Reactions of carbohydrates:
Hydrolysis:
● Carbohydrate hydrolysis is a process by which large sugar molecules such as starch,
glycogen and cellulose are broken down into simple sugars such as glucose and fructose.
● This process is important in the breakdown and absorption of dietary carbohydrates as
well as a wide range of biological processes.
● Carbohydrate hydrolysis is an important metabolic process, and its products are used in
many industries such as food, pharmaceutical and chemical industries for a variety of
applications. It is also used in the production of biofuels and for the production of
alcohols and other chemicals.
Cyclincreaction:
● carbohydrate molecules do exist to a very small extent in their open chain form, they
generally prefer to be in their cyclic form.
● This is because the cyclic form of carbohydrates is lower in energy and
thermodynamically more stable than the open chain counterparts.
● In order to transform an open chain carbohydrate into its ring form, an intramolecular
nucleophilic addition reaction must take place.
● In this reaction, a hydroxyl group on the open chain sugar nucleophilically attacks the
carbon of the carbonyl of that same sugar.
● Typically, sugars such as aldohexoses (i.e. glucose) exist as a six-membered ring while
ketohexoses (i.e. fructose) may exist as either five-membered or six-membered rings.
Ribose is an example of an aldopentose that exists only as a five-membered ring.
Thermal degradation:
● It may occur by two different major reaction pathways: Maillard reaction, which takes
place in the presence of amino acids. Caramelization, that occurs when simple sugars are
heated at high temperatures.
● Caramelization is the common name for a group of reactions that occur when
carbohydrates are exposed to high temperatures with no amino groups involved.
● Caramelization commonly occurs when sugars are heated, dry or in concentrated
solution, either alone or with certain additives
● The Maillard reaction, named after L. C. Maillard, is also known as non- enzymatic
browning. It is an extremely complex process and is the reaction between reducing sugars
and proteins by the impact of heat.
Caramelization
● Caramelization is a complex group of reactions that take place when sugars, either dry or
in concentrated aqueous solutions, are subjected to high temperatures to form unique
brown colors and flavors
● Caramelization is a poorly understood series of pyrolysis reactions.
● Most sugars can caramelize and the temperature necessary for caramelization varies with
the type of sugars.
● Fructose, for example, requires an initial temperature of 150℃ while maltose caramelizes
at 180℃
Gelatinization:
● It is a process of breaking down intermolecular bonds of starch molecules in the presence
of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and
oxygen) to engage more water. This irreversibly dissolves the starch granule in water.
Water acts as a plasticizer..
● The gelatinization temperature of starch depends upon plant type and the amount of water
present, pH, types and concentration of salt, sugar, fat and protein in the recipe, as well as
starch derivatization technology are used
ENZYMATIC BROWNING
● Enzymatic browning takes place when the enzyme polyphenol oxidase or other enzymes
catalyze the oxidation of phenols in the fruit to form compounds called quinones. The
quinones can then polymerize to form melanins, which cause the brown pigments
● Enzymatic browning of fruits and vegetables is usually not desirable and creates heavy
economic losses for growers.
● Enzymatic browning is beneficial for: Developing flavor in tea (here the reaction is
incorrectly called fermentation) Developing color and flavor in dried fruit such as figs
and raisins.
Sweetness
● Sweetness is a basic taste most commonly perceived when eating foods rich in sugars.
Sweet tastes are generally regarded as pleasurable.
● In addition to sugars like sucrose, many other chemical compounds are sweet, including
aldehydes, ketones, and sugar alcohols.
● Some are sweet at very low concentrations, allowing their use as non-caloric sugar
substitutes. Such non-sugar sweeteners include saccharin and aspartame. Other
compounds, such as miraculin, may alter perception of sweetness itself.
