Quality Properties of Subcritical Water Extracts of Onion and Onion Juice Product

Freeze-dried onion was extracted with subcritical water at different temperatures and times, and the individual phenolic compounds, total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity [2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and ferric reducing antioxidant power (FRAP)] were measured. The total individual phenolic compounds in the subcritical water extract (SWE) (120°C/10 min) was 1.50-fold higher than that in the hot water extract (HWE) (100℃/10 min). The quercetin, quercetin-3,4'-diglucoside, quercetin-4'-glucoside, and protocatechuic acid contents in the SWE (120℃/10 min) were 2.19-, 1.28-, 1.52-, and 1.31-fold higher, respectively, than those in the HWE. TPC and TFC were also 1.48- and 1.53-fold higher in the SWE (120℃/10 min), respectively, than those in the HWE. DPPH and FRAP increased with increasing extraction temperature and time, and were 1.82- and 1.63-fold higher in the SWE (120℃/30 min), respectively, than those in the HWE. A trial product (TP) of onion juice was made with subcritical water extracts of raw onion and onion skin (88:12) to prepare onion juice with a high phenolic content, and its quality was compared with that of the commercial product (CP). The total individual phenolic compounds and quercetin levels in the TP were 3.1- and 5.4- times higher than those in the CP, respectively. On the other hand, TPC, TFC, DPPH, and FRAP in the TP were only 1.22, 1.21, 1.37, and 1.15 times higher than those in the CP, and other chemical compounds in the CP affected those properties. The 5-hydroxymethylfurfural content, which is a heat damage index, of the CP was 84.7 times higher than that of the TP. These results indicate that onion juice made by subcritical water extraction at 120℃/10 min was high in phenolic compounds and high in antioxidant activity without significant heat damage.