[bertm]

It looks like the powder is made by a physical process. Just as water absorbs into a towel. Ethanol apparently absorbs into dextrin powder and stays as a dry solid.

From the patent:

EXAMPLE II A 150 g. amount of the bulked 5 DE. MOR-REX dextrin of Example I was blended with 250 g. of 90% by weight water) ethanol in a Hobart mixer at speed No. 2 with a wire whisk for 2 minutes. A dry-tothe-touch free flowing powder was obtained that contained 56.2% ethanol and 8.3% moisture. The bulked ethanol containing flowable powder was packaged in air-tight foil envelopes and after 6 months storage had retained its flowability characteristics and its original alcohol content.

EXAMPLE III A teaspoonful (3 g.) of the product of Example II, when added to 10 ml. of water produced a clear, sparkling low-viscosity alcoholic solution with a nonsweet strong alcohol taste.