Enzymatic browning by tyrosinase and nonenzymatic browning (Maillard reaction) are responsible for producing new compounds which contributes considerably to the undesirable of aroma, taste and colour in food. In this study, tyrosinase activity and browning intensity in quince juice were inhibited by treating them with antioxidant compounds vanillin, glutathione and melatonin as compared to the control. The maximum inhibition percentage of Tyrosinase were 83, 68.8 and 90.3 % and the maximum inhibitory effect of browning intensity were 13.9, 9.7 and 29.8% respectively. For untreated quince juice which heated to 5 hours at 100 o C, the results showed increasing in A294, Browning intensity (A420) and reducing power, as well as decreasing in reducing sugar, free amino group, phenolic compounds contents and inhibition percentage of lipid peroxidation. The decreasing of A294, browning intensity and increasing in reducing power were showed after treating with (10mM) vanillin and (8mM) glutathione compared to control. Conversely to glutathione, the addition of vanillin to quince juice revealed increasing in inhibition percentage of lipid peroxidation, reducing sugar and phenolic contents and decreasing in free amino acid content compared to control. Inhibition Mode of tyrosinase by melatonin was noncompetitive. The Km value remained constant (1.96mM), while Vmax reduced from 228.13 to 114.06 U.ml-1 .min-1. Inhibition constant Ki value was 6.9 mM.

Background: Because it tends to cause deterioration in the quality of food and appearance, food browning is unacceptable. Tyrosinase, which catalyzes the transformation of mono phenolic compounds into oquinones, has been associated with this phenomenon. Natural anti-browning agents were used to help avoid the enzymatic browning that occurs in many foods. Methods: Tyrosinase of Jerusalem Artichoke tubers was purified through (NH4)2SO4 sedimentation, dialysis, chromatography, and finally gel electrophoresis. The purified enzyme was characterized and inhibited by rosemary extracts. Results: Purification of tyrosinase from Jerusalem Artichoke tuber were accomplished. The specific activity at the final step of purification increased to 14115.76 U/mg protein with purification fold 32.89 using CM-Cellulose chromatography. The molecular mass was evaluated by electrophoresis and found to be 62 KDa. Maximum tyrosinase activity was found at 30 °C, pH 7.2, and higher affinity towards Ltyrosine. Inhibition percentage of heated extracts for leaves and flowers on tyrosinase activity was better than nonheated with 29.65% and 23.75%, respectively. The kinetic analysis exposed uncompetitive inhibition by leaves and flowers heated extracts. Conclusions: In this study, we concluded the usage of natural anti-browning inhibitors like rosemary extract be able to be castoff to substitute the chemical agents which might be dangerous to social healthiness. Natural anti-browning agents can be used to prevent the browning of many foods.