PREFACE The writers experience as a health officer and as a teacher has led him to believe that there is need for a book telling something about milk, how it is produced and how it is transported and delivered to city milk consumers and of the methods of control adopted to insure its purity. The subject is a broad one and in its divisions touches several specialized fields which are the domain of experts so that necessarily the work of these men has been drawn on for material. The attempt has been made to give them due credit by lists of sources at the end of each chapter. To the many dairymen and city milk dealers who have given cordial welcome to their farms and plants, to those who have supplied photo- graphs and to the friends who have helped with advice in the preparation of the manuscript and in revision of the proof the author gives hearty thanks. BOSTON, MASSACHUSETTS, January, 1917. . . CONTENTS CHAPTER I PAGE v MILK 1 Importance of the Dairy Industry Consumption of Milk in the United States Milk a Valuable Food Definition of Milk Breeding of the Cow Average Chemical Composition of Milk Chemical Composition of Skimmilk and Whey The Composition of Milk in Detail Colostrum Color of Milk Opacity of Milk Specific Gravity of Milk Freezing Point of Milk Electric Conductivity of Milk Refractive Index of Milk Fat Globules in Milk Cellular Content of Milk The Enzymes of Milk Water in Milk Butterfat Proteins Casein Lactalbumin Lactglobulin Carbohydrates Lactose Hexose Sugar Salts in Milk Gases Consumption and Flow of Milk Variable Effect of Maltreating Cows Factors Influencing the Milk Flow Effect of Milking Variation in Milk from Milking to Milking Individuality of the Cow and the Production of Butterfat Effect of the Condition of the Cow at Calving Changes in Character of Milk during the Lactation Period Seasonal Variation in Composition Breed of Cow and Production of Butterfat Bacteria in Milk Classification of Bacteria Found in Milk Importance of the Various Classes of Bacteria Stages in the Bacterial Decomposition ofMilk FirstStage SecondStage ThirdStage Fourth Stage Bacterial Decomposition of Milk Usually Harmless Dirty Milk Infected Milk Grading of Milk Modified Milk Milk Beverages Condensed Milk and Milk Powder. CHAPTER II DISEASES COMMUNICABLE IN MILK 28 Milk a Vehicle of Infection Discovery ol Disease Germs Classification of Milk-borne Diseases Tuberculosis Nature of Tuberculosis Entrance of Tuberculosis Germs into the Body Passage of Tuberculosis Germs from the Body Infection of Milk by Germs of Tuberculosis in the Feces Fre- quency of the Infection of Milk with Tuberculosis Length of Life of Tuberculosis Germs Outside the Body Mode of Infection of the Herd The Diagnosis of Tuberculosis Physical Examination Tuberculin Subcutaneous Test Tuberculin Test as a Criterion for Excluding Milk from the Market The IntradermalTest TheOphthalmic Test Value ofthe Tuberculin Test Results of Tuberculin Testing Tuberculin Testing in Wisconsin Tuberculin Testing in Hawaii Tuberculin Testing in North Carolina Tuberculin Testing in Savannah, Ga. Tuberculin Testing in Chester County, Pennsylvania Tuberculin Testing in Minnesota Tuberculin Testing in Montclair, N. J...