Carbohydrates are essential in foods as an energy source (starch is the main source of human calories), a flavouring (simple sugars are usually sweet) and as a functional ingredient (sucrose allows ice cream to be soft in the freezer; xanthan gum thickens a low-fat salad dressing). Carbohydrates are a type of macronutrient found in many foods and beverages. Most carbohydrates are naturally occurring in plant-based foods, such as grains. Food manufacturers also add carbohydrates to processed foods in the form of starch or added sugar. As with all our approaches to food ingredients/constituents we will first examine the structure of carbohydrates and then try to elucidate how their structures allow them to function as they do. As their name suggests, carbohydrates basically made up from sugar and water, i.e. Cx(H2O)y, although this ratio is often not strictly true and occasionally other atoms may be present. The carbons are arranges in a chain (most often 5-6 atoms) functionalized with alcohol groups. The terminal carbon either carries either an aldehyde or a ketone functional group.
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Carbohydrates are classified based on size of base carbon chain, number of sugar units, location of C=O and stereochemistry. Classifications of carbohydrate are monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharide is the smallest possible sugar unit. Examples include glucose, galactose or fructose. When we talk about blood sugar we are referring to glucose in the blood; glucose is a major source of energy for a cell. In human nutrition, galactose can be found most readily in milk and dairy products, while fructose is found mostly in vegetables and fruit. When monosaccharides merge together in linked groups they are known as polysaccharides. Disaccharide is two monosaccharide molecules bonded together. Polysaccharides are polymers. A simple compound is a monomer, while a complex compound is a polymer which is made of two or more monomers. Disaccharides are polysaccharides – “poly…” specifies any number higher than one, while “di…” specifies exactly two.
Examples of disaccharides include lactose, maltose, and sucrose. If you bond one glucose molecule with a fructose molecule you get a sucrose molecule. Sucrose is found in table sugar, and is often formed as a result of photosynthesis (sunlight absorbed by chlorophyll reacting with other compounds in plants). If you bond one glucose molecule with a galactose molecule you get lactose, which is commonly found in milk. Starch, glycogen, dextran and cellulose are polysaccharides. Polysaccharides differ not only in the natural of their component monosaccharides but also in the length of their chains and in the amount of chain branching that occurs. Polysaccharides function as storage materials, structural components, or protective substances.
Thus, starch ( which exists in two forms: amylose and amylopectin ), glycogen and other storage polysaccharides, as readily metabolizable food, provide energy reserves for cells. Chitin and cellulose provide strong support for the skeletons of arthropods and green plants, respectively. In this experiment those activity that had been carried out means to determine the carbohydrate class of an unknown by carrying out a series of chemical reactions with the unknown and known compounds in each class of carbohydrate such as the Molisch test (general CHO), Barfoed’s test (monosaccharides), Fehling’s test (reducing sugars), Benedict’s test (reducing sugars) and Iodine test (amylose).