In an IA on how storage temperature affects the acid content of oranges, this is my theory, is it correct?
In terms of Le Chatelier's principle, the temperature change shifts the equilibrium hence increasing or decreasing moles hydrogen ions. These changes persists only when the temperature remains steady. Oranges normally consumed at room temperature irrespective of how they were stored before. So under these new conditions, the equilibrium is bound to shift according to room temperature. Hence, my hypothesis is that storage temperature will not affect the hydrogen concentration in the oranges on a permanent basis and hence it will not affect the acid content.
Your understanding how how Le Châtelier's Principle applies is correct. Citric acid and ascorbic acid, both present in orange juice, are weak acids. Their degrees of ionization are represented by acid dissociation constants, Ka's. Equilibrium constants vary with temperature. This difference can be calculated for any two temperatures if one knows the molar enthalpy of ionization. This would be difficult to calculate for orange juice without knowing the concentrations of each acid, but the point is that temperature does affect the equilibrium as you stated. You're correct that the [H+] concentration won't be affected by a decrease in temperature followed by a return to the original temperature, as the equilibrium values for the higher temperture will be re-established.
There are some other issues with storage that could affect the concentration of [H+]. For example, if the oranges are cut open such that the juice is exposed to oxygen the acids will be oxidized. Changes that take place in the oranges over time will affect the water content and therefore the [H+].