My mage's hallow accumulates Tass in the form of mushrooms, and yes, it takes time for the edible tass to convert to mana. Also, the mushrooms spoil quite fast unless consumed soon-ish! (The wiki says a week for food).
Unlike other forms of tass, edible tass cannot be stored for long. I guess if you have prime & life 2 you can cast something to prolong the "shelf life" of edible tass but that's not what my mage has (or plans to have).
As such, he has tass in the form of mushrooms that spoils. Even if we assume (and it's not covered in the rules) that mushroom-tass can be preserved a bit longer with fermentation or preservatives and cold temperature, the fact remains that the tass is spoiling sooner or later.
Thus, it's wasted Tass unless you use it and according to the wiki-rules, it is 3 mana for one OR 2 people.
In my understanding, that means that about half the mana of Corn's Hallow spoils. Which is a waste.