Effect of peroxyacetic acid treatment and bruising on the bacterial community and shelf-life of baby spinach

The importance of leaf integrity, i.e. the effects of bruising (mechanical damage), and sanitisation with peroxyacetic acid (PAA) on bacterial communities of ready-to-eat baby spinach remains unclear. Two shelf-life studies were conducted at 4 °C to investigate the effect of bruising and sanitisation on the growth of spoilage microorganisms. In the first experiment, both bruising treatments (100% and 40% of leaves) halved shelf life to 12 d, whereas intact leaves had a shelf-life of 23 d. Bruising had no influence on bacterial diversity during shelf-life, though some differences in the relative abundance of minor genera were observed. <i>Pseudomonas</i> and <i>Pantoea</i> were the most dominant bacterial genera, regardless of bruising treatment. High throughput amplicon sequencing also identified other spoilage bacteria including <i>Chryseobacterium</i>, <i>Stenotrophomonas</i>, <i>Bacillus</i>, <i>Sphingobacterium</i>, <i>Erwinia</i> and <i>Flavobacterium</i>. In the second experiment, washing of intact baby spinach with a sanitiser (80 mg/L: PAA) reduced microbial load as determined by aerobic plate count but did not immediately affect the presence/relative abundance of most of the genera of spoilage bacteria observed. During shelf-life, the bacterial diversity of sanitised leaves was significantly lower than on water-washed leaves. Although sanitisation resulted in a higher initial log reduction in microbial load compared to control (portable tap water), sanitisation did not extend the shelf life of baby spinach (23 d). Sanitised spinach had reduced bacterial diversity however, by the end of shelf life, both sanitised and water-washed spinach was dominated by <i>Pseudomonas</i> and <i>Pantoea</i> spoilage bacteria. This study demonstrated for the first time that the shorter shelf life of bruised leaves was related to faster microbial growth rather than changes in bacterial diversity or community composition.