Usually, when you increase the temperature on anything, the rate of "whatever" increases as well. When we talk about pH, we are talking about the availability of hydrogen ions (H+). In juices, depending on the juice, the pH can range from 3 to 6. We are on the acidic side of the pH scale, so we have to understand when we talk about increasing the strength of the acid indicator, we will go down the scale, to the left of the "7" mark, which is an indicator of neutral territory between acids and bases. Greater temperature will cause the availability of hydrogen ions to increase, which will indicate a stronger acid, which will result in lower numbers of acid strength on the pH scale. It should be noted there can be a lot of confusion over this concept. A lower number on the pH scale indicates a stronger acid. So the higher the temperature, the lower the resulting pH number.
As the temperature increases, the pH decreases. There is an explanation for why this occurs. A chemical reaction is caused by the temperature change. The citric acid in fruit juices begins to dissociate. As the acid dissociates, the number of hydrogen ions present in the juice increases. pH is measured by the concentration of hydrogen ions using the formula pH = -log[H+]. With more hydrogen ions present, the pH is lower. In the same way, a lower temperature prevents this reaction from occurring.