Is there such a word as fermentable, the student was trying to use as an adjective?

Hi, yes, it's an adjective indeed. It ends with a suffix (as you know a suffix is a word added at the end of a base word which gives it a different meaning).

An adjective suffix, now usually in a passive sense; forms adjectives meaning possible, or fit.

able to be

movable: able to be moved

amendable, able to be amended

fit to be

blamable, fit to be blamed

salable, fit to be sold

relevant or suitable to, in accordance with

fashionable

seasonable

expressing capacity or worthiness in a passive sense

indicating obligation; as in payable, reportable, taxable.

Hope this helps

Good luck, God bless you

Yes. but i dont think that can't be used as an adjective.

fermentable - 3 dictionary results

–noun 1. Also called organized ferment. any of a group of living organisms, as yeasts, molds, and certain bacteria, that cause fermentation.

2. Also called unorganized ferment. an enzyme.

3. fermentation.

4. agitation; unrest; excitement; commotion; tumult: The new painters worked in a creative ferment. The capital lived in a political ferment.

–verb (used with object) 5. to act upon as a ferment.

6. to cause to undergo fermentation.

7. to inflame; foment: to ferment prejudiced crowds to riot.

8. to cause agitation or excitement in: Reading fermented his active imagination.

–verb (used without object) 9. to be fermented; undergo fermentation.

10. to seethe with agitation or excitement.

What the heck how in the world could that be used as an adjective? The suffix able is a verb!

of course there is. it's the the act of being able to be fermented. many words can have the suffix "able" added to them.

It appears to be an adjective.