New Year's secrets: Harmony brings wealth, and peace brings life

28. Let the dough rise. The yeast gets angry and the buns swell up.

[○･｀Д´･ ○]

Written by Li Qingchao (Shandong Normal University)

Time flies so fast, and in a blink of an eye, the Chinese New Year is coming in a few days. It feels like a dream, and I am really afraid that the Chinese New Year holiday will be over when I wake up (Ĭ ^ Ĭ). It has been several years since my supervisor taught me how to knead dough: the dough should be smooth, the bowl should be smooth, and the hands should be smooth, without any sticky dough; the dough should also be smooth, with a flat and smooth surface. It can be seen that I am not a child who likes to work at home. This kind of common sense of life was not learned until I was a doctoral student from a virologist (someone: PhD supervisors also need to teach how to knead dough? Are the requirements for PhD supervisors so high now?).

Today is the 28th day of the twelfth lunar month. According to folk customs, dough is kneaded and fermented on this day, and steamed buns are steamed the next day, which symbolizes getting rich and thriving, and people hope that the coming year will be full of money and a prosperous life. So, today I would like to answer a question for everyone, please feel free to ask:

How did the noodles rise?

Figure 1: Shandong Intangible Cultural Heritage: Jiaodong Flower Cake

Source: http://www.jdxzhuabobo.com/

Fermentation

Steamed buns and dumplings need to be fermented, but dumplings and noodles do not need to be fermented. Everyone can feel it when eating noodles. Fermented noodles are soft and delicious, with many pores in the noodles; while dumpling wrappers or noodles are chewy, without pores, and look relatively transparent. What causes the difference between the two? Unfermented noodles are called "dead noodles", and fermented noodles are called "fermented noodles". What makes the noodles not "dead" but fermented?

The key here is whether "starter" is added when kneading the dough, also known as yeast or old dough. Different dialects call it differently, but it is actually the same thing, which is the small dough left over from the last steamed buns. Because it has a little history, starter appears to be darker, and you may be able to pull strings when you dig it open. It smells a bit like wine or sour. In essence, starter is dough with yeast strains. If you don't steam buns often at home, you can directly use fresh yeast blocks or dry yeast powder. Extracting yeast is fresh yeast, and after drying and granulation, it becomes dry yeast. Fresh yeast is a fresh yeast block in an active state, with a short storage time and suitable for refrigeration, but it has strong fermentation ability; yeast powder is a freeze-dried yeast powder in a dehydrated dormant state and has a long storage time. In terms of the principle of dough fermentation, starter, fresh yeast, and dry yeast are all the same: after the yeast is inoculated into the dough, it grows and reproduces in the dough, producing carbon dioxide gas. Therefore, after the dough ferments, the volume of the dough becomes larger and pores can be seen. During the steaming and baking process, carbon dioxide gas overflows and expands, creating more and larger pores, making the pasta soft and delicious (baking soda NaHCO3 can also achieve a similar effect, which is not discussed in this article).

Figure 2: The starter (upper left) that has been fermenting for a long time, the fresh yeast block (upper right), the dry yeast powder (lower left), and the fermented dough. 丨Picture from the Internet

yeast

In daily life, the word yeast generally refers to the bacteria used for dough and alcohol fermentation. In microbiology, yeast refers to single-cell fungi. Currently, more than 1,500 species of yeast have been discovered, accounting for 1% of the known fungal species [1] (fungi also include molds that mainly exist in the form of hyphae, and mushrooms that can produce large fruiting bodies). Humans have been using yeast to make wine, bake, and make pasta for a very long time. As early as the Neolithic Age, humans were already able to make wine.

However, a true understanding of these fermentation processes had to wait until 1857.

At that time, French scientist Pasteur discovered that alcohol fermentation is not a simple chemical reaction, but is caused by biological participation. The little cutie that plays a role in this process is mainly brewer's yeast (Saccharomyces cerevisiae), which is also known as beer yeast and baker's yeast. It is also commonly used in Chinese steamed buns (if it was discovered by the Chinese, it might be called "white wine yeast" or "baking yeast"). Unless otherwise specified, the term "yeast" generally refers to single-cell fungi in a broad sense, or brewer's yeast in a narrow sense.

Figure 3. Colonies and names of various yeasts [2]

The earliest yeasts were isolated from grape skins (most yeasts prefer an environment with high sugar content). The colonies are off-white, moist, and have a wine aroma. Yeast is a eukaryotic single-cell organism, very small, about 5-10 microns (about 150 yeasts can be placed in one millimeter), and spherical or ellipsoidal.

Figure 4. Observation of brewer's yeast under an optical microscope | Source: Wikipedia

The surface of yeast is covered by its cell wall and is relatively smooth, but under an electron scanning microscope, some round scars can be seen, which are the traces left by brewer's yeast after budding reproduction. Unlike most bacteria or human somatic cells that undergo equal binary fission, during the division process of brewer's yeast, the daughter cells are relatively small, like small buds growing from the mother cell. When the bud is large enough to separate from the mother cell, the pedicle scar or bud scar is formed at the break.

Figure 5. The morphology of brewer's yeast under an electron microscope | Source: Wikipedia

Fermentation

So what did this chubby little cutie do in the dough?

To put it simply, they are "angry"! If they are not angry, the buns will not swell.

Technically speaking, it is "fermented". Yeast breaks down sugars under anaerobic conditions to produce carbon dioxide and alcohol. This process is part of yeast metabolism, the purpose of which is to obtain energy, reducing power and certain compounds necessary for life processes.

The term "fermentation" is commonly used in life and in the field of microorganisms. In a broad sense, fermentation refers to the process by which people use the life activities of microorganisms under aerobic or anaerobic conditions to prepare microbial cells themselves, direct metabolites of microorganisms, or secondary metabolites of microorganisms.

Figure 6. Yeast breaks down sugars to release energy, produce alcohol and carbon dioxide**[3]**

So, how does yeast ferment sugar into alcohol and carbon dioxide?

In 1897, German chemist Eduard Buchner ground yeast until there was no living yeast left. He found that this extract without living yeast could also break down sugar into alcohol and carbon dioxide. (Buchner didn't know that a Japanese company later made a lot of money by grinding yeast. Their yeast extract was called "magic water" when it came to China.)

Buchner called this substance contained in yeast Zymase and won the 1907 Nobel Prize in Chemistry[4].

Now we know that alcoholic fermentation is actually the result of the combined action of a series of enzymes, and alcohol fermentation is actually a mixture of enzymes.

Enzyme is a biological macromolecule with catalytic effect, usually protein. Enzyme has another ancient name: enzyme, which we have abandoned. Pay attention to the blackboard and highlight it: the simplified Chinese text that repeatedly uses "enzyme" is misleading, please be aware of it.

So what enzymes are involved in the fermentation process and what chemical reactions are catalyzed? This is one of the most difficult parts of the entire undergraduate study at the School of Life Sciences: biochemical metabolism (in order to avoid this part, I chose a postgraduate major that does not require biochemical metabolism. Later I found that I had completely forgotten that microbiology also taught this part until I needed to teach this part.ლ(ٱ٥ٱლ)).

Whether it is starch or sucrose, any sugar must be converted into glucose and enter the glycolysis pathway for degradation. Glycolysis is a metabolic pathway that converts glucose C6H12O6 into ① pyruvic acid CH3COCOOH. The glycolysis process releases free energy, which is phosphorylated at the substrate level to form ② high-energy molecules: adenosine triphosphate (ATP). At the same time, the glycolysis process also generates ③ "reducing power": reduced nicotinamide adenine dinucleotide (NADH+H+).

Glycolysis occurs in the cytoplasm and does not require oxygen. The most common glycolysis pathway is the EMP pathway (Embden–Meyerhof–Parnas, the name of the three main scientists who discovered this process), which requires ten enzymes and ten steps.

Figure 7. The overall reaction and specific process of glycolysis | The author adapted from Wikipedia

The ATP produced by glycolysis can provide the necessary energy for life activities, while pyruvate and reducing power have different arrangements. Under aerobic conditions, pyruvate enters the tricarboxylic acid cycle, decomposes, and produces carbon dioxide. In this inheritance, in addition to the energy produced by phosphorylation of the afternoon bill, more reducing power (NADH+H+ and FADH2) is also produced. Reducing power produces more ATP through the process of oxidative phosphorylation, and hydrogen ions eventually combine with oxygen to produce water. Under anaerobic conditions, pyruvate in yeast produces carbon dioxide and acetaldehyde through pyruvate decarboxylase. As an endogenous organic electron acceptor, acetaldehyde can receive the "nowhere to put" NADH+H+ reducing power and is eventually reduced to ethanol.

This process in which NADH+H+ produced in the glycolysis reaction under anaerobic conditions combines with endogenous organic electron acceptors and produces energy only through substrate-level phosphorylation is called fermentation, which is the narrowest sense of "fermentation".

Tips

Adenosine triphosphate (ATP) is the most widely used "energy currency" in all known life, directly providing energy for life activities. There are two ways to produce ATP. The first is substrate level phosphorylation, in which the energy of compounds containing high-energy phosphate bonds is directly transferred to ATP with the participation of enzymes. The second way is that ATP synthase uses proton motive force (PMF) energy to produce ATP in a chemical osmotic coupling manner (the concentration of hydrogen ions on both sides of the plasma membrane is different, and when hydrogen ions flow back from the high-concentration side to the low-concentration side, ATP synthase is driven to synthesize ATP). The source of proton motive force is mainly formed by the electron transport chain driven by biological oxidation processes and photosynthesis.

Conclusion

Wow, although yeast is just "angry" and produces a little carbon dioxide gas, I didn't expect that such a complicated process goes on inside the cell. In fact, in addition to its wide and long-standing application in the food field, yeast is also an important model organism, playing an important role in the research of cytoskeleton, cell cycle, meiosis and other fields. This is because yeast is almost as easy to raise as E. coli (the star bacteria of genetic engineering, which has sent away many master's and doctoral students!), but yeast is closer to humans than E. coli because it is a eukaryotic organism.

Figure 8: Yeast is a eukaryotic organism | Lallemand Brewing

In addition, yeast is also an important microbial engineering bacteria. For example, hepatitis B vaccine is produced using yeast.

Drink some wine with steamed buns, thank you yeast!

Wait, there are also types of yeast that can cause diseases. The most common pathogenic yeasts are Cryptococcus, Torulopsis, and Candida (Candida), which usually cause opportunistic infections or infect patients with weakened immune systems (I won’t post the pictures, I wish you all a happy new year).

References

[1] https://en.wikipedia.org/wiki/Yeast

[2] Alexander N. Glazer, Hiroshi Nikaido, MICROBIAL BIOTECHNOLOGY, Fundamentals of Applied Microbiology, Second Edition

[3] https://socratic.org/questions/how-do-the-products-of-fermentation-in-animals-differ-from-yeast

[4] https://en.wikipedia.org/wiki/Zymase

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