Expiration Date of Ready-to-Eat Salads: Effects on Microbial Load and Biochemical Attributes

Abstract

When minimally processed vegetables reach their expiration date, expose an increased microbial load. This includes mainly spoilage microorganisms but also foodborne pathogens, thus affecting the quality and safety of highly consumed ready-to-eat salads. A total of 144 ready-to-eat salads from the Cypriot market were analyzed in an attempt to determine the effects of the expiration date on the microbial load and plant metabolic variables of the salads. Possible correlations between them were also investigated for the first time. Furthermore, the impacts of the season (winter, summer), salad producing companies and type of salad and/or their interactions with the tested parameters were investigated. Results revealed that the microbial load (mainly spoilage microorganisms, such as Pseudomonas spp., yeasts and molds) increased towards the end of the shelf life. The microbial load was differentiated among the five salad producers and/or the salad types, highlighting the importance of a common and safe sanitation-processing chain in the preparation of ready-to-eat salads. Summer was the season in which Escherichia coli counts were found to be higher for plain lettuce, while Staphylococcus spp. was increased numbers for the lettuce+endive/radicchio, lettuce+rocket and lettuce+chives type of salads. Additionally, an increased Staphylococcus spp. was observed for plain rocket salads in winter. All samples examined were found negative for Salmonella enterica and Listeria monocytogenes. Moreover, carbon dioxide production and damage indexes (hydrogen peroxide and lipid peroxidation) increased on expiration date on both winter and summer seasons, indicating plant tissue stress at the end of shelf life. These findings indicate that the expiration date and relevant shelf life of processed vegetables are important parameters to be considered when postharvest management is applied to these products, ensuring safety and quality.

Keywords: food safety; foodborne pathogens; polyphenols; processes salads; respiration.

Figure 1

Effect of season on the microbiological quality (A–I) of ready-to-eat salads collected in winter () and summer () among salad producers/packagers (A–E). Results include all samples for each microorganism tested and are the mean value ± standard deviation. Each box contains 50 percent of cases, and whiskers represent the rest. The line across the inside of the box represents the median value. *, ** and *** indicate significant differences at p ≤ 5%, 1% and 0.1%.

Figure 2

Microbiological quality (A–I) of different types of ready-to-eat salads collected in winter () and summer (). Results include only positive samples for each microorganism tested and are the mean value ± standard deviation. Other = lettuce + 2 or more ingredients. Each box contains 50 percent of cases, and whiskers represent the rest. The line across the inside of the box represents the median value. *, ** and *** indicate significant differences at p ≤ 5%, 1% and 0.1%.

Figure 2

Microbiological quality (A–I) of different types of ready-to-eat salads collected in winter () and summer (). Results include only positive samples for each microorganism tested and are the mean value ± standard deviation. Other = lettuce + 2 or more ingredients. Each box contains 50 percent of cases, and whiskers represent the rest. The line across the inside of the box represents the median value. *, ** and *** indicate significant differences at p ≤ 5%, 1% and 0.1%.

Figure 3

Effect of season on the total phenolic content, antioxidants, % CO₂ and damage index (H₂O₂ and lipid peroxidation) (A–G) of ready-to-eat salads collected in winter () and summer () among salad producers/packagers (A, B, C, D, and E). Results include all samples for each microorganism tested and are the mean value ± standard deviation. Each box contains 50 percent of cases, and whiskers represent the rest. The line across the inside of the box represents the median value. * and *** indicate significant differences at p ≤ 5% and 0.1%.

Figure 4

Effect of season on the total phenolic content, antioxidants, % CO₂ and damage index (H₂O₂ and lipid peroxidation) (A–G) of ready-to-eat salads collected in winter () and summer () among types of salads. Results include all samples for each microorganism tested and are the mean value ± standard deviation. Other = lettuce + 2 or more ingredients. Each box contains 50 percent of cases, and whiskers represent the rest. The line across the inside of the box represents the median value. *, ** and *** indicate significant differences at p ≤ 5%, 1% and 0.1%.