From Bhagavad Gita 4.24 and 15.14

Brahmaarpanam Brahma Havir
Brahmaagnau Brahmanaa Hutam
Brahmaiva Tena Gantavyam
Brahma Karma Samaadhinaha 

[This is 24th verse from the 4th chapter of Bhagavad Geetha] The act of offering is Brahman. The offering itself is Brahman. The offering is done by Brahman in the sacred fire which is Brahman. He alone attains Brahman who, in all actions, is fully absorbed in Brahman. (As we chant this prayer we are offering the different types of food to Brahman). 

Aham Vaishvaanaro Bhutva
Praaninaam Dehamaashritha
Praanaapaana Samaa Yuktaha
Pachaamyannam Chatur Vidam 

[This is 14th verse from the 15th chapter of Bhagavad Geetha] This sloka is a sort of acknowledgement and assurance to us from Brahman. "I am Vaishnavara, existing as fire God in the bodies of living beings. Being associated with ingoing (prana) and outgoing (apaana) life breaths, I will digest all the four different types of food (that which we bite and chew; that which we masticate with the tongue; those which we gulp; that which we swallow) and purify them." 

Harir Daatha Harir Bhoktha
Harir Annam Prajaapatih
Harir Vipra Shareerastu
Bhoonkte Bhojayathe Harih. 

Oh Lord Hari, You are the food, You are the enjoyer of the food, You are the giver of food. Therefore, I offer all that I consume at Thy Lotus Feet.

Swami's explanation of this prayer

We should partake food with a Sathwic mind. Our ancestors recommended the offering of food to God before partaking. Food so partaken becomes "Prasad" (consecrated offering). Prayer cleanses the food of the thre  impurities; caused by the absence of cleanliness of the vessel, cleanliness of the food stuff, and cleanliness in the process of cooking. It is necessary to get rid of these three impurities to purify the food; for, pure food goes into the making of a pure mind. It is not possible to ensure the purity of the cooking process, since we do not know what thoughts rage in the mind of the man who prepares the food. Similarly, we cannot ensure the cleanliness of the food ingredients as we do not know whether it was acquired in a righteous way by the seller who has sold it to us. Hence, it is essential on our part to offer food to God in the form of prayer, so that these three impurities do not afflict our mind. The prayer:

Brahmaarpanam Brahma Havir
Brahmaagnau Brahmanaa Hutam
Brahmaiva Tena Gantavyam
Brahma Karma Samaadhinaha
Aham Vaishvaanaro Bhutva
Praaninaam Dehamaashritha
Praanaapaana Samaa Yuktaha
Pachaamyannam Chatur Vidam 
Harir Daatha Harir Bhoktha
Harir Annam Prajaapatih
Harir Vipra Shareerastu
Bhoonkte Bhojayathe Harih.

"The food thus offered to God is digested by 'Vaishwanara" in the digestive system. Since God exists in the form of fire as Vaishwanara, He digests the food along with the impurities. So, man will not be affected even i  the impurities enter the food."

- Baba
Summer Showers in Brindavan 1993 Page 86

Potency of Brahmaarpanam Mantra

When i was in Brindavan in May this year, i had the opportunity to attend the morning talks at Sai Krishan Kalyana Mandap. On one of the days Uncle Prahlad Rao gave a talk and he covered the potency of the food mantra stated below.

In the early days (70s if i recall) Swami had asked all the Brindavan campus college boys to join Him for lunch. At that time only about 100 boys were there. Kirlian photographer, Dr Baranowski was also there and was invited for lunch. The sumptuous food was laid out on plates in front of the boys. The boys were waiting for Swami to come and join them. Dr Baranowski then used his Kirlian camera and took photos of the food laid out on the plates. There was no aura found around the plates of food.

Later Swami arrived and joined the boys for lunch. The boys recited the Brahmaarpanam mantra before partaking the food. After the food mantra was recited, Dr Baranowski again took pictures of the food. This time, blue aura was found around the plates of food. He was excited but yet having a scientific mind asked Swami on the reason for the blue aura around the food. Swami said that the Brahmaarpanam mantra is a very powerful mantra and that after recitation, the food is converted to prasadam, as it has been offered to God. It further purifies the food and all items around it. Thus, the blue aura was found around the plates of food after the mantra was recited.

FOOD MANTRA

Brahmaarpanam Brahma Havir
Brahmaagnau Brahmanaa Hutam
Brahmaiva Tena Gantavyam
Brahma Karma Samaadhinaha
Aham Vaishvaanaro Bhutva
Praaninaam Dehamaashritha
Praanaapaana Samaa Yuktaha
Pachaamyannam Chatur Vidam

The meaning of Brahmaarpanam Mantra. It is as under:

Brahmaarpanam Brahma Havir
Brahmaagnau Brahmanaa Hutam
Brahmaiva Tena Gantavyam
Brahma Karma Samaadhinaha

The whole creation being the gross projection of Bramham, the Cosmic Consciousness itself; the food too is Bramham, the process of offering it is Bramhan; it is being offered in the fire of Brahman. He who thus sees Brahman is action, reaches Bramham alone.

Aham Vaishvaanaro Bhutva
Praaninaam Dehamaashritha
Praanaapaana Samaa Yuktaha
Pachaamyannam Chatur Vidam

I, the Supreme Spirit, abiding in the body of living beings as the Fire (Vaiswanara) in their stomach I am associated with their Praana and Apaana, digest the four type of foods (solids, fluids, semi-fluid and liquid) which they eat.