Outlines of dairy bacteriology - A concise manual for the use of students in dairying

Edwin George Hastings, H. L. (Harry Luman) Russell

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• Publié par: ormi
• Publié le: 08 décembre 2010
• Nombre de lectures: 13
• Langue: English
• Poids de l'ouvrage: 1 Mo

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The Project Gutenberg EBook of Outlines of dairy bacteriology, by H. L. Russell and E. G. Hastings This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.net Title: Outlines of dairy bacteriology A concise manual for the use of students in dairying Author: H. L. Russell E. G. Hastings Release Date: May 14, 2010 [EBook #32367] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK OUTLINES OF DAIRY BACTERIOLOGY *** Produced by Stacy Brown, Peter Vachuska, Julia Miller and the Online Distributed Proofreading Team at http://www.pgdp.net OUTLINES OF DAIRY BACTERIOLOGY A CONCISE MANUAL FOR THE USE OF STUDENTS IN DAIRYING BY [Pg 1] H. L. RUSSELL DEAN OF THE COLLEGE OF AGRICULTURE UNIVERSITY OF WISCONSIN AND E. G. HASTINGS PROFESSOR OF AGRICULTURAL BACTERIOLOGY UNIVERSITY OF WISCONSIN TENTH EDITION MADISON, WISCONSIN H. L. RUSSELL 1914 COPYRIGHT 1914 BY H. L. RUSSELL AND E. G. HASTINGS [Pg 2] PREFACE TO THE TENTH EDITION. This text was originally the outgrowth of a series of lectures on the subject of dairy bacteriology to practical students in the winter Dairy Course in the University of Wisconsin. The importance of bacteriology in dairy processes has now come to be so widely recognized that no student of dairying regards his training as complete until he has had the fundamental principles of this subject. The aim of this volume is not to furnish an exhaustive treatise of the subject, but an outline and sufficient detail to enable the general student of dairying to obtain as comprehensive an idea of the bacteria and their effects on milk and other dairy products as may be possible without the aid of laboratory practice. When possible the dairy student is urged to secure a laboratory knowledge of these organisms, but lacking this, the student and general reader should secure a general survey of the field of bacteriology in relation to dairying. In this, the tenth edition, the effort has been made to include all of the recent developments of the subject. Especially is this true in regard to the subject of market milk, a phase of dairying that has gained greatly in importance in the last few years. The changes in the methods of handling market milk have been marked. The results of these changes in influencing the quality of milk offered [Pg 3] to the consumer are fully discussed. H. L. R. E. G. H. CONTENTS Structure, Growth and Distribution of Bacteria Methods of Studying Bacteria Contamination of Milk Infection of Milk with Pathogenic Bacteria Fermentations of Milk Preservation of Milk Bacteria and Butter Making Bacteria and Cheese Making Bacteria in Market Milk 7 20 28 62 82 113 136 161 189 [Pg 4] CHAPTER I. STRUCTURE, GROWTH AND DISTRIBUTION. Relation of bacteriology to dairying. The arts which have been developed by mankind have been the outgrowth of experience. Man first learned by doing, how to perform these various activities, and a scientific knowledge of the underlying principles which govern these processes was later developed. The art of dairying has been practiced from time immemorial, but a correct understanding of the fundamental principles on which the practice of dairying rests is of recent origin. In working out these principles, chemistry has been of great service, but in later years, bacteriology has also been most successfully applied to the problems of modern dairying. Indeed, it may be said that the science of dairying, as related to the problems of dairy manufacture is, in large degree, dependent upon an understanding of bacteriological principles. It is therefore essential that the student of dairying, even though he is concerned in large measure with the practical aspects of the subject, should acquire as complete an understanding of these principles as possible. [Pg 7] While bacteriology is concerned primarily with the activities of those microscopic forms of plant life known as the bacteria, yet the general principles governing the life of this particular class of organisms are sufficiently similar to those governing the molds and other types of microscopic life that affect milk and its products to make it possible to include all of these types in a general [Pg 8] consideration of the subject. consideration of the subject. Nature of bacteria. The vegetable kingdom to which the bacteria belong consists of plants of the most varying size and nature. Those of most common acquaintance are the green plants varying in size from those not visible to the naked eye to the largest trees. Another class of plants known as fungi or fungous plants do not contain chlorophyll, the green coloring matter, but are usually colorless and, as a rule, of small size; among them are included such forms as the mushrooms, smuts, rusts and mildews, as well as the molds and yeasts. The bacteria are closely allied to this latter class. When first discovered they were thought to be animals because of the ability of some forms to move about in liquids. The bacteria, like other kinds of living organisms, possess a definite form and shape. They are the simplest in structure of all the plants, the individual organism consisting of a single cell. The larger and more highly organized forms of life are made up of many microscopic cells, and the life of the individual consists of the work of all the cells. The bacteria are very comparable to the single cells of the higher plants and animals, but in the case of the bacteria the single cell is able to exist apart from all other cells and to carry out all of its life processes including reproduction. Forms of bacteria. With the multicellular organisms much variation in form is possible, but with these single-celled organisms the possible variation in form is greatly limited. Three well marked types occur among the bacteria: the round or coccus form (plural cocci); the rod-shaped or bacillus (plural bacilli); and the [Pg 9] twisted or spirillum type (plural spirilla). Most organisms of special significance in dairying belong to the coccus or bacillus group. Size of bacteria. The bacteria, as a class, are among the smallest of living objects. None of them are individually visible to the naked eye, and they can be so seen only when clumps or masses are formed in the process of growth. FIG . 1.—FORMS OF B ACTERIA. A, coccus; B, bacillus; C, spirillum. While there is considerable relative variation in size, yet in actual dimensions, this difference is so small as to make careful microscopic determinations necessary. An average diameter may be taken as about one thirty-thousandth of an inch, while the length varies naturally several fold, depending upon whether the type under observation is a coccus or a bacillus. It is very difficult to conceive of the minuteness of the bacteria; the following may give some idea of their size. In a drop of cream ready for churning may be found as many as 10,000,000 and in a piece of fresh cheese as large as a cherry there may be as many living bacteria as there are people on our earth. While the bacteria are very minute, the effect which they exert in milk and other dairy products is great on account of their enormous numbers. Manner of growth. The cells of which all plants and animals consist increase in numbers by the division of each cell into two cells through the formation of a [Pg 10] division wall across the cell. The new cells divide and the plant or animal continues to grow. The same cell division occurs in the bacteria but since the bacteria are single celled, division of the cells means an increase in numbers rather than growth as in the higher forms of life. FIG . 2.—D IVISION OF B ACTERIA. The bacteria increase in numbers by the division of each cell into two cells. (After Novy.) In the case of those bacteria that have a greater length than diameter, the new wall is formed at right angles to the long axis of the cell. As soon as the division is complete each cell is a complete individual, capable of carrying on all of its life processes. The cells may, however, cohere and thus form distinctive groupings that may serve to identify certain types. Some of the cocci form long chains and the term streptococcus is applied to such. Other groupings may be similar to a bale of twine or they may be massed in clusters with no regularity distinguishable. Spores. Just as ordinary plants form resistant structures, known as seeds, capable of retaining vitality under conditions unfavorable for growth thereby perpetuating the species, so with certain of the bacteria, definite structures, known as spores, that are analogous in some respects to the seeds of the higher plants, are produced within the mother cell. The spores are exceedingly resistant to the influence of an unfavorable environment, such as heat, cold, [Pg 11] drying, and even chemical agents. It is this property of the spores which makes it so difficult to destroy the bacterial life in the process of sterilizing milk. The property of spore-formation is fortunately confined to a comparatively small number of different species of bacilli. Movement. Many of the bacteria are provided with vibratory organs of locomotion, known as cilia (singular cilium) which are variously distributed on the surface of the cell. By the movement of these relatively long, thread-like appendages the individual cell is able to move in liquids. It must be remembered, when these moving cells are observed under the microscope, that their apparent rate of movement is magnified relatively as much as their size. Conditions for growth. All kinds of living things need certain conditions for growth such as food, moisture, air and a favorable temperature. The bacteria prefer as food such organic matter as milk, meat, and vegetable infusions. Those living on dead organic matter are