They are quite elaborate and long Sanskrit mantras, I'm sure there are some that use much shorter mantras.
This mantra is offering the food that is about to be consumed to the five pranas. You repeat this mantra then sip some water before eating. And then infuse you food with blessed energy.
śraddhāyāṁ prāṇe niviṣṭo'mṛtaṁ juhomi
śraddhāyāmapāne niviṣṭo'mṛtaṁ juhomi
śraddhāyāṁ vyāne niviṣṭo'mṛtaṁ juhomi
śraddhāyāmudāne niviṣṭo'mṛtaṁ juhomi
śraddhāyāɱ samāne niviṣṭo'mṛtaṁ juhomi
brahmaṇi ma ātmāmṛtatvāya
It translates into english as follows:
Firm in my religious faith, I have offered this oblation of ambrosia into Prana with reverence: O Prana, increase the power of my in-breath by this food! Firm in my religious faith, I have offered this oblation of ambrosia into Apana with reverence: O Apana, increase the power of my out-breath with this food! Firm in my religious faith, I have offered this oblation of ambrosia into Vyana with reverence: O Vyana, increase the power of my diffused breath with this food! Firm in my religious faith, I have offered this oblation of ambrosia into Udana with reverence: O Udana, increase the power of my up-breath with this food! Firm in my religious faith, I have offered this oblation of ambrosia into Samana with reverence: O Samana, increase the power of my middle breath with this food!
This mantra is from the Mahanarayana Upanishad.