I recall a teshuva of R Moshe (which I can’t find and may be remembering wrong) that says that an esrog that was stored in a vacuum from last year is not kosher even though it’s perfectly yellow and beautiful.
Where I bought an esrog they also gave me a paper that said “DO NOT REFRIGERATE YOUR ESROG” (bold is theirs). This got me wondering is the issue halachik that any preservation techniques invalidate an esrog or are they simply giving what they feel to be good advice?