The sweetness that you can't let go of, is it all the taste of sugar?

Author: Ju Peng, Zhang Ling (Kunming Institute of Botany, Chinese Academy of Sciences)

Planning: Yangmei

The article comes from the Science Academy official account (ID: kexuedayuan)

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In the hot summer, a cup of iced lemon tea or iced cola will refresh your mind with its sweet taste. But do you know how long it takes to burn off the drinks you drink for the sake of pleasure? Shouting "Burn my calories" will not work.

(Image source: pixabay, self-made)

Where does the sweet taste that makes people love and hate it come from? Where will the piece of fat in my body go? How can I enjoy the sweet taste without losing my collarbone?

Does everything that tastes sweet always taste like sugar?

Sweetness is defined in neurobiology as a response to sufficient soluble carbohydrates in the mouth. Sugar is a class of compounds, which are polyhydroxy aldehydes or ketones composed of C (carbon), H (hydrogen), O (oxygen), and their condensation products or derivatives. But not all sugars produce sweetness, and sweetness is not necessarily produced by sugar. In short, there is not a one-to-one causal relationship between sweetness and sugar.

How does the perception of sweetness come about? Humans have come up with a set of "sweetness theory" because of their own "soul-searching". To pick out the key points, a molecule that presents sweetness must be a miniature "battery structure" with positive and negative poles, with a donor group that provides protons and an acceptor group that can accept protons. There is a certain distance between the two groups, and the molecule should have a certain shape and size - if you replace protons with electrons, doesn't it look like a battery? Of course, if you insist on asking whether it is a No. 5 battery, a No. 7 battery or a button battery, I can only say, young man, you have a big imagination, and it depends on your wisdom to pursue the answer to this question.

Shallenberger sweetness molecular mechanism

With this "rough" theory, people speculate that not all sugars in nature are sweet. The most well-known sweet sugars from natural sources are glucose and fructose, which are monosaccharides with only one basic module. They are abundant in the fruits of plants and are the main sweet substances and energy sources. The soft white sugar, white sugar, brown sugar, rock sugar, etc. that we usually eat are all different forms of disaccharide sucrose with different purities or crystallization methods. They are formed by the dehydration condensation of the hemiacetal hydroxyl of one molecule of glucose and the hemiacetal hydroxyl of one molecule of fructose.

(Image source: veer gallery, pixabay)

Sugarcane and beets have a high content of sucrose, so they are often used as raw materials for sugar production and are also the most widely used natural sweeteners in the world. In ancient times when trade was not well developed, tropical crops such as sugarcane and beets were not available, so the source of sugar was different from that of today. Maltose, a disaccharide made from two molecules of glucose condensed by grains such as wheat and glutinous rice, has become an important component of sweeteners. The "Yi" in "Ganzhiruyi" refers to maltose syrup. The sweetness of maltose is about one-third of that of sucrose, and it tastes smooth and mild. It is delicious even when eaten alone. The "Dingding Sugar" sold by street vendors is this kind of sugar. There is also a kind of lactose in the disaccharide, which is a unique sugar substance in milk. It is even sweeter and is formed by the condensation of galactose and glucose. It helps the body absorb calcium. In general, the food industry will choose a mixture of glucose, maltose, etc. made by incomplete hydrolysis of starch, that is, invert syrup, for large-scale food addition.

Of course, "honey" produced by the hard work of bees was also an important sweetener in ancient times. It is a mixed sugar source of sucrose, glucose, fructose, etc. secreted by the "nectar glands" of plant flowers. After being collected by bees, enzymes are injected to decompose the sucrose into glucose and fructose, which is concentrated and processed. It can be regarded as a mixed sweetener processed by animals.

D-Glucose D-Fructose Sucrose

Records of sugar production in ancient China: Sugar Frosting Spectrum (left) and Tiangong Kaiwu (right)

What makes us fall in love with sweetness?

Humans' need for sweetness may stem from plants' need to reproduce. Plants convert solar energy into chemical energy through photosynthesis and store it in their bodies to maintain the energy consumption necessary for survival. The most efficient, convenient, safe and universal form of chemical energy storage may be glucose. Glucose is a polyhydroxy aldehyde that is most widely distributed in nature and is the main energy source and metabolic intermediate for organisms. When plants cannot reproduce and spread with the help of external forces (such as wind and water power), glucose gives them the opportunity to cooperate with animals. Animals cannot directly use solar energy, so it has become a necessity for herbivores to survive by decomposing and extracting energy substances in plants through a good "stomach and intestines".

Sugar thus became the "currency" for transactions between animals and plants: plants paid "sugar" for animals to run errands for them and complete the necessary "mobile services", such as pollination (cultivating more next generations) or spreading seeds (grabbing territory in new worlds); animals obtained high-energy sugars and could save time on foraging and enjoy life (combing their feathers or falling in love).

"Division of labor changes the world, and trade changes life" is simply the natural version of "The Wealth of Nations".

Energy transfer in the food chain (Image source: veer gallery, pixabay)

Not all sugars are sweet, but some easily available "sweet" sugars bring energy; what our ancestors pursued was not "sweetness", but the exhilarating energy behind this "sweet label". The benefits brought to them by these energy substances make people remember this taste, forming a positive feedback in the body, and then looking for it. A recent study published in Nature answered people's questions about sugar and sweetness. Scientists' research shows that there is a sugar-specific pathway that is different from the sweetness-specific pathway, which can independently produce a preference for sugar and stimulate sugar consumption. Current research has confirmed that in animal models, even if the functional sweet taste transmission pathway is missing, they still show a preference for sugar.

I tried to verify that sugar addiction is a common hobby by feeding the dog sugar water and seeing that it wagged its tail more often, but I found that it wagged its tail much more often when it smelled meat than when it drank sugar water. The conclusion I came to at the time was that the ancestors of these animals had discovered a better way to obtain energy and gave up on sugar intake, thus embarking on a completely different development path from our ancestors. This experiment, which now seems "unscientific and imprecise", has become a seed of curiosity buried in my young heart, leading me along the way to discover more stories about sweetness and sugar.

OMG, I can eat sweet things without gaining weight

At this point, you might be thinking: I know so much about sweetness but I still can’t control myself. I still want to eat sweet things. My scale may be broken but I can’t stop. ! ! !

Don’t worry, we will introduce some sweet flavors that won’t make you fat~

The most unique monosaccharide is xylose. Although its sweetness is comparable to that of glucose, it does not generate calories. In other words, taking xylose gives you a sweet taste without burning calories. It is the favorite of thousands of girls, which is why xylose products are so popular in today's society. In addition to the "sugars" in plants that are sweet and can provide energy, there are also some "non-sugar" molecules from natural sources that can provide sweetness even though they cannot provide energy.

Licorice, a leguminous plant that is famous for its ability to "harmonize all herbs", is a "multi-talented" herb in the entire Chinese medicine industry. It can not only treat a variety of diseases, but also has a special sweetness due to the liquiritin it contains. The sweetness of liquiritin is 100-300 times that of sucrose, and it also has a flavor-enhancing effect. The sweetness of liquiritin develops slowly and lasts for a long time, giving a better sweetness experience than sucrose, so it is often used in combination with sucrose.

Another Asteraceae plant, Stevia, contains a chemical component called stevioside, which is even sweeter. Its aglycone is the diterpenoid component steviol, and its glycosides are sophorose and glucose. The sweetness of stevioside is 200-300 times that of sucrose, and its properties in all aspects are similar to those of sucrose. It is very suitable for diabetics to use as a sweetener, and can also be used as a low-energy food additive.

As an alternative plant in the Cucurbitaceae family, the fruit of monk fruit contains mogroside, a triterpenoid chemical component, which is 300 times sweeter than sucrose and is also a low-calorie, high-sweetness sweetener. Monk fruit also has a relatively high amino acid content and is a plant that can be used as both medicine and food. It is often used to treat lung heat, dry cough, sore throat, and aphonia.

Designed by Ju Peng (Image source: veer Gallery (middle))

Although there are non-sugar sweeteners in plants, the most widely used are still various sugar sweeteners. The two functions of "sweetness" and "energy" are like two sides of a coin, which have gradually become a problem in contemporary society. How can we enjoy the pleasure brought by sweetness cheaply without the obesity caused by excess "energy storage"? Humans are ready to use technology to split the "coin" in half, and artificial sweeteners are born.

Although synthetic non-sugar sweeteners have many disadvantages, they are cheap and low in calories. For example, saccharin, which was once widely used, is a sodium salt of o-benzoylsulfonyl imide, which is 300-500 times sweeter than sucrose, but has a slightly bitter taste; the much-criticized cyclamate, called sodium cyclamate, is not as sweet, only 30-50 times sweeter than sucrose, and also has a slightly bitter taste; cyclamate, also known as aspartame, is aspartame methyl ester, which is 100-200 times sweeter than sucrose and has low stability.

As artificial sweeteners are used more and more widely, they are becoming more and more controversial. But so far, there are no definite research results on the carcinogenicity of artificial sweeteners, so it is still impossible to draw a conclusion. The latest study conducted by researchers from the Yale University School of Medicine and the French National Center for Scientific Research shows that it may not be the sweeteners that are harmful, but the combination of sweeteners and carbohydrates. The sweetener used in this study is sucralose (sucralose), which is a sweet substance formed by chemical synthesis modification of natural products. It is structurally modified on the parent nucleus of the natural product sucrose to introduce substituent chlorine atoms, thereby increasing its sweetness to 600 times that of sucrose, but does not provide energy. Seeing that humans have successfully split this coin in half through technology, while holding the "sweet" side complacently, they find that there is another side behind it - safety issues. People seem to have solved the problem, but they have also created new problems.

(Photo source: Food Safety Centre of the Government of the Hong Kong Special Administrative Region)

Sweetness may hide more secrets in nature

Looking at the natural world, the world of sweetness is far more vast than we imagine: for example, chloroform (trichloromethane, chloroform), as a commonly used and dangerous "big brother" among chemical solvents, tastes sweet, and it was once used as an anesthetic until people found that it could cause liver damage; the sweetness of lead, beryllium, and chlorine, the basic elements that make up the material world, has also been sorted out; some mineral waters, because they are rich in a variety of trace elements, also show sweetness, but once the types or proportions of these free ions change, the sweetness will disappear; some plant proteins also show sweetness, coexisting with the sugar in them; the sweetness of some animal proteins is accompanied by umami...

What is on the back of these “sweet” coins?

The world is so big and time is so long, it is worth our exploration with curiosity.