Chemical Engineering in Daily Life - Question 4 - Standard Answers Available
Thread Content
This post was last edited by yjqin1 on 2015-8-20 at 18:05. In the 1970s, there was a slogan: Advance the revolution while boosting production; bake good steamed buns and use proper amounts of alkali. The trade name for sodium carbonate is pure soda (a name given by the Tianjin Soda Factory in the early days); it is commonly referred to as soda, and in the north it is also called alkali powder ； Sodium bicarbonate is commonly known as baking soda; people in the north call it alkali flour. When I was young, I would watch my mom mix dough, let it rise, and then knead it to make steamed buns (I have a good eye for observation; otherwise, how could I become a scientist?) ). In winter, the dough doesn’t rise well; mom adds some baking soda to it, and the steamed buns turn out okay. In the summer, mom would knead the dough before going to work; by the time she came home at noon to cook, it had turned into a large batch full of bubbles and had an acidic taste. At that point, mom would add soda (alkali) to it, and the steamed buns and baked bread made from this dough were extremely delicious. Unafraid of offending anyone; it’s tastier than the steamed buns from Qingdao. When I’m abroad, based on my memories of my mother’s cooking back then, I steam steamed buns once or twice every seven or eight years. But once things get out of control, I don’t know what to do. I didn’t buy soda abroad; I only bought baking soda. The dough rose too much this time; everything wasn’t working as it should. My wife said to add some baking soda, just a little more. I didn’t listen and went around the supermarket again, but there was no soda. Adhering to the principle of not giving in easily, I wanted my wife to know that adding baking soda didn’t work, so I deliberately added more of it. (50 words omitted) As a result, the first batch of steamed buns turned yellow and didn’t rise – how could that be? With the perseverance of a chemist, I boiled a second pot of steamed buns, using less baking soda this time. As a result, a large pot of plump, round steamed buns was produced, which tasted delicious and had a sweet flavor. The above is the background. Question: The pH of a sodium carbonate solution is around 11.5, while the pH of a sodium bicarbonate solution is around 7.7. The pH of dough after fermentation is around 4. Why can steamed buns be made using either sodium carbonate or sodium bicarbonate? What is the principle behind this?
During the fermentation process, anaerobic respiration takes place within the dough; over time, this leads to the accumulation of acidic substances. Therefore, soda (with a pH of 11.5) is used to neutralize the pH and bring it to a neutral level, resulting in delicious dough when steamed； Adding sodium carbonate can be considered a two-step reaction: first, sodium bicarbonate is formed, and then carbon dioxide is produced. The above are my personal opinions.
Since they are all experts and no one dares to speak, I’ll take the liberty of saying something. I know a little about this: there are two methods for preparing the dough for steamed buns. First, as for the ingredients, the main component of flour is starch, right? The main active components in yeast are yeast cells, as well as probiotics such as lactic acid bacteria; there’s also phytase. One of the most traditional methods for making steamed buns with leavened dough is to use yeast that has been activated. The yeast and starch undergo fermentation, producing carbon dioxide, which causes the dough to expand. Of course, biochemical reactions proceed slowly and require an appropriate temperature in order for microorganisms to grow and carry out these reactions. If the leavening process isn’t controlled properly, an acidic taste may develop, possibly due to lactic acid or acetic acid. In such cases, a little soda or baking soda can be added to neutralize the acidity; this also helps produce more carbon dioxide, which is beneficial for the leavening process ； Another method is to use soda or baking soda to leaven the dough; carbon dioxide is what makes the dough rise, creating more air pockets during steaming. There’s one more thing: the sweetness must be maltose converted by fungi.
Standard answer: Good steamed buns can be made using both baking soda and soda ash. After the dough ferments, carbonic acid (carbon dioxide) and some carboxylic acid compounds are produced. As an acid, carbonic acid has pKa1=6.2 and pKa2=10, while the pKa of a typical monobasic carboxylic acid is around 4.8 (with formic acid having a value of 3.8). For dibasic carboxylic acids, both pKa1 and pKa2 lie between 4.3 and 5.3 (with oxalic acid being an exception, having pKa1=1.2 and pKa2=4.2). Therefore, under acidic conditions, carbonic acid exists almost entirely in the form of carbon dioxide; when its saturated vapor pressure exceeds 1 atm, it escapes and forms many bubbles. When sodium carbonate is added, it acts as a salt of a strong acid and a weak base; the pH of its aqueous solution is typically around 11.5. As a result, it reacts with the organic acids present in the dough to produce sodium salts of organic acids and carbon dioxide. This reaction occurs more vigorously at the high temperatures used during steaming, resulting in porous steamed buns. The sweetness of steamed buns comes from the products of fermentation. When sodium bicarbonate is added, it acts as a salt of a strong acid and a weak base; the pH of its aqueous solution is generally around 7.8. Therefore, it reacts with the organic acids present in the dough to produce sodium salts of organic acids and carbon dioxide. This reaction occurs more intensely at the high temperatures used during steaming, resulting in porous steamed buns. The sweetness of steamed buns comes from the products of fermentation. Please note that when using sodium bicarbonate instead of sodium carbonate, a larger amount is required. This is because sodium bicarbonate decomposes when heated: 2NaHCO3 → Na2CO3 + H2O + CO2; therefore, 2 moles of sodium bicarbonate are required to have the same effect as 1 mole. That’s when I got confused right here, and I even had a argument with my wife. Later, while working on projects in China and participating in a project related to the thermal-humid decomposition of mixed solutions of sodium carbonate and sodium bicarbonate at a natural soda ash plant, I came to understand this principle.
One more thing to add: In the past, it was also very cold indoors in the countryside during winter, so the dough wouldn’t rise. In this case, the mothers also need to make leavened steamed buns. Especially when guests come unexpectedly. Just add some baking soda to the dough that has been mixed already. The steamed buns that are made turn out a bit fluffy, but they turn yellow and have an alkaline taste. The fluffiness comes from the thermal decomposition of sodium bicarbonate, 2NaHCO3 → Na2CO3 + H2O + CO2, while the alkaline taste is due to the sodium carbonate produced as a by-product of this decomposition.