It loses a little mass due to water boiling off, but other than that the mass has to stay exactly the same (barring nuclear reactions, which typically don't make for good quinoa).
Hot quinoa is slightly heavier than cold quinoa.
why?
Heat is energy, so hot quinoa has more energy, which means more mass. All internal energy of a system contributes to its rest mass.
I'm not sure if special relativity applies in the context of quinoa. No scale is going to show a difference in the mass of quinoa caused by relativistic effects of heat differences.
No weighing scale ... sounds like a challenge!
I don't want to be extremely pedantic. But even regular chemical reactions change mass just as nuclear reactions.
It's very small, but it's there. Any change in internal energy will cause a mass change, regardless of the specific fields that are affected.