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
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used in most bread formulas in this country, to a greater extent than elsewhere. Other familiar materials are sweetened condensed milk, malt extracts, sirups, and various hydrolyzed products of starch, among which is a dried product of recent interest containing over 90 per cent of reducing sugars as dextrose or glucose. The amount of sugar initially contributed by the flour is comparatiyely insignificant. Subsequently, as fermentation proceeds, the amount of sugar available from the flour depends largely either upon its own diastatic power or upon the presence of active diastases from other enzyme-bearing materials such as malt extract. The colloidal condition of the starch is probably of some importance here. The importance of the diastatic content and activity of wheat flour in this connection has been recently pointed out by Rumsey, especially in regard to the importance of sugar production in the later stages of fermentation, particularly in proofing, and during the first few minutes of exposure to the heat of the oven, while the effects of the addition of diastatic products have been investigated by Collatz, who has noted the desirable effects of supplementing the diastatic content in relation to gas production with certain types of flours.
EFFECT OF TEMPERATURE The influence of temperature on fermentation is well recognized in commercial practice as one of the requirements of successful bread-making. The temperature coefficient of alcoholic fermentation by living yeast cells has been shown by Slator to be of the same order as for many chemical reactions, though varying considerably, an increase in temperature corresponding with a diminution of the coefficient. Aberson's results, X t + 10/Kt = 2.72, which represents the mean coefficient for 10 degrees between 12" and 33" C., agree well with Slator's observations. Gas production in the fermentation of doughs, as influenced by temperature under various conditions, has been investigated ever since there was any scientific interest in the subject, and considerable study has also been made of the optimum temperature for various en5ymic hydrolyses, especially of the diastases. I n commercial practice straight doughs are usually set a t about 26" to 27" C. Dough rooms are maintained a t about 27.80" C. The temperature of the proof is considerably higher-32 " to 37" C., for example-which greatly increases bacterial, yeast, and enzymic activity. On baking a t the usual oven temperature, 230" to 248" C., the center of an 18-ounce dough may reach a temperature of over 63" 0. in about 15 minutes, The optimum temperature for diastatic activity appears to be about 63" to 65" C. Collatz has recently found an optimum of 65' C. for malt flour. The presence of bacteria, such as the butyric acid group, which may produce undesirable changes, has received considerable attention in the literature. It does not seem very probable that, with the usual temperature employed and the increased hydrogen-ion concentration which develops during proofing, a n y appreciably injurious effects will occur from these sources.
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EFFECTOF H-Iop CONCENTRATION The influence of the hydrogen-ion concentration on the fermentation of doughs has been a subject of much interest in recent years. This has been due primarily, no doubt, to the more general recognition of the importance of the hydrogen-ion concentration in relation to chemical reactions and biochemical phenomena generally, but more particularly in baking to the inspiration from the well-known investigation of Jessen-Hansen on the baking value of wheat flour. H e stated in this paper that "for the dough of any wheat
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flour, there exists a determined concentration of hydrogen ions with which the bread produced from this flour will be the most successful, and this concentration is greater than that which is found in a dough made from the flour in question freshly milled and prepared with pure distilled water or fresh milk. This optimum concentration corresponds approximately to the exponent of the hydrogen ion p H = 5.0; and that the hydrogen-ion concentration was a little higher for choice flours, and slightly lower for inferior flours, and that the different artificial methods used for increasing the baking qualities of inferior flours are of no value other than increasing the hydrogen-ion concentration of the dough." While the acidity of the dough had long been recognized as of great importance in bread-making, consideration regarding it had been confined before Jessen-Hansen to the gross effects of titrable acidity. Since 1911, however, the effect of acidity in terms of the hydrogen-ion concentration generally determined by eIectrometric methods has led to a better understanding of the phenomena involved and the limitations of our present knowledge of its influence on the question of the proper length of the fermentation period with various kinds of flour and other components of bread doughs. Some investigators have apparently considered that, if a bread dough is fermented to about the hydrogen-ion concentration of N or pH = 5.0, the problem is solved, and the time elapsed from the original hydrogen-ion concentration to the development of this optimum is the correct period of fermentation. Unfortunately, however, this does not always follow. Bailey and Sherwood have recently stated, after a long and painstaking investigation in which they followed the changes in hydrogen concentration as pH from the mixing of the dough to the baked bread in a number of large commercial doughs under'plant conditions, that they "do not feel that the control of the hydrogen-ion concentration in dough fermentation necessarily implies entire control in practical results." Their summary of the significance of the hydrogen-ion concentration is of much interest. In their opinion the chief reasons are that the isoelectric point of the gluten is a t pH = 5.0, yeast fermentation apparently reaches a maximdm a t pH = 5.0, flour and malt diastases have their optimum activity, other things being equal, a t pH = 5.0, and rope-producing organisms are apparently almost inactivated and their activities reduced in mediums more acid than pH = 5.0. Since patent flour doughs are much less than pH = 5.0, it follows, as far as the foregoing phenomena are concerned, that they all tend to approach an optimum as the dough progressively increases toward pH = 5.0. "Whether some of the phenomena, such as proteolysis, are effected in the same direction has not been determined as yet." Bailey and Shenvood's work emphasizes that, in the estimation of the probable rate of fermentation of bread doughs for their optimum hydrogen-ion concentration, there are a number of variables which must be taken into account, such as grade of the flour, consistency of the dough, temperature, weight of the dough batch, and the influence of materials, the composition and properties of which may influence the rate and direction of the change. An important point is made in reference to calculating the period of fermentation from initial hydrogen-ion concentration of the dough-that this should be done in relation to the grade of the flour, or that doughs made with lower grades be deliberately acidified a t the outset. The addition of acids and acid-reacting or producing materials is of much interest in this connection. Additions of organic acids such as lactic acid to bread doughs under certain conditions of temperature and proper adjustment of the formula have been found to have a desir-
December, 1923
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able effect in rapidly bringing the dough to the optimum p H for baking. Aiiother important influence of the optimum hydrogenion concentration of bread doughs is in suppressing the activities of certain bacteria other than those of the mesenteric group responsible for rope. OTHERCONSIDERATIONS There are many other aspects of panary fermentation that might be reviewed which are concerned with the effects of various agents, such as free oxygen, electrolytes, fat and other compounds, on yeast and enzyme action, as well as the interrelation of fermentation changes with the physical properties of the gluten for the retention of the gas. The study of lactic acid bacteria suitable for introduction into the dough batch for increasing the hydrogen-ion concentration is now being the subject of considerable investigation. As the tendency in baking has been to shorten the fermentation period ever since it began to adapt itself to large-scale production demands, the future probably will bring forth improvements in this direction both from the mechanical and biochemical sides. The subject in its entirety is such a broad one that many points of interest have necessarily remained unmentioned. If the impression has been created that panary fermentation offers a broad field of study for the chemist and biologist, it will have served its purpose.
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calories of the total 47 million million consumed. Sherman4 estimates that in the temperate zone from 40 to 75 per cent of the total calories consumed by the entire Ibopulation is furnished by cereals and breadstuffs. It is because of this. that the nutritive properties of bread should be carefully studied and efforts made to improve its nutritive qualities..
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The Role of Bread in Nutrition By Worth Hale HARVARD MEDICAL SCHOOL, BOSTON, MASS
HE importance of an adequate diet in its relation to health and growth of the laboratory animals is clearly understood. So far as animals are concerned, fairly accurate minimum requirements of fat, carbohydrate, amino acids, inorganic substances, and accessory food factors have been established. The adequacy of many individual foodstuffs has been determined and such as have been deficient have been supplemented by the diet as a whole. But among children, where the rate of growth is relatively very slow, the effects of undernourishment are less clearly defined. As a result no one attempts to establish a minimum of the individual food substances required for adequate nutrition, but rather to provide a very considerable excess as a margin for safety. Nevertheless, it is becoming more and more evident that there are many children in this country below the average in weight, height, and health, in some cases because of inherited factors, possibly more often because of faulty hygiene or because of a diet insufficient in amount or in quality. Natural foods are constantly being replaced by prepared foods, either as a matter of choice or as is necessary under conditions involving transportation, preservation, and :storage of food in immense quantity. Economic conditions limit the amount and the kind of food. “There are some children the largest item of whose diet is bread. There are numbers of children whose only breakfast is bread and coffee.”l The role of bread in nutrition is an important one. During the period 1915 to 1920 the ten-year average per capita wheat consumption for the United States rose from 5.3 to 5.8 bushels.2 This is nearly 1 pound of wheat per day per person. Some of this represents flour and cereals other than bread, but by far the larger amount is eaten as bread. I n England3 wheat and barley represent 16 million million
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Holt, “Food, Health and Disease,” 1922. World’s Almanac. Sarling, “The Feeding of Nations,” p 143.
