Effect of Various Pretreatments Methods under Physicochemical and Nutritional Properties of Onions

The pre-thermal treatment, or blanching, is an important process to minimize quality changes during main cookingor storage. In this study, three types of pre-thermal treatments were applied to onions to observe quality changes intheir physicochemical, nutritional, and microbial properties. The washed and sliced onions were treated by hot-waterimmersion (100oC, 1~10 min), steaming (100oC, 1~10 min) and stir-frying with oil (180oC, 10~80 sec). The onionswere cooled and wiped with gauze. The results showed that the total color difference of the treated sample increasedto 18-23 units compared to that of the fresh control. The pH value of the onion was not significantly different withthe exception of the stir-fried samples. The hardness of the onion tended to decrease depending on the treatmenttime. The micro-structures of onions treated by hot-water immersion and steaming were damaged above 2 minwhereas stir-fried onions showed structural damage above 210 sec. In the analysis of the nutritional changes in theonion, the ascorbic acid content of the blanched treatments were lower than that of fresh onion and the free sugarcontests showed a similar pattern excluding the stir-fried onions. In the organic acid content assay, the succinic acidcontent of the treated onion sample had significantly decreased compared with other organic acid contents (p0.05).The peroxidase activities of the blanched treatments by hot-water immersion and steaming were inactivated by morethan 92%, whereas only 76% inactivity was achieved by the stir-frying treatment. The mesophilic and psy-chrotrophic bacteria counts of the treated onion were not detected or decreased to near a 3-log scale, and yeast wasdetected only the sample treated by the stir-frying for 1 min. In particular, neither bacteria nor yeast counts in allonion samples by steam treatment were detected. Consequently, the results demonstrated that steam treatment for 2min could be the optimal pre-thermal condition for onions to minimize the quality changes and increase safety.