AN INTRODUCTORY STORY OF BAKING POWDERS 0. S.
RASK
TEE JOHNS HOPKINS UNIVERSITY, BALTIMORE, MARYLAND
This paper outlines briefly the history and principles of leavening in general and of baking-powder leavening in particular. The commonly used bakingp d e r ingredients, their characteristics, and interactions are discussed. T k formulation, keeping qualities, baking cltaracteristics, and healthfulness of uarious types of powders are also considered.
. . . . . .
General and Historical A considerable amount of air and also a little carbon dioxide will invariably be contained within a dough prepared in an ordinary manner from only flour and water. A portion of these gases is dissolved in the dough and the rest is held mechanically in the form of bubbles of various sizes. If such a dough is subjected to oven heat the entrapped bubbles will expand in accordance with the gas laws and the dissolved gases will gradually be thrown out of solution, after which they also will expand in a like manner. In this expansion the gases will obviously expand or leaven the surrounding or enclosed dough to a degree varying with its extensibility and gasretaining capacity. As a result, the dough will have a slightly porous and aerated structure by the time heat coagulation of its proteins transforms it into a baked product having the characteristic resilient structure. However, the leavening action so produced is usually less than that popularly desired. In order to obtain additiorial leavening effects i t is necessary to incorporate into the dough, along with i4s other ingredients, something which will function as a source of the necessary quantity of leavening gas. The practice of so doing, which is now a highly developed art, had its origin in the ancient so-called "sour-dough" and "salt-rising" methods of leavening, which are essentially the same. In the sour-dough method the leavening agent or source of leavening gas is a small portion of dough usually saved from a previous baking and allowed to turn sour by being kept in a warm place for a few days. In the salt-rising method the leavening agent is a small portion of slack dough prepared from flour, salt, and warm milk and then allowed to stand in a warm place for such time as may be necessary for it to "sour" properly. Although primitive man understood very well the practice of these methods he had no understanding of their principles. We now know that these doughs, as they sour, function as cultures of C02-producing microorganisms, among which are wild yeasts and bacteria of the Bacillus Welchi type. The quantity of microorganisms added in this manner is usually sufficient for generating within the dough such volumes of carbon dioxide as are necessary for a fairly satisfactory leavening prior to and during the earlier stages of its subsequent baking. 1340
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Both of these methods of leavening are empirical and unscientific. Like all other methods of this type they yield uncertain and variable and therefore more or less unsatisfactory results. But out of them have been evolved the modem and scientific methods of leavening. Modern Methods of Leavening
A11 modem methods of leavening are like the ancient ones in that carbon dioxide is the leavening gas. It seems to be the most suitable and the most readily available gas for leavening or abating baked foods. All modern means of leavening are therefore processes for generating carbon dioxide within a dough prior to and during the earlier stages of its baking. These processes are of three general types which may be characterized as follows: 1.
Yeast action on fermentable carbohydrates, whereby COX is generated, usually called "yeast leavening." 2. Decomposition by heat of substances yielding carbon dioxide as one of the decomposition products. 3. Acid action on soda. The first type, uiz., yeast leavening, is the most important process by which bread is leavened. It involves incorporation into the dough of a pure culture of a strain of yeast selected and developed especially for its COz-producing capacity in bread doughs. Yeast leavening is therefore a direct outgrowth of salt-rising and sour-dough methods of leavening. The second type is of minor importa&e. The only known substances which will decompose to liberate carbon dioxideunder baking conditions are ammonium bicarbonate and acetone dicarboxylic acid. The former, win., NH4HC03,is used to a limited extent in commercial bakeries and the latter has not as yet been developed beyond the experimental stage. The third type may be divided into two sub-divisions. In one, the carbon dioxide is generated by the action of acids contained in sour milk when i t is incorporated into the dough along with the soda. In this article our interest is entirely in the other sub-class in which carbon dioxide is generated from a previously prepared dry mixture containing dry sodium bicarbonate and a dry, edible, solid, acid-reacting material. Such a dry and previously prepared mixture containing sodium bicarbonate and a solid, edible acid or acid-reacting material is known in the trade and to the consumer as baking powder. Baking Powder Baking powder is in all probability not a new subject to anybody. Most of us have known baking powders since childhood days when we regarded the kitchen and the pantry as especially profitable fields for investigations. As our intellectual horizons broadened most of us inevitably encountered
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discussions and treatises on baking powders, since they occur in such varied fields as domestic science, baking technology, elementary chemistry, industrial chemistry, and trade wars. This extensive baking-powder literature includes both technical and popular discussions in which may be found almost any desired information on the subject. Another article may, therefore, seem superfluous. However, this article represents an attempt to collect and condense such baking-powder data as may be of special interest to students and teachers of chemistry. Defmition The U. S. Department of Agriculture has " . . .as a guide for the officials of this department in enforcing the food and drug act. . ." defined baking powder as follows: Baking powder is the leavening agent produced by the mixing of an acid-reacting- material and sodium bicarbonate, with or without starch or flour. It yields not less than 12 per cent. of available carbon dioxide. The acid-reacting materials in baking powder are: (1) tartaric acid or its acid salts, (2) acid salts of phosphoric acid, (3) compounds of aluminum, or (4) any combination in substantial proportions of the foregoing. This definition is highly technical and is intended primarily to distinguish legal and illegal baking powders froin one another. In its construction it conforms to the pattern of a technical regulatory system of federal govemment "food definitions and standards" of which it is a part and which is understood properly only by those familiar with the enforcement of food control laws. Chemists in general will probably find greater usefulness in a definition reading somewhat as follows: Baking powder is a leavening agent consisting essentially of a dry and reasonably stable mixture of powdered sodium bicarbonate and a chemically equivalent quantity of a suitable and powdered acid-reacting material composed of one or more acids, acid salts, or acidreacting salts, the mixture being diluted with dry starch so as to contain usually from 13 to 14 per cent. of "available" carbon dioxide which will evolve as a gas in a baking dough. The acids and acid-reacting materials most commonly used in baking powders are the following: cream of tartar (KHC4H40s),tartaric acid (H2CIH106),mono-calcium phosphate or "phosphate" [Ca(H,PO,),], and sodium aluminum sulfate [Na304Alr(S04)a],commonly designated in the trade as S. A. S. Phosfihute Bakiig Powder.-Sometimes only one acid-reacting material is used in a baking powder, and such a powder is known to the trade as a
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"straight" powder. Practically the only nationally advertised "straight" powder on the retail market is that made with calcium acid phosphate, known as "phosphate baking powder." Tartrate Baking Powder.-A "straight" cream of tartar baking powder is manufactured by one or two wholesale houses. All nationally advertised powders containing cream of tartar also contain tartaric acid; and such powders are called tartrate powders: which name most clearly states the similar nature of the acid ingredients. Combination Baking Powder.-Another type of powder, containing both calcium acid phosphate and sodium aluminum sulfate, is known as combination baking powder* or a "double acting" powder. This last type represents a large proportion of the baking powder manufactured in the United States.** The name "combination powder" clearly calls.attention to the union of ingredients of diierent nature and different action. The reactions which take place in a dough between these several acids and NaHC03 may be the following:
+ + + +
+ + + + + + + + + + +
KHC,H40a NaHC03 = KNaC4HlOa H20 CO* H2C,H408 2NaHCOa = NaAHdOe 2He0 2CO> 2Ca(HnP0J2f 4NaHCOs = 2CaHPO4 2NazHPOd 4H20 4-4COr 3Ca(H1POd)r 4NaHCOa = Caa(PO& 4NaH1P04 4H20 4C02 = Cas(PO4X 4NaeHPO< 8HsO 8COz ~ C ~ ( H . P O A )8NaHCQ ~ NanSO4AIz(SO& 3H20 6NaHCOa = 4Na9S04 2Al(OH)s 6Hs0
+
+
+ + +
(1) (2) (3) (4)
+ 6CO.
(5) (6)
The reaction between Ca(HzPO& and WaHC03 is not understood fully. There are reasons for believing that these two s+stances can react with one another in three different ways as indicated by the equations (a), (4), and (5). All three reactions may take place simultaneously to different degrees depending on conditions in the dough.
Standardization It is logical to standardize baking powder on the basis of its content of "available" carbon dioxide since it is the active and, as yet, the only leavening principle known or recognized as useful in the product. Furthermore, as a matter of convenience to the housewife, baking powder is standardized to a conventional and fairly constant strength so that a definite quantity of powder, such as a teaspoonful, will always represent a correspondingly definite activity. As specified in the definition, this strength is 13 to 14 per cent. available carbon dioxide. This particular value has been selected as a result of practical considerations which will he explained in a succeeding section on baking-powder formulas. The government specification of "not less than 12 per cent. of available carbon dioxide"
* See Classification, Circular No. 138, U. S. Dept, of Agr., Revised 1931. p. 5. ** lbid., p. 7.
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is the minimum limit tolerated in a legal product for interstate commerce. Baking-powder manufacturers invariably adjust their powders to a somewhat higher concentration of available carbon dioxide, usually from around 13.5 to around 14.5 per cent. which they designate as factory strengths. Such factory strengths allow for a loss of 1.5 to 2.5 per cent. of carbon dioxide as a result of deterioration which may take place between the time the powder is mixed and the time it is delivered to the ultimate consumer. This standardization is accomplished by dilution with starch as indicated in the above definition, and as explained further in the following section on formulas. Formulas The desired available carbon dioxide content of a baking powder is the starting point for calculating its formula, and determines the required sodium bicarbonate in accordance with the following proportion: Combining weight of COz : Combining weight of NaHCOa :: Desired percentage of available COz : Percentage of required NaHCO?, 44 : 84 :: 14 : 26.73 or in case 14 per cent. is the required available carbon dioxide. Accordingly in making one hundred pounds of baking powder which will yield 14 per cent. of available carbon dioxide the manufacturer may be conceived of as starting with 26.73 pounds of NaHC03. To this weight of NaHC03 he will add an equivalent weight of acid-reacting material, make up to a hundred pounds with starch; and mix. Combining weight of NaHC08 : Comhinigg weight of acid :: 26.73 : Pounds of acid required in 100 pounds of baking powder, 84 : 188 ::26.73 : 59.8 or in case the acid used is cream of tartar of which the combining weight is 188. These two materials, viz., 26.73 pounds of NaHCOa and 59.8 pounds of cream of tartar, are made up to 100 pounds with dry starch and then mixed. Accordingly, the formula for a cream of tartar baking powder containing 14 per cent. of available COI will he: NaHCOs.. . . . . . . . . . . . . . . . . . . 26.73% Cream of tartar.............. 59.80% Starch.. . . . . . . . . . . . . . . . . . . . 13.471T0 100.00
According to similar calculations the formula for a tartaric acid powder containing 14 per cent. of COPwill he: NaBCOs . . . . . . . . . . . . . . . . . . . . 26.73 Tartaric add. . . . . . . . . . . . . . . . 23.87 Starch ...................... 49.40
and the formula for a S. A. S. powder containing 14per cent. of COzwill he:
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NaHCOa... . . . . . . . . . . . . . . . . . 26.73 S . A . S...................... 25.65 Starch. . . . . . . . . . . . . . . . . . . . . . 47.62 1W.00
The theoretical formula for a monocalcium phosphate powder cannot be calculated with the same certainty as those for the other acid-reacting materials since there are three different ways in which Ca(H,PO& and NaHC03 react with one another, as indicated by equations (3), (4). and (5). A combination of these three equations will be 4Ca(H#04)2 8NaHCOs = CaHPO* ?- Cas(PO& -C 3NapHP04 f 2NaHZO4 f
+
8C01
+ 8Hs0
(7)
This combined equation represents the reaction between Ca(HzPO& and NaHC03 only if reactions represented by equations .(3), (4), and ( 5 ) take place to equal extents, which seems improbable. Some authorities believe that equation (5) represents most closely the reactions which take place. The two corresponding formulas for a monocalcium phosphate powder containing 14 per cent. of carbon dioxide will be:
Starch
As has already been stated, calcium acid phosphate powder is the only straight powder of any retail commercial significance a t the present time. Nearly all of the others contain two acid ingredients, viz., cream of tartar and tartaric acid (tartrate baking powder) or calcium acid phosphate and sodium aluminum sulfate (combination baking powder). Combination powders of the cream of tartar-tartaric acid type are, or may be conceived of as, equivalent to the two straight cream of tartar and tartaric acid powders mixed in the desired proportions, although that is probably not the way they are actually prepared in any factory. The cream of tartar and the tartaric acid are more probably added in the desired ratio and in the required quantities to the same lot of soda and then mixed with the required amount of starch. There seems to be no simple rule or principle governing the ratio of cream of tartar to tartaric acid in the type of combination powder containing these two acids. Presumably, the ratio used in any particular powder has been selected as a result of considerations of several factors such as baking performance and keeping qualities of the resulting powder, and relative costs and efficiencies of the two acids. In some instances this ratio may be one which, by a process of trial and error, has been found to result in a powder conforming most closely to that which its manufacturer regards as ideal.
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A different andmore difficultproblem arises in selecting the ratios of phosphate to S. A. S. in that type of combination powder. The exact reactions in this type of combination powder are not known, but they are believed to be rather complex and probably also somewhat variable. It would therefore be misleading to attempt to represent them by any chemical equations. Nevertheless, the following entirely hypothetical equation is interesting partly because it indicates possible reactions and partly because it indicates the ratio of phosphate to S. A. S. required in order that all of the aluminum may occur as insoluble AIPOl in the reaction residue. NaSO+4I.(SOA
+ Ca(HzPO& '+ 4NaHC08 = 2AIPO4 + CaSO, + 3NanSOc + 4C01
+ 4Hn0
(8)
According to this equation, the formula for a phosphate-S. A. S. combination powder containing 14 per cent. of COzwould be as follows: NaHCOa.. . . . . . . . . . . . . . . . . . . . . 26.73 S . A . S. . . . . . . . . . . . . . . . . . . . . . . . 38.51 Ca(HzPO&. . . . . . . . . . . . . . . . . . . . 18.64 16.12 Starch. . . . . . . . . . . . . . . . . . . . . . . . 100.00
The end product, AIPOd,suggests that S. A. S. has not functioned a t all as an acid in equation (8) since the conversion of S. A. S. into Alp01 does not involve the release of any acidity. On the other hand, the acid capacity of Ca(HZPO&has been doubled since its conversion into Alp04 has released all of its four hydrogens including its two tertiary hydrogens which are not released or displaced in the other and preceding reactions of Ca(H2P0& with NaHC03. The reaction between the S. A. S. and the Ca(HzPOa)zis probably that represented by the following equation (9) which can be regarded as an intermediary step in the reaction represented by equation
In this reaction S. A. S. causes a precipitation of the phosphate radical in the form of the tribasic phosphate, AIP04,and thereby releases or displaces the two tertiary hydrogens of Ca(HzP04)2,as weU as its two secondary hydrogens, all in the form of 2HSOa which then react with the soda. Accordingly, in equation (9), S. A. S. functions as an acid only indirectly by doubling the neutralizing capacity of the Ca(HzP04)n. It is interesting to consider and compare the ratios of soda to acid material in equations (5), (6), and (8). These are, respectively, 0.957, 1.042, and 0.468. The lowest neutralizing capacity is therefore that of the when these react as a mixequimolecnlar mixture of S. A. S. and Ca(HzP04)2 ture according to equation (8). However, as has already been stated, equation (8) is only hypothetical. The actual reactions are probably represented by some combination, as yet unknown, of equations ( 5 ) , (6), and (8). I t is,
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therefore, impossible t o derive by any calculations a practical formula for an S. A. S.-phosphate combination powder. Manufacturers of this type of powder probably use empirical formulas which have been developed by processes of trial and error. None of these are known with any exactness to the writer. However, Bailey (U. S.Dept. Agr. Circ., No. 138) gives the following formula for this type of combination powder: NaHCO*.. . . . . . . . . . . . . . . . . . . . . 26.73 Ca(H9P04)1.. . . . . . . . . . . . . . . . . . . 13.28 S.A.S. . . . . . . . . . . . . . . . . . . . . . . . 19.92 Starch.. . . . . . . . . . : . . . . . . . . . . . . 40.07 100.00
In this formula the ratio of soda to the S. A. S.-phosphate mixture is 0.805 which is not so low as that in equation (8) but nevertheless considerably lower than the average (0.9995) of those of equations (5) and (6). Apparently, therefore, S. A. S. and phosphate even though mixed react only to a partial extent according to equation (9) and t o some appreciable extent as they would individually and separately according to equations (5) and (6). The soda-decomposingcapacity of the S. A. S.-phosphate mixture is low to the extent to which i t reacts according to equation (9) and conversely its soda-decomposing capacity approaches the average of those in equations (5) and (6) t o the extent to which S. A. S. and phosphate react individually according t o equations (6) and ( 5 ) , respectively, when mixed with one another. Since S. A. S. and phosphate do in all probability react to some extent according t o equation (9) it ma& be assumed that they are not so well utilized in a mixture as when used separately. However, the lower utilization of the mixture is compensated by the better performance of the combination, at least according to the judgments of the manufacturers and consumers of this type of powder. Otherwise this type of powder would not exist as a commercial commodity. The advantages of the S. A. S.-phosphate type of combiiation powder will he discussed in a following section. All of the foregoing formulas are theoretical and, excepting (S), are practical to the extent that baking powders made according to them are quite satisfactory. Furthermore, some of them may bave been the original formulas used by the industry during the earlier stages of its development, but they are probably not the exact formulas used a t the present time. Every baking-powder manufacturing organization now appears to bave its own practical factory formula which is usually a trade secret. However, it seems safe to assume that factory formulas can only be slight modifications of theoretical formulas. Probably the most common and important but not necessarily the only dierence between factory and theoretical formulas is that indicated by the presence of a slight excess of soda in most com-
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mercial brands of baking powder. Accordingly, factory formulas call for a slight excess of soda over and above the stoichiometric quantity specified in the theoretical formulas. Factory formulas are the results of commendable and more or less successful efforts on the part of manufacturers to increase the usefulness of their products. In the past these efforts have consisted mainly of experiments of the trial-and-error type by which only empirical results are possible. Present-day factory formulas are therefore empirical to the extent to which they are the results of such experimentation. However, in recent years many baking-powder menufacturers, if not all, have adopted more scientific methods of research as a means of improving and making more scientific, and therefore less empirical, their processes and products. As yet very little is known on the outside concerning the details of this research or of any of its results because this research, like that of most other commercial organizations, does not find its way into the literature as quickly as that conducted by educational, governmental, and the more strictly research institutions. Baking-Powder Constituents Since baking powder is a physical mixture its properties and characteristics are essentially those of its constituents. Brief descriptions of these will therefore probably portray best the different types of baking powder The acid constituents seem most logical to begin with because they are responsible for the different types of baking powder, the other constituents, viz., starch and soda, being common to all. The more significant cbaracteristics of the baking-powder acids are listefl and evaluated in Table I, and discussed further in following sections. TABLE I S i-d c a n t Prooerties of Bakine-Powder Acids Toricily
Ratio of Acid lo Sodo
Tartaric acid, HGH
