"a food-hygienically acceptable substance": Grammatical syntax?

"food-hygienically" is a compound adverb, which functions like a regular adverb. Sources say that they can be written as a single word (e.g. "overnight"), hyphenated ("in-house", "self-consciously"), or as multiple words ("early on", "upside down") - (sources: ThoughtCo, Capstone Editing [Australia]). A phrase such as "self-evidently stupid idea" or even "brain-meltingly stupid idea" has a similar structure but would be uncontroversial.

So grammatically it's ok, but elegance and easy comprehensibility are different things. It's fairly unambiguous what is meant, but it's perhaps not instantly comprehensible: the compound adverb "food-hygienically" is not common and requires thought to consider the relation of the two halves. Personally, I'd prefer something more explicit and without the long sequence of words jammed together, such as "a substance acceptable under food hygiene regulations". If the context is unambiguous, you could realistically drop the word "food" and say a "hygienically acceptable substance".