Fructose: Simple sugar and component of sucrose

It is a monosaccharide component of sucrose present in fruits as fruit sugar (laevulose). It is used in beverages, fruit juices, pulps. Fructose is also found in ‘table sugar.’

It provides similar amount of energy as sucrose. Fructose is termed a ‘slow sugar’ as it is metabolized slower than sucrose. In a well-controlled diabetic, the fructose metabolic pathway demonstrates a positive flux towards formation of glycogen from fructose i.e. glycogenesis.

Sources of dietary fructose include agave, the richest natural source of fructose, with 85% of carbohydrate in this form; honey, with approximately 50%; and fruit juices.

Fructose is very sweet and is often made into high fructose corn syrup, used in soft drinks and processed foods. HFCS is made from acid- or amylase-treated corn starch and contains 42–55% fructose. The most popular HFCS formulations used for food applications are the HFCS-42 and HFCS-55. HFCS-55 consists of ~55% fructose and ~45% glucose. HFCS-55 is mainly found in soft drinks and other sweetened beverages.

A 16-ounce bottle of apple juice may have more than 30 grams of fructose and a 20-ounce bottle of soda can have up to 40 grams.

Fructose is associated in epidemiologic studies with greater weight, triglyceride, blood pressure, and insulin resistance levels and in animal and human feeding studies with small dense LDL cholesterol, nonalcoholic fatty liver disease, and greater levels of protein glycation. An increased intake of fructose may cause hypertriglyceridemia, especially in patients with uncontrolled diabetes.

Hereditary fructose intolerance is a very rare genetic disorder. This is when the liver is not able to help the body break down fructose. Symptoms can be more serious. This disorder requires more than just limiting fructose.

Fructose: Simple sugar and component of sucrose

The fruit sugar of fructose

Sugars are naturally occurring sweeteners, the most common in our nutrition being sucrose, fructose, and glucose. Fructose and glucose are monosaccharides present in small amounts in fruits and honey.

Fructose is a monosaccharide found in nature primarily in fruits. It’s eaten as sucrose (common table sugar), which is composed of equal parts of glucose and fructose. It is the sweetest of the naturally occurring nutritive (caloric) sweeteners and has many unique functional and nutritional properties that make it a valuable food ingredient. Fructose is a hexose, with a chemical formula C6H12O6 identical to that of glucose.

When fructose is attached to a sugar called glucose, it forms sucrose or ‘table sugar.’ Long chains of fructose are called fructans and are found in certain vegetables, wheat, and other foods.

 Fructose is also found in sucrose (table sugar), honey, agave nectar, fruit juices, fruit juice concentrates, pure crystalline fructose and high fructose corn syrup (HFCS).
The fruit sugar of fructose

Simple sugars or monosaccharides

The two main types of sugars are monosaccharides and disaccharides. Monosaccharides consist of a single sugar molecule.

Monosaccharides may contain from three to nine carbon atoms although most of them contain five or six. A three carbon monosaccharide is called a triose; one containing four carbons is called a tetrose; five, a pentose; six, a hexose; seven, a heptose; eight an octose and nine a nonose.

*Pentoses (Arabinose, ribose, xylose)
The pentose sugars, deoxyribose and ribose, are essential components of the genetic material DNA and RNA. E.g. glucose and fructose: (C6H12O6); arabinose and xylose: (C5H10O5).

Arabinose is found in gums and when several arabinose molecules are joined together, a pentosan formed.

*Hexoses
-Aldohexoses – galactose, glucose
-Ketohexose - fructose
All hexoses have the same chemical formula C6H12O6, but slightly different structures. In nature, only fructose and glucose occur in free form. Galactose joined with glucose forms the disaccharide lactose.

Glucose is a hexose. Octoses and nanoses are quite rare. Glucose, the main source of energy for body cells, is found in most sweet fruits and in blood.
Simple sugars or monosaccharides

Glycosidic bond

Polysaccharides chains are generally formed by a repeated sequence of monomers or oligomers. Few monosaccharides exist in the free state. They generally occur linked by glycosidic bond to one another or to an aglycones.

The term ‘glycosidic bond’ is used for the bond between the anomeric C-atom and the glycosidic O (N-, S,) atom.

While the term ‘glycosidic linkage' is used to define a set of all bonds that connects to monosaccharide rings.

It is a bond that links two sugar units together; it is formed between the free carbonyl group of one sugar and a hydroxyl group of another sugar; the orientation (α or β) and position (eg. 1,4) of the link must be specified. The structure of a di-, oligo- or polysaccharide sugar is based on the bonding of two monosaccharides.

In the case of a disaccharide, two monosaccharides are joined with loss of one molecule of water. This joining occurs between two –OH groups, one of each monosaccharide.
Glycosidic bond

Disaccharides in food

Disaccharides consist of two monosaccharides linked together with a special linkage, called a glycosidic bond. The following disaccharides are important in human nutrition: sucrose, lactose and maltose.

Sucrose or table sugar is the most common and contains glucose and fructose. It is commonly known in households and is found in many plant fruits and saps.

Chemically sucrose is one glucose molecule and one fructose molecule linked together.

Galactose seldom found free in nature, galactose is part of lactose, the sugar found in milk. Lactose is composed of one molecule of glucose and one molecule of galactose.

About 5% of fluid milk is lactose, or milk sugar. Human milk has a higher concentration of lactose than cow’s milk, so human tastes sweeter than cow’s milk.

Maltose is formed by the enzymatic hydrolysis of starch and is an important component of the barley malt used to brew beer.

 Maltose also primarily used in the production of breakfast cereal and in some infant formula. It is a homogenous disaccharide consisting of two units of glucose.

Disaccharides in food

Monosaccharides

Carbohydrates are major functional constituents of living system. Carbohydrates may be classified as monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides are simple carbohydrates containing between three and eight carbon atoms, but only those with six and five or six carbon atoms are common.

Monosaccharides consist of a single sugar molecule – mono meaning “one” and saccharide “sugar”.

Monosaccharides are often classified by both their number of carbon atoms and their functional group.

For example, an aldohexose is a monosaccharides that contains a total six carbon atoms including that of the aldehydes in its structure. Similarly, a ketopentose has five carbons in its structure including the one in the keto group.

Three of the most important ones in foods are the six carbon sugars glucose, fructose and galactose.

All the three monosaccharides have the same number and kinds of atoms but in different arrangements.

Glucose is known as an aldose sugar because it contains an aldehydes group (CHO) located on the first carbon atom of the chain.

Most cells depend on glucoses for their fuel to some extent, and the cells of the brain and the rest of the nervous system depend exclusively on glucose for their energy.

Fructose is a six carbon sugar, like glucose but it is a ketose sugar, not aldose , because it contains ketone group and not an aldehydes group.

Fructose occurs naturally in fruits, honey, and saps, Other source include soft drinks, ready to eat cereals and other products sweetened with high-fructose-corn-syrup.

Galactose rarely occurs as a monosaccharide in food. It usually chemically bonded to glucose to form lactose the primarily sugar in milk and dairy product.

During digestion galactose is freed as a simple sugar.

Monosaccharides are also often called sugar. Hexose are six-carbon sugars, pentoses five carbon sugar and so on. The word ‘sugar’ associated with ‘sweetness’ and most (but not all) monosaccharides, have a sweet taste.
Monosaccharides