Lord Kṛṣṇa is offered, according to the Bhagavad-gītā (Bg. 9.26), vegetables, flowers, fruits, grains, milk, etc. In this way, a person in Kṛṣṇa consciousness becomes automatically trained not to accept food not meant for human consumption, or which is not in the category of goodness.
Bhagavad-gītā 6.17, Purport (Excerpt)