[OhyesOhno69]

Hi, I would like to ask how do you check water content and ash content in a fruit? For water content, can you leave the fruit in a 100 degree oven overnight and just let it cool then weigh it? thx!

[Arkcon]

That is a reasonable way.  It is susceptible to some error, you can't assume you've gotten every last bit of moisture out of every cell, even if you freeze dried it.  A better, and faster, way is to perform a Karl Fisher titration, but that has technical problems of it's own. 

Dry ashing of foods probably has procedures written up in food science references, you'll want to throughly burn all organics, and not lose some ash as flyaway or volatiles.