Limited information about the survival of Salmonella and E. coli O157 on strawberries and basil is available. As strawberries and basil are often consumed raw, they can pose a potential risk of foodborne illness for consumers. Therefore, the survival of both pathogens was assessed during one week storage at cold and ambient temperatures.
Strawberries (100 ± 5 g in a perforated box) and basil leaves (25 g in a closed tray), were inoculated with a mix of 2 strains of E. coli O157 or Salmonella (Thompson and Typhimurium) to obtain an initial inoculum of 104 – 105 cfu/g. Strawberries’ samples were stored at 4°C, 10°C, 15°C, and 22°C and basil leaves at 7°C, 15°C and 22°C for 7 days (or less if visual spoilage occurred before or the pathogens dropped below the detection limit i.e. 50 cfu/g). For each temperature/time condition 2 independent storage experiments were performed (study A and B) (with triplicate sampling at defined time points). Samples were analysed by plating on Chromocult agar (+ 50 µg/ml nalidixic acid; E. coli O157) and XLD (Salmonella). In addition, the visual quality was assessed during storage.
No growth of E. coli O157 or Salmonella was observed in case of strawberries stored at 4°C, 10°C, 15°C and 22°C. E. coli O157 and Salmonella survived during 7 days at 4°C but with a decrease of 2,5 to 3,8 log units being observed and a final recovery rate from 2 and 5 out of 6 samples respectively. At 10°C, no E. coli O157 was detected anymore after 6 days (0/6), in contrast to Salmonella which could be detected after 7 days in 5/6 samples with a similar decrease as for 4°C (2,5 to 3,9 log units). The numbers of pathogens dropped below the detection limit after 4 and 5 days at 15°C for E. coli O157 and Salmonella in study A respectively, while in study B a reduction of 3,0 to 4,0 log units was observed after 6 days, which was the end of shelf life due to the growth of molds. Storage of strawberries at 22°C resulted in growth of molds from day 2 (A) or 3 (B) whereby a reduction of pathogens was noticed of 1 to 2 log units after 2 days. In contrast, reduction of pathogens below the detection limit was not yet reached after 7 days storage of basil at different temperatures and only at 22°C some samples needed to be discarded due to the growth of molds and quality defects. Storage of basil leaves for 7 days at 7°C resulted in a decline of maximum 1,8 log cfu/g for both pathogens. Similar results were obtained at 15°C for Salmonella, whereas no decline was observed in case of E. coli O157. Also at 22°C restricted increase/decrease (≤ 1 log cfu/g) was observed for both pathogens after 7 days. Thus, avoiding contamination in particular at cultivation (and (post-)harvest) is important as both pathogens survive during storage, washing has only a limited effect and both strawberries and basil are consumed after minimal processing, excluding an inactivation treatment