Disaccharides, Chemistry tutorial


Disaccharides are the carbohydrates which are formed whenever two monosaccharides are combined. The formation of a disaccharide comprises the union of two monosaccharides which undergo a procedure in which a water molecule is eliminated as part of the union. Similar to the two monosaccharides which join to form the single disaccharide, the carbohydrate is sweet to taste and tends to dissolve in water by means of relative ease.

The procedure whereby disaccharides are formed is termed as dehydration synthesis or condensation reaction. This signifies to the extraction of the water molecule throughout the joining of the two monosaccharides. What is left behind is a dry substance which can be stored by relative ease, and employed as an ingredient in the formation of many various kinds of foods.

Disaccharides belong to one kind of some various chemical groupings or combinations of carbohydrates. The other illustrations comprise polysaccharides and oligosaccharides. The categorization of a specific combination of carbohydrates tends to based on the kind of molecules employed and the number comprised in the formation of the group.

One of the best acknowledged disaccharides is sucrose. In this specific disaccharide formula, a fructose molecule is combined by a glucose molecule. The outcome is a sweet substance which can be employed in baking and the formation of numerous various kinds or processed sweets. Cane sugar is an illustration of sucrose.

The other popular kind of disaccharides is lactose. As well termed as milk sugar, this specific disaccharide is formed by joining a glucose molecule having a galactose molecule. Dissimilar to some other disaccharides, lactose does cause an allergic reaction in a few people, which has led to the formation of more sugar and dairy alternatives which rely on other substances to sweeten the product.

There are even several kinds of disaccharides which are employed as they cause less of a spike in blood sugar. This crafts them ideal for people who suffer with type 2 diabetes, as this lets them to enjoy in moderation a few foods which they would or else have to exclude from their diets altogether. Maltose is often employed in the creation of soft candies, like chocolates and fruit-based treats. As glucose molecules are still present, they tend to make less of a spike in blood sugar levels and are generally absorbed to the body easier than the carbohydrates which compose regular table sugar. Still, diabetics are generally advised to limit their utilization of these safer disaccharides, as too many of such alternative treats eaten at similar time can cause some degree of gastronomical distress, comprising diarrhea.

How are Disaccharides formed?

Disaccharides are prepared whenever two monosaccharides are joined altogether and a molecule of water is eliminated. This is termed as 'dehydration synthesis or condensation reaction'. The left over dry substance in the synthesis of disaccharide can be stored by relative ease, and employed as an ingredient in the formation of many various kinds of foods.

How are Disaccharides classified?

There are two kinds of Disaccharides. They are as illustrated below:

A) Reducing Disaccharide:

In this kind of disaccharide, the reducing sugar is a free 'hemiacetal' unit. Hemiacetal are compounds which are derived from the aldehydes and ketones correspondingly. Aldehydes are basically organic compounds. This functional group, having the structure R-CHO, comprises of carbonyl center bonded to hydrogen and an R group. The -CHO is known as the aldehyde or formyl group. Most of the fragrances are aldehydes. Ketones are as well organic compound having the structure RC (=O) R' in which the C=O is the ketone group. Illustrations of reducing disaccharide are maltose and Cellobiose.

B) Non-reducing Disaccharides:

In this kind, monosaccharides encompass no free hemiacetal unit. Illustrations of non-reducing disaccharides are Sucrose and Trehalose.

What are the physical properties of Disaccharide?

Based on the monosaccharide constituents, disaccharides comprise the given properties:

  • Crystalline
  • At times water-soluble
  • At times sweet-tasting and sticky-feeling

What is the role of Disaccharides in human health?

Excess of disaccharides causes spike in the blood sugar and leads to a disease known as 'Diabetes'. However, some kinds of disaccharides are employed as they cause fewer spikes in the blood sugar and are favored by 'type 2 diabetes' patients. Example: Maltose. As glucose molecules are still present, they tend to make less of spike in blood sugar levels and are absorbed to the body simpler than regular table sugar. However, too much of these safer disaccharides can cause diarrhea.

Common Disaccharides:

The three disaccharides are present in human diet. Lactose is naturally found in the milk of mammals and includes glucose and galactose. Sucrose is found in sugar cane and beets. Sucrose, as well termed as table sugar, includes glucose and fructose. Maltose is a disaccharide which includes two molecules of glucose. Maltose prepared all through the breakdown of some starches, like barley, throughout food manufacturing. Glucose is the simple sugar cell which your body utilizes for energy. Your body generally transforms galactose and fructose, as well termed as fruit sugar, into glucose.

A) Maltose:

Maltose doesn't encompass a specific function in the body. Manufacturers transform maltose to a disaccharide sugar alcohol termed as maltilol for use as a bulk sweetener in powder and syrup and added to numerous sucrose-free and diabetic foods, comprising chocolates, chewing gum, candies, bakery goods, ice cream and jam. Your body slowly absorbs just 50 to 60 % of maltilol, having the remainder excreted or fermented in the large intestine.

Maltose takes place to a limited extent in sprouting grain. This is formed most frequently via the partial hydrolysis of starch and glycogen. In the manufacture of beer, maltose is discharged by the action of malt (that is, germinating barley) on starch; for this cause, it is frequently termed to as malt sugar. Maltose is around 30% as sweet as sucrose. The human body is not able to metabolize maltose or any other disaccharide directly from the diet as the molecules are too large to pass via the cell membranes of the intestinal wall. Thus, an ingested disaccharide should first be broken down via hydrolysis to its two constituent monosaccharide units. In the body, these hydrolysis reactions are catalyzed via enzymes like maltase. The similar reactions can be taken out in the laboratory by dilute acid as a catalyst, however in that case the rate is much slower, and high temperatures are needed. Whether it takes place in the body or a glass beaker, the hydrolysis of maltose generates two molecules of D-glucose.

maltose → (H+ or maltase) → 2D-glucose

Maltose is a reducing sugar. Therefore, its two glucose molecules should be connected in such a manner as to leave one anomeric carbon which can open to make an aldehyde group. The glucose units in maltose are connected in a head-to-tail fashion via an α-linkage from the first carbon atom of one glucose molecule to the 4th carbon atom of the second glucose molecule (that is, an α-1, 4-glycosidic linkage). The bond from the anomeric carbon of the first monosaccharide unit is directed downward, that is why this is termed as α-glycosidic linkage. The OH group on the anomeric carbon of the second glucose can be in either α or β position, as illustrated in the figure shown below.

924_Equilibrium Mixture of Maltose Isomers.jpg

Fig: Equilibrium mixture of Maltose Isomers

B) Lactose:

Lactose, as well termed as milk sugar, is the nutritional source of energy for infants throughout nursing. Lactose prepares milk tastes sweet and is an ingredient in numerous processed foods which include dairy. Manufacturers add whey, a byproduct of the dairy production which includes lactose, to some food products, like cookies, breads, cakes, doughnuts, breakfast bars and ice cream. Lactose needs you encompass an enzyme termed as lactase to digest the disaccharide. Most of the people are lactose intolerant and don't produce adequate amounts of lactase to digest lactose, causing these symptoms as diarrhea, nausea, gas, abdominal cramps and bloating. You can take a lactase supplement to assist you digest lactose and ameliorate the symptoms.

Lactose is identified as milk sugar as it takes place in the milk of humans, cows and other mammals. However, the natural synthesis of lactose takes place merely in mammary tissue, while most of the other carbohydrates are plant products. Human milk includes around 7.5% lactose, and cow's milk includes around 4.5%. This sugar is one of the lowest ranking in terms of sweetness, being around one-sixth as sweet as sucrose. Lactose is prepared commercially from whey, a by-product in the preparation of cheese. It is significant as an infant food and in the production of penicillin.

Lactose is a reducing sugar comprised of one molecule of D-galactose and one molecule of D-glucose combined through a β-1,4-glycosidic bond (that is, the bond from anomeric carbon of the first monosaccharide unit being directed upward). The two monosaccharides are acquired from lactose through acid hydrolysis or the catalytic action of the enzyme lactase:


Fig: Lactose

Most of the adults and a few children suffer from a deficiency of lactase. Such individuals are stated to be lactose intolerant as they can't digest the lactose found in milk. A more severe problem is the genetic disease galactosemia that yields from the absence of an enzyme required to transform galactose to glucose. Some bacteria can metabolize lactose, making lactic acid as one of the products. This reaction is mainly responsible for the 'souring' of milk.

C) Sucrose:

Sucrose is the disaccharide presents most in the human diet. The function of sucrose is to sweeten foods for more taste appeal. Manufacturers add sucrose as a sweetener to candies, ice cookies, cream, cakes, sauces, breads, ketchup and canned goods. The benefit of employing sucrose as a sweetener is that it is stable in both liquid and crystallized solid states.

Manufacturers substituted sucrose as a sweetener in most of the food and beverage products having high-fructose corn syrup, a sweetener which includes a mixture of glucose and high concentrations of fructose and is less expensive than sucrose.

Sucrose, probably the biggest-selling pure organic compound in the world, is known as beet sugar, cane sugar, table sugar or simply sugar. Most of the sucrose sold commercially is acquired from sugar cane and sugar beets (that is, whose juices are 14% to 20% sucrose) via evaporation of the water and recrystallization. The dark brown liquid which remains after the recrystallization of sugar is sold as the molasses.

The sucrose molecule is exclusive among the common disaccharides in having an α-1, β-2-glycosidic (that is, head-to-head) linkage. As this glycosidic linkage is made by the OH group on the anomeric carbon of α-D-glucose and the OH group on the anomeric carbon of β-D-fructose, it ties up the anomeric carbons of both fructose and glucose.


Fig: Sucrose

This linkage provides sucrose some properties that are fairly dissimilar from those of maltose and lactose. As long as the sucrose molecule remains unbroken, neither monosaccharide 'uncyclizes' to prepare an open-chain structure. Therefore, sucrose is incapable of mutarotation and present in just one form both in the solid state and in solution. Moreover, sucrose doesn't undergo reactions which are typical of aldehydes and ketones. Thus, sucrose is a non-reducing sugar.

The hydrolysis of sucrose in dilute acid or via the action of enzyme sucrase (as well termed as invertase) provides an equimolar mixture of glucose and fructose. This 1:1 mixture is termed to as invert sugar as it rotates plane-polarized light in the opposite direction as compare to sucrose. The hydrolysis reaction has several practical applications. Sucrose readily recrystallized from a solution; however invert sugar consists of a much greater tendency to remain in solution. In the preparation of jelly and candy and in the canning of fruit, the recrystallization of sugar is undesirable. Thus, conditions leading to the hydrolysis of sucrose are used in these methods. Furthermore, as fructose is sweeter than sucrose, the hydrolysis adds to the sweetening effect. Bees accomplish this reaction whenever they make honey.

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