Determination of phytochemical properties of dried onion slices (Allium cepa L. var. Violet of galmi)

The high water content in fresh onion bulb make it difficult to preserve. In order to remedy these situation, it can be converted into dried slices. The objective of this work was to optimize the phenolic compounds and antioxidant properties of dried onion slice. Box Behnken design was performed to determine the effect of drying temperature (25-45 degrees C), water activity (0.2-0.6) and drying time (12-24 h) on the moisture content, total phenolic, flavonoids content, DPPH free radical scavenging and total reducing power on dried onion slice. Desirability was fixed to obtain the best possible combination of factors to a maximum values of total phenolic and flavonoids content, DPPH free radical scavenging, total reducing power and a low level of moisture content. Data analysis showed that the factors significantly (p < 0.05) affected the responses variables. Desirability function showed that the optimal dry conditions were 20 h for drying time, drying temperature of 28 degrees C and water activity of 0.48. At this optimum point the moisture content, polyphenol and flavonoids content, DPPH free radical scavenging and total reducing power were respectively: 9.53%, 1060.45 mg gallic acid/100 g DM; 342.61 mg quercetin/100 g DM; 52.18 mg Trolox/100 g DM and 78.14 mg Vitamin C/100 g DM. In this overall optimum point desirability was 0.9. No significant difference (p < 0.05) was found between the experimental and predicted values of the response variables at optimum point.