Any milk which is sold or exchanged or offered for sale or exchange shall be deemed to be sold, exchanged or offered as of standard quality, unless otherwise expressly stated at the time of such sale, exchange or offer. Milk of standard quality shall contain not more than eighty-eight and one-half per cent of watery fluid, not less than eleven and one-half per cent of milk solids, not less than eight and one-quarter per cent of solids not fat and not less than three and one-quarter per cent of butterfat, and the certificate of the director of the Connecticut Agricultural Experiment Station or chemist in charge thereof, or of the director of the laboratory of the Department of Public Health, shall be evidence of the composition of any milk.