A leaven or leavening agent is something which will lighten a mixture. Flour mixtures are usually leavened by steam, by evolution of carbon dioxide, or by the expansion of inclosed and heated air. White of egg or cream may be said to be " leavened ' when " whipped," in which case the leavening 180 LABORATORY MANUAL OF FOODS AND COOKERY consists simply in the mechanical incorporation of air bubbles. In some bakeries (especially in England) dough is leavened by incorporating air under pressure, which when the pressure is released will expand even without application of heat. When leavening is to be accomplished by means of carbon dioxide the latter is usually produced either : (1) by yeast fermentation, producing carbon dioxide from a portion of the carbohydrate of the mixture ; (2) by interaction of sodium bicarbonate (" cooking soda ") with an acid substance (either free acid or acid salt) in the presence of moisture. Sour milk, buttermilk, vinegar, molasses, and some fruit juices contain free acids. Many fruit juices contain acid salts. Acid salts used as such to liberate carbon dioxide from sodium bicarbonate - - in other words used as the " acid constituent ' of baking powders - - are chiefly : (a) acid potassium tartrate (cream of tartar), (b) acid phosphate, (c) some form of alum or aluminum sulphate ; hence the classification of baking powders in three main groups - - the " tartrate," the " phosphate," and the " alum " powders. EXPERIMENTS WITH LEAVENS AND LEAVENING AGENTS Egg, air, steam 169. - - Measure the unbeaten white of an egg in a tablespoon. Beat with a Dover egg beater and measure in a tablespoon. Result? 170. - - Whip an unbeaten egg white with a wire whisk. What is the difference in volume? To what is this due? Notice the dif- ference in size of air spaces where Dover beater is used as compared with wire whisk. 171. - - Perform 169 and 170 each with an egg yolk. Why is the increase in volume less noticeable ? FLOUR MIXTURES 181 What is the marked difference in the composition of the egg yolk as compared with the egg white? 172. - - a. Use the unbeaten egg to make popovers. b. Use the beaten egg to make sponge cake. c. Use the whipped egg to make Foamy Omelet. Compare texture of each product. Explain. d. Which egg white at time of using contains most water? Sodium bicarbonate 173. - - Have ready for this and the succeeding experiments, the following : baking soda molasses cream of tartar chocolate lemon juice coffee vinegar hot water sour milk tomato limewater Test with litmus and record reaction of each. 174. - - Carbon dioxide precipitates the lime in a solution of lime and water. This can be shown by breathing through a tube, one end of which is immersed in a beaker of limewater. The white, chalky deposit which soon appears on the bottom of the beaker is carbonate of lime from the lime of solution and the carbon dioxide of the breath. 175. - - Arrange 9 test tubes with perforated corks or rubber stop- pers fitted with a bent tube to conduct gas from test tube into beaker of limewater. In each dry test tube put \ tsp. dry baking soda. Arrange per- forated corks or stoppers with bent tube and have ready to adjust at once and conduct the gas given off into limewater (equal volume in each beaker).

AI-modernized reading of the original text

A leaven or leavening agent is something which will lighten a mixture. Flour mixtures are usually leavened by steam, by evolution of carbon dioxide, or by the expansion of inclosed and heated air. White of egg or cream may be said to be " leavened ' when " whipped," in which case the leavening consists simply in the mechanical incorporation of air bubbles. In some bakeries (especially in England) dough is leavened by incorporating air under pressure, which when the pressure is released will expand even without application of heat. When leavening is to be accomplished by means of carbon dioxide the latter is usually produced either : (1) by yeast fermentation, producing carbon dioxide from a portion of the carbohydrate of the mixture ; (2) by interaction of sodium bicarbonate (" cooking soda ") with an acid substance (either free acid or acid salt) in the presence of moisture. Sour milk, buttermilk, vinegar, molasses, and some fruit juices contain free acids. Many fruit juices contain acid salts. Acid salts used as such to liberate carbon dioxide from sodium bicarbonate - - in other words used as the " acid constituent ' of baking powders - - are chiefly : (a) acid potassium tartrate (cream of tartar), (b) acid phosphate, (c) some form of alum or aluminum sulphate ; hence the classification of baking powders in three main groups - - the " tartrate," the " phosphate," and the " alum " powders. EXPERIMENTS WITH LEAVENS AND LEAVENING AGENTS Egg, air, steam 169. - - Measure the unbeaten white of an egg in a tablespoon. Beat with a Dover egg beater and measure in a tablespoon. Result? 170. - - Whip an unbeaten egg white with a wire whisk. What is the difference in volume? To what is this due? Notice the difference in size of air spaces where Dover beater is used as compared with wire whisk. 171. - - Perform 169 and 170 each with an egg yolk. Why is the increase in volume less noticeable ? What is the marked difference in the composition of the egg yolk as compared with the egg white? 172. - - a. Use the unbeaten egg to make popovers. b. Use the beaten egg to make sponge cake. c. Use the whipped egg to make Foamy Omelet. Compare texture of each product. Explain. d. Which egg white at time of using contains most water? Sodium bicarbonate 173. - - Have ready for this and the succeeding experiments, the following : baking soda molasses cream of tartar chocolate lemon juice coffee vinegar hot water sour milk tomato limewater Test with litmus and record reaction of each. 174. - - Carbon dioxide precipitates the lime in a solution of lime and water. This can be shown by breathing through a tube, one end of which is immersed in a beaker of limewater. The white, chalky deposit which soon appears on the bottom of the beaker is carbonate of lime from the lime of solution and the carbon dioxide of the breath. 175. - - Arrange 9 test tubes with perforated corks or rubber stoppers fitted with a bent tube to conduct gas from test tube into beaker of limewater. In each dry test tube put \ tsp. dry baking soda. Arrange perforated corks or stoppers with bent tube and have ready to adjust at once and conduct the gas given off into limewater (equal volume in each beaker).

A laboratory manual of foods and cookery