Srila Prabhupada never took grains on Janmastami , after midnight on the very first Janamstami in ISKCON he served only cut fruits .
Actually sastra does never allow us to take Mahagrains untill the next day.There is an article I have written about this on http://www.dandavats.com/?p=4883
In the Vayu Purana it is stated, “If one desires to become free from all sin, he or she should accept some rice Mahaprasada of Lord Jagannatha (or other grains of Mahaprasadam) on the day after Janmastami day.”
Lord Brahma instructs Narada in the Visnu Rahasya, “Anyone who eats grains on Janmastami day becomes filled with all categories of sin performed in the three worlds, and takes birth as a snake in the forest.”
Brahma Vaivarta Purana:
On a pure Janmastami (not mixed with the saptami) one should not eat even fruit or betel nuts, or even drink water. Eating these is like eating stool or cow’s flesh, or like drinking urine. What, then, can be said of eating rice?
One who eats on Lord Krishna’s birthday is lowest of mankind. His sinful reaction is like that of having raped his mother and murdered a hundred brahmanas.
YE NA KURVANTI JANANTAH KRSNA JANMA ASTAMI VRATAM TE BHAVANTI MAHA PRAJNA VYALA MAHATI KANANE
(HARI BHAKTI VILASA 15/275 from SKANDA PURANA)
Oh greatly intelligent, any person, even with knowledge of Krsna’s birthday
(Janmastami), who does not fast, he takes his next birth as a snake in a thick forest.
SAMPARKEN API YAH KURYAT KASCIT JANMASTAMI VRATAM VISNU LOKAM AVAPNOTI SO’PI PARTHA NA SAMSAYAH
(HARI BHAKTI VILASA 15/295 from SKANDA PURANA)
If someone fasts on Janmastami day, with devotion, he attains the abode of Lord Sri Visnu.
TASYAM VISNU TITHAU KECID DHANYAH KALIYUGE JANAH YE’BHYARCAYANTI DEVESAM JAGRATAH SAMUPOSITAH
(HARI BHAKTI VILASA 15/307 from BRAHMA PURANA Suta Gosvami speaks)
This day (Janmastami day) removes all kinds of sins. Specifically in Kali-yuga, if someone fasts on this day, remains awake overnight and worships Lord Sri Hari properly, those people are worshipable.
I pray all devotees may have a beautiful celebration on Janmastami for the pleasure of Sri Krishna.