Food that shreds many tiny particles when you bite it. For example, cookies or bread toast.
crumbly or crumby
Oxford Living Dictionaries carries the following entry:
Consisting of or easily breaking into small fragments.
‘the cheese has a sharp flavour and is crumbly and moist’
Crumby also has your intended meaning:
1 Resembling or covered in crumbs.
2 variant spelling of crummy
whereas crummy is:
crummy (also crumby)
1 Dirty, unpleasant, or of poor quality.
‘a crummy little room’
‘I've been feeling pretty crummy the last few days’
So, crumbly is unproblematic, crumby works, though it does also have another meaning, but crummy (pronounced identically to crumby) should be avoided for your use case.