Food Chem Lecture Notes

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Food Chemistry Lecture Notes

Transcript of Food Chem Lecture Notes

Properties of selected Carbohydratesi. sugarMono- and oligosaccharides and their corresponding sugar alcohols, with a few exceptions, are sweet. -D-Mannose has a sweet-bitter taste, and some oligosaccharides are bitter.Chain and Ring forms simple sugars can exist in a chain form or a ring form ring form is favored in aqueous solutions Isomers, which differ only in their configuration about their carbonyl carbon atom are called anomers.

-D-Glucose -D-Glucose Stereochemistry- study of the arrangement of atoms in three-dimensional space Stereoisomers - compounds in which the atoms are linked in the same order but differ in their spatial arrangement Enantiomer - one of two stereoisomers that are mirror images of each other that are non-superposable (not identical)Polysaccharides are polymers of simple sugars. Many polysaccharides, unlike sugars, are insoluble in water. Dietary fiber includes polysaccharides and oligosaccharides that are resistant to digestion and absorption in the human small intestine but which are completely or partially fermented by microorganisms in the large intestine.Polysaccharides are widely and abundantly distributed in nature, fulfilling roles as:

Structure-forming skeletal substances (cellulose, hemicellulose and pectin in plants; chitin, mucopolysaccharides in animals). Assimilative reserve substances (starch, dextrins, inulin in plants; glycogen in animals).

Water-binding substances (agar, pectin and alginate in plants; mucopolysaccharides in animals).

ii. StarchStarch is composed of a mixture of two substances: amylose, an essentially linear polysaccharide, and amylopectin, a highly branched polysaccharide. Both forms of starch are polymers of -D-Glucose. Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble.

Amylosemolecules consist typically of 200 to 20,000 glucose units which form a helix as a result of the bond angles between the glucose units.

Amylose

Amylopectin differs from amylose in being highly branched. Short side chains of about 30 glucose units are attached with 16 linkages approximately every twenty to thirty glucose units along the chain. Amylopectin molecules may contain up to two million glucose units.

Amylopectin

The side branching chains are clustered together within the amylopectin molecule

Starches are transformed into many commercial products by hydrolysis using acids or enzymes as catalysts.

Hydrolysis is a chemical reaction in which water is used to break long polysaccharide chains into smaller chains or into simple carbohydrates.iii. Hemicellulose polysaccharide components of plant cell walls other than cellulose comprise almost one-third of the carbohydrates in woody plant tissue. The chemical structure of hemicelluloses consists of long chains of a variety of pentoses, hexoses, and their corresponding uronic acids. Though indigestible, they can be fermented by yeasts and bacteria. found in fruit, plant stems, and grain hulls. The polysaccharides yielding pentoses on hydrolysis are called pentosans.Xylan is an example of a pentosan consisting of D-xylose units with 14 linkages

Xylan

iv. Cellulose a polymer of -D-Glucose Cellulose is the major structural material of plants. Cellulose can be hydrolyzed to its constituent glucose units by microorganisms that inhabit the digestive tract of termites and ruminants. Cellulose may be modified in the laboratory by treating it with nitric acid (HNO3) to replace all the hydroxyl groups with nitrate groups (-ONO2) to produce cellulose nitrate (nitrocellulose or guncotton) which is an explosive component of smokeless powder. v. Pectin Acts as a cementing material in the cell walls of all plant tissues. The white portion of the rind of lemons and oranges contains approximately 30% pectin. Pectin is the methylated ester of polygalacturonic acid, which consists of chains of 300 to 1000 galacturonic acid units joined with 14 linkages. The Degree of Esterification (DE) affects the gelling properties of pectin. Pectin is an important ingredient of fruit preserves, jellies, and jams.

Pectin is a polymer of -Galacturonic acid with a variable number of methyl ester groups Commercial pectins are galacturonoglycans [poly(a-D-galactopyranosyluronic acids) with various contents of methyl ester groups (Table 2). Native pectins found in the cell walls and intercellular layers of all land plants are more complex molecules that are converted into commercial products during extraction with acid.

Commercial pectin is obtained from citrus peel and apple pomace. Pectin from lemon and lime peel is the easiest to isolate and is of the highest quality. Pectins have an unique ability to form spreadable gels in the presence of sugar and acid or in the presence of calcium ions and are used almost exclusively in these types of applications.

Note: Pomace is the solid remains of grapes, olives, or other fruit after pressing for juice or oil. It contains the skins, pulp, seeds, and stems of the fruit.

Glycosidic Linkage A covalent bond which bonded a carbohydrate molecule to another molecule is called as glycosidic bond.

Generally glucose circulate in bloodstream in the form of glucose or form disaccharides, in which two Monosaccharides linked with glycosidic linkage which formed by a condensation reaction between the hydroxyl group of the first-position carbon and the hydroxyl group of the fourth-position carbon of each monosaccharide units. During polymerization, carbonyl group of one monosaccharide unit gets condenses with alcoholic unit of another monosaccharide to form disaccharide unit.

vi. Gum Arabic an exudate of acacia trees, of which there are many species distributed over tropical and subtropical regions

a heterogeneous material, but generally consists of two fractions. One is about 70% of the gum, composed of polysaccharide chains with little or no nitrogenous material, and molecules of higher molecular weight that have protein as an integral part of their structures.

The polysaccharide structures, both those attached to protein and those that are not, are highly branched

Gum Arabic dissolves easily when stirred in water. It is unique among the food gums because of its high solubility and the low viscosity of its solutions.

The soft drink industry consumes about 30% of the gum supply as an emulsifier and stabilizer.

More than half the world's supply of gum arabic is used in confections such as caramels, toffees, jujubes, and pastilles. Its functions in confections are to prevent sucrose crystallization and to emulsify and distribute fatty components

vii. Algal Polysaccharide Algal polysaccharides, or seaweed gums, are derived from brown seaweeds, such as alginates, and red seaweeds, such as agar and carrageenan, which is also known as Irish moss. Alginates form insoluble gels that are used as emulsifiers, thickeners, and binders in food production.

Agar Agar is a gelatinous substance, obtained from algae and discovered in the late 1650s or early 1660s by Minoya Tarozaemon in Japan, where it is called Kanten. Agar is derived from the polysaccharide agarose, which forms the supporting structure in the cell walls of certain species of algae, and which is released on boiling. Agar is actually the resulting mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin

The structure of an agarose polymer.

Agar forms a gel that is soluble in hot water but not cold water. Agar is used as a culture medium for microbes as well as a stabilizer in many food items. Carrageenan

are a family of linear sulfated polysaccharides

Carrageenan can be broken into components that do and do not form gels. Carrageenan is used in many food items to thicken them and create a smooth texture. extracted from red seaweeds with a dilute alkaline solution; the sodium salt of a carrageenan is normally produced.

Also prepared and used is an alkali-modified seaweed flour called processed Euchema seaweed (PES) or Phillippine natural grade (PNG) carrageenan

Chitin (C8H13O5N) is a long-chain polymer of a N acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g., crabs, lobsters and shrimps) and insects, the radulae of molluscs, and the beaks and internal shells of cephalopods, including squid and octopuses. The structure of chitin is comparable to the polysaccharide cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has also proven useful for several medical and industrial purposes.

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