XV. LEAVENING AGENTS A leavening agent is a substance which, when put into a dough, usually forms carbon dioxide. This gas "lightens" the dough. Yeast is the oldest leavening agent. It changes starch and sugar to carbon dioxide and alcohol. Baking soda is sodium bicarbonate, NaHCO3. It may be used in dough with some substance that contains an acid, such as sour milk or molasses. Baking powder is a mixture of powdered sodium bicarbonate with a powdered acid or acid principle, such as tartaric acid or an alum, with starch to keep the mixture dry. EXPERIMENT 54 Products of Yeast Fermentation MATERIALS. Yeast cake (compressed), molasses, limewater. APPARATUS. 250 cc. flasks, test tube, delivery tube, distilling flask, condenser, thermometer, liter beaker. A. Carbon Dioxide. Note: Two students work together. 1. Mix about one fourth of a cake of compressed yeast with 15 cc. of water in the evaporating dish. Stir till a smooth mixture is formed. Pour the yeast mixture into a flask containing 25 cc. of molasses and 100 cc. of water. 2. Note the odor and taste of the mixture. 3. Fit a one-holed stopper and a delivery tube to the flask containing the mixture. Let the other end of the delivery tube dip into a flask containing limewater. 4. Put the flasks in the sun or a warm place. Begin Experiment 55. 5. At the end of two hours examine the liquid and limewater. Is the yeast working? What gas caused the change in the limewater? Write the equations and name all sub stances: CO2+Ca(OH)2 - CaCO3+H2O 6. What effect has a very low temperature upon fermentation? Why are fresh vegetables, fruits, milk, and butter kept in a refrigerator? 7. Boil 10 cc. of the fermenting yeast mixture. Cool again to room temperature. Does fermentation continue? Why? Why do jars of canned fruit sometimes ferment? If these fruits are reheated soon after fermentation begins, they may be used. Why? B. Alcohol. 1. At the end of 12 hours examine the mixture. Has fermentation ceased? Why? 2. Note the odor and taste of the mixture. What new substance is present? The sugar in the molasses is changed to alcohol and carbon dioxide. Write the equation and name all substances. C6H12O6+yeast-2 C2H5OH+2 CO2 Note: Empty the mixture in your flask into the liter beaker provided. The teacher will distill half of this for alcohol, the other half should be labeled and set aside for a week or more for part C. 3. (Instructor's experiment) : Distill about 500 cc. of the fermented molasses, using a water bath. Collect the fraction that comes over below 79° C. What is this distillate chiefly? 4. Note the odor of the distillate. 5. Apply a lighted match to 1 cc. of it. Does it burn? 6. What is formed when fruit juices containing sugar ferment? Where does the yeast that causes the fermentation come from? 7. How does yeast leaven bread dough? C. Vinegar. 1. Note the odor and taste of the yeast molasses mixture that has been allowed to stand for several weeks. What is the substance? 2. How could vinegar be made at home from fruit parings? 3. What is "sweet cider"? What is "hard cider"? What is "cider vinegar"? MATERIALS. EXPERIMENT 55 Baking Soda, Baking Powder Dilute hydrochloric acid, sulfuric acid, sodium bicarbonate, molasses, sour milk, powdered tartaric acid, cream of tartar, acid calcium phosphate H,Ca(PO4)2, ammonium alum, any baking powder (composition unknown to student), 6 common baking powders with labels, icdine solution, barium chloride solution, ammonium molybdate solution. APPARATUS. Test tubes, beakers, funnel, filter paper. A. Baking Soda (Sodium Bicarbonate, NaHCO3). 1. Put 15 cc. of dilute hydrochloric acid in a beaker. Add 1 gram of sodium bicarbonate. What gas is evolved? Write the equation, naming all substances: HCI+HNaCO3 -NaCl+CO2+H2O 2. Dissolve 5 cc. of molasses in 15 cc. of water in a beaker. Add 1 gram of sodium bicarbonate. Result? What is the acid in molasses? 3. Repeat (2), using sour milk. Result? What is the acid in sour milk? 4. How is "soda" used to leaven a dough? B. Baking Powders. 1. Tartrate baking powders. a. Make a tartrate baking powder by mixing a gram of tartaric acid (H2C4H4O6) with a gram of sodium bicarbonate (NaHCO3). Add 15 cc. of water. Result? What is the gas evolved? Write the reaction and name all substances: H2C4H4O6+2 NaHCO3 NaзC4H4O6+2 H2O+2 CO2 b. Make a tartrate baking powder by mixing cream of tartar (acid potassium tartrate, HKC4H4O6) with the soda. Add water as before. Result? Write the equation and name all substances: HKC4H4O6+HNaCO3 → KNaC4H4O6+H2O+CO2 2. Phosphate baking powders. Repeat B, 1, using acid calcium phosphate, H1Ca(PO4)2, with the soda. Write the equation and name all substances: H1Ca (PO4)2+2 HNaCO3 →→→→ HNа2PO4+HCaPO4+2 H2O+2 CO2 3. Alum baking powders. Repeat B, 1, using, instead, powdered ammonium alum, NHAI (SO4)2, with the soda. Write the equation and name all substances: 2 NHAI (SO4)2+6 HNaCO3 →→ 2 Al(OH)3+ (NH4)2SO4+3 Na2SO4+6 CO2 4. Why is baking powder such a useful leavening agent? 5. Which baking powder is considered most efficient of the three mentioned? Why? C. Tests for Baking Powders. A baking powder may be a tartrate baking powder, or an alum powder or a phosphate powder or a mixture of two or three of these powders. Test an unknown baking powder to determine its nature as follows: 1. Put 15 grams of the baking powder into a beaker and pour over it 50 cc. of water. Stir until no more gas is evolved, then filter carefully. 2. Starch is insoluble in cold water and will remain on the filter paper as a white residue. Make the usual starch test on this residue. Result? 3. If the baking powder contains alum, the filtrate will contain sulfates. To 5 cc. of the filtrate add 5 cc. of hydrochloric acid and 5 cc. of barium chloride solution. A white precipitate indicates sulfates. Is the sample an alum baking powder? 4. If the baking powder is a phosphate baking powder, the filtrate will contain acid calcium phosphate. Test 5 cc. of the filtrate for a phosphate by adding a few drops of nitric acid, then heat nearly to boiling and add a few drops of this hot mixture to 5 cc. of ammonium molybdate solution. A |