The word apavitra anna refers to food that is unacceptable for a Vaiṣṇava. In other words, a Vaiṣṇava cannot accept any food offered by an avaiṣṇava in the name of mahā-prasādam. This should be a principle for all Vaiṣṇavas. When asked, “What is the behavior of a Vaiṣṇava?” Śrī Caitanya Mahāprabhu replied, “A Vaiṣṇava must avoid the company of an avaiṣṇava [asat].” The word asat refers to an avaiṣṇava, that is, one who is not a Vaiṣṇava. Asat-saṅga-tyāga, — ei vaiṣṇava-ācāra (Cc. Madhya 22.87). A Vaiṣṇava must be very strict in this respect and should not at all cooperate with an avaiṣṇava. If an avaiṣṇava offers food in the name of mahā-prasādam, it should not be accepted. Such food cannot be prasādam because an avaiṣṇava cannot offer anything to the Lord. Sometimes preachers in the Kṛṣṇa consciousness movement have to accept food in a home where the householder is an avaiṣṇava; however, if this food is offered to the Deity, it can be taken. Ordinary food cooked by an avaiṣṇava should not be accepted by a Vaiṣṇava. Even if an avaiṣṇava cooks food without fault, he cannot offer it to Lord Viṣṇu, and it cannot be accepted as mahā-prasādam. According to Lord Kṛṣṇa in the Bhagavad-gītā (9.26):

patraṁ puṣpaṁ phalaṁ toyaṁ yo me bhaktyā prayacchati
tad ahaṁ bhakty-upahṛtam aśnāmi prayatātmanaḥ

“If one offers Me with love and devotion a leaf, a flower, a fruit or water, I will accept it.”

Kṛṣṇa can accept anything offered by His devotee with devotion. An avaiṣṇava may be a vegetarian and a very clean cook, but because he cannot offer Viṣṇu the food he cooks, it cannot be accepted as mahā-prasādam. It is better that a Vaiṣṇava abandon such food as untouchable.