Nothing contained in this chapter shall be construed to prohibit the sale of milk, cream, skim milk, or milk products to any milk plant, either at wholesale or pursuant to contract, by weight and test and/or upon the basis of milk fat content and/or on the basis of bacterial counts or other sanitation requirements; provided, that in the case of milk or milk products of a particular description or grade, nothing in this section shall be construed to authorize a sale of any milk or milk product which does not at least conform to the requirements of that description or grade.