It is the practice of the Hindus to offer Food to God before taking food.

The cooked Food is placed before the image /photo of God in a vessel,normally reserved only for this purpose.

The vessel should be covered with a lid and a portion of the lid is to be open with the steam from the freshly cooked food,coming out.

Then Water in a vessel is taken and in a specially designed Spoon and sprinkled around the vessel three times with the chanting of these Mantras.

OM

‘Praanaaya swaahaa…

Apaanaaya swaahaa…
Vyaanaaya swaahaa…
Udaanaaya swaahaa…
Samaanaaya swaahaa…
Brahmane swaahaa !

Amruthostharanamasi”

Food that can be offered as Naivedyam or Thiruvaaaradhanam.

Only Vegetarian.

Fruits,Nuts,Dry Fruits,Milk,Honey,Coconut,Betel Leaves with Arecanut.

Of cooked foods ,any thing can be offered.

Recommended is Cooked Rice in full(no part food in a plate) with Celery(Toor Dal and Ghee)

The food should be prepared after taking bath.

It is advisable to offer prayers before offering .

If not possible, offering should be done before we partake food.

Food that has been eaten or remnants should not be offered.

Honey is the Best offering as it is said to be beyond impurity.

Reasons for Offering to god.

We eat things from the bounty of Nature.

Offering to God is our  way of thanking Him.

Note the Lord’s Prayer.

‘”Our Father in heaven,hallowed be your name.Your kingdom come,your will be done,on earth as it is in heaven.Give us this day our daily bread,and forgive us our debts,as we also have forgiven our debtors.And lead us not into temptation,but deliver us from evil.”

Latin Liturgical version.

Pater noster, qui es in caelis:sanctificetur Nomen Tuum;adveniat Regnum Tuum;fiat voluntas Tua,sicut in caelo, et in terra.Panem nostrum cotidianum da nobis hodie;et dimitte nobis debita nostra,sicut et nos dimittimus debitoribus nostris;et ne nos inducas in tentationem;sed libera nos a Malo”

Hindu Sastras declare that eating Food without offering to God first amounts to Stealing.

Another reason is that food which is made of natural Resources contain impurities.

( now we have chemicals as well)

The food we eat is the result of the toils of many.

We do not know what they have gone through to produce and we are not sure whether they have been adequately compensated.

Their anguish will affect the their produce.

When we offer food to God, He takes away the impurities and the consequences born out of anger and anguish of the producer.

After all, we are because of what we think and  what we think is because of what we eat.

‘

Naivedhya (Sanskrit: नैवेध्य) a Sanskrit word meaning supplication, is food offered to a Hindu deity as part of a worship ritual, before eating it. As such, tasting during preparation or eating the food before offering it to God is forbidden. The food is placed before a deity and prayers are offered. Then the food is consumed as a holy offering. The offerings may include cooked food, sugar cones, and fruits. Mostly, vegetarian food alone is offered to the deity and later distributed to the devotees who are present in the temple. Great care is taken when food is cooked for the deity. For some tribal-gods like Kali, animals, such as goats or roostersare slaughtered in the temple precincts and offered. Many orthodox Hindus offer cooked food or some fruits to a picture or idol of a deity before they eat it.

Naivedhya is not necessarily a food offering to God and actually means ‘offering to God’ in the stricter sense of the words. It could be any offering, tangible or intangible. A resolution, a promise or even a willingness to do, perform or restrict from certain things can also be connoted as offering to God.

God, prayers and wishes are more a belief and hence an offering to God is an extension of this belief. However, one needs to differentiate Naivedhya from ‘Prasad‘. Prasad is actually what one get from the God. The meaning of these words is usually attributed to food as we invariably offer to and receive from the house of Gods in the form of eatables.

http://en.wikipedia.org/wiki/Naivedyam

Importance of Bhagavad ArAdhanam by Srimad ParavAkkOttai Andavan, Sri Gopaladesika