Place the adverb before or after "to"?

Take for instance these two sentences:

[...] to confirm to clients that they are eating food free from contaminants and disease-causing bacteria, but also to identify the properties of the dishes.

[...] to confirm to clients that they are eating food free from contaminants and disease-causing bacteria, but to also identify the properties of the dishes.

Which of the above sentences is using correct grammar?

This depends on how formal you wish to be. By the content it sounds like you wish to be relatively formal.

The construction ''to identify'' is an infinitive in English. Traditional orthodoxy has it that infinitives should not be split, which is to say that no adverb should be interposed between ''to'' and the verb form (in this case ``identify''). If you are concerned about grammatical niceties it's probably best to be on the safe side and to go with your first formulation ''also to identify''. That is also the sentence that rings more naturally to my ears.

However, the prohibition on split infinitives doesn't always follow oral usage and there are contexts where it will read better to split the infinitive. My own preference would generally be to split the infinitive here if the only alternative sounds forced or unnatural. That said, when I'm writing formal papers I will generally look for a work-around rather as far as possible in preference to infinitive-splitting.

In your example ''but to also identify the properties of the dishes'' not only splits the infinitive but seems awkward to me, so I would strongly recommend the first sentence. The key is that, in my opinion, this is not an inviolable grammatical rule unless you are in a very particular context.