Factors determining the mechanical properties of banana fruit skin during induced ripening

Abstract

Postharvest peel splitting in banana impairs visual quality and exacerbates mass loss, fruit fly infestation and microbial infection. The aim was to determine the mechanistic basis of peel splitting. We therefore monitored the change in physical and mechanical properties of the peel as affected by different storage conditions. Peel splitting occurred axially, in the fruit corners. It began 4 d after ripening induction. Splitting was preceded by radial cracks in the inner peel, that propagated towards the outside. In fruit stored at 98 % relative humidity (RH), splitting occurred in 38 % of fruit; there was no splitting in fruit stored at 14 or 60 % RH. The percentages of fruit with radial cracks in the inner peel were 63 %, 100 %, and 100 % at 14 %, 60 % and 98 % RH, respectively. The fracture force (Fmax) of the peel decreased rapidly from 53.0 ± 1.6 N to 2.1 ± 0.2 N during ripening. Simulated radial cracks (cuts) in the inner peel decreased Fmax from 22.8 ± 2.2 N to 15.9 ± 0.9 N Paring off the outer peel decreased Fmax from 18.9 ± 1.7 to 4.4 ± 0.8 N, but paring off the inner peel had no effect. Osmotic water uptake by peel discs incubated in glucose solutions decreased with increasing concentration. Water was lost to hypertonic glucose solutions causing discs to shrink. Epidermal and hypodermal cell wall thickness both increased due to cell wall swelling. Our results show: (1) the outer peel layers represent the mechanical backbone of the peel, (2) radial cracks in the inner peel are the first stage of peel splitting and (3) peel splitting is caused by local stress concentrations due to radial cracking and is exacerbated by cell wall swelling. Key to reducing peel splitting at retail is prompt removal of fruit from the high RH environment of the plastic-lined cartons.