Where any product for which exemption is claimed under Section 12-21-1880 is found to contain less than thirty per cent by volume of natural fruit or natural vegetable juice or any product is found to contain less than thirty per cent by volume of natural liquid milk, the tax imposed by this article shall apply to all sales of such product, and all such products offered for sale and not properly stamped shall be subject to confiscation as provided by Section 12-21-2870.