The Cooling Conclusion
Thayiramuthu (Tamil: thayiru = curd/yogurt + amuthu = nectar; 'the nectar of curd') is the devotional name for curd rice (dadyodana in Sanskrit; thayir sādam colloquially) prepared and distributed as prasādam. It is always the final item in a traditional prasādam meal — the cooling, satisfying conclusion after the hot dishes.
Connection to Kṛṣṇa
Curd rice has a deep devotional association: Kṛṣṇa is celebrated in the Bhāgavatam and the Āzhvārs' pāsurams as a lover of all milk products — butter, yogurt, and milk. Offering curd rice to Kṛṣṇa-form deities is especially beloved. Several Divya Deśams have specific festivals where thayir sādam is the principal prasādam.
Symbolism
Just as curd rice cools the body after a hot meal, the tradition teaches that the final 'taste' of Bhagavān's grace in liberation is cooling and satisfying — the soul's long restlessness in saṃsāra ends in the peaceful, fulfilled rest of Vaikuṇṭha. Thayiramuthu symbolizes this final arrival.