The common form of chemical leavening involves a food acid and soda (sodium bicarbonate). The soda dissolves readily in the aqueous phase of the dough or batter. When the acid dissolves, the hydrogen ion reacts with bicarbonate ion, releasing carbon dioxide (C02), which expands the baking piece. Water-vapor pressure increases as the internal temperature of the bakery item increases. Air incorporated during dough mixing also expands. Finally, certain materials, notably ammonium bicarbonate, decompose on heating and generate gases that leaven the product.
During dough mixing air is incorporated as numerous small bubbles (1,2). The actual amount varies, but with an ordinary horizontal mixer a bread dough contains about 15 to 20% air by volume. If yeast fermentation is occurring, this volume shrinks by 20% owing to oxygen depletion (3), and during baking, the nitrogen remaining expands by 17% owing to temperature rise, so the net leavening contribution due to air is nil. In the absence of yeast, thermal expansion is still only 17%, so the overall effect is small.
In cookie doughs and cake batters the amount of air retained is variable. In a standard sugar cookie dough, for example, the dough contains about 10% air by volume (4). Cake batter specific gravity may vary from 0.60 to 1.10 g/mL, depending on the formulation and emulsifier system used. The density of air-free batter is about 1.30 g/mL (5), so the amount of air in the batter may range from 15 to 55% by volume. Even at the highest level, however, the thermal expansion of this air contributes only slightly to the final volume of the cake.
Nevertheless, it is important to get good air incorporation in the dough or batter during mixing, because the air bubbles serve as nuclei for other leavening gases. The internal pressure produced by the surface tension of a gas bubble is inversely related to bubble diameter. If a small bubble and a large bubble are near each other, the gas diffuses from the small one to the large one, and the small bubble disappears. If no gas bubbles are already present in, say, a cake batter when baking is begun, then the C02 that is formed by the chemical reactions in the batter has no place to go, because a new bubble cannot form (with a zero radius the surface tension is infinite). If the cake batter contains relatively few air bubbles, the C02 diffuses to these, and the resulting cake has a coarse, open grain, often accompanied by tunnels. With many small air nuclei, the grain is fine and close.
Surfactants aid incorporation and subdivision of air into dough (2). Emulsifiers help incorporate air into shortening during the preparation of cookie doughs and cake batters. In all cases the presence of many air bubbles for nucleation is an important part of any leavening action.
The formation of steam by evaporation of water would seem to be important only when the temperature in the baking piece exceeds 100°C in items such as cookies, crackers, and snacks such as tortilla chips. Also, it has been suggested that the water present in roll-in margarine, used in making puff pastry, evaporates and helps to give the characteristic open structure of this product. Extruded goods, of course, depend almost entirely on steam for their expansion.
The vapor pressure of water increases with increasing temperature, and this increased pressure can expand the gas bubbles in cake batter and in bread. In calculations of the expansion of sponge-cake batter (which contained no soda for chemical leavening), it was found that the theoretical curves (which took into account the thermal expansion of the air) fitted the experimental measurements within a few percent (5). When theoretical expansion for bread dough in the oven (see "Oven Spring") was calculated, it was concluded that water evaporation accounted for 60% of the total volume increase (6).
Ammonium bicarbonate is often used as a supplementary leavening agent for cookies and snack crackers. At room temperature, dissolved in the dough water, it is stable, but when the temperature reaches 40°C (104°F) in the early stages of the oven, it decomposes:
One mole of ammonium bicarbonate (79 g, or 2.8 oz) gives 2 mol of gas (44.8 L, or ca 1.6 ft3).
This potential for extensive gas formation means that ammonium bicarbonate must be uniformly distributed throughout the dough; the presence of small undissolved pieces would give rather large blowouts in the product. Ammonium bicarbonate is usually dissolved in a few quarts of warm water and then added to the mixer along with the other water.
Because ammonia is water soluble, this leavener is applicable only in low-moisture products. If the finished moisture is above 5% (say, in a soft cookie), the water will retain some of the NH3 and the product has a characteristic ammonia taste. In contrast to this general rule, ammonium bicarbonate is sometimes added to eclair and popover doughs (pâté de choux). In this case the combination of high baking temperatures, thin walls, and a large internal cavity allows enough space for ammonia to diffuse out of the baked good.
Baking soda (sodium bicarbonate) has been the workhorse chemical leavener of baked goods for well over a century. In most cases it is combined with a leavening acid to form C02, but soda itself undergoes thermal decomposition:
This reaction usually requires rather high temperatures (>120°C) to happen at an appreciable rate, so using soda as a sole leavening agent is restricted mainly to cookies and snack crackers in which the internal temperatures can approach this range.
The more usual reaction is with hydrogen ions from leavening acids:
Soda is readily soluble in water (saturation solubility of 6.5% at 0°C, 14.7% at 60°C) and dissolves in the dough or batter water during mixing. The rate of reaction is governed by the rate of dissolution of the leavening acid (see "Leavening Acids"); no H+ is present until the acid dissolves and ionizes. Of course, the soda can react with acidic ingredients in the formula such as buttermilk and even flour itself. Chlorinated cake flour, which is more acidic than ordinary pastry flour, will neutralize about 0.27 g of soda per 100g of flour. Soda is a mild alkali; the pH of an aqueous solution is about 8.2. Increasing the amount of soda in a formula raises the pH of the dough, for example, in raising the dough pH of saltine crackers to 7.4. Excess soda (more than that neutralized by available leavening acid) is added to devil's food cake batter to raise the pH of the finished cake crumb to about 7.8 and generate the dark chocolate color desired (cocoa is lighter colored in mild acid, darker colored in mild alkali).
Sodium bicarbonate is available in various granulations (Table 1); it is important to choose the correct one for the application. If the ingredient is added at the mixer, and the dough or batter will be processed fairly quickly, then rapid dissolution is wanted and No. 3 (fine powdered) or No. 1 (powdered) would be appropriate. At the other extreme is the inclusion of soda in a dry mix (eg, cake, cake doughnut) that is packaged and stored for a period of time before use. In this case thermal decomposition of the dry powder becomes a factor. Grade No. 1 (powdered) decomposes 50% faster than grade No. 5 (coarse granular) in this situation. A loss of 2 to 4% per week at 50°C (122°F) and 0.5 to 1% at 30°C has been reported. A common practice is to add 5 to 10% more than needed when the dry mix is blended, in the hope that when the mix is used (with uncontrolled variables of age and storage temperatures), the amount of soda will be adequate to perform as expected. It would be preferable to use grade No. 4 (granular) or No. 5 (coarse granular) for this application and reduce the amount of overage in the dry mix.
Recently this material has become commercially available as a substitute for sodium bicarbonate; it is functionally equivalent but lowers the sodium content of the finished product. As shown in Table 1 the granulation grade currently offered is intermediate between No. 4 (granular) and No. 5 (coarse granular) sodium bicarbonate. The effect on pH and the reactions to C02 are the same. Because the molecular weight is greater (100.11 for KHC03 vs 84.01 for NaHC03), 19% more is required to get the same effect; 10 lb of sodium bicarbonate is replaced with 11.9 lb (11 lb 14-1/4 oz) of potassium bicarbonate.
A review of bicarbonates for leavening has been published (7).
The traditional acidulants for baking were vinegar (acetic acid), lemon juice (citric acid), cream of tartar (potassium acid tartrate), or sour milk (lactic and acetic acids). In all cases the acid reacted with the soda as soon as they were mixed and the baker relied on batter viscosity to retain the C02 until the cake or cookie was baked in the oven. In 1864 monocalcium phosphate hydrate was patented for use in making a commercial baking powder. Later (ca 1885) sodium aluminum sulfate began to be used; this compound has a low solubility in water at room temperature and essentially none in the aqueous phase of a batter, so it does not release C02 until late in the baking cycle. The combi
Table 1. Sodium Bicarbonate Granulation
Grade number Cumulative % retained, minimum-maximum
Table 1. Sodium Bicarbonate Granulation
Grade number Cumulative % retained, minimum-maximum
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