Influence of Key Processing Technology on the Quality of Freshly-squeezed Lettuce Juice

In order to realize the effects of blanching (95 ℃/2 min), separation (6000×g/10 min) and sterilization (600 MPa, 25 ℃/2 min) on the processing quality of freshly-squeezed lettuce juice, hydroponic green rosa lettuce were chosen as the test material. Changes of microorganisms, color, physical and chemical parameters, enzyme activities and antioxidant capacity were evaluated. The results suggested that the total aerobic bacteria of freshly-squeezed lettuce juice was less than 2 lgCFU/mL and the mold and yeast were less than 1.3 lgCFU/mL after sterilization process, all of which were within the national standard limit (GB 7101-2015). Separation process significantly affected the color and chlorophyll content of lettuce juice. The L^*, a^*, b^* and C^* values of fresh clear lettuce juice were significantly increased (P<0.05), and the clear juice was brighter, lighter in green, deeper in yellow and higher in saturation, and the total chlorophyl content decreased by 56.44%. Blanching process significantly decreased the a^* value (P<0.05), and the green color became brighter. The inactivation rates of polyphenoloxidase, peroxidase and catalase were 83.76%, 84.87% and 90.80% after blanching, respectively. Blanching and sterilization both affected the active components and antioxidant capacity of freshly-squeezed lettuce juice, the soluble sugar content retention rate was 91.24% and 88.3%, the soluble protein retention rate was 64.22% and 60.60%, the total phenolic content was decreased by 53.31% and 56.28%, the antioxidant capacity decreased by 82.61% and 73.91%, respectively. This study could provide technical reference for processing of freshly-squeezed vegetable juice.