Nirmalya is the dried rice Mahaprasad of Lord Jagannath Temple Puri. Cooked Mahaprasad in all its best conditions can at best be preserved for one day. Nirmalya can be preserved for a good number of days. When the cooked rice Mahaprasad stock remains unutilized for the day, the said stock is taken to a place called 'Nirmalya Khala' in Koili Baikuntha in the temple garden and dried there in the hot Sun. When the said rice is completely dried up, the dried form of rice becomes Nirmalya and it is made available to the devotees in red cloth pockets. This is also known as 'Kaibalya'.
In spiritual recognition Nirmalya is equally important as Mahaprasad. The people of Odisha preserve it to take even an iota of Nirmalya after performing daily Worship to God. Even in Odisha household it is required at the time of nirbandha (marriage negotiation). Even the bride and bridegrooms promise by holding Nirmalya that they will stick to this negotiation and not to betray each other. Moreover at the time of death, the Nirmalya and Holy Basil (Tulsi) with water is poured in the tongue and throat of the imminent deceased person with the belief that he is sure to have a place for himself in the heaven after his death following atonement of all his sins. In these ways, Mahaprasad and Nirmalya are very auspicious.
Sources: Odisha Review Article 1, Odisha Review Article 2