The cultivated strawberry is a popular fruit worldwide with desirable flavor, texture, and visual appeal. In the United States, the value of total strawberry production was $2.2 billion in 2010 [U.S. Department of Agriculture (USDA), 2011]. California accounts for 90% of national production. Fresh fruit are typically transported by refrigerated truck trailers up to 3000 miles from growing areas to markets throughout North America (Mitcham and Mitchell, 2002). Strawberry fruit are a highly perishable commodity, and shipment at low temperature (i.e., 32 °F) is critical for maintaining postharvest quality (Maxie et al., 1959).
The shelf life of strawberry fruit is often limited by their high rates of respiration, softening, and water loss (Kader, 1991). Fruit are also very susceptible to mechanical damage and decay (Sommer et al., 1973). Botrytis cinerea and Rhizopus stolonifer, the causal agents of gray mold and rhizopus rot, respectively, are the main pathogenic fungi to infect strawberry fruit (Maas, 1998; Wells, 1970). With gray mold, infection often begins in the field when the pathogen colonizes senescing flower petals and grows into the fruit (Powelson, 1960). The infection typically remains latent until fruit are exposed to favorable conditions such as physical wounding, high temperature, and high relative humidity (RH) that are sometimes encountered during postharvest handling (Sommer et al., 1973). Symptoms of disease are usually first visualized as light brown lesions on the fruit surface (Maas, 1998). These lesions eventually enlarge and become covered by masses of gray to tan-colored fungal mycelium and spores that render fruit unmarketable.
The application of chemical fungicides and proper cultural practices (e.g., field sanitation) during strawberry production have long been relied on to control fungal pathogens (Maas, 1998). Thereafter, nonchemical treatment strategies are typically used to control fruit decay during postharvest handling. For example, prompt (i.e., within 1 h of harvest) precooling and maintenance of fruit at low temperature during postharvest operations is widely recommended to slow metabolic reactions associated with fruit senescence and pathogen development (Maxie et al., 1959; Sommer et al., 1973). Exposure to a modified atmosphere (MA) of elevated CO2, reduced oxygen (O2), or both can also retard the growth of fungal pathogens (Brooks, 1932; Couey et al., 1966; Couey and Wells, 1970). These fungistatic atmospheres complement proper temperature management in reducing decay, although the additive benefits are reportedly modest below 5 °C because fungal growth is already suppressed at these low temperatures (Harvey et al., 1966; Sommer et al., 1973).
Shipping and storing strawberries in a MA at low temperature is widely practiced by commercial operators in the United States to reduce decay and maintain fruit quality (Kader, 2002). The most commonly used MA system, Tectrol (TransFresh, Corp., Salinas, CA), involves enclosing an entire pallet of strawberries in a plastic cover that is sealed to a plastic sheet on the pallet base. A partial vacuum is established within the pallet and CO2 gas is then injected to establish an atmosphere of 10% to 15% CO2 and 5% to 10% O2 (Harvey et al., 1980; Kader, 1991). An alternative MA system, CO2 West (CO2 West, San Luis Obispo, CA), uses pads containing sodium bicarbonate and citric acid that react with water vapor to liberate CO2 inside the plastic pallet cover (Finnegan and Boldt, 2007), which is open at the pallet base. Two additional pallet cover systems, PEAKfresh (PEAKfresh USA, Lake Forest, CA) and PrimePro (Destiny Packaging, Monterey, CA), simply involve placing a plastic cover over pallets. Both of these MA systems remain open at the pallet base and rely on strawberry fruit respiration to increase CO2 and decrease O2 concentrations.
The pallet cover systems listed above are claimed to extend the postharvest life and enhance the quality of strawberry fruit. However, there is limited publicly available data comparing the efficacy of these systems. In the present study, we evaluated the capacity of the four proprietary pallet cover systems (CO2 West, PEAKfresh, PrimePro, and Tectrol) to reduce decay and maintain quality of strawberry fruit during and after six transcontinental shipments from Watsonville, CA to Atlanta, GA or Jacksonville, FL. Fruit were also evaluated after a 2-d simulated retail display life to determine if there were residual effects of the intransit treatments.
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