"The kitchen department should be very clean and things should not be wasted. This is the first consideration."
Srila Prabhupada Letter to Aksayananda, 01-06-76
When preparing food for offering to Lord Krsna, cleanliness is a most important principle. Nothing impure should be offered to the Lord, so the kitchen environment and all gear and utensils must be kept very clean. Foodstuffs being prepared for offering must be used only for that purpose:
"That which is meant to be offered in yajna cannot be tasted by anyone before being offered to the Deity. In our temples, this regulation is in effect. One cannot eat food from the kitchen unless it is offered to the Deity. If something is taken before being offered to the Deity, the entire preparation is polluted and can no longer be offered. Those engaged in Deity worship must know this very well so that they may be saved from committing offenses in Deity worship."
Srila Prabhupada also instructed that prasadam should never be taken in the kitchen where cooking is going on. If space is limited, part of the kitchen may be partitioned off as a prasadam taking area, but this activity must be done separately from the cooking area.
"One should never eat within the kitchen, there is ample place to eat so why should one eat in the kitchen? Kitchen should be considered as good as the Lord's room, and nobody should wear shoes in the kitchen, smelling and tasting of foods being prepared for the Lord should never be done, talking within the kitchen should be only what is necessary for preparing the prasadam, or about the Lord, and dirty dishes (those taken from kitchen and eaten from) should not be brought back into the kitchen (but if there is no other place to wash them, then they should be put into sink and washed immediately.), hands should always be washed when preparing prasadam, and in this way, everything shall be prepared very cleanly and pure.)"
Srila Prabhupada Letter to Aniruddha, 06-16-68
"It is advisable that food being offered to the Deity be covered when taken from the kitchen to the Deity room. In that way, others may not see it. Those who are not accustomed to following the advanced regulative devotional principles may desire to eat the food, and that is an offense. Therefore no one should be given a chance to even see it. However, when it is brought before the Deity, it must be uncovered."
Caitanya-caritamrta, Madhya lila 4:124
At all times, animals must be kept out of the kitchen area, and away from foodstuffs being prepared. Animals may not be permitted to see foodstuffs as they're being prepared or carried to the altar.
Whenever possible, it is best to have completely separate cooking facilities: a Deity kitchen, and a devotee kitchen. Of course, it is especially difficult for many householders to arrange separate kitchen spaces. At the least, one should maintain cooking pots, utensils, etc. that are only use when cooking for the Lord. Serving spoons must be kept only for Krsna's use, so that foodstuffs are not contaminated while being placed onto the serving dishes.
"Formerly it was the custom of brahmanas to worship Lord Visnu daily at home and cook food in new pots. This system is still going on in Jagannatha Puri. The food would be cooked in earthen pots, all fresh and new, and after cooking, the pots would be thrown away. By the side of the house there was generally a big pit where such pots were thrown."
Caitanya-caritamrta, Adi lila 14:7