Osmotic dehydration of CO₂-treated persimmon slices: a microscopic approach to mass transport phenomena

Persimmon is worth processing for its important nutritional value, linked to its high carotenoids, polyphenols and fiber content. Osmotic pre-treatments could be applied to improve fruit product quality together with postharvest treatments that remove astringency without concomitant softening. The aim of this work was to analyse persimmon tissue structure at a microscopic level to define the influence of osmo-dehydration time on mass transfer rates. Ten mm thick persimmon slices ('Kaki Tipo'), CO₂-treated and not, were osmodehydrated for 120 and 240 min at 25°C in 60% (wt/wt) sucrose solution and solid liquid exchanges were calculated. Histocytochemical analyses, performed under light microscopy on semi-thin sections of both fresh and embedded samples, showed the polymerization of soluble tannins in crystalline bodies, accounting for the lower astringency of CO₂-treated slices. Polymerization, by freeing intercellular spaces, and thereby allowing sucrose intake, could explain the higher solid-liquid exchanges of CO₂-treated persimmon. Moreover, osmosis induced soluble tannins polymerization. Results confirmed the utmost importance of fruit cellular structure analysis in optimizing the osmo-dehydration process.