Evaporative Cooling of Apples: Sunburn Suppression Benefits and Influence on Microbial Survival

Sunburn is an important disorder when evaluating apple external quality and economic value. In the State of Washington at least 10% of cullage is attributed to sunburn each year. Use of overhead evaporative cooling (EC) can contribute to the suppression of sunburn. Direct water application onto the apple can diminish apple surface temperatures, thus decreasing the chances of sunburn development. However, applying open source irrigation water directly to the apple surface can pose a serious threat to human health if water is contaminated. To evaluate this risk, an apple field inoculation examination was conducted in 2014 to assess generic Escherichia coli (E. coli) die-off rates under realistic apple growing conditions and orchard settings. Weather data were collected to investigate possible relationships between environmental conditions and bacterial die-off rates. Secondly, sunburn incidence for Gala, Golden Delicious and Fuji were observed in 2012 and 2013, for two treatments (EC and untreated control) and two canopy locations (high and low). Packout analyses were conducted based on sunburn data results, for the three varieties in both years, using different crop load scenarios. In both years, the use of EC diminished sunburn incidence, except for Fuji in 2013. In 2012, EC decreased sunburn incidence by 8.1% for Fuji, 18.3% for Gala and 0.7% for Golden Delicious. In 2013 untreated Golden Delicious and Gala apples had 10.2% and 1.6% more sunburn overall than treated fruit, respectively. Untreated Fuji apples had 3.5% less sunburn overall than treated. The higher percentage of sunburn on Fuji treated with EC is believed to be caused by the discontinuation of water application too early prior to harvest. Overall higher canopy location presented more sunburn than fruit in lower canopy positions. Packout analysis results show that EC increased net returns for all varieties in both years, and in all crop load scenarios. Both EC treated and untreated fruit showed   approximately a 2.5 log reduction 8 hours after inoculation (in the absence of light), and a further 1.0 log reduction between 8 and 24 hours. This indicates that overall 99.99% of generic E. coli did not survive the first 24 hours. Evaporative cooling had positive effects regarding generic E. coli die-off rates for Golden Delicious and did not negatively impact microbial die-off, considering the weather and water application rates used in this experiment. The possible implications for growers related to new food safety regulations under the Food Safety Modernization Act will be discussed.