1. No category

Carbohydrate (2)
CHEMICAL PROPERTIES
•
•
•
•
•
•
•
•
Oxidation
Reduction
Fermentation
Polymerization
Hydrolisis
Browning non enzymatic
Caramelization
Osazone formation
OXIDATION OF GLUCOSE
• The reaction is commonly used for quantitative
determination of sugars
• Fehling solution is an alkaline solution of copper(II)
that oxidizes an aldose to an aldonate and in the
process is reduced to copper(I), which precipitates
as brick-red Cu2O.
OXIDATION OF GLUCOSE
O
O
C
H
H C OH
HO C H
H C OH
C OH
Benedicts
reagent
+ Cu
H C OH
CH2OH
D-glucose
Glucose is a reducing
sugar
2+
Glucose is oxidized to
a carboxylic acid
H C OH
[O]
HO C H
H C OH
H C OH
CH2OH
D-gluconic acid
+ Cu2O(s)
Cu+ (reduced
form)
Reduction
• Reduction : Reduction of carbonyl group gives
thecorresponding alcohol e . g . glucose gives
sorbitol , ribose gives ribitol , galactose gives
galacticol.
Reduction
• Hydrogenation is the addition of hydrogen
to a double bond
• Sorbitol can be obtained by hydrogenation
of D-glucose
• D-Mannitol can be obtained by
hydrogenation of D-mannose.
• Xylitol is produced from hydrogenation of
D-xylose obtained from hemicelluloses
Reduction
Fermentation
• Fermentation : Fermentation is the action of
bacterial or Yeast enzymes on carbohydrate.
• Yeast : saccharomyces cerevisae
Polymerization
• Polymerization reactions are building reactions
• These reactions build polymers from monomers
and lose water in the process
• Polymerization reactions between
carbohydrates leads to the formation of
– Disaccharides
– Oligosaccharides
– Polysaccharides
• Polymerization reactions:
– Occur between intermolecular hydroxyl groups
– Result in a covalent glycosidic bond linkage
Polymerization
Browning Reaction
• Maillard browning
reducing sugar + amine  brown pigments +
flavors
• Caramelization
high temps
sugar  brown pigments + flavors
• Enzymatic browning
polyphenoloxidase
phenolics  brown pigments + flavors
Browning non enzymatic
• Under some conditions, reducing sugars produce
brown colors that are desirable and important in
some foods.
• Other brown colors obtained upon heating or during
long-term storage of foods containing reducing
sugars are undesirable.
• Common browning of foods on heating or on
storage is usually due to a chemical reaction
between reducing sugars, mainly D-glucose, and a
free amino acid or a free amino group of an amino
acid that is part of a protein chain.
• This reaction is called the Maillard reaction
(nonenzymic browning)
Browning non enzymatic
• Maillard browning products, including soluble
and insoluble polymers, are found where
reducing sugars and amino acids, proteins,
and/or other nitrogen-containing compounds
are heated together, such as in soy sauce and
bread crusts.
• Maillard reaction products are important
contributors to the flavor of milk chocolate. The
Maillard reaction is also important in the
production of caramels, toffees, during which
reducing sugars also react with milk proteins
Step Maillard
• Condensation : reducing sugar + primary
amine glucose glycosyloamine + water
• Amadori rearrangement : glucose
glycosyloamine  intermediet (1-amino-1-αdeoxy- α -D-fructopyranose)  flavor + color
• Polymerization : intermediet  melanoidin
(dark brown) + flavor
Browning non enzymatic
• Desirable:
 Flavor - coffee, cocoa, meats
 Color - bread crust, syrup, meat
 Antioxidants
• Undesirable
 Color - changes in color during storage
 Flavor - changes during processing and storage
 Nutritional loss - essential amino acids,
 Vitamins (vit c)
 Toxicity/mutagenicity
Caramelization
• The formation of the caramel pigment can be
considered a nonenzymatic browning reaction in the
absence of nitrogenous compounds.
• When sugars are subjected to heat in the absence of
water or are heated in concentrated solution, a
series of reactions occurs that finally leads to
caramel formation.
• Heating of carbohydrates, in particular sucrose
reducing sugars, without nitrogen-containing
compounds effects a complex group of reactions
termed caramelization. Reaction is facilitated by
small amounts of acids and certain salts.
Caramelization
• Mostly thermolysis causes dehydration of the
sugar molecule with introduction of double
bonds or formation of anhydro rings.
• Conjugated double bonds absorb light and
produce color.
• Brown caramel color made by heating a sucrose
solution with ammonium bisulfite is used in cola
soft drinks, other acidic beverages, baked
goods, syrups, candies, pet foods, and dry
seasonings
Caramelization
Caramelization
• At high temperatures, sugar reactions are
accelerated
 Isomerization
 Water elimination
 Oxidation
• Caramelization occurs at
 High temperatures (~150°C)
 Low water content/high sugar
• Formation of
Caramel flavors and
 Enediols
pigments
 Dicarbony
Caramelization
Sugar
Fructose
Galactose
Glucose
Maltose
Sucrose
T (oC)
110
160
160
180
160
Osazone formation
• Osazone formation : Osazone are characteristic
crystals resulting from the reaction of sugars with
phenylhydrazine . All sugars having free carbonyl
group can form osazone crystal
• Phenylhydrazine (C6H5NHNH2) reacts with
carbons #1 and #2 of reducing sugars to form
derivatives called osazones. The formation of these
distinctive crystalline derivatives is useful for
comparing the structures of sugars
Osazone formation
• Firstly glucose with phenylhydrazine gives us
glucosephenylhydrazone by elimination of a
water molecule from the functional group.
• The next step involves reaction of one mole
of glucosephenylhydrazone with two moles
of phenylhydrazine (excess).
Osazone formation
SUGAR VS
ARTIFICIAL
SWEETERNER
•
•
•
•
•
Aspartame
Saccharin
Sucralose
cyclamate
Stevia
STEVIA
• Active compounds: group of diterpene
(steviol) glycosides
– dulcoside A, rebaudiosides A-E , steviolbioside,
&stevioside
• Rebaudioside A (3.8%) and stevioside (9.1%)
are the sweetest, most abundant, & most
important commercially
Chemical structure of stevioside (left) and
rebaudioside A (right)
Extraction & Commercial Uses
• Processing of dried leaves from Stevia
produces a powdery white substance
(stevioside), which is three hundred
times sweeter, by weight, than sucrose
derived from sugar cane
• Water extraction process followed by
crystallization techniques to isolate the
steviol glycosides
Nutrition
• The dried leaves are 30 to 45 times as sweet
as sucrose
• Non-nutritive, three hundred times sweeter
than sugar, heat stable, non-fermentable,
as well as an anti-plaque and anti-carie
• More studies must be conducted on
bulking agents to make it easy to replace it
for sugar
• saccharin may not be as sweet as they taste, it
contains anthranilic acid, nitrous acid, sulfur
dioxide and chlorine fused with ammonia.
• Sucralose, which is sugar chemically modified
by the addition of chlorine
• The key is a molecule known only as GIV3727
that specifically targets and inhibits a handful
of human bitter taste receptors,