A good approach is to solve this partly in-camera and partly in post.

For capture, several answers suggest using a bright backlit surface such as glass, with the background intentionally blown to pure white while keeping the powder slightly darker by underexposing. You can also make two exposures: one normal shot for the look you want, and one strongly backlit/underexposed shot that gives a clean silhouette of the powder. In Photoshop, use the silhouette shot as a mask for the powder and the normal shot to preserve the natural shadow.

If you already have a usable image on a subtly colored background, a Photoshop-only method can work too: convert to Lab color, place a white fill layer underneath, inspect the channels, and use the channel with the best contrast (for example the blue/yellow channel) with Blend If to separate the powder from the background. Fine-tune any color cast with curves on the A/B channels.

In short: the most reliable path is controlled lighting with a backlit white background, ideally with a second exposure for masking, then compositing in Photoshop.