27 C.F.R. § 25.262 – Restrictions and conditions on processes of concentration and reconstitution.

(a) Conditions on concentration. A brewer may not employ any process of concentration which separates alcohol spirits from any fermented substance.
(b) Conditions on reconstitution—(1) The process of reconstitution of beer will consist of the addition to the concentrate of carbon dioxide and water only.
(2) A brewer may not employ any process of concentration or reconstitution unless the beer upon reconstitution will, without the addition of any substance other than carbon dioxide and water, possess the taste, aroma, color, and other characteristics of beer which has not been concentrated.
(3) The process of reconstitution shall provide for the addition of sufficient water to restore the concentrate to a volume not less than, and an alcohol content not greater than, that of the beer used to produce the concentrate.
(Sec. 201, Pub. L. 85-859, 72 Stat. 1315, as amended, 1388, as amended (26 U.S.C. 5002, 5401))