2019 ASHS Annual Conference

The objectives of this study are to assess the fate of generic Listeria on apple surfaces stored under regular atmosphere (RA, 33°F) and controlled atmosphere (CA, 33°F 1% CO₂, 2% O₂) with and without continuous low doses (~90 ppb) of ozone (O₃); examine survival of natural microorganisms on apple surfaces under above mentioned storage conditions and evaluate impacts of ozone on final fruit quality after storage. A 3-strain L. innocua cocktail prepared via mixing equal numbers of each respective strain suspension was used to inoculate unwaxed and unbruised Granny Smith apples at commercial maturity in 2017. Granny Smith apples were established to 6.09 ± 0.07 Log₁₀ CFU/apple and held at room temperature for 24 hours, before being subjected to cold storage in a typical commercial apple facility. Inoculated Granny Smith apples were randomly separated into three groups and subjected to three different storage conditions: RA, CA and CA with a low dose ozone for up to 30 weeks. Apples under different storage conditions were sampled at 0, 1, 3, 6, 12, 18, 24 and 30 weeks of storage to analyze the survival of L. innocua on apple surfaces. Fruit maturity and quality measurements such as firmness, total soluble solids and titratable acidity were performed at harvest and after 30 weeks in storage. The 30 weeks quality evaluation was conducted after a week at room temperature. External disorders were evaluated after 1 day and 1 week at room temperature. During the first 3 weeks of cold storage, L. innocua was reduced by 1.0-1.4 Log₁₀ CFU/apple. A single log reduction is equal to 10-fold or 90% reduction in Listeria population on apple surface. Die-off rate significantly decreased during the subsequent storage period. After 30 weeks of cold storage in a commercial RA or CA storage environment L. innocua on Granny Smith apple surface was reduced by 2.9, 2.2 and 4.6 Log₁₀ CFU/apple in RA, CA, and CA + O₃, respectively. Surprisingly, apples subjected to RA had significantly less L. innocua compared to those stored under CA conditions. The natural bacteria level remained stable during 12-week storage. It slightly increased in both RA/CA during subsequent storage, while remaining similar or slightly decreased in apples under CA with continuous low dose ozone. Overall, ozone treatment did not affect fruit quality.