In a recently released report (U.S. Department of Agriculture, 2018), the United States was ranked the largest producer of blueberries in the world in 2016, with cultivated and wild blueberry total market values of $720.2 million and $27.7 million, respectively. The short postharvest shelf life of fresh fruit like blueberries requires efficient technologies for postharvest cooling, handling, and storage to keep losses to a minimum. Blueberries, like many other small fruits that are consumed raw, have the potential for foodborne illness due to microbiological contamination (Macori et al., 2018). Outbreaks associated with blueberries have involved organisms such as Salmonella (Miller et al., 2013) and hepatitis A (Calder et al., 2003). Intervention strategies such as hydrocooling (Sargent et al., 2017) can be used to reduce the microbial load on the surface of fruit that presents an increased risk of cross-contamination and reduction in quality (Sreedharan et al., 2015; Tokarskyy et al., 2015).
Fruit quality depends on many variables such as cultivar, preharvest practices, climacteric conditions, maturity at harvest, harvesting methodology, and postharvest conditions (Sousa-Gallagher et al., 2016). These make predicting the shelf life a difficult task compared with predicting that of other food products. Postharvest temperature management in the blueberry industry is a key factor that contributes to fruit quality and shelf life (Sargent et al., 2017). Postharvest abiotic and biotic deterioration can be controlled by reducing the storage temperature and respiration rate, and by modifying the atmosphere surrounding the product (Van Hoorn, 2004).
Blueberries are climacteric fruit (Ban et al., 2007; El-Agamy et al., 1982; Ismail and Kender, 1969; Lipe, 1978; Windus et al., 1976). To obtain the best flavor and appearance, they should be harvested during the fully ripe stage (Sargent et al., 2009). Ripe blueberries are easily damaged by rough handling (Demir et al., 2011; Xu et al., 2015; Yu et al., 2012) and adverse temperatures (Sousa-Gallagher et al., 2016). Blueberry softening is known to be influenced by cell wall modifications during ripening of the fruit, although these changes are largely complete by the time of harvest (Paniagua et al., 2013). Freshly harvested blueberries constantly lose water to the surrounding environment by transpiration (Wills et al., 1998); therefore, rapid cooling and good temperature management are vital if ripening and deterioration processes are delayed (Sousa-Gallagher et al., 2016; Van Hoorn, 2004).
Temperature management, including keeping the harvested fruit under shade and/or transferring them in a precooled vehicle for transportation, starts in the field, and deterioration is a function of time and temperature; however, faster cooling retains fruit quality and can significantly extend the shelf life (Jackson et al., 1999). Brecht et al. (2003) associated forced-air precooling to 1 to 2 °C immediately after harvest with improved preservation of natural blueberry quality during storage, distribution, and export. Florida blueberries are typically subjected to forced-air cooling (FAC) to an intermediate temperature of ≈16 °C in field lugs soon after harvest. Then, they are kept overnight at that temperature before being sorted and packed in clamshell containers the following day and undergoing FAC for ≈1 h to a final storage temperature of 1 to 2 °C. Less typically, the fruit are packed and cooled on the day of harvest.
As previously mentioned, blueberries have been associated with Salmonella-related outbreaks (Miller et al., 2013; Wu et al., 2017); therefore, further research considering the likelihood of the persistence and survival of bacterium is warranted. Moreover, during postharvest processing, including hydrocooling, blueberries may be bruised or damaged such that the survival of bacterium on fruit is affected. Intervention strategies such as hydrocooling can be used to reduce the microbial load on the surface of fruit. However, the increased risk of cross-contamination and reduced fruit quality must be considered before application of these strategies (Sreedharan et al., 2015; Tokarskyy et al., 2015). Salmonella has been reported to survive but not proliferate in low pH environments, including damaged fruit such as blueberries (Bassett and McClure, 2008; Nguyen et al., 2014).
Because the consumption of Salmonella-contaminated blueberries can cause foodborne disease outbreaks, it is necessary to evaluate the effects of hydrocooling on the quality of blueberries, the potential damage it could cause to the fruit, and its ability to facilitate Salmonella proliferation.
The objectives of this study were to compare the efficacy of FAC to that of hydrocooling with sanitizer and of hydrocooling without sanitizer to reduce Salmonella on inoculated blueberries and to assess the effects of these treatments on the shelf life and quality of fruit.
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