Microorganisms are harder to grow or sustain below 20C/70F, above 50C/120F, and/or in dry, and/or soaked in salt/sugar/acid/base. Your grandma is spot on about

Microorganisms are harder to grow or sustain below 20C/70F, above 50C/120F, and/or in dry, and/or soaked in salt/sugar/acid/base. Your grandma is spot on about unflavored rice from morning. Not 100% sure about Tamarind rice after a whole day, but that make some sense too(I'd Tupperware and refrigerate immediately or at least reheat occasionally).