It is often mentioned that eating certain food or drinking certain liquids would affect one's singing. However, it seems to me that neither food nor drinks ever touch the vocal folds since the latter are part of the repiratory system. Looking for clarification on this matter. Thank you.
There is far more to singing than just the vocal folds. Much of the sound of a singers voice is shaped and influenced by muscles in the throat and face (including your nasal passages). So even if food does not come into contact with the vocal folds themselves, certain foods can have an effect on other parts of your vocal tract that can affect your singing.
For example, eating high fat dairy products can thicken the mucous in your throat and other parts of the vocal tract (including your nasal tract) and have an impact on your singing. Certain acidic foods or spicy foods can cause a powerful reaction in your throat that can constrict the airway and affect your voice. Foods (substances) that dry out your throat such as alcohol and caffeine can make it more difficult to sing properly. Have you ever tried to sing when your throat was dry?
I always avoid cold beverages prior to singing because it seems that cold beverages may tighten or constrict the muscles in your throat. You want your throat and all the muscle tissue surrounding your vocal folds to be warm prior to singing.
Of course it's a good idea to be well hydrated prior to singing for many reasons. One reason is that the vocal folds will respond better and be less prone to injury, if your body is in an overall state of proper hydration. Proper hydration starts well in advance of a singing performance. It actually takes many hours of proper water intake in the absence of excess diuretic foods or substances (salty foods, caffeine, alcohol) to achieve proper hydration.
Being even mildly dehydrated can affect your singing by making your throat and other parts of your vocal tract dry, and can have a deleterious effect on your vocal folds as well.
So even though food does not come into direct contact with your vocal folds, the foods and substances you ingest the day before and immediately prior to a singing performance, can make a big difference in how you sound, and how well your body tolerates the stress of a performance.