Sugar is a class of organic substances with a wide distribution in nature and the richest content, which is generally composed of three elements: carbon, hydrogen and oxygen, and together with proteins, nucleic acids and lipids, it constitutes the most important four types of biological macromolecules.
Sugars and carbohydrates
Originally, chemists thought that the ratio of hydrogen to oxygen in the molecules of carbohydrate compounds was the same as that of water except carbon, so they regarded it as various compounds formed by carbon and water, collectively called, expressed in the general formula Cn(H2O)m.
Later, it was found that some compounds are similar in chemical structure to sugars, but their composition cannot be expressed by the above formula, such as rhamnosus and deoxyribose and other molecules, the ratio of hydrogen to oxygen atom number is not 2:1; in addition, it was also found that many carbohydrates may contain other elements in addition to carbon, hydrogen and oxygen. Therefore, it is incorrect to say the term "carbohydrate" strictly, but it has been conventionally used today.
A member of the sugar family
Sugars are generally divided into monosaccharides and glycans, and glycans are divided into oligosaccharides (oligosaccharides) and polysaccharides. Monosaccharides and oligosaccharides are mostly crystals, soluble in water, and have a sweet taste. Most polysaccharides are amorphous, non-sweet white solids, and most of them are insoluble in water.
Monosaccharides are the smallest structural units of sugars, which cannot be hydrolyzed and then divided into smaller sugar molecules, and can be directly absorbed and utilized by the human body without digestion. There are about hundreds of monosaccharides found in nature, and the monosaccharides that play an important role in nutrition are glucose, fructose and galactose.
The sugars synthesized by the dehydration and condensation of multiple monosaccharide molecules are polysaccharides, and the starch, glycogen and cellulose in the polysaccharides play an important role in nutrition. Starch and glycogen are polygonal sugars that can be digested and absorbed by the human body, and cellulose cannot be digested and absorbed by the human body due to the lack of corresponding enzymes, but they are very important dietary components.
Generally, the composition of less than 10 sugar units is called oligosaccharides. There is no strict boundary between oligosaccharides and polysaccharides, and when the molecular weight is high to a certain extent, the physical and chemical properties gradually change. The most important of the oligosaccharides is disaccharides (composed of two monosaccharides), and the disaccharides that are closely related to human life are sucrose, maltose and lactose.
Widely distributed in living organisms
Carbohydrates are common in various animals, plants and microorganisms, and they exist in the form of free sugars or in combination with proteins, lipids, etc., and have a variety of physiological functions in the organism.
The most widely distributed of these is glucose, which contains glucose or polysaccharides composed of glucose in most biological cells and blood, which releases heat when oxidized. Therefore, the normal regulation of glucose metabolism is very important for the movement of life.
Lactose, mannose, ribose, and deoxyribose are also widely distributed. The distribution of some oligosaccharides and polysaccharides carries a certain degree of ethnic specificity. For example, cellulose is mainly found in the cell walls of plants and fungi, while animals do not have cellulose.
As an important bioenergy, carbohydrates release energy through biological oxidation to meet the needs of life activities. 50% to 60% of the total calories that humans consume from food come from sugars. Sugars can also be converted into other substances necessary for life, such as proteins and lipids, through various metabolic pathways.
Carbohydrates also have a supportive and protective effect on living organisms. For example, cellulose is the main component of plant cell walls and plays a supporting role in most plant bodies. Chitin is also a polysaccharide that forms the shell of insects and crustaceans and acts as a mechanical protector.
In mammals, birds and other animals, some sugar and protein-bound substances have the functions of lubricating joints and protecting mucous membranes. Complex sugar compounds on the surface of cells or in body fluids also have the function of information transmission, and the sugar chain parts of these sugar complexes play a key role in the process of information recognition and transmission.
Sugar doesn't have to be all sweet
The vast majority of sugars have a sweet taste, and the sweetness is due to the hydroxyl (-OH) functional group in the carbohydrate compound at work. In general, the more hydroxyl groups, the sweeter the substance. The level of sweetness is called sweetness (or sweetness), and it cannot be quantitatively determined by physical or chemical methods, but can only be judged by people's taste senses. Sensory identification is not only affected subjectively, but also by objective conditions such as material concentration, humidity, and the presence of other components.
Therefore, sweetness is a relative value, usually with sucrose as the benchmark, generally with 10% or 15% aqueous solution of sucrose at 20 ° C as a sweetness as 1, the sweetness of other sugars compared to it. For example, if sugar is the sweetest sugar in common natural sugars, the sweetness is about 1.5, the sweetness of glucose is about 0.75, the sweetness of maltose is about 0.33, and the sweetness of lactose is about 0.27. Therefore, fructose-based fruits with the same sugar content taste sweeter than glucose-based fruits.
But there are also no sweet sugars, such as starch. Although starch also contains hydroxyl groups, the hydroxyl parts form glycoside bonds, so there is no sweetness in itself. Starch will be hydrolyzed under the action of amylase to produce maltose, which will produce sweetness. We contain this amylase in our saliva, which is why rice and pasta are sweetened after chewing.
What is sweet doesn't have to be sugar either
Sugar-free chewing gum, sugar-free drinks, etc. can be seen everywhere in life, but there is sweetness without sugar because of the addition of sugar substitute sweeteners.
There are many types of sweeteners given to food, which can be divided into natural sweeteners and synthetic sweeteners according to the source; according to nutritional value, it can be divided into nutritional (caloric) sweeteners and non-nutritive (no calories) sweeteners; according to chemical structure and properties, it can be divided into sugar sweeteners and non-sugar sweeteners.
Sugar alcohol sweeteners such as maltitol, sorbitol, etc., are synthetic sweeteners. Its sweetness and calorific value are similar to or lower than that of sucrose.
Steviol glycosides and glycyrrhizinate monokalum and tripotassium glycyrrhizinate are natural non-nutritive sweeteners, which are extracted from stevia and licorice, respectively, and are about 200 times sweeter than sucrose.
Saccharin, cyclamate, aspartame, etc. are synthetic non-nutritive sweeteners.
Sugar sweeteners such as sucrose, glucose, fructose, fructose syrup, etc., are commonly referred to as sugars in China and are regarded as foods, and only sugar alcohols and non-sugar sweeteners are managed as food additives.
Nutritional sweeteners are represented by xylitol
Xylitol is found in naturally in plants such as raspberries, strawberries, cauliflower, spinach, etc., but in very low amounts. Xylitol in industrial production is usually obtained by a reduction reaction of xylose. Xylose is widely present in a variety of plants and can be extracted from plants such as birch, raspberry, and corn. Xylitol is as sweet as sucrose, but the calories are only 60% of sucrose.
Xylitol has other advantages in addition to its low calories. For example, it cannot be broken down by bacteria, so it has anti-caries and anti-caries properties. It does not need insulin to participate in metabolism to enter the cells directly, the metabolism rate is fast, it will not cause blood sugar to rise, and it is suitable for making food for diabetics.
In addition, xylitol crystals absorb heat when dissolved, have a cool feeling in the mouth, and can increase the flavor of mint, lemon, and spearmint foods, so xylitol is often added to chewing gum.
Non-nutritive sweetener representative - aspartame
Aspartame (methyl aspartylphenylalanine) is not present in nature and is a peptide derivative that chemists inadvertently synthesize when synthesizing ulcer-suppressing drugs, and is a white crystalline powder at room temperature. Aspartame has a refreshing, sucrose-like sweetness, which is about 200 times sweeter than sucrose, which is more durable than the sweetness of sucrose. Although it also produces heat, the heat generated can be ignored because the dose is extremely small when used to make people feel sweet.
Aspartame is unstable to heat, and the hydrolyzed sweetness disappears at high temperatures, so it cannot be used for baking, frying and other foods that require high temperature treatment, and are currently mainly added to beverages, vitamin lozenges or chewing gum. Aspartame contains phenylalanine and is not suitable for people with phenylketonuria, so foods containing aspartame must be marked as "containing phenylalanine".
aspartame
Sugar is the energy necessary to maintain life, when people eat sweets, they will activate dopamine neurons in the brain, so that the brain is excited and produce a sense of pleasure, so many people can not refuse desserts. However, excessive intake of sugar can bring a burden to the body, causing various diseases such as high blood pressure and osteoporosis. The World Health Organization clearly proposes that in order to maintain health, in addition to the natural sugar in food, the daily free sugar should be controlled at 5% of the total energy demand, which is no more than 25 grams per day.
Free sugar (defined as added sugar in China) refers to sugars and syrups that are added to foods during the production and preparation of food. Common ones include white sugar, brown sugar, corn syrup, high fructose corn syrup, molasses, honey, juice concentrates, and glucose.
Although sugar substitute products solve the calorie problem, it does not mean that unlimited intake can be obtained. Excessive intake of sweeteners is still harmful to health, so you can't indulge yourself in the habit of sweetening.
This article is the original of the Encyclopedia of China database, and its reproduction must be authorized.