The phrase "bitter coating" refers to a situation where something may seem good or appealing on the outside, but actually turns out to be unpleasant or disappointing. Full definition
It has a natural protective bitter coating called saponin, so be sure to wash it thoroughly before cooking. (mindbodygreen.com)
Because there is a detergent - like bitter coating on each quinoa seed, called saponin, which naturally protects the quinoa from birds and insects. (noshandnourish.com)
While most quinoa sold today has had this bitter coating removed, an extra rinse is a good idea to remove any residue. (wholegrainscouncil.org)