seeing through the eyes of scriptures
dear Madhavananda Prabhu
While previously discussing fasting you made the objection how about if one got some grains stuck in his teeth before fasting on Visnutattva apperance days and next days swallows this, so I told you the teeth should be nicely cleaned.This is also the instruction of Krishna in regards to this Utpanna ekadasi and we can easy understand this is true for all fasting days where one should not take grains.
"The Supreme Lord Krishna replied, 'O Arjuna, in the beginning of winter (northern hemisphere), on the Ekadasi that occurs during the dark fortnight of the month of Margasirsha (November-December), a novice should begin his practice of observing a fast on Ekadasi. On Dasami, the day before Ekadasi, he should clean his teeth nicely
Bhavisya Uttara Purana
We learn as children to daily brush our teeth and here the Supreme father Lord Krsna also tells us :" Go brush your teeth before fasting".
All ekadasis and Visnutattva days should be without grains , still some have doubts due to lack of faith in Srila Prabhupada and sastra
that one should not take grains on Visnutattva apperance days, this is also true for Advaita Acaryas apperance days and Nityananda Tryodasi etc
So the day before these fasting days brush the teeth nicely and do not take grains this is the instruction.
And best is to do full fasting if we can and are motivated.
One temple places sandesh on Lord Narasimha and then on Narasimha Caturdasi serve this "Maha prasadam" before dusk, actually the real fasting is Nirjala fasting as explained by Lord Narasimha in Narasimha Purana.Due to the influence of Kali yuga there is then 2 offences made smearing Lord Narasimha with Sandesh that is not allowed by sastra and braking the fast before Dusk (which is the official ISKCON time for breaking the fast with ekadasi prasadam, anukalpa prasadam)