Grades of milk and milk products as defined in this chapter shall be based on the respectively applicable standards contained in this chapter, with the grading of milk products being identical with the grading of milk, except that bacterial standards are omitted in the case of cultured milk products. Vitamin D milk shall be only of grade A, certified pasteurized, or certified raw quality. The grade of a milk product shall be that of the lowest grade milk or milk product used in its preparation.