We are making some avocado rolls and want to put them in the fridge for eating tomorrow. We will of course wrap them very well but how to avoid the avocado turning black or darkening?
You can try dipping it into acidulated water (water with acid added, either lemon juice or vinegar, or even a powdered vitamin c tablet). This should delay the browning, but I am not sure it will last long enough to hold the rolls overnight.
You will have the additional problem that the nori will begin to soften overnight.
Placing whole avocado (with intact skins) in a water bath at 43°C for a couple of hours will deactivate the enzyme that causes browning.
See Avocado Tests
Ditto the suggestions on dipping the avocado slices in some sort of citric acid. But trying to hold the rolls isn't the best idea. As stated, the nori will become soft, the avocado with brown and turn mushy and the rice will dry out. If you insist on holding it try this method. Wrap the entire cylindrical roll tightly (no air) in plastic wrap, store in the fridge, then unwrap and slice them up right before serving. You might want to consider omitting the avocado.
Best of luck to you!