Ascorbic Acids or vitamin C is an excellent ANTIOXIDANT which help retain the freshness of juice/fruit its present in. For example, addition of a spoonful of lemon juice can help you keep the fresh coriander leaf paste green for longer time, otherwise it turns brownish very soon.
If all other ingredients (Sugar, Mineral ions, etc contents) of various fruit juice samples kept constant, the greater the amount of ascorbic acid present , greater will the self life of the juice.
In five juice samples of various fruit like grape, apple, pomegranate, orange and mango there are various factors like sugar content (reducing and non-reducing sugar), mineral ions (Oxidation- reduction), carotenes (also may have antioxidant properties) that may affect their self life.
So, I would prefer to start with taking the amount of Ascorbic acid as the single variable.
Take 5 aliquots (equal amount of same sample) of the juice of same fruit, say pomegranate, in different test tubes. keep one test tube as control (add nothing), in another add 1 ml of distilled water. In rest of 3 test tubes add 1 ml of ascorbic acids (x mol/ ml , 2xmol/ ml, 4xmol/ml concentration). Incubate the sample at room temperature for few hours. After the incubation period, note the change in texture of the samples like taste (sourness), smell, color change, etc.
Furthermore, you can take 5 different juice samples in duplicates. Add 1 ml of ascorbic acid (1 spoonful of lemon juice) to one and keep another as control (no additive). Do the same for all 5 juice, each pair with 1 control and another with same quantity of ascorbic acids added. Incubate for few hours and note the change in their texture compared with the respective control.
However, if u are a UG/ PG student, you may calculate the ascorbic acids content in fruit juice with simple colorimetric or titration methods. You can proceed with the juices as in immediate previous para. After incubation you can determine the amount of reducing sugar remaining. Going beyond that like measuring the concentration of various components before and after incubation of samples with ascorbic acid added vs the control will require a sophisticated lab.