Postharvest ethanol vapor treatment reduces fungal load of Medjool dates

Medjool dates are in high demand but the level of microbial load can exceed thresholds set by some countries. The objective was therefore to identify methods to reduce the microbial load of Medjool dates after harvest without compromising its quality. Medjool dates were obtained after harvest from the same orchard over 2 seasons or from a packing house after 6 month of storage. The dates were treated with ethanol vapor, cold-plasma applicator of hydrogen peroxide and a commercial formulation of peracetic acid and stored at 0°C for one month or −18°C for 6 month. In all experiments, treatment with ethanol vapor for 20 hours reduced fungal load by at least 2 orders of magnitude but had lower efficacy with respect to bacterial load. The ethanol vapor treatment was also effective when carried out after 6 month of storage at −18°C. Treatment with hydrogen peroxide reduced bacterial load but showed potential for skin damage. Similar levels of external and total microbial load were measured suggesting that external contamination was dominant in asymptomatic fruit. Sequencing of microbial DNA from the date tissue showed that Aspergillus was the predominant fungal genus while Bacillus was the dominant bacterial genus after shelf life. The treatment with ethanol vapor changed the volatile profile of the dates but did not have an apparent influence of their flavor. The ethanol vapor treatment can be a useful tool to reduce microbial load in relevant postharvest systems.