Whenever a state institution purchases milk or cream pursuant to competitive bids based on the butterfat contents of the milk or cream supplied or to be supplied pursuant to such bid or bids, the tests to determine the butterfat contents thereof shall be made pursuant to and in compliance with the testing requirements provided under this subchapter. Whenever any bid has been accepted by any state institution to supply milk or cream or both of a required percentage of butterfat, the successful bidder shall be deemed to be in compliance with the requirements of the bid whenever any tests of the milk or cream shall be made, and such tests to determine the amount of the butterfat in such milk or cream or both shall be made pursuant to and in compliance with the provisions of this subchapter. The successful bidder shall not be required to satisfy any other tests made by any other sources than those prescribed under this subchapter.