Carbohydrates are the main source of energy for most populations throughout the world. Plant foods supply most of the carbohydrates eaten by adults since most foods of animal origin contain negligible amounts. The exception to this is lactose, or milk sugar, an important energy source for babies but less important in the adult diet. Dietary carbohydrates range from simple sugars to complex molecules
such as starches or non-starch polysaccharides, and are classified according to the number and configuration of the single sugar units (monosaccharides) joined together.
Simple sugars the monosaccharides and disaccharides are sweet, and foods containing these are often seen as particularly palatable because of this.
Monosaccharides
All carbohydrates are made up of carbon, hydrogen, and oxygen, and depending on the number of carbon atoms in the backbone may be trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), or heptoses (7C). The most important dietary carbohydrates are the 5 and 6 carbon sugars the pentoses and hexoses. Some of these exist as monosaccharides...
But more commonly are found joined as chains of various lengths. Glucose is a hexose, and the main form in which carbohydrate is absorbed into the bloodstream. It is rarely found free in natural foods (small amounts in some fruits and vegetables), although some occurs in honey. Fructose is found in honey, fruits, and some vegetables. Another hexose, galactose, is a characteristic component of lactose or milk sugar, and also a common component of some of the complex storage carbohydrates in plants such as legumes. Pentose sugars such as ribose and deoxyribose are components of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), and so are present in minute amounts in all foods.
Disaccharides
These are combinations of two simple sugars. Sucrose, a combination of glucose and fructose, is the most common disaccharide; extracted from sugar cane and sugar beet, it is present naturally in some fruits and vegetables. Other common disaccharides are lactose the sugar found in milk (glucose and galactose) and maltose (glucose plus glucose), which is formed when barley sprouts during the brewing process. Malt extracts and malted milk drinks will contain maltose.
Oligosaccharides
Oligosaccharides are short chains of sugars with three to nine sugar molecules. The most common of these (raffi nose, stachyose, and verbascose) are found in legumes and
are not digested by human digestive enzymes they are digested by bacteria in the large intestine, producing gas.Fructo-oligosaccharides (FOS), especially inulin, found in Jerusalem artichokes, and others found in onions, garlic
and some cereals, have recently drawn interest as they appear to function as prebiotics (see later), encouraging the growth of particular, supposedly beneficial bacteria the so-called probiotics – in the large intestine.
Polysaccharides
This group of carbohydrates covers a wide range of compounds all containing long chains of sugar molecules joined together. They may contain only one type of sugar e.g. starch. Starches are the major polysaccharide and carbohydrate in the human diet, and contain only glucose.Others the non-starch polysaccharides usually contain at least two different sugars and may contain several different types. For example, there might be a backbone of one type of sugar such as galactose, with side chains ofanother, e.g. mannose such a compound would be called a galactomanna.
There are many different types of non starch polysaccharides, depending on the plant source
and the function within the plant. Cellulose is a common non-starch polysaccharide, and non-cellulosic polysaccharides such as pectins, and plant gums such as gum arabic and guar gum, are other examples.The way in which the molecules are joined together affects their structure and properties. There are different types of linkages between the sugar molecules that may result in the compound being a straight chain or having branches. The types of linkages also affect their availability to human digestive enzymes. Starch is made up of two types of chains of glucose molecules: amylose is an unbranched form while amylopectin is highly branched.
Starches from different plant sources differ in the proportions of amylose and amylopectin, and this also affects the availability of the glucose from the starch in the food, especially after cooking and cooling. Raw starch is indigestible,and must be cooked with some water in order to gelatinize it and make it available to the amylase enzymes in the gut.
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