This is a little confusing. Some sites recommend yogurt, some say kefir is ok too, some recommend whey, and I think some recommend just water or acid. I plan on starting with the millet, but still haven’t finished researching options.
A good overview from Nourished Kitchen:
First hit that came up on Google:
“You can lacto ferment whole grains before cooking them by soaking them in tepid water with a couple of teaspoons of whey (from strained yogurt) or kefir. keep for 24 hours at room temperature, strain and then cook in the usual way. This can also be done with lemon juice or vinegar in place of the whey. Porridge is also best made this way with whey/kefir, as it has the highest phytate content of all grains.
Recipes containing grain flour, bean flour or nut meal can also be soaked in this way. Simply add the liquid ingredients to the flours, including a teaspoon of lemon juice, two of whey, or some actual yogurt in the recipe. Leave for 24 hours at room temperature and add the rest of the ingredients. You’ll find that breads are fluffier and more toothsome, and raising agents work better this way.”
A page on teff:
“Teff only requires 36 hours or less to sprout – the shortest time of any grain.”