Survival of Salmonella spp. on Dried Fruit Held at Varying Temperatures

Introduction: Dried fruits can be contaminated with pathogenic microorganisms at any stage of growth or processing.  Commonly, there is no heat step employed prior to being added directly to ready-to-consume products, creating a potential food safety hazard.  While the inherent antimicrobial activities of dried fruit such as raisins, cranberries, blueberries, and cherries have been reported, scientific data remains limited or inconclusive. 

Purpose: This study determines the survival of a Salmonella cocktail on 12 types of dried fruit stored at temperatures ranging from 4°C to 45°C. The results of this study serve as the scientific basis for industry microbiological classification of those dried fruit and for potential pathogen reduction steps.

Methods: Dried fruit was inoculated using either a wet or dry technique. A_w, moisture, titratable acidity, and pH were measured throughout the inoculation process and incubation.  Following inoculation, fruit and culture controls were held at 4, 10, 18, 25, 35 and 45°C. Samples (25 g) were analyzed for Salmonella initially and weekly or bi-weekly until 3 sequential negative results were obtained. Microbial analysis was conducted using plate count on trypticase soy agar and confirmed using VIDAS or FDA BAM. 

Results: For both wet and dry inoculated fruit, a significant log reduction (4-7 logs) was observed by week 3 at room temperature (25°C) and higher.  At 4 and 10°C, the pathogen survived for over three months, with levels dropping by 2–3 logs.  All runs were conducted in triplicate. 

Not all dried fruit behaved the same under similar conditions. 

Significance: This study indicates that under refrigerated storage conditions following harvesting, Salmonella, if present, will survive for up to 3 months on 11 of 12 dried fruit types.   In addition, for most dried fruit stored at temperatures greater than 25°C, pathogen levels decrease by 4–5 logs within 3 weeks.