Leavening Equivalence Value

nation of the two leavening acids gave rise to double-acting beiking powder. During the first third of the twentieth century a number of phosphate compounds were studied, and the range of leavening acids available today to formulators was developed. These acids have the advantage of controlled and predictable rates of C02 release and account for essentially all commercial leavening acids sold today. Several good reviews on the properties and uses of leavening acids have been published (8-12) that give more details about various specific applications.

Table 2 lists the leavening acids and the chemical reactions that occur when they dissolve in water to liberate H+. Of use to the formulator are the neutralizing value (NV) and the equivalence value (EV). Suppliers characterize their products by NV, which is defined as the pounds of sodium bicarbonate that are neutralized by 100 lb of the acid. When developing product formulas, it is more convenient to use EV; for example, if 2% soda is included in the formula, 2.78% sodium acid pyrophosphate (SAPP) is added to achieve exact equivalence.

Neutralizing value is determined by experimental titration (9,10). The yield of H+ shown in Table 2 corresponds to NV, although there are some discrepancies. For instance, dicalcium phosphate dihydrate (DCP.Di) should yield 2.15 H+ to have an NV of 35. One that is even more puzzling is the reaction of SAPP. The pyrophosphate ion has four ionization constants, the first two of which are neutralized during manufacture of disodium pyrophosphate. The additional ionization constants for the dihydro-gen dianion formed on solution in water are 5.77 for pKt and 8.22 for pK2. In a cake batter at pH 7 the dianion should yield only 1.1 H+, equivalent to an NV of 42, but in actual fact NV is 72 as shown in the table. Obviously other, unrecognized, reactions occur that increase the acidifying potential of SAPP. Neutralizing value (and EV) are for practical guidance; the reactions are for interest and edification.

Reaction Rates. Two kinds of reaction rates are of interest to bakers: (1) immediate, at room temperature; and

(2) during baking. Both are functions of the solubility of the leavening acid. The first rate, called the dough reaction rate (DRR), is measured by mixing all the dry ingredients (cake flour, nonfat dry milk, shortening, salt, soda, and leavening acid), adding water, stirring briefly, and then measuring the evolution of gas at 27°C for 10 min (13). In the absence of leavening acid there is a blank reaction of about 20% of the soda due to acidity of the flour and dry milk. Monocalcium phosphate monohydrate (MCP.H20) reacts about as quickly as cream of tartar; that is, it is a very fast acting acid. Anhydrous MCP is about 80% as reactive. Dicalcium phosphate dihydrate (DCP.Di) is essentially unreactive under these conditions, as are sodium aluminum phosphate (SA1P) and sodium acid pyrophosphate for refrigerated dough (SAPP-RD). The various grades of SAPP (see the following discussion) show increasing extents of reaction, with the reaction rate for sodium aluminum sulfate (SAS) being somewhere in the middle of the SAPPs (9) and dimagnesium phosphate (DMP) slightly slower than SA1P.

Baking (temperature-dependent) reaction rates measure the rate of C02 release from a dough at various temperatures. The rate is actually governed by the solubility of the acid. A slow acid such as SA1P is nearly insoluble at room temperature, so it cannot ionize to give H+ for reaction with soda; as the temperature rises, the material dissolves and starts to perform its function. Figure 1 shows the rates for several leavening acids (8,9,14). The ranking of acids is the same as that given by the DRR test, but Figure 1 is useful in choosing acids for giving lift at various points in the baking cycle. For instance, MCP hydrate generates C02 in a cake batter during mixing, whereas SA1P starts to leaven the cake only midway through the baking cycle. DCP.Di is effectively unreactive (insoluble) until the temperature reaches about 80°C, near the end of the bake cycle. However, it has value in triggering a late release of C02, which helps prevent dipped centers or fallen layer cakes.

Sodium acid pyrophosphate is provided in various grades ranging from fast to slow baking reaction rates. Al-

Table 2. Reactions of Leavening Agents

Leavening acid

NV"

EV6

Reaction

Monobasic calcium

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Responses

  • Beatrice
    How are the equivalence value and neutralizing value related for leavening?
    8 years ago

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