Carbohydrates
Carbohydrates are an important source of energy in all human diets. The amount in the diet varies according to the economy, the range being 40–80% of calorie intake. Carbohydrates are synthesised from carbon dioxide and water. The primary structures are monosaccharide sugars, which may dimerise to disaccharides, e.g. sucrose, or polymerise extensively to form polysaccharides, e.g. cellulose, pectin and hemicellulose. In the human, carbohydrates are central to energy utilisation as glucose and to storage as starch.
1. Monosaccharides, are either polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses) with optically active chiral centres. The two forms of the molecule are described as dextrorotatory (D or +) or laevorotatory (L or –) depending on the direction that they rotate plane-polarized light. A mixture of equal amounts of the d and l forms is known as a racemic mixture.
2. Monosaccharides include a -d-glucose, fructose, mannose, xylose and galactose. Monosaccharides form intramolecular hemiacetals, so the resulting compounds form a five (furanose) or six-membered (pyranose) ring. Carbohydrates that can reduce Fehling’s, Benedicts’s or Tollen’s solution (complexed cupric solutions) are known as reducing sugars.
3. Monosaccharides can link through glycosidic bonds to form disaccharides and larger carbohydrate structures. Disaccharides include sucrose (glucose and fructose); lactose (glucose and galactose); maltose (glucose and glucose).
4. The commercial interest in sucrose results in its being available in a variety of forms. There are many commercially advantageous effects of sugar in prepared foods.
Polysaccharides
In biology most carbohydrates exist as high molecular weight polymers. Polysaccharides are simple sugars connected by glycosidic bonds. The sugars involved are d-glucose, d-mannose, d- and l-galactose, d-xylose, l-arabinose, d-glucuronic acid, d-galacturonic acid, d-mannuronic acid, d-glucosamine, d-galactosamine and neuraminic acid.
Cellulose and glycogen
Cellulose is a major structural polysaccharide, providing approximately half of plant carbon, and consisting of polymeric glucose joined by a b – (1–4) type glycosidic linkage. D-Glucose, in the chair form of a pyranose ring, is a rigid structure, and is characteristic of cellulose as an extended polysaccharide chain. Cellulose has a molecular weight of 50 000 Da or greater, forming chains which join together as bundles, with a diameter of 100–250 Å with approximately 2000 chains in such a bundle.
The major polysaccharides for energy storage in animal cells . Glycogen is an a -(1–4) homopolymers, highly branched with an a -(1–6) linkage occurring every 8–10 glucose units along the backbone with short side chains of approximately 8–12 glucose units each
Starch and glycogen
The major polysaccharide for energy storage in plant cells is starch.
Starch
Starch is an a -(1–4) homopolymers with occasional a -(1–6) linkages at branch points. They differ in their chain lengths and branching patterns. Starch occurs both as unbranched amylose and as branched amylopectin
Amylose is a linear unit with glucoses joined by a -(1–4) linkages and is insoluble in cold water. Amylose consists of between 200 and 2000 glucose units, depending on the source.
Amylopectin This is a branched structure with glucose joined by a -(1–6) linkages at the branch point, and a -(1–4) linkages in the linear sections The polymer contains between 10 000 and 100 000 glucose units, but no helical structure exists.
Gelatinisation
As a starch granule is heated in water the water is absorbed, the granule structure is altered by the loss of crystallisation of amylopectin, and the starch takes up a random formation. This is followed by swelling, hydration and solubilisation.
Rapidly digestible starch. Heating starch in water results in a soluble preparation.. This gelatinised digestible starch is readily and rapidly digested by the enzymes of the gastrointestinal tract.
Slowly digestible starch consists of physically inaccessible amorphous starch and raw granules with type A and C crystalline structures.
Retrogradation
Starch retrogradation is a process which occurs when the molecules of gelatinised starch begin to associate again in an ordered structure. In its initial phases two or more starch chains may form a simple junction point, which may then develop into a more extensively ordered region.
Resistant starch
resists intestinal and pancreatic enzymatic digestion. Such starch may be resistant because
the starch is physically inaccessible to enzymatic digestion eg grains, seeds and dense, processed starch
the granular nature of the starch
retrograded amylose formed during cooling.
All of these whilst being resistant to human digestive processes are susceptible to colonic bacterial attack.
The development of resistant starch may occur during food processing, in the baking of bread, cooling and storage. Amylose forms a strong retrograded starch, stable to 120°C, while amylopectin retrograde starch can be disrupted by gentle heating.
Resistant starch resists intestinal and pancreatic enzymatic digestion. Such starch may be resistant because
the starch is physically inaccessible to enzymatic digestion eg grains, seeds and dense, processed starch
the granular nature of the starch
retrograded amylose formed during cooling.
All of these whilst being resistant to human digestive processes are susceptible to colonic bacterial attack.
The development of resistant starch may occur during food processing, in the baking of bread, cooling and storage. Amylose forms a strong retrograded starch, stable to 120°C, while amylopectin retrograde starch can be disrupted by gentle heating.
Sugar
In the UK it is recommended that the average intake of non-milk extrinsic sugar (i.e. other than milk lactose ) should not exceed 60 g/day or 10% of the total dietary intake.
Starches
It has been recommended that starches should provide the balance of dietary energy not provided by the whole protein, fat and non-milk extrinsic sugars; that is, on average 37% of total dietary energy for the adult population. The same principle should be applied to children over 2 years old.
Key points
1. In biology most carbohydrates exist as high molecular weight polymers. These polysaccharides are simple sugars connected by glycosidic bonds.
2. Cellulose is a major structural polysaccharide and accounts for approximately half of the carbon found in plants. Cellulose consists of polymeric glucose with a glycosidic linkage of the b -(1–4) type.
3. The two major polysaccharides for energy storage are starch in plant cells and glycogen in animal cells. They differ in their chain lengths and branching patterns. Glycogen is highly branched with an a -(1–6) linkage occurring every 8 to 10 glucose units along the backbone with short side chains of approximately 8 to 12 glucose units each. Starch occurs both as unbranched amylose and as branched amylopectin.
4. The chemical and physical changes associated with gelatinisation and retrogradation have important consequences for food production and intestinal absorption.
5. Starch is hydrolysed by the enzyme amylase. Starches which are not hydrolysed by amylase are resistant starches.
6. Starch, proteins or lipids may interact to form aggregates.
7. Starches should provide the balance of dietary energy not provided by the whole protein, fat and non-milk extrinsic sugars; that is, on average 37% of total dietary energy for the adult population.
energy.