Only Krishna Prasadam

First and foremost, one should eat only prasädam i.e. food that has been offered to Lord Kåñëa with devotion.

Part of surrendering to Kåñëa is being careful not to eat anything that has not been offered to Him. That should be our determination. [The Laws of Nature]

Our life in Kåñëa consciousness means to be always the servant of Kåñëa. As the servant eats remnants of food left by the master, we, servants of Kåñëa, eat remnants of food left by Kåñëa. That food is called prasädam, or the Lord’s mercy. [TQE]

One should eat only food cooked by devotees.

The word apavitra anna refers to food that is unacceptable for a Vaiñëava. In other words, a Vaiñëava cannot accept any food offered by an avaiñëava in the name of mahä-prasädam. This should be a principle for all Vaiñëavas.

A vaiñëava must be very strict in this respect and should not at all cooperate with an avaiñëava. If an avaiñëava offers food in the name of mahä-prasädam, it should not be accepted. Such food cannot be prasädam because an avaiñëava cannot offer anything to the Lord.

Even if an avaiñëava cooks food without fault, he cannot offer it to Lord Viñëu, and it cannot be accepted as mahä-prasädam.

Kåñëa can accept anything offered by His devotee with devotion. An avaiñëava may be a vegetarian and a very clean cook, but because he cannot offer Viñëu the food he cooks, it cannot be accepted as mahä-prasädam. It is better that a Vaiñëava abandon such food as untouchable.

[Excerpts from Çré Caitanya-caritämåta Madhya-lélä 9.53 Purport]

Çréla Prabhupäda strictly forbids taking food prepared in restaurants or hotels or in airplanes. One should not accept even water from a non-devotee unless there is a dire necessity.

One should carefully avoid eating food offered by nondevotees. Indeed, a devotee should be very strict in not accepting food from a nondevotee, especially food prepared in restaurants or hotels or on airplanes. [Çré Caitanya-caritämåta Madhya-lélä 12.195]

We are prohibited to take prasädam anywhere and everywhere. Because if there is no Deity worship, if we take food from their house, then we partake the sin, resultant action of their sinful activities. We should be careful. Unless it is prasädam, if we accept invitation from anywhere and everywhere, that means we are accepting the sinful activities, resultant action of the sin, person. [Sannyasa Initiation Lecture, Calcutta, January 26, 1973]

So far accepting water, no, practically from one who is not initiated we should not accept any eatables or drinkables. This should not be discussed, but you should remember it. But if there is dire necessity, there is not question of prohibition. [Letter to: Bhavananda, Honolulu, May 9, 1972]