Improving the shelf-life of baby leafy salad vegetables

Baby leafy salad vegetables have a limited shelf-life because they are fragile, have a high respiration rate, neutral pH, high aw, and upon mechanical damage, release nutrients that support microbial growth. Sanitisation, handling (e.g. leading to damage), packaging systems, storage temperature, and high relative humidity (90-100%) are postharvest factors known to influence microbial loads and microbial growth potential and shelf-life. This thesis investigates how handling and processing of baby leafy salad vegetables influences shelf-life, quality and the composition of bacterial spoilage communities. This knowledge offers opportunities for longer shelf-life and, consequently, wider market access of leafy green vegetables. Peroxyacetic acid (PAA) is a commercial organic sanitiser reported to extend shelf-life of some fresh produce including leafy salad vegetables. The effect of PAA sanitisation on the bacterial community of baby leafy salad vegetables during shelf-life has rarely been studied. Results showed that despite reducing total microbial load, PAA (80mg/L) did not influence bacterial diversity on intact baby spinach leaves on day-0 nor did washing with tap water only, and that spoilage bacteria were not eliminated, but were somewhat reduced. Sanitised baby spinach had lower bacterial diversity index (2.3) compared with water-washed leaves (2.8) at 4 ¬¨‚àûC storage. Relative abundance of Pseudomonas on PAA-treated intact (i.e. undamaged) baby spinach was >50% from day-6 until the end of shelf-life and was higher than in water-washed spinach. Pseudomonas ranged from 24-49% relative abundance from day-9 until the end of shelf-life on water-washed samples however, Pantoea, Paenarthrobacter, Exiguobacterium, and Flavobacterium were also prevalent. The shelf-life (23 d) of PAA-sanitised intact baby spinach was, similar to water-washed intact baby spinach. Thus, PAA treatment alone did not extend shelf-life of bagged baby-spinach leaves. Changes in microbiome composition did not appear to influence shelf-life either. Mechanical damage (bruising‚ÄövÑvp) of leafy salad vegetables can occur during harvesting, transporting and processing, and is known to reduce shelf-life. However, the effect of 'bruising' (mechanical damage) on the bacterial community of leafy salad vegetables during shelf-life has not been rigorously explored. In this thesis, I studied the shelf-life and bacterial community of baby spinach of three 'quality' categories namely; 100% bruised leaves, 40% bruised + 60% intact (bruised + intact‚ÄövÑvp, i.e., undamaged leaves), and 100% intact leaves. All categories of leaves were sanitised with 80 mg/L PAA. Bruising halved the shelf-life of baby spinach: intact leaves had a shelf-life of 23-d compared to 12-d for bruised or bruised + intact leaves. The relative abundance of Pseudomonas, Sphingobacterium, Chryseobacterium Flavobacterium, and Janthinobacterium, which are mostly recognised as spoilage bacteria, increased during shelf-life (days 1-15) on the bruised and 'bruised + intact' leaves. The bacterial diversity differences between the quality categories were not significant, though some differences in relative abundance of minor genera were observed. The bacterial community was dominated by Pseudomonas spp. and Pantoea, regardless of leaf quality and treatment, and similar to the observations in earlier experiments investigating the effects of sanitiser. During commercial processing, most added water is removed from baby leafy salad vegetables after washing and partial sanitisation in disinfectant baths. Drying systems, are not completely efficient. Using leaves without surface moisture, this study demonstrated that addition of wash water as 1, 2 or 5 mL PAA (80 mg/L) to 60-g OPP bags (190 mm * 250 mm) of dry baby spinach leaves significantly reduced shelf-life. Two and five mL additions reduced shelf-life by 17 and 35%, respectively, in an initial trial (Trial 1). One mL added wash water reduced shelf-life by 13%, whereas 2 and 5 mL added wash water reduced shelf-life by 38% in a subsequent trial (Trial 2). Baby spinach leaves with no added wash water had the longest shelf-lives: 23-d in Trial 1 and 16-d in Trial 2 and retained normal quality attributes until the end of shelf-life. The presence of grit reduces the eating quality of baby salad vegetables. The efficacy of a food grade anionic surfactant, sodium dodecyl sulphate (SDS), alone (0.025, 0.05, and 0.1% SDS), and in combination (0.05% SDS) with 40 mg/L PAA on grit removal, shelf-life, quality, and sensorial attributes of baby spinach was also studied. With SDS addition, grit levels were significantly reduced (21%) without reduction in quality attributes (colour, electrolyte leakage, visual quality and taste) nor reduction in shelf-life. This research has provided new insights on postharvest factors that may influence the shelf-life and quality of leafy salad vegetables. Shelf-life studies demonstrated that excess wash water and bruising significantly reduced shelf-life of bagged baby spinach leaves. Although surfactants did not improve shelf-life, they could be used in industry to improve grit removal and product quality without compromising other quality attributes of baby spinach leaves or sanitiser efficacy. These studies also improved our understanding of the role of spoilage microorganisms in the shelf-life of leafy salad vegetables. Specifically, the survival and growth of the most dominant spoilage microorganism, Pseudomonas, was not affected by sanitiser treatment (PAA, 80 mg/L). Surprisingly, while bruising greatly reduced shelf-life, it did not influence bacterial diversity, although relative abundance of other spoilage microorganisms increased during storage. Future research should focus on optimising drying conditions for baby leafy salad vegetables and managing moisture accumulation in packages of leafy green vegetables during storage and distribution to extend shelf-life, and on reducing bruising during harvest and processing. Explanatory note on thesis structure This thesis contains a combination of peer reviewed publications, and articles undergoing peer-review or revision. Accordingly, some repetition may occur between chapters. Chapter 1 consists of a general introduction about the research topic and ends with the thesis objectives. Chapter 2 is the literature review which explores available information on factors affecting the shelf-life of baby leafy focussing mainly on postharvest factors with emphasis on processing. Chapters 3 and 4 are experimental chapters written in the form of scientific publications and are being prepared for publication in the international refereed literature. Elements of Chapter 5 have already been published in the Journal of food quality. Chapter 6 is a general discussion which ends with conclusions and recommendations for future work. The Appendices contains supplementary tables and figures and preliminary studies conducted as part of this research.