Researchers have found that the enzyme polyphenol oxidase (PPO) in certain fruits like bananas can reduce the absorption of heart-healthy flavanols, especially when combined with flavanol-rich ingredients like berries. The study suggests choosing ingredients with low PPO activity, such as pineapple or oranges, when making smoothies to optimize flavanol absorption.
My experience is from preparing sliced apples to eat later. Dip them in slightly salted water for less than a minute and they’ll hold for quite some time. No need to carry water in the container. The salt must help create an insulation membrane against oxidation. This is all practical experience and I haven’t seen research on it.