Your favorite/most hated sour food hides the secret of how life obtains energy

Food that is not stored properly can easily go bad and cause diarrhea if eaten.

If you don’t take a shower, you will be sour and smelly.

What makes food sour? How do microorganisms produce acid? Why do humans taste sour?

Written by Li Qingchao (Shandong Normal University)

If food is not stored properly, it will easily go bad and produce an unpleasant sour taste. However, sourness is not necessarily a bad thing. People also like to make sour foods, such as yogurt, pickles, vinegar, etc. Artificial fermentation can be traced back to yogurt in North Africa in 10,000 BC[1], and evidence of winemaking in China as early as 7,000 BC[2].

What causes food to go sour? One of the most famous examples of food spoilage or fermentation in history is the sourness of wine. According to the pre-Qin Han Feizi, Yang Qian, a Song Dynasty scholar, proposed the theory of "fierce dogs and sour wine". He said that the dogs at the door of the wine shop were too fierce, so people were afraid to buy wine and the wine could not be sold and turned sour.

Will wine turn sour after being stored for a long time? People generally believe that the longer wine is stored, the more fragrant it becomes. What is going on?

Microbial fermentation produces organic acids

In 1856, French scientist Louis Pasteur was asked by a student's parent to study the problem of wine turning sour. Pasteur proposed that the fermentation process was caused by living microorganisms (fermentation theory). Pasteur used a microscope to find that round microorganisms (yeast) existed in normal wine fermentation, while smaller rod-shaped microorganisms (bacteria) existed in abnormal fermentation. Therefore, abnormal fermentation, such as wine turning sour, was caused by contamination by "bad" microorganisms. Based on these theories, Pasteur invented pasteurization (short-term heating) to kill these microorganisms, which could prevent the wine from continuing to ferment and turn sour, thereby extending the storage period.

Now we know that bacteria and yeast produce different metabolites after degrading sugars. Yeast fermentation produces alcohol, while bacteria fermentation can produce alcohol and organic acids such as lactic acid and acetic acid. This is related to the metabolic pathway of microorganisms. In other words, long storage time is not the reason why wine turns sour, and a sour body odor is not caused by not taking a shower. The real culprit is the fermentation of specific microorganisms.

Figure 1 Pasteur studied the problem of wine turning sour. Normal fermentation is caused by yeast, while abnormal fermentation is caused by bacteria.

Now we know that microorganisms can cause food to spoil. In the process of food spoilage, sourness is an early phenomenon that can be detected. Its mechanism is that organic acids are produced in the metabolic process of microorganisms breaking down sugars to release energy. We can inhibit the proliferation of microorganisms through low-temperature storage, preservatives, pickling, drying, etc., and heating can kill microorganisms to achieve the purpose of storing food.

Figure 2 Organic acids produced by microbial metabolism. Acetic acid or lactic acid produced by bacteria is the main reason why food becomes sour, while the source of sweat odor is mainly propionic acid produced by bacteria. [3]

However, if we want to get to the bottom of it, don't organisms produce carbon dioxide and water when they decompose sugars? Why do they produce organic acids?

Organic acids are intermediate products of biological oxidation

The process of organisms completely oxidizing glucose to produce energy is similar to the chemical reaction of glucose combustion, so the decomposition of sugar will produce carbon dioxide and water.

However, life is not a fire. The decomposition of sugars does not burn them all, but produces energy and intermediate metabolites. The energy circulates in the form of ATP (adenosine triphosphate) and can be used for various life activities; while the intermediate metabolites can be used to synthesize other important substances, such as amino acids, ribose, fat, etc.

Figure 3 ATP is the universal energy “currency” of living things.

Unlike violent redox reactions such as combustion and explosion, biological oxidation occurring in living organisms is a step-by-step, controllable, relatively slow chemical reaction catalyzed by enzymes.

Figure 4 Unlike chemical reactions such as combustion, biological oxidation is step-by-step and controllable.

In the redox reaction, the reducing agent loses electrons while the oxidizing agent gains electrons. The gradual oxidation process of organic matter can be intuitively understood as the process of oxygenation and dehydrogenation. In biological oxidation, these processes are fully developed and stripped: the oxidation process of organic matter requires multiple steps of oxygenation, dehydrogenation, and decarboxylation. These are urgently transferred and removed.

Where is the acid? Glucose produces pyruvate through glycolysis. After pyruvate is converted into acetyl-CoA, it enters the tricarboxylic acid cycle (TCA, or citric acid cycle) to produce a large number of organic acids. The intermediate metabolites of the tricarboxylic acid cycle are very important to organisms. Among them, citric acid is known as the first edible acidulant and has a wide range of uses in the food industry. It is the world's largest organic acid produced by biochemical methods (fermentation), with an annual output of millions of tons. It is one of the pillar products of the fermentation industry [4].

Figure 5 Krebs cycle.

The transfer and transmission of hydrogen ions is accompanied by the gain and loss of electrons (in the respiratory chain or electron transport chain). The electron transfer drives the transport of hydrogen ions against the concentration gradient, generating proton potential energy (concentration gradient) on both sides of the membrane (including the inner membrane of mitochondria in eukaryotes or the plasma membrane of prokaryotes). The reflux of hydrogen ions driven by proton potential energy drives ATP synthase to synthesize ATP. This process of transferring [H] and electrons through the respiratory chain and coupling ATP synthesis is called oxidative phosphorylation. The metabolic pathway containing oxidative phosphorylation is called respiration (Cellular respiration, note that it is not the "breathing" of exhalation and inhalation).

Figure 6 The oxidative phosphorylation process is a process in which [H] and electrons produced by redox reactions gradually release energy and generate proton potential energy during the transfer process, thereby driving ATP synthase to synthesize ATP.

The TCA cycle is a widespread metabolic pathway, so why aren't all our foods acidic? The reason is that the intermediates in the TCA cycle generally do not accumulate in large quantities.

In order to achieve production goals, in the fermentation industry, it is often necessary to increase the activity of synthases and reduce the activity of degradative enzymes in order to accumulate specific organic acids. For example, when producing citric acid, Aspergillus niger is mainly used, which can increase the activity of citrate synthase tenfold, while the activity of other enzymes that degrade citric acid (aconitase, isocitrate dehydrogenase) is reduced. However, the increase in citric acid is more likely due to enhanced biosynthesis rather than inhibition of degradation. In addition, pyruvate carboxylase, which converts pyruvate into oxaloacetate, is also a key enzyme in citric acid production.

So how does acid accumulate under non-production conditions?

Organic acids are the end products of fermentation

Life does not produce or destroy elements, they are just transit stations for material metabolism; life does not produce energy out of thin air, they are just converters, storers and consumers of energy. Similarly, the [H] produced during the decomposition of sugars will not disappear out of thin air, it needs a place to go. In the process of oxidative phosphorylation, [H] can eventually be given to oxygen to produce water, a process called aerobic respiration. In the absence of oxygen, [H] can also be given to oxidizing inorganic substances such as nitrates and sulfates, or to exogenous fumaric acid, this type of respiration is called anaerobic respiration.

What should we do if [H] does not enter the respiratory chain and cannot be transferred to oxides through the respiratory chain?

Not allowing acetyl-CoA to enter the tricarboxylic acid cycle will produce more [H], and the [H] produced during glycolysis (glucose is degraded into pyruvate) will be returned to the intermediate metabolites. The latter metabolic pathway that returns [H] to endogenous organic matter is called fermentation (in a narrow sense). Here, fermentation means that there is no oxidative phosphorylation catabolic energy-generating process, and only substrate level phosphorylation exists (another new concept, which refers to the direct transfer of phosphate groups from substrates to ADP or GDP to produce ATP or GTP without the need for ATP synthase).

In the process of sugar decomposition to produce pyruvate, [H] and ATP are produced. If [H] does not enter the respiratory chain, it will find a way to return it to pyruvate or the metabolites of pyruvate.

Pyruvate accepts [H] and is reduced to produce lactic acid.

Figure 7 Reduction of pyruvate to lactate.

In some glucose decomposition processes, instead of producing two pyruvic acids, it is split into four-carbon sugar (later converted into five-carbon sugar) and acetyl phosphate (HK pathway), or decarboxylated to produce five-carbon sugar and then split into 3-phospho-glyceraldehyde (later converted into pyruvic acid) and acetyl phosphate (PK pathway). Pyruvic acid can produce lactic acid, while acetyl phosphate can accept [H] to produce ethanol, or be oxidized to produce acetic acid.

Figure 8 Heterolactic fermentation, which produces lactic acid and other products.

If pyruvate is decarboxylated to acetaldehyde and then reduced, it becomes ethanol.

Figure 9 Alcohol fermentation: pyruvate produces acetaldehyde and then accepts [H].

Under oxygen-rich conditions, ethanol can continue to undergo incomplete oxidation metabolism and be oxidized to acetic acid under the action of acetic acid bacteria: ethanol is first oxidized to acetaldehyde by alcohol dehydrogenase, then acetaldehyde is hydrated to form acetaldehyde hydrate, and finally the latter is acted on by acetaldehyde dehydrogenase to form acetic acid. In addition, pyruvate can be reduced to propionic acid under the metabolism of propionibacterium.

It seems that organic acids are a type of substance that is easily produced during microbial fermentation. Why do humans need to perceive the existence of this substance?

The evolutionary significance of sour taste

Taste can bring survival advantages in evolution. By pursuing pleasant tastes and avoiding unpleasant tastes, species can gain survival advantages. For example, the much-loved sweet taste represents sugar and energy, while the unpleasant bitter taste often indicates toxicity. There is relatively little discussion about sour taste. This is because the preference or aversion to sour taste is not absolute. It depends on the degree of sourness and the combination with other tastes.

Sour taste is caused by organic acids (including lactic acid, citric acid, malic acid and acetic acid) and inorganic acids (such as hydrochloric acid, nitric acid and sulfuric acid). For vertebrates, the taste of sour is quite conserved in evolution. Studies have found that almost all vertebrates - including primitive fish such as lampreys - have a sour taste. In contrast, some birds have lost their sweet taste, and whales cannot taste bitterness.

For fish, sour taste can remind them to pay attention to the acidity and alkalinity of the water. For other animals, the information represented by sour taste is relatively complex. Sour fruits often represent immaturity, but they also represent rich vitamin C. The effect of sour taste in preventing animals from eating unripe fruits is far less than astringency and bitterness.

Another meaning of acid is fermentation. Food fermentation brings two-way effects, depending on the type of food and the strain of bacteria that causes fermentation. Sour fermented products mixed with foul smells and moldy smells generally mean toxic metabolites. On the other hand, human civilization has also mastered the skills of artificial fermentation and production of sour delicacies. The reason why people prefer relatively pure sour fermented products is that fruits and vegetables fermented mainly with yeast and lactic acid bacteria often make food better - more free amino acids and vitamins are produced after fermentation, and fiber and certain plant toxins are also decomposed. At the same time, organic acids are produced, the pH value decreases, and the growth of harmful bacteria is also inhibited. In addition, the presence of alcohol makes fermented fruits more popular. And jungle animals also know these secrets.

Figure 10 New Zealand Animal of the Year 2018: A drunk New Zealand wood pigeon hanging upside down in a tree.

The weather is hot, so be careful with your diet.

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