8 The Rise of Self-Rising Flour A Recipe for Success

Malcolm McCamish University of Queensland, St. Lucia. Queensland, Australia 4067

As educators become increasingly aware of the similarities between the laboratory and the kitchen ( I ) , there is a grow- ing advocacy for the use of culinary experiments to explain and reinforce chemical principles (2,3).

The advent of affordable gas and high-performance liquid chromatography equipment ( 4 ) has provided a host of quan- titative underaraduate experiments including determina- tion of the amount of caffeine in beverages (5). which has come to rival the more standard extraction of caffeine from coffee (6) , tea (7) or cola (8). Atomic absorption spectropho- tometers have enabled the quantitation of trace elements, such as calcium in fruit juice (9). Unfortunately, due to the obviaus expense, similar experiments are not used in most secondarv schools. Subseauentlv. kitchen chemistrv exueri- ~~~~ ~~ , . . ments at the introductory level have remained almost exclu- sivel\, uualitative 13. 10. 1l).'l'his article describes one inter- - . . , . . esting and inexpensive quantitative experiment which has been successfully used a t the secondary level.

Baklng Aerators The chemistry of baking powder makes an interesting

demonstration topic, and when the study is seen in the perspective of thelong "evolutionary" development of bak- ing aerators, it provides an ideal paradigm for the incremen- tai approach tdproblem-soivingtbat characterizes so much "real" chemistry.

Three common raising agents are used in baking: (1) wa- ter, which expands on conversion to steam, for example, in chou oastrv. cream uuffs. eclairs: (2) tranoed air bubbles. which'expand on he&g,'for example, whfpped egg whites in meringues; and (3) carbon dioxide, which can be generat- ed from yeast or sodium bicarbonate. Though each agent provides some fascinatina chemistrv, this article will be con- cerned only with sodiumbicarbona~e.

The culinary requirement and the chemical problem in causing a cake to rise is to achieve a slow release of some but not all of the carbon dioxide while the batter is being mixed. This forms small, uniform gas cells into which the remainder of the carbon dioxide can be gathered as i t is released. This is accomplished during the initial stages of heating, before the hatter has dried and set. At this stage, the batter is still sufficiently elastic to accommodate the additional volume that results as additional gas is trapped and as the tempera- ture increases. There are the additional consumer reauire- ments of acceptable appearance and palatability.

Manufactured self-risinn flour. esueciallv that which is - designed for a specific purpose, for example, cake mix and pizza dough mix, often contains a variety of ingredients in a secret formulation, tailored for the specific product. It is therefore impossible to detail the complex chemistry of an individual mix; rather, our goal is to investigate the function and the reactions of the individual ingredients.

The use of bakine soda. sodium bicarbonate. bv itself " introduces problems. First of all, no gas is evolved during the mixine stare. Second. onlv half of the "available" carbon . . dioxile is &imately released. The rest is converted into the sodium carbonate salt,

This decomposition does not begin until the batter has reached about 50 "C. Because considerable heat is required to attain this temperature, partial setting and loss of elastic- ity of the batter occurs before the gas has evolved. Moreover, sodium carbonate (washing soda) can produce an unwanted color and a characteristic, strong alkaline taste, which can render the product unpalatable. In spite of these facts, bak- ing soda enjoys limited use in products such as soda scones (baking soda biscuits), pancakes, and waffles. The high cooking temperature, generally 450 O F (230 'C) (cf. cakes, 350°F (175 O C ) ) causes the carbon dioxide to be liberated before the dough has set. Highly flavored sweet fillings are often used in such products to mask any soapy taste that results.

The problems typically attributed to baking soda can be overcome by using baking acid rather than heat to liberate the carhon dioxide: the historv of self-risine flour is reallv ~ - - ~ ~~~~ ~ ~ ~~ ~

the search for a suitable baking acid. The use of a mineral acid such as hydrochloric acid introduces its own difficulties.

Flavor considerations demand accurate stoichiometry; it is generally desirable to use exact equivalents of acid and soda since an excess of either reagent results in unpalatability and consumer resistance (however, the thought of titrating a cake does seem ludicrous). This process results in the rapid, effective generation of the available carbon dioxide; unfortu- nately, this occurs long before the batter has reached the oven.

The stoichiometric problem could be solved by the manu- facturer through inclusion of an appropriate solid acid, the sodium salt of which does not impart an unpleasant taste. These criteria are satisfied by tartaric acid.

CHOHCOOH ZNaHCO, + 1 -

CHOHCOONa I + 2C0, + 2Hz0 CHOHCOONa

The organoleptic properties of the salt are acceptable, and the acidity of tartar acid (pK1 = 2.93, pKz = 4.24) is suffi- cient for both acid groups to be used (apparent pK, H2C03 = 6.36), which makes economic sense. Unfortunately, the solu-

710 Journal of Chemical Education

1 bility of tartaric acid (139 g1100 g HzO at 20 O C ) causes a faster evolution of carbon dioxide on mixing than can be tolerated.

The next evolutionary step involved replacing tartaric acid with its potassium acid salt. Potassium hydrogen tar- trate, or cream of tartar, is only sparingly soluble in cold water (0.37 g1100 g Hz0) hut usefully soluble in hot water (6.1 g/lOO g HzO). For most domestic use, this reaction is slow enough a t rnnm temperature to be effective. Unfortu- nately, commercial users report that, because their produc- tion methods often reauire a lone waitine time between mixing and baking, almost all of the carbonhioxide is liber- ated before the cake mix starts to warm in the oven. There are also considerable economic considerations. The high cost of cream of tartar and its low efficiency make i t virtually unused today as a baking acid.

Economic considerations led to the development of phos- phate aerators, the first of which was calciim dihydiogen phosphate.

The beneficial effects of low solubility a t room temperature (1.8 g1100 g H20) are further enhanced by the acidity of the phosphate (pK HzPO; = 7.2), which caused the series equi- librium to lie to the left (see eq 1). I t is only the last essential- ly irreversible step driven forward by increasing the tem- perature (Henry's Law) that upsets the series of equilibria. Thus the cake rises as a result of Le Chatelier's Principle. The reaction is, however, more complicated than this. (See Demonstration 8).

The story could be complete but for the bitter taste of tbe uroduct. The sodium analoaue is nnfortunatelv too soluble. However, advantage can b;taken of phosphate's ability to link to itself. Sodium pyrophosphate, which has an accept- able solubility and which yields an acceptably tasting prod- uct, has become the "acid" of choice,

NaHC03 + Na2H2Pz07 - NazHPzOI + Cost + H20

Today, almost all nmmercial baking puwder, are based on sodium dihydrogen pyruphosphate. In general, such hak- ine oou,ders liherate 30 40-c of the available carbon dioxide oGkixing, 10% on standing more than 10-15 min, and 50- 60% during bakine. Often thev are mixed with a small amount of-the m o k a l c i u m dchydrogen phosphate to in- crease or decrease the rate of release of carbon dioxide. Of course, the reactions will be more complex than those "aque- ous reactions" shown here since many side reactions are possible with ionizable calcium, proteins, and other ingredi- ents.

Bicarbonate and pyrophosphate are premixed by the manufacturer in the correct stoichiometric proportions, and the slight tendency to absorb water is accommodated by the vast excess of flour, or in free baking powders by the addition of about 30% starch. As a bonus-the advertisers can an- nounce that the product is phosphate enriched.

Demonstrations

The above story is rich in demonstration potential, several of which are outlined below.

1. Determination of the solubilities of uarious baking acids. A simple way to achieve this is to titrate a sample of a saturated solution of each of the samole hakina acids with sodium bicarbonate . solution.

2. O~momtmtion of the diminishingsolubility of carbon dioxide with increasing temperature. This can be shown with any carbonat- ed beverage, either by coupling it with the lime water experiment or by tasting it after heating.

Taste Sensations and the Tongue's Response Sites

Example Taste Sensetion Site of Serntian

sugar cubes sweet tip of tongue rock selt sell tip and edge lemon wedges sour edge grapefruit bitter back of tongue

3. Detect the flouor of uarious bakingproducts. Though tasting chemicals is generally to he discouraged in chemistry classes, the baking acids and their products can be sampled to determine their flavor. At the beginning of this demonstration, show the four major taste sensations by distributing some simple foodstuffs (as listed in the table) to the class and have them rub over their tongues. Once the reference tastes are established, the baking samples are distrib- uted. The sour taste of acids is reinforced before the cream of tartar is oasred around. The class is asked if thev think the samole would . ~~~~

be ar~dic enough tu Iherarc carlrm dioxid~. So programmed arr we LO tasre wi th the tip 1.1 the tongue ruhich d u c ~ nor detert arid1 that there is an almost unanimous re3ponse rhar the eompwnd ir not acidic.

4. Show the effect of solubility andpH on the rate of liberation of carbon dioxide. This can readily be shown without having to prepare a cake batter which will retard solution. In a simple experi- ment 4 g of sodium bicarbonate (-1 L potential COz) is mixed with the stoichiometric amounts of tartaric acid, cream of tartar, and potassium dihdyrogen phosphate and placed in a Buchner flask fitted with a dropping funnel which contains 100 mL of water. Plastic tubing connects the side arm to a 2-L graduated cylinder filled with water and inverted in a plastic bucket of water. The contents of the flask are magnetically stirred. (See Fig. 1.)

The tap of the dropping funnel is securely stoppered so that evolved gas can not escape. When the tap is opened the amount of eas can he measured at eiven time intervals. No attemot is made to - ~~~~ ~

cdmpeniate lor soluhilitir~, err. The results are shown in Figure 2. The rate of gas ev~~lutim can he measured wrrh a slop wnrrh from this data.

5. Determine the amount of immediately ouailable earbon diox- ide in bahing powder. Water can he added to a known weight of baking powder (e.g., 5 g) in an apparatus described above (in Dem- onstration 4). The evolved gas can then be collected over water as before. It must he recognized that the percentage liberated from solution will be greater than that liberated from a batter during mixing.

After 5 min, excess hydrochloric acid is added to liberate all residual carbon dioxide. This is equivalent to the amount of gas

Figure 1. Setup for solubility and pH experiment.

Volume 64 Number 8 August 1987 711

RATE of C 0 2 EVOLUTION

Figure 2. Rate of evolution of COI with different acids

evolved during baking. A series of synthetic baking powders can be prepared and the characteristics of their gas evolution investigated.

6. Determine the efficiency of the batter to trap the gas. Having determined the amount of C02 liberated by heating, some measure of the efficiency of gas entrapment within the batter can bemade by calculating the volumes of batter and risen cake, given that 500 g of flour contains about 8 g of bicarbonate (i.e., about 2 L of potential

Cod.

7. Calculate the relatiue costs of producing I L (or I mol) of carbon dioxide. All too often the economies of the chemical industry are overlooked. Our example shows half a dozen ways of achieving the same reauired result. An insight into the economies of scale can be gdined hy nmrrasting t h r unit costs c,f plait) 2nd self-rising flour merutor ndded b y the mnnufarrurer) with rhr cost of using baking pvwder (nerarnr added by the consumer). ,+ facror of >10 is nor uncommon.

8. Comment on the products of the reaction of eolcium dihydro- gen phosphate with bicarbonate and to perform the appropriate calculations. Acid-base considerations indicate that the products should be carbonic acid (initially) and hydrogen phosphate ion (HPOj7 with the equilibrium favoring the reagents ( K 0.14). Despite the minute concentration of PO:- (-lo-% it is sufficient to exceed the solubility product of calcium phosphate, which therefore precipitates and so drives the reaction to completion.

Literature Clted 1. Selinger. R. Chemistry in the Morhef Place, 3rd ed.: Harcourt: Sidney, 1986; Chapter

a. 2. Hoskttler, J. D. J. Cham. Edue. 1383.60.1037. 3. Gr-r, A.E. J. Chem.Educ. 1384.61.362. 4. McCerniah,M.;Cannell,G. R.;Rrethertan,L. J. Chem.Educ. 1982,59,249. 5. DiNunrio, J. E. J. Chom. Educ. 1385.62.446. 6. Pavia, D. L.; Larnprnsn, G. M.; Skriz, G., Jr. Introduction lo Organic Leborolory

Techniqwa. 2nd ed.; Saunders: Philadelphia, 1982; p 64. 7. Adams, R.; Johnson, J. R.: Wilcox, C. F.. Jr. Labomtory Ezpdments in Organic

Chemimy, 7th ed.: Msernillsn: London. 1979, p 114. 6. Helrnkamn. G. K.: Johnson. H. W.. Jr. Selacled Exoarimanls in Orcanic Chemirlrv.

2nd ed.[~reernsn: Ssn Francisco; 1968: p 158. 9. StrahLA. N. J. Chem.Educ. 1385.62.343.

10. Martino..l..I. Cham.Edue. 19R1.60. IOOd. ,~ ~ ~

11. Mebane, R. C.: Rybolf,T. R. J. Chem Educ. 1985.62.285

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