Arrowroot powder is a flavorless powder used as a thickener for sauces, soups, and more. Arrowroot powder is available at most major grocery stores. A common substitute for arrowroot powder is tapioca or cornstarch. However, arrowroot powder is not necessary for making this keto broccoli recipe. It only serves as a thickener for the beef and broccoli sauce and provides no extra flavor.