Development of Ozone-Based Processes for Decontamination of Fresh Produce to Enhance Safety and Extend Shelflife

Although good agricultural practices (GAP) and good handling practices (GHP) may reduce the incidence of pathogens in fresh produce, there is no known defense against contamination due to wildlife or surface water. Additionally, current strategies designed to minimize the risk of contamination during post-harvest operations of fresh leafy greens are not completely effective. Contaminations of fresh fruits with spoilage microorganisms also create economical losses by decreasing products shelflife. Therefore, it is necessary to develop sound mitigation strategies to minimize the health hazards associated with fresh produce.

The main objectives of this study are to enhance the safety of fresh produce in general, and baby spinach in particular, by integrating ozone-based sanitization steps into existing processing practices, while targeting Escherichia coli O157:H7 as the pathogen of concern, and to assess the feasibility of using ozone, carbon dioxide or their combinations, for reducing natural microbiota, and extending the shelf-life of strawberries.

A pilot-scale system has been successfully assembled that allows vacuum cooling and ozone treatment of fresh produce simultaneously. Combinations of vacuum cooling and ozonation (SanVac) successfully inactivated up to 2.4 log E. coli O157:H7 CFU/g spinach. Contribution of important treatment variables (ozone concentration, pressure and treatment time) to process lethality was also investigated using response-surface methodology. Parameters for the optimized SanVac process are of 1.5 g O₃/kg gas-mix (935 ppm, vol/vol), 10 psig holding pressure, and 30 min holding time; these conditions achieve 1.8 log inactivation against E. coli O157:H7 with no apparent damage to quality of baby spinach. A long-term, low-ozone process was also developed to treat fresh produce during transportation or temporary refrigerated storage. This treatment, termed “SanTrans”, involves sparging moist gaseous ozone at 16 mg/kg (10 ppm, vol/vol) for up to 3 days. The process inactivated up to 1.4 log E. coli CFU/g spinach, and the optimum process resulted in 1.0 log inactivation with minimal effect on product quality. In order to maximize inactivation on E. coli O157:H7, contaminated spinach was sequentially subjected to optimized SanVac (at 1.5 g/kg for 30 min and 10 psig) and SanTrans (at 16 mg/kg) processes, using freshly-harvested, unprocessed spinach that was shipped directly from California fields. These sequential treatments inactivated 4.1 to ≥ 5.0 log E. coli O157:H7, depending on the treatment time.

An ozone-based treatment system was designed and constructed to enhance the storage life of fresh strawberries (Fragaria x ananassa). The O₃/CO₂ combination treatments showed synergistic effect in delaying mold growth and quality deterioration of the strawberries. When samples were treated with O₃/CO₂ for 4 h and stored at 4°C, the initiation of visual mold appearance was delayed until the 16^th day of storage; an 8-day shelflife extension, compared to untreated samples. These novel technologies are promising alternatives to conventional processes and should enhance the safety and extend the shelflife of fresh fruits and vegetables. Additionally, the new treatments should be relatively easy to integrate into existing fresh produce processes and practices.