Dehydrofrozen sweet organic cherry : the influence of an osmotic pre-treatment and cultivar on colour stability

Sweet organic cherry (Prunus avium) is worth processing for its important nutritional value, linked to high anthocyanin content. Osmo-dehydrofreezing, a combined process where freezing is preceded by an osmotic dehydration step in concentrated sugar solution, could be the method of choice in order to maintain the original quality of fruit. The purpose of the present work was to evaluate the influence of sweet cherry cellular structure, linked to cultivar, on solid-liquid exchanges and on colour modification during osmotic dehydration. Moreover the effect of the osmotic step on composition parameters and colour of sweet cherry during frozen storage was studied. Nine cultivars of sweet organic cherries were processed: two grown in the South and seven in the North of Italy. According to literature data, osmotic dehydration was carried out for 45 min at 25°C in a 65% (wt/wt) sucrose solution. Freezing was carried out at – 50°C air temperature and 4m/s air speed. Frozen and osmodehydrofrozen sweet cherries were stored at – 20°C up to 9 months. Solid-liquid exchanges were quite low, as already cited in literature, and they were influenced by the cultivar. The faint lightening of colour, caused by the process and related to a decrease of both a* (redness) and b* (yellowness) values, was linked to solid-liquid exchanges: the higher the exchange, the greater the difference in colour, except for Giulietta and Adriana. These cultivars behaved in an ideal way showing a fairly high concentration index with very limited colour modification. During frozen storage the osmotic pre-treatment helped towards keeping the original cherry colour: the extent of the protection was linked to the cultivar. The results, besides confirming the utmost importance of fruit tissue structure, evidenced that it is only by a strict interaction between two factors, the choice of the right cultivar and the correct treatment, that high quality processed sweet organic cherry can be obtained.