Carbohydrates in Foods

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Carbohydrates in Foods
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Carbohydrates

• Aldehyde or ketone compounds with multiple
  hydroxyl (-OH) groups Cm(H2O)n
• One of the four major classes of biomolecules
• Make up most of organic matter on earth
• Have multiple roles in living organisms
• Energy source (starch in plants, glycogen in
  animals)
• (ATP is phophorylated sugar)
• Metabolic intermediates
• Part of RNA and DNA (ribose and deoxyribose
  sugars)
• Cell wall of bacteria and plants (cellulose in
  plants)
• Linked to many proteins and lipids (e.g.,
  glycoproteins)

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Monosaccharides

• The simplest carbohydrates
• Aldehydes or ketones with 2 or more OH groups
• (CH2O)n
• n 3 (trioses ? 3 C) is the smallest
• - glyceraldehyde ? aldose containing an
  aldehyde group
• - dihydroxyacetone ? ketose containing a keto
  group

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• Glyceraldehyde has a single asymetric carbon
• ? 2 stereoisomers (D- and L- configuration)
• tetroses ? 4 C
• pentoses ? 5 C
• hexoses ? 6 C (e.g., glucose and fructose)
• heptoses ? 7 C
• Consider D- and L- at the farthest C from from
  the aldehyde or ketone group.
• aldose ? C1 at CHO
• ketose ? C1 at CH2OH
• If n asymmetric C or chiral carbon atom, the
  number of stereoisomers 2n

ketose
aldose
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• Aldose - one terminal carbonyl (CO) group
• Ketose - one non-terminal carbonyl group
• Enantiomers stereoisomers which are mirror
  image of each other (i.e., D-form and L-form)
• Diastereoisomers stereoisomers which are not
  mirror image of each other (e.g., D-erythrose and
  D-threose)
• Epimers stereoisomers which are different only
  at a single asymetric atom (e.g., D-glucose and
  D-mannose at C2)
• Anomers stereoisomers which are different only
  at an anomeric carbon atom (e.g., ?-D- and
  ?-D-glucose at C1 ?-D- and ?-D-fructose C2)

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• The predominant forms of glucose and fructose are
  not open chains.
• The open-chain forms can cyclize into rings.
• - glucose ? reaction of the OH group at C5 with
  CHO group at C1 forming a six-member ring called
  pyranose
• - fructose ? reaction of the OH group at C5 with
  CO group at C2 forming a five-member ring called
  furanose
• - fructose can also be present as pyranose
  (predominant in free state)
• - Additional asymetric C is created ? OH group
  can be up (?) or down (?)

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• In water, ?-D-glucose and ?-D-glucose
  interconvert (called mutarotation) through the
  open-chain form to give an equibrium mixture (?
  33, ? 66, and open-form 1).
• The pyranose ring is not flat but can be in chair
  (predominant) and boat conformations
• The furanose ring is not flat but can be in
  envelope conformations

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Disaccharides

• Formed by formation of a glycosidic linkage/bond
  between two monosaccharide molecules
• ROH R'OH ? R-O-R' H2O
• involving -OH bonded to anomeric carbon of a
  cyclic sugar
• C12(H2O)11
• Glycosidic bonds (C-O-C) between monosaccharide
  units are the basis of oligosaccharide and
  polysaccharide formation
• ?- or ?- anomers can be bonded to any OH on the
  other sugar

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• The carbons which participate in a glycosidic
  bond are numbered.
• For example, two molecules of ?-D-glucose can be
  bonded by ?(1?4) or ?(1?6)
• Non-reducing end and reducing end can occur.
• Common disaccharides are sucrose, maltose, and
  lactose.
• Matose, lactose ? reducing sugars
• Sucrose ? non-reducing sugar
• Maltose has ?(1?4) glucose and glucose
• lactose has ?(1?4) galactose and glucose
• sucrose has ?,?(1?2) glucose and fructose

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• Sucrose can be hydrolyzed by
• a) warm diluted acid
• b) sucrase or invertase
• to glucose and fructose (invert sugar the
  mixture of glucose and fructose).
• The hydrolysis of sucrose inversion
• Sucrose () rotation (dextrorotatory)
• glucose (weakly )
• fructose (strongly -)
• invert sugar (-) rotation (levorotatory)
• Jam processing, yeast fermentation ? inversion

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Oligosaccharides

• Sucrose from cane, beet
• Lactose from milk, hydrolyzed by lactase in
  humans and ?-galactosidase in bacteria
• Matose from starch hydrolysis, hydrolyzed by
  maltase
• Containing 2-20 monosaccharide units linked by
  glycosidic bonds
• Disaccharides, trisaccharides (maltotriose)
  (raffinose Gal-Glu-Fru),
  tetrasaccharides (stachyose Gal-Gal-Glu-Fru)

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Polysaccharides

• Condensation polymers of monosaccharides
• High MW macromolecules, colloids
• General fomular (C6H10O5)n
• Glucose is the commonest monosac. unit.
• 2 classes
• - homopolysaccharides one type of
  monosaccharides
• - heteropolysaccharides more than one type of
  monosaccharides

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• Structure of polysaccharides vary in
• - type of monosac. units
• - order or sequence of monosac. units
• - type of glycosidic linkages (e.g., ?- for
  structural cellulose, and chitin ?- for storage
  starch and glycogen)
• 1. Starch only in plants, low osmotic potential
• - occurs in 2 forms - ?-amylose and amylopectin
  different in degree of branching
• - MW 5000-500,000 - only ?-D-glucose
• - amylose linear, linked by ?-1,4 glycosidic
  bonds, forms helical coils hydrated with water (6
  glucose units per turn), blue complex with iodine
  

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• - amylopectin highly branched, length of each
  branch 25-30 glucose units, linked by ?-1,4 in
  linear and ?-1,6 glycosidic bonds at branching
  points, reddish violet complex with iodine
• 2. Glycogen only in animals (liver, muscle),
  low osmotic potential
• - similar to amylopectin
• - branched-chain polymer of ?-D-glucose
• - linked by ?-1,4 in linear and ?-1,6
  glycosidic bonds
• - but degree of branching (length of each
  branch 10 glucose units) and MW are higher than
  amylopectin

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• 3. Dextran
• - storage polysaccharide in yeast and bacteria
• - only glucose residues
• - but nearly all linkages are ?-1,6
• branching linkages are ?-1,2 or ?-1,3 or
  ?-1,4
• 4. Cellulose
• - structural componet in plant cell wall
• - only glucose residues no branching
• - linked by ?-1,4 forming straight chain (each
  unit is 180 to each other, H-bonds form) ? high
  tensile strength for fibers

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• - mamals have no cellulase, ruminants have
  bacterial cellulase in digestive tracts
• - fungi also produce cellulase
• 5. Chitin
• - in exoskeletons of insects and crustacea
• - similar to cellulose
• - only N-acetyl- ?- D-glucosamine residues
  (glucose substituted with N-acetylamino group for
  OH group at C2) ? amino sugar
• - linked by ?-1,4
• - H-bonds form in each chain ? mechanical
  strength

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• 6. Pectin
• - in plant cell wall
• - mostly D-galacturonic acid residues
  (galactose in which OH at C6 has been oxidized to
  a carboxyl group) ? sugar acid

Lignin - nonpolysaccharides - a polymer of
coniferyl alcohol - very tough and durable
material in wood - part of dietary fiber
and crude fiber
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7. Other Polysaccharide Hydrocolloids

• hydrocolloids ? viscosity, gel
• polysaccharide hydrocolloids most are
  heteropolysac.
• 7.1 Plant exudates
• - gum arabic or gum acacia
• - gum tragacanth
• - gum karaya
• 7.2 Seed gums
• - Locust bean gum
• - Guar gum
• 7.3 Seaweed extracts
• - Carrageenan
• - Algin or Alginate
• - Agar

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Gum arabic
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Gum karaya
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Locust bean gum
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Guar gum
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Guar gum
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Carrageenan
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Agar agar
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Alginate
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• 7.4 Microbial gums
• - Xanthan gum
• - Gellan gum
• - Nata de coco
• 7.5 Cellulose derivatives
• - Carboxymethylcellulose (CMC)
• - Methylcellulose
• - Hydroxypropylcellulose

Inulin - fructo-oligosaccharide or oligofructose
fructans - mostly ?(2-1)
fructosyl-fructose linkage, chicory -
glucose can be found at the end of the fructose
chain ? G(F)n , a chain length 2-20 fructose
units - health benefits (soluble fiber,
prebiotic)
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Nata de coco
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Starch

• Polymer of glucose amylose and amylopectin
• Stored in form of starch granules in plant cells
• Granule size, shape, and ratio of amylose and
  amylopectin depends on type of plant
• Degree of polymerization (DP) of amylose and
  amylopectin depends on type of plant (high DP ?
  high MW)
• Starch properties depend on type of plant

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• Properties of starch
• 1. Gelatinization (not gelation)
• - occurs when starch granules are heated in
  water
• - heat vibrates H-bonds between starch
  molecules ? water molecules can form H-bonds with
  starch
• - then, starch granules swell less free
  water, friction viscosity increases
• - clarity increases
• - loss of birefringence
• - after reaching the highest swelling ?
  granules break down ? viscosity decreases

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• 2. Retrogradation
• - after gelatinization starch molecules are
  dispersed in water forming H-bonds
• - upon cooling ? starch molecules rearrange to
  form H-bonds with themselves instead of water ?
  3D-structure
• - at low concentration slow cooling ?
  sedimentation
• - at high concentration fast cooling ? gel
  network, increased viscosity
• - water molecules are squeezed out ? can cause
  syneresis
• - amylose tend to show stronger retrogradation
  than amylopectin

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• - retrogradation ? bread staling, product
  texture
• Modified starches
• - to improve native starch properties by
• 1. Physical methods heat, milling
• 2. Chemical methods chemicals, enzymes
• 3. Biotechnology genetic engineering
• 4. Combination
• - some examples of starch modification
• 1. Pregelatinization
• 2. Substitution esterification (starch
  acetate, monophosphate), etherification
  (hydroxypropyl starch)

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• 3. Cross-linking links between starch chains
  (distarch phosphate)
• 4. Acid-thinning
• 5. Oxidizing
• Starch hydrolysis
• 1. Dextrinization or pyroconversion heat
  starch and acid in dry conditions ? dextrin
• 2. Liquefaction hydrolysis of gelatinized
  starch by acid and/or enzymes ? maltodextrin (DE
  lt20)
• 3. Saccharification high degree of hydrolysis
  glucose syrup
• DE Dextrose Equivalent content of reducing
  groups as glucose by weight 100/DP

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Pectin

• Present in plant cell wall with cellulose
• Polymers of D-galacturonic acid (?-1,4)
• COOH can be esterified by CH3 ? methoxyl ester
  COOCH3
• Protopectin high methoxyl (high DM, high DE)
• pectinic acid some methoxyl
• pectic acid no methoxyl
• High-methoxyl pectin (gt 50) ? acid, high sugar
• - rapid-set pectin (gt70)
• - slow-set pectin (50-70) forms gel at lower
  T
• Low-methoxyl (lt50) ?Ca2, low sugar

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• Sugar alcohols
• - sugar derivatives
• - polyhydric alcohols, polyols
• - CHO is substituted by CH2OH by hydrogenation
  or fermetation of sugars
• - e.g., sorbitol, mannitol, xylitol
• - lower calories, slowly absorbed, reduced
  tooth decay, cooling effect
• - chewing gums, confectionery
• Sugar sweetness
• - depends on type of sugar

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• LAB
• Starch extraction cell disruption ? centrifuge
• Microscopy of starches
• - 1 of starch suspension (1 water 1 glycerol)
• - iodine solution
• Record
• - characteristics of starch granules (e.g.,
  size, shape, iodine complex)