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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 15, No. 12
these the following may be mentioned: ( a ) ash content, The work referred to has convinced the writer that the (b) pentosan content, (c) fat content, ( d ) fiber content, ( e ) physical properties of the gluten are, in many instances a t titrable acidity of water extracts, (f) percentage of soluble least, the determining factor as to whether or not a given proteins, (9) specific conductivity of water extracts, (h) lot of flour will produce a satisfactory loaf of bread. It buffer action of water extracts, (i) catalase activity, ( j ) would appear that the physical properties of the gluten relative proportion of branny particles, and (IC) number of are in a large measure determined at the time the wheat wheat hairs per unit of flour. proteins are laid down in the endosperm of the wheat berry, All the foregoing bear a negative correlation t o the grade with the result that the strong flours are strong, not beof flour-that is, the percentage or index increases with cause they differ in acid or salt content from weak flours, decreasing grade or degree of refinement and commercial but because the gluten is capable of forming a more tenacious value. I n addition to the items mentioned, flour grade is gel than is the gluten from a weak flour. The question as generally correlated with the color or visual appearance of to why glutens differ in tenacity is still unsolved. Neverdry flour, of dough made therefrom, and of the baked bread. theless, the fact remains that such differences do occur. The appearance of the crumb of baked bread is extensively It would appear as if the flour strength problem were closely used in cereal laboratories as an index of flour grade. The allied to the rubber problem. Why does rubber produced color which results on wetting the flour is used in a crude in different localities and by different methods differ so way by millers and milling chemists in what is known as enormously in tensile strength and in value? It appears the Pekar test. This test has been made more or less quan- probable that rubber is a polymer of isoprene. It may be titative by measuring the relative extent of color change that chemically the rubbers are indistinguishable from per unit of time in the freshly wetted material. each other, but this does not preclude a wide difference in physical properties. A solution of the rubber problem may solve the strong and weak flour question, and perhaps the cereal chemist may solve the problems vexing the rubber manufacturer. It is a long cry from bread t o automobile tires; nevertheless, in this problem of tensile strength they as Systems’ meet on common ground. I n an investigation having for its object the question as By Ross Aiken Gortner t o which of the proteins of wheat flour was responsible for the variation in the physical properties of the gluten, it was UNIVERSITY OB MINNESOTA, ST. PAUL, MI“. found3 that the alcohol-soluble protein apparently showed HEAT flour, the only one of the cereal flours which uniform physico-chemical properties in both strong and is adapted to the manufacture of yeast-leavened weak flour; the glutenin, however, differed widely from bread, owes its desirable properties to the nature Aour t o Aour in its physico-chemical properties. It appears, of the proteins of the wheat. These proteins have been therefore, that the question of flour strength is dependent extensively studied and comprise a prolamine gliadin, a primarily on the physical properties of the glutenin. Viscosglutelin glutenin, an albumin leucosin, a globulin, and a ity measurements on flour-in-water suspensions brought to a pH of 3.0 by the addition of lactic acid have been found proteose. The gliadin and glutenin are present in the dough in a to serve admirably as a means of evaluating gluten quality, rather intimate physical or physico-chemical mixture which and it has been found possible by such measurements to is known as the gluten. Whether or not gluten is formed in give for the first time a numerical value to gluten quality. the process of dough manufacture or whether it already ex- Inasmuch as viscosity is such an important property to the ists in the endosperm of the wheat berry is still an open emulsoid colloids, i t appears probable that the colloidal properties of the gluten gel are the determining factors question. The gluten plays its role in bread manufacture in that it in flour strength. It is possible to destroy the desirable absorbs water and forms an elastic gel, which stretches baking qualities of wheat flours by altering the physical under the teniion of the carbon dioxide produced by yeast properties of the gluten gel. For example, doughing up a fermentation and thus “raises” the loaf. The tenacity flour with 70 per cent alcohol and immediately evaporating with which gluten particle adheres to gluten particle varies the alcohol at room temperature will ruin a wheat flour widely from flour to flour. Those flours which have a high for bread-making. On the other hand, such treatment is ratio of carbon dioxide production to carbon dioxide diffusing without effect upon rye flour. Both wheat and rye flour through the gluten are desirable for bread-making purposes, contain the same prolamine gliadin, but rye flour contains and are known as “strong” flours. Those flours that form no glutenin, and these experiments are regarded as addidoughs which fail to retain a large proportion of the carbon tional evidence that it is the physico-chemical properties of dioxide are known as “weak” flours, and are more suited to the protein glutenin which cause the variation in the baking the manufacture of pastry and crackers than to yeast- qualities of wheat flour. (Variations in baking quality may be caused by factors other than gluten quality. The disleavened bread. Many investigators have concerned themselves with a cussion in this paper is limited to gluten quality and is not study of the factor$ which may be involved in flour strength, intended to include factors influencing yeast activity or varibut it has been only recently that the newer theories and ations due to different amounts of gluten.) Colloid phenomena are likewise concerned in the baking methods of colloid chemistry have been applied. These studies give promise of assisting materially in a solution of the processes and in the baked loaf. During the baking process the proteins are heat-coagulated, causing changes in their problem.2 water-holding capacity. A part of the starch grains are 1 Abstract of paper. The complete paper expanded to cover a someruptured, causing the gelatinized starch to absorb moisture, what broader field will appear a s a chapter in a book on colloid chemistry the net result of such changes being that the consistency of which is to be issued under the editorship of Jerome Alexander. 1 For a review of the literature dealing with flour strength and the the dough is altered to t h e rather firm texture of the baked original papers on the relation of flour strength to colloidal properties, see loaf. When such a loaf is stored at ordinary temperature
Flour and Bread
Colloid
W
J . d g v . Research, 13, 389 (1918);J P h y s . Chem., 26, 101 (1922);27, 481, 567, 674,771,in press (1923).
3
J . P h y s . Chem., 27, 674 (1923).
December, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
it becomes “stale,” the stale condition being characterized by a crumbly texture and a feeling of dryness. Apparently, the staling of bread is not due primarily to a loss of water from the loaf, but rather to changes in the protein-starch gel. Several workers have produced evidence that the staling of bread is caused by metastable gelatinized starch reverting t o a more insoluble form, with coincident changes in the colloidal properties of the loaf, such as differential syneresis and imbibition in different portions of the loaf.
Panary Fermentation By C. Brewster Morison AMERICAN ZNSTITUTE OF BAKING, CHICAGO,
ILI,.
H E process of bread-making may be divided conveniently into three principal stages which are well defined in plant operation. These are, in sequence, the mixing of the raw materials, the fermentation of the resulting dough, and baking. In commercial practice the fermentation period is generally understood to be the total time which has elapsed from the completion of mixing until the dough is ready for scaling or dividing. Actually, however, fermentation changes are initiated during mixing, and continue through the subsequent mechanical operations of making up, receiving an additional impetus from the high temperature of the final rising, or proof, and again at the beginning of baking until further enzymic activities are terminated by the destructive temperature which is reached after a certain period of exposure to the heat of the oven. The successful control of fermentation, especially in relation to the determination of the time required for the proper development of the dough, so that it can be made up, given the final proof, and baked, is perhaps the outstanding problem of the bread-making process. It involves not only the effective coordination of practically all plant opemtive conditions from the mixer to the oven, but fundamental considerations relative to the flour, particularly its strength, the properties and composition of the many other ingredients used in bread, and their adjustment or arrangement in formulas. It is thus a problem complicated by a number of variables in any given set of conditions, which include shop operative factors, character of flour and raw materials, and the inherent complexity of the physical and biochemical changes which take place in heterogeneous systems such as a bread dough. The essential purpose of panary fermentation, expressed in rather general terms, is the effective aeration of dough by the uniform dispersion of a gas throughout the system, which must be occluded and retained by the gluten, so that a well-risen loaf of bread will result on baking, of required volume, grain, texture, taste, flavor, and certain other desirable characteristics. This process must be so conducted that no injury or weakening is done to the colloidal or physical properties of the wheat protein or gluten that determine the capacity for what is commonly termed “gas retention” in the literature of flour strength. Aeration is primarily accomplished by the use of various leavoning agents from biological or chemical sources, of which culture yeast is now the most important. Compressed yeast produced from pure culture strains of the organism physiologically adapted to gas production in the complex environment of dough has practically supplanted the old methods using natural leavens, barms, ferments, and brewer’s yeasts, except in certain instances where special types of bread are required; but even here biological culture materials
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are coming into use, such as the well-known salt-rising ferment of Kohman and the recent “sauerteig” culture of Beccard, for the fermentation of sour rye doughs. Chemical methods of aeration, such as carbon dioxide under pressure, or a saturated solution of this gas in water, have now little more than historical interest, and chemically reacti.ng compounds which evolve gas in the presence of moisture or heat find their chief application in biscuit and cracker doughs, cake mixes, and self-rising flours. I n American bakery practice bread generally refers to the yeast-risen product. The history of yeast as a leavening agent is of fascinating interest, but cannot be reviewed here. Malouin, who wrote the first book of any value on baking in the eighteenth century, commented enthusiastically upon the introduction of yeast into bread-making, though at that time the varieties used were chiefly impure brewers’ yeast. He recognized it at that time as epoch-making in the history of the baker’s art. The classical researches of Pasteur, Hansen, and Lindtner, which were to come later, on pure culture yeasts for the fermentation industries have very probably been the most significant scientific influence on the advancement of baking. This statement can readily be appreciated if we imagine what might happen if culture yeast were eliminated from modern commercial practice. Perhaps the future may bring forth synthetic catalysts which will displace yeast. This is obviously a matter of speculation, though of considerable interest t o the physical chemist. The general purpose of panary fermentation has been previously indicated and reference made to the complicated nature of the problem. The remainder of this discussion will be confined to a review of some of the biochemical changes that take place, which appear to be of suggestive interest. BIOCHEMICAL CHAKGES The fermentation of a bread dough is characterized in its general aspects as a complex phenomenon which may be largely attributed to the presence of enzymes associated, not only with the physiological activities of living cells, yeast, and bacteria, but with the flour and other products from biological sources. The typical biochemical changes involved are of the general nature of reactions catalyzed by enzymes, of which the fermentation of hexose sugars b y Buchner’s zymase, a veritable battery of enzymes, is the most prominent. There appears to be good evidence that the fermentation of sugar by living yeast cells is R typical enzyme reaction. Other changes are the hydrolysis of the disaccharide sugars, sucrose, and maltose, by yeast sucrase and maltase, with the production of the hexose sugars, glucose, and fructose, and that of starch by the diastatic enzymes of wheat flour and other materials such as malt extract, with the production oi maltose which may be converted into available glucose. Some hydrolysis of the protein constituents of doughs, particularly the gluten of wheat flour, probably takes place, though there is little information in the literature regarding it, either as to the extent of such changes, the determining factors involved, or to the sources of the enzymes. This is a problem of immediate interest in connection with fermentation control, particularly in relation to the gas retention properties and coagulation of the gluten, which should include a bacteriological study of the flour and other sources of bacteria. EFFECTOF CARBOHYDRATES The carbohydrate materials used in bread formulas, either initially or subsequently available for alcoholic fermentation, are rather extensive. Cane sugar is generally
