The quantitative and qualitative changes of sugars in sour cherry fruit subjected to osmo-convective drying

Analizowano zmiany zawartości składników cukrowych w trakcie odwadniania osmotycznego wiśni w celu określenia proporcji między zawartością cukrów w produkcie końcowym a ilością cukrów pochodzących naturalnie z surowca. Owoce ośmiu odmian wiśni poddawano odwadnianiu osmotycznemu w 60-procentowym roztworze sacharozy i suszono konwekcyjnie. Analizę ilościową i jakościową cukrów wykonano metodą HPLC w surowcu mrożonym, odwodnionym oraz w suszu. Dominującymi cukrami w surowcu była glukoza (46–61%) i fruktoza (34–39%). Wiśnie zawierały też 12–20% sorbitolu. Proces odwadniania spowodował przyrosty fruktozy oraz spadki zawartości sorbitolu; zmiany zawartości glukozy nie były jednoznaczne. W sezonie 2009 w owocach odwadnianych stwierdzono obecność 2–20 g·100 g–1 s.s. sacharozy, która podczas suszenia konwekcyjnego w całości uległa hydrolizie. Zmiany pozostałych składników na etapie suszenia były relatywnie niewielkie. W ogólnym ilościowym bilansie składników cukrowych stwierdzono zaledwie 11-procentowy wzrost ich zawartości w suszu w stosunku do surowca, wykazano natomiast znaczące zmiany składu jakościowego. Z surowca tracony był sorbitol na rzecz znacznego wzrostu zawartości fruktozy.Osmotic treatment of plant tissue prior to drying results in a water content reduction that shortens drying time. Simultaneously, the infused osmotic agent modifi es the taste profi le of the treated tissue. In the case of sour cherries, osmotic dehydration causes an improvement in sensory characteristics, which enables direct consumption of dried product. Among different osmotic agents, sucrose has shown to be one of the most popular, as not only does it display the above characteristic, but additionally has been found to have a protective effect on bioactive compound retention. Unfortunately, recent literature seams to focus on the negative connection between high sugar consumption and the development of chronic diseases. In respect of this, trials have been undertaken to ascertain the precise qualitative and quantitative changes in sugar components provoked by osmotic dehydration in sour cherry tissue during osmo-dried fruit production. Additionally, the proportion between added and native sugar content was calculated. Fruits of 8 cultivars of sour cherries collected in 2009 and 2010 seasons were subjected to osmotic treatment (60° Brix sucrose solution, 1 hour at 40°C) and then convectively dried for 8 hours (3 m·s–1, 60°C). At each stage of processing (raw material, osmo-treated fruit, dried fruit) the precise analyses of sucrose, glucose, fructose and sorbitol contents were carried with using the HPLC method. Experiments were performed in two technological repetitions for both seasons. Irrespectively of cultivar and season glucose was quantifi ed as the predominant sugar component accounting for 46–61%. The second abundant was fructose ranging between 34–39%. The sour cherry fruit contained also substantial amount of sorbitol (12–20%). As it was expected, the applied osmotic dehydration treatment led to signifi cant changes in sugar components pattern. During 1 hour process the fructose contents increased (13–91%), which was associated with simultaneous sorbitol losses (64–85%). At this stage the signifi cant decrease of glucose/fructose ratio was observed. Further convective drying stage hardly affected sugar component composition, excluding losses of small sucrose amounts found in the osmo-treated samples form 2009 season trials. Although 60° Brix sucrose solution was used as the osmotic agent for pre-drying treatment, no sucrose in the fi nal dried fruit products were found. From the results of the experiments undertaken it can be confi rmed that on average, only 11% more sugar was present in dried product than in the raw material form. What must be noted, however, are the substantial changes in singular sugar constituents proportions, especially when it is considered from the dietetic point of view. Irrespectively of season and cultivar investigated, during the process, the sorbitol (known for its reduced caloric content and laxative properties) was drastically lowered, while the fructose amount signifi cantly increased