Mustard seeds are small round seeds obtained from the mustard plant. Seeds from the mustard plant, most commonly available in the brown version, which has the best flavor. The black mustard seed has a sharper, stronger flavor and the yellow and white seeds have a milder flavor. Both its seeds and leaves are edible, making it a versatile addition to your diet. Mustard paste is a low calorie way to add flavor and a dash of nutrients to your meals. Ground mustard seeds contain the toxic compound, isothiocyanate, that causes airway irritation and edema similar to black pepper (known to be lethal with aspiration).