Effect of Washing, Waxing and Low-Temperature Storage on the Postharvest Microbiome of Apple

Abstract

There is growing recognition of the role that the microbiome plays in the health and physiology of many plant species. However, considerably less research has been conducted on the postharvest microbiome of produce and the impact that postharvest processing may have on its composition. Here, amplicon sequencing was used to study the effect of washing, waxing, and low-temperature storage at 2 °C for six months on the bacterial and fungal communities of apple calyx-end, stem-end, and peel tissues. The results of the present work reveal that tissue-type is the main factor defining fungal and bacterial diversity and community composition on apple fruit. Both postharvest treatments and low temperature storage had a strong impact on the fungal and bacterial diversity and community composition of these tissue types. Distinct spatial and temporal changes in the composition and diversity of the microbiota were observed in response to various postharvest management practices. The greatest impact was attributed to sanitation practices with major differences among unwashed, washed and washed-waxed apples. The magnitude of the differences, however, was tissue-specific, with the greatest impact occurring on peel tissues. Temporally, the largest shift occurred during the first two months of low-temperature storage, although fungi were more affected by storage time than bacteria. In general, fungi and bacteria were impacted equally by sanitation practices, especially the epiphytic microflora of peel tissues. This research provides a foundation for understanding the impact of postharvest management practices on the microbiome of apple and its potential subsequent effects on postharvest disease management and food safety.

Keywords: Empire apples; Malus domestica; carposphere; foodborne pathogens; fruit microbiome; microbial composition; microbiota; plant microbiota; postharvest management.

Figure 1

Schematic representation of the different apple tissues i.e., stem-end, calyx-end, and peel sampled and analyzed in the present study.

Figure 2

Box plots showing the bacterial fungal diversity (Shannon index) in the apple calyx-end, peel, and stem-end tissues. Superimposed on the box plots are the horizontally jittered raw data points. Values indicate p values of the results of pairwise comparison using nonparametric t-test.

Figure 3

PCoA based on Bray Curtis dissimilarity metrics, showing the distance in the bacterial (a) and fungal (b) communities between apple calyx-end, peel, and stem-end tissues.

Figure 4

Relative abundances of the most prevalent bacterial taxa >1%. (a) a heat tree showing the overall taxonomic hierarchy from phyla to order level. The size and color of nodes and edges are correlated with taxa abundance across all the investigated samples. (b) bar plot showing the distribution of the bacterial phyla among apple tissue types (calyx-end, peel, and stem-end tissues), and (c) at the lowest identifiable taxonomical level. Taxa with relative abundance less than 1% were merged into “Other” group in grey.

Figure 5

Relative abundances of the most prevalent fungal taxa >1%. (a) a heat tree showing the overall taxonomic hierarchy from phyla to order level. The size and color of nodes and edges are correlated with taxa abundance across all the investigated samples. (b) bar plot showing the distribution of the fungal phyla among apple tissue types (calyx-end, peel, and stem-end tissues), and (c) at the Lowest identifiable taxonomical level. Taxa with relative abundance less than 1% were merged together into “Other” group in grey.

Figure 6

Box plots showing the bacterial and fungal diversity (Shannon index) in the apple calyx-end (a,d), stem-end (b,e), and peel (c,f) tissues. Superimposed on the box plots are the horizontally jittered raw data points colored according to the sampling points (0, 2 m, 4 m, and 6 m). Different letters indicate significant differences between groups (p < 0.05).

Figure 7

Temporal variations in the bacterial and fungal diversity based on Shannon index at different sampling points (0, 2 m, 4 m, and 6 m) during cold storage of apple calyx-end (a,d), stem-end (b,e), and peel (c,f) tissues.

Figure 8

PCoA based on Bray Curtis dissimilarity metrics, showing the distance in the bacterial and fungal communities between postharvest treatment (UW = unwashed, W = Washed, and WW = washed and waxed) in apple calyx-end, peel, and stem-end tissues.

Figure 9

PCoA based on Bray Curtis dissimilarity metrics, showing the distance in the bacterial (a) and fungal (b) communities between sampling points (0, 2 m, 4 m, and 6 m) in different postharvest treatments (UW = unwashed, W = Washed, and WW = washed and waxed) in apple calyx-end, peel, and stem-end tissues.

Figure 10

Relative abundances of the most prevalent bacterial and fungal genera present in different sampling points (0, 2 m, 4 m, and 6 m) in different postharvest treatments (UW = unwashed, W = Washed, and WW = washed and waxed).