Every person testing or grading milk or cream to determine the milk fat as a basis of fixing the purchase price or to determine the acceptability of such milk or cream shall secure a tester’s license from the department and shall make such tests and grading only by such process as has been approved by the department, and no person shall make such test and grading without such license, and other than by such process. Each applicant for such license shall be required to submit to examination or by actual demonstration show competency in testing and grading cream and milk according to the regulations prescribed by the department. The fee for each licensee shall be twenty-five dollars ($25.00). The license shall be valid for three (3) years and must be renewed by December 31 of the third year. With the approval of the department any licensee may appoint a substitute to act for a period not to exceed fourteen (14) days.