Why the banana though?
Oddly enough, I just searched for bananas and vocal cords and one of the first links debated that :\
https://www.openmicuk.co.uk/advice/are-bananas-good-or-bad-f...
My recommendations are drink plenty of water, avoid alcohol, and practice.
Other folk remedies for voice include flat, room temperature Coca-Cola. I don't think anyone has demonstrated efficacy of any of these techniques, but another big factor in effective singing is mental state. If a placebo ritual helps get you in the correct frame of mind to sing, then it might just be worth doing. The destructive alternative is to get intoxicated before a performance.