1. Your protein powder is made with protein isolates.
Most protein powders have a chalky aftertaste because they are made with protein isolates. Protein isolates, which are listed on the ingredient list as “pea protein” and “whey protein” as opposed to “peas” and “whey,” have been stripped of the nutrients that make food taste good.
2. Your protein powder is made with artificial sweeteners or stevia.
A pinch of artificial sweetener can be used to achieve the same amount of sweetness as several tablespoons of sugar, with none of the calories or sugar. For this reason, protein powder companies love to use artificial sweeteners in their products. Despite their obvious benefits, however, artificial sweeteners are bad news for your taste buds. They are either way too sweet, or have a bitter aftertaste. This is due to the fact that they activate bitter taste receptors in addition to sweet receptors. Stevia, a popular natural sweetener, also activates bitter receptors. This bitterness, which many say reminds them of licking aluminum foil, can linger in your mouth for hours.
3. Your protein powder is made with gritty plant proteins.
Some protein powders, especially plant-based protein powders, have a gritty texture because they are minimally-processed. Although minimally-processed foods are good for you, large particles have a smaller specific surface area (surface area ÷ mass) than small particles, and therefore take longer to dissolve. Think about how long it would take to dissolve a sugar cube in water versus how long it would take to dissolve and an equal amount of granular sugar. A sugar cube would dissolve slower because the combined surface area of all of the sugar granules is much greater than the surface area of the sugar cube. More surface area means more contact with the water molecules, which allows the granular sugar to dissolve more quickly.
Another reason why some plant-based protein powders have a gritty texture is their high insoluble fiber content. Plants contain insoluble fiber, and the more insoluble fiber a plant contains, the more that it will taste like dirt. The plant proteins with the most fiber tend to be those made with seeds (pumpkin seeds, chia seeds, etc.).