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.