|God in His Bhagavad Gita has broadly indicated that for true service, purity of food is necessary. As is the food, so is the mind. The food we eat should be regulated. How much food we consume and not what food we take, is more important. It should be in right measure and should be clean. Food dear to (wo)men of pure heart enhances longevity of life, a cheerful spirit, strength, health, happiness and delight.
A true devotee will refuse to accept any food that is not offered to God. Eatables which are offered to God are warmly accepted as His grace by devotees, in as much as they, being blessed by God, is holy. When the heart is made pure, perennial contemplation is possible. One of the steps adopted even today by many families is to place daily food preparations before the deities they worship and recite shlokas, the belief being that God is pleased with the dedication displayed
Before we partake daily meals we first sprinkle water around the plate as an act of purification. Five morsels of food are placed on the side of the table acknowledging the debt owed by us to the :
Divine forces (devta runa) for their benign grace and protection.
Our ancestors (pitru runa) for giving us their lineage and the family culture.
The sages (rishi runa) as our religion and culture have been "realised" maintained and handed down to us by them.
Our fellow beings (manushya runa) who constitute society without the support of which we could not live as we do and
Other living beings (bhuta runa) for serving us selflessly.
Sources : http://www.the-hindu.com/1996/07/25/THREL.html
There after the Lord, the life force, who is also within us as the five life - giving physiological functions, is offered the food. The five life-giving functions are praanaaya (respiratory), apaanaaya (extretory), vyaanaaya (circulatory), udaanaaya (reversal) and samaanaaya (digestive). After offering the food thus, it is eaten as prasad - blessed food.