberry fruit will be uniformly red in color, firm, flavorful, and free of defects and disease. Maturity is based on surface color. Sugar content does not increase after harvest; therefore, strawberries should be harvested fully ripe for best flavor. The U.S. minimum is ½ or ¾ of berry surface showing red or pink color, depending on grade. U.S. No. 1 grade consists of strawberries with the cap attached and which are firm, not over-ripe or undeveloped, of uniform size, and free of decay and damage. U.S. No. 2 grade consists of strawberries free from decay or serious damage and with at least ½ of each fruit showing pink or red color. Strawberries are extremely perishable, and it is important to begin cooling within one hour of harvest to avoid loss of quality and reduction in amount of marketable fruit. Temperature management is the single most important factor in minimizing strawberry deterioration and maximizing postharvest life. Forced-air cooling is highly recommended, although room-cooling is used in some cases. The optimum storage condition for strawberries is 0 °C (32 °F) with 90 to 95% RH. Strawberries can be stored for up to seven days at 0 °C (32 °F), depending on disease pressure.

Strawberries are often packaged by pickers in the field into either 1-dry-pt or 1-dry-qt open mesh baskets, or clear clamshell containers. The mesh baskets or clamshells are held in a corrugated fiberboard tray holding about 4 to 5 kg (9 to 11 lb. Studies show that with increasing numbers of ventilation openings in packaging, transpiration losses (water loss) increase and the consistency and color stability of the fruit decreases. The uneven ground surface of packaging materials made of polyethylene terephthalate and polyactic acid actually increases the puncture pressure on the strawberry, which leads to pressure marks and injuries of the fruit’s surface.

Potential strawberry markets include those available through direct-marketing methods such as U-pick or pick-your-own (PYO), roadside stands, and farmers’ markets. Fruit harvested for wholesale market is normally picked firmer and cooled rapidly to reduce perishability during shipping and to ensure good shelf life. Strawberry fruits are not sensitive to chilling temperatures and should be stored as cold as possible without freezing.

For more information regarding the production and distribution of Strawberries please visit the Produce Point of Origin Database.

Food Safety

Photo by: Dr. William E. Keene
Photo by: Dr. William E. Keene

Animal manure and human fecal matter represent significant sources of contamination and have been documented in strawberry-associated outbreak investigations. Therefore, growers should follow GAPs. This involves using only composted manure, ensuring worker health and hygiene, using safe water sources for irrigation, maintaining the sanitation of the fields, and taking measures to reduce contamination from wildlife or domestic animals. Further information regarding GAPs specifically regarding strawberries can be found on the California Strawberry Commission website. Farming, harvesting and processing practices are the important control points to reduce contamination. Farm workers, who must touch each berry to harvest it, can be a significant source of contamination. The use of sanitizing solutions or vigorous washing is not completely effective at cleansing strawberries and is unlikely to eliminate bacterial surface contamination. Therefore, do not wash berries before storing them since wet fruit will encourage bacterial surface fungal growth. Store them dry and wash only before using.

Regardless, safe handling of strawberries by consumers should also be recognized and promoted. Strawberries should be inspected for freshness. Strawberries suitable for eating are bright red with a green cap, while soft, moldy, brown-capped, or white or green-colored strawberries should be discarded. In addition, strawberries should be stored in the refrigerator (below 40º) and kept dry until ready to prepare. They should then be washed thoroughly with cold water immediately prior to serving and sliced on a sanitized surface with a clean knife. Washing berries removes dirt, insects, and can help reduce microbial contamination.

Consumption

Strawberries are most often consumed fresh; however, they can also be frozen, dried, or made into preserves, jams, and jellies. Strawberries are commonly added to dairy products to make yogurts, milkshakes, ice cream, and smoothies, and they frequently used in pastries and pies. Americans consume about eight pounds of strawberries every year; approximately 75% of that amount is fresh strawberries and the remainder is frozen.

Per the FoodNet 2006-2007 Atlas of Exposures, 45% of Americans report eating strawberries in the past seven days and over 53 percent of seven to nine-year-olds picked strawberries as their favorite fruit.

Information on how to keep strawberries stored properly visit FoodKeeper App.

Nutrition

Strawberries are an excellent source of vitamins and other essential nutrients. One cup of fresh strawberries has about 50 calories and contains 160% of the RDA of vitamin C. Strawberries are also a good source of potassium, manganese, folic acid, and fiber.

Because they are now available in fresh form all year long, strawberries have become one the most widely-consumed fruits in the United States. Their increased popularity may also be due, in part, to mounting evidence that consuming diets rich in fruits and vegetables has a positive effect in lowering the risk for chronic diseases such as cancer, cardiovascular disease, and stroke. More consumers are becoming aware that strawberries contain a variety of bioactive compounds that are reported to have potent antioxidant power, and which benefit the aging brain.

References

  1. Black B, Hill R, Cardon G. Strawberry Irrigation [Internet]. Utah State University Cooperative Extension; 2008. Available from: http://col.st/1l1gaF6
  2. California Strawberry Commission. About Strawberries [Internet]. California Strawberries. 2014 [cited 2014 Jun 17]. Available from: http://col.st/1l1dyai
  3. California Strawberry Commission. California Strawberry Commission Food Safety Program [Internet]. California Strawberry Commission; 2005. Available from: http://col.st/1l1iKLc
  4. Centers for Disease Control and Prevention (CDC). Population Survey Atlas of Exposures. [Internet]. Foodborne Active Surveillance Network (FoodNet). 2006. Available from: http://col.st/wAoCY
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  6. Dannehl SH-K. Storage of strawberries with consideration of different packaging materials. Erwerbs-Obstbau [Internet]. 2008;50(2):49–61. Available from: http://col.st/1l1hesD
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  8. Hancock JF. Strawberries [Internet]. First edition. Oxon, UK ; New York, NY, USA: CABI; 1999. 237 p. Available from: http://col.st/1l1eI5C
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  11. Laidler MR, Tourdjman M, Buser GL, Hostetler T, Repp KK, Leman R, et al. Escherichia coli O157:H7 infections associated with consumption of locally grown strawberries contaminated by deer. Clin Infect Dis [Internet]. 2013 Oct;57(8):1129–34. Available from: http://col.st/1l1ifAP
  12. Made D, Trubner K, Neubert E, Hohne M, Johne R. Detection and Typing of Norovirus from Frozen Strawberries Involved in a Large-Scale Gastroenteritis Outbreak in Germany. Food Environ Virol [Internet]. 2013 [cited 2014 Jun 17];5:162–8. Available from: http://col.st/1l1iPyH
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  17. Pritts M. Dr. Marvin Pritts – Food Source Information Wiki – Strawberry Page Review. Food Source Information Wiki; 2015.
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  20. Van Laanen P, Scott A. Safe Handling of Fresh Strawberries [Internet]. Texas Cooperative Extension; Available from: http://col.st/1l1eafT
  21. Whitaker V, Boyd N, Peres N, Smith H. Vegetable Production Handbook of Florida. Chapter 15. Strawberry Production [Internet]. 2015. Available from: http://edis.ifas.ufl.edu/pdffiles/cv/cv13400.pdf
  22. 2007 Census of Agriculture: National, State and County Tables [Internet]. 2009. Available from: http://www.agcensus.usda.gov/mwg-internal/de5fs23hu73ds/progress?id=C7ta6poDWkq0RY_rB0gFMHBW76PWsF-EgBWQQPIr19I,
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Externally Reviewed by: Marvin Pritts, PhD 
Affiliation: Cornell University - College of Agriculture and Life Sciences 
Reviewed on: 21 July 2015
Externally Reviewed by: Hillary Booth
Affiliation: Lead Foodborne Research Analyst, Oregon Public Health Division
Reviewed on: 19 August 2015

Authors

Milena Guajardo

Milena Guajardo

Claudia Meister

Claudia Meister

Marisa Bunning

Marisa Bunning

Leslee Warren

Leslee Warren

Current MS Epidemiology student at the Colorado School of Public Health and a Graduate Assistant at the Colorado Integrated Food Safety Center of Excellence.

David Dekevich

David Dekevich

Florida Integrated Food Safety Center of Excellence Liaison at the Florida Department of Health