[Black Poppy Seed | White Poppy Seed; Khas Khas (India) | Papaver somniferum of order Ranunculales]
There are two kinds of poppy seeds sold on the spice markets, Black and White. They are imperfectly interchangeable due to color, taste and other characteristics. Both come from the infamous opium poppy, but contain only tiny amounts of the psychoactive alkalis morphine and codeine. Those have to be gathered from the sap long before the seeds are mature enough to harvest. Drug tests, however, may come out positive from eating poppy seed rolls, muffins or bagels. Poppy seed is also used as an oil seed, but in North America it's used for slow drying artist's oil paints rather than for cooking.
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Black Poppy Seeds are commonly used in Europe and the U.S. sprinkled on baked goods and in baked goods stuffings. The photo specimens were 0.020 inch diameter.
White Poppy Seeds are milder than the black. They are always called for in Indian cooking, where they are used roasted and ground. They serve as both a flavoring and as a thickening agent in light colored sauces. They come from a line of poppies specifically bred for these mild light colored seeds. The photo specimens were 0.020 inch diameter.
In India, these used to be cheap and plentiful, so many older recipes call for a lot of them. Today, the poppy crop is tightly regulated and much smaller, so they are too expensive to use in traditional quantities.