الملخص

Abstract: 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 percentages of Tyrosinase were 83, 68.8 and 90.3 % and the maximum inhibitory effects of browning intensity were 13.9, 9.7 and 29.8% respectively. For untreated quince juice which heated to 5 hours at 100 oC, 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 is non-competitive. The Km value remains 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.