Survival of Salmonella spp. and Escherichia coli O157:H7 on Fresh and Frozen Strawberries

Abstract

For maximum shelf life, fresh strawberries are harvested directly without washing into retail containers. Frozen berries are usually hulled in the field and washed prior to freezing, sometimes with the addition of sucrose. To determine survival of potential bacterial contaminants, cut or intact surfaces of fresh strawberries were spot inoculated with five- or six-strain cocktails of Salmonella or Escherichia coli O157:H7 (log 7.0 CFU/sample). Inoculated strawberries were dried for 1 h at 24°C and were stored in closed containers at 5 or 24°C. Sliced strawberries with or without added 20% sucrose were inoculated with one of two strains of E. coli O157:H7 and frozen at −20°C. An initial population reduction of approximately 0.5-log cycles was observed on intact but not cut berries after the 1-h drying period. During storage at 24°C for up to 48 h, populations of Salmonella and E. coli O157:H7 did not decline further. When strawberries were stored at 5°C for up to 7 days, populations of both pathogens remained constant on cut surfaces but decreased by 1- to 2-log cycles on intact surfaces. After 30 days of frozen storage, the population of E. coli O157:H7 had declined by 0.7- to 2.2-log cycles (with and without sucrose, respectively). Results of this study indicate that E. coli O157:H7 and Salmonella are capable of survival but not growth on the surface of fresh strawberries throughout the expected shelf life of the fruit and can survive in frozen strawberries for periods of greater than 1 month.