Osmoconcentration as a Method of Supplementing the Loss of Fructooligosaccharides in Long-Stored Onions

Onion bulbs are a primary source of health-promoting fructooligosaccharides (FOS), which significantly degrade during long-term storage. This study investigated the potential of osmotic dehydration in a concentrate containing FOS (55.3% DM) to restore FOS levels in stored onions. The available data lack information on the use of osmoconcentration for this purpose. Four cultivars (Alonso F1, Hysky F1, Hystore F1, and Red Lady F1) were evaluated after four months of storage. Two 40% hypertonic solutions, a saccharose control and a fructooligosaccharide concentrate were used. Mass migration indicators, including water loss, weight reduction, and solid gain, were determined alongside HPLC-RI analysis of the saccharide profile. Results showed that storage caused an approximately 50% reduction in the initial FOS content (approx. 4 g/100 g). Osmotic dehydration using the FOS concentrate increased FOS levels approximately threefold compared to the saccharose treatment, effectively restoring or exceeding post-harvest concentrations. Red Lady demonstrated the highest water loss (28.9%), while Hystore exhibited the greatest solid gain (9.8%). In conclusion, osmotic dehydration with FOS-rich solutions is an effective method for the functionalization of stored onion raw material. This approach successfully compensates for physiological losses during storage and allows for precise modification of the nutritional profile of the bulbs without compromising osmoconcentration efficiency.