The experiment was conducted on a gac–papaya juice to determine the effects of cooking conditions (open boiler system and closed system) on product quality and antioxidant activity. The samples were subjected to cooking for different lengths of time for each of the processing conditions. The main effect of cooking condition (time, temperature, type of cooking) was determined by using ANOVA. The physicochemical properties and antioxidant activity of the product were measured. The physicochemical properties (Brix, pH and viscosity) of gac-papaya juice changed significantly during heating time. The lycopene content significantly increased (p < 0.05) when the period of boiling increased (at different cooking conditions) between 0 to 5 minutes. However, both the b-carotene and the Vitamin C content decreased significantly (p < 0.05) as boiling period increased between 0 to 5 minutes. Processing of juice in open boiler system led to a significant decrease in ascorbic acid and beta-carotene whilst vacuum cooking preserved these compounds. The degradation of these compounds followed a first-order model. The antioxidant activity (AA) of these products (i.e., boiling in closed and open system) was also determined. In both boiling conditions, significant increasing antioxidant activity was found.