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CARBOHYDRATES

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CARBOHYDRATES DEFINITION CONFIGURATION SUGAR CLASSIFICATION CHEMICAL REACTIONS POLYSACCHARIDES GUMS * * * Chemical reactions DEHYDRATION Favored at acid pH Occurs ... – PowerPoint PPT presentation

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Title: CARBOHYDRATES


1
CARBOHYDRATES
  • DEFINITION
  • CONFIGURATION
  • SUGAR CLASSIFICATION
  • CHEMICAL REACTIONS
  • POLYSACCHARIDES
  • GUMS

2
Importance of carbohydrates
  • We use them as our major energy source (4 kcal/g)
  • Humans starch, sucrose and fructose
  • 80 of our energy intake (average)
  • We use them for their sweet taste
  • We use them to provide structure and texture in
    food products
  • Bread pudding (starch) Dextrin (soft drinks)
    Pectin (jellies)
  • We use them to lower water activity of food
    products and also influence ice crystallization
  • Intermediate moist foods Ice cream

3
Importance of carbohydrates
  • We use them as fat substitutes
  • Modifies starches celluloses, and gums
  • We use them to impart desirable flavors and
    colors for certain food products
  • Maillard browning
  • We use them as an energy source in fermentation
    reactions
  • Yogurt
  • We use them for their reported health benefits
  • Dietary fiber

4
Definition of a carbohydrate
  • The word originates from carbon and hydrate
    or hydrates of carbon
  • Cx(H2O)y
  • The empirical formula showed equal numbers of
    carbons and water
  • X6 and Y6 for glucose, galactose and fructose
  • Simple carbs. are polyhydroxy aldehydes (aldoses)
    ketones (ketoses)
  • By definition carbs. are aldoses, ketoses and
    compounds derived from these via condensation,
    hydrolysis, reduction, oxidation and substitution

5
Classification of carbohydrates
  • Monosaccharides
  • The simplest of the CHO forms
  • Building blocks of other higher carbohydrates
  • Disaccharides
  • Two monosaccharide units
  • Oligosaccharides
  • 2-10 monosaccharide units
  • Polysaccharides
  • gt10 monosaccharide units

6
Monosaccharide classification
  • 1. The number of carbons (3-9)
  • triose, tetrose, pentose, hexose.

Fischer projection of monosaccharides
7
Monosaccharide classification
(simplest of all sugars)
  • 2. Configuration
  • Sugars have asymmetric (chiral) carbons and
    therefore can exist in two forms (enantiomers)
  • D-sugar vs. L-sugar, or (R) vs. (S)
  • Based on the location of the OH group of the
    highest asymmetrical center (right D left L)

8
Monosaccharide classification
  • 3. Type of carbonyl group
  • ALDOSE Aldehyde group
  • Glucose, galactose and mannose most common in
    foods
  • KETOSE Ketone group
  • Fructose most important

Aldehyde
Ketone
isomers
9
Sugar ring formation
  • Most sugar units of carbohydrates in nature (and
    thus foods) have ring structures
  • Formed by a reaction between the aldehyde or
    ketone group and an OH group of the sugar
  • This results in ring structures called
  • Hemiacetal (aldoses)
  • Hemiketal (ketoses)
  • These can further react to create di-, oligo- and
    polysaccharides (condensation reactions) and
    react with alcohols

10
Formation of ?- and ?-anomers of D-glucose A
new asymmetric center is created and the carbon
at that center is known as the anomeric carbon
(labeled ) If the OH is facing down at C then
we have the ?-anomer If the OH is facing up at
C then we have the ?-anomer
11
The most common sugar ring forms
  • Pyranose
  • Six-member rings
  • More thermodynamically favorable
  • Most common
  • Furanose
  • Five-member rings
  • More kinetically favorable

12
The more correct representation of the ring form
  • The pyranose and furanose rings are not flat
  • For pyranose rings the chair and boat forms are
    better representations of their actual structures
  • The furanose rings are present as either envelope
    or twist conformations

Which is the more stable form?
13
Other important monosaccharides
14
Sugar alcohols
  • No carboxyl group
  • Can be produced by reducing monosaccharides
  • Unusual sweet taste (cool)
  • Popular in sugar free applications
  • Slowly absorbed
  • Contribute calories
  • 100g Extra gum 60g sugar alcohols 165 kcal
  • Can have laxative effect ?
  • Humectants ? lower aw
  • Used to protect proteins in freezing and drying
    applications
  • Safe and non-browning

15
Disaccharides
  • Classified by many as the smallest
    oligosaccharides
  • Formed by a condensation reaction between 2
    monosaccharide units forming a glycosidic bond
  • Most common
  • Sucrose
  • Lactose
  • Maltose

16
Sucrose (table sugar)
Note that Fructose has been flipped and that it
is in the ?-position
  • Naturally present
  • Popular ingredient in foods (very large daily
    consumption)
  • Used widely in fermentation
  • Different commercial forms
  • Composed of glucose and fructose
  • The glycosidic bond is formed between the
    anomeric carbons of Glu and Fru
  • This renders the anomeric carbons non-reactive
    and the sugar is therefore called a NON-REDUCING
    sugar

?-1-2
  • The bond can be broken by hydrolysis
  • Enzyme (fructosidase invertase)
  • Acid/heat
  • Product called invert sugar

17
Maltose
  • 2 units of glucose
  • Forms from the breakdown of starch during malting
    of grains (barley) and commercially by using
    enzymes (?-amylase)
  • E.g. malt beverages beer
  • Used sparingly as mild sweetener in foods
  • Very hygroscopic
  • OH-group can be reactive and we term this as a
    REDUCING SUGAR
  • Is free to react with oxidants

?-1-4
Reducing end
18
Lactose
  • Galactose and glucose
  • The only sugar found in milk
  • 4.8 in cows
  • 6.7 in humans
  • The primary carbohydrate source for developing
    mammals
  • Stimulates uptake and retention of calcium
  • Food products
  • Milk
  • Unfermented dairy products
  • Fermented dairy products
  • Contain less lactose
  • Lactose converted to lactic acid

?-1-4
Reducing end
Cleaved by lactase (enzyme)
19
Lactose
  • Problems with lactose in foods
  • A) Crystallization during drying
  • Appearance of glass in milk powder
  • Sandy texture in ice cream
  • Sometimes dissolved while other times it will not
    dissolve
  • ?-D-lactose VERY INSOLUBLE (5 gm/100 ml)
  • Causes the glass-like appearance in foods
  • ?-D-lactose MORE SOLUBLE (45 gm/100 ml)
  • If ?gtgt? more ? will form
  • Limits amounts of milk solids one can use in
    formulations
  • Quick drying ? get non-crystalline lactose
    (amorphous) ? no crystalline form
  • Slow drying or concentration ? more crystalline
    lactose

20
Lactose
  • B) Color and flavor
  • Lactose is a reducing sugar
  • Can react with proteins and form undesirable
    color and flavors
  • Problem with dairy product and dairy ingredients,
    especially during drying, concentration and
    heating
  • C) Lactose intolerance
  • Some lack enzyme lactase
  • Age and ethnic group related
  • Lactase ? lactic acid problem for the
    intestines
  • Gas, bloating, diarrhea, acid buildup
  • Several ways to prevent or minimize this problem

21
Tri- and tetrasaccharides
  • Galactosylsucroses
  • Raffinose (3) and Stachyose (4)
  • Found primarily in legumes
  • Poorly absorbed in small intestine and
    indigestible
  • We cant hydrolyze the ? 1-6 linkage
  • Bacteria in intestines use it and produce gas ?
    Cause of flatulence
  • Flatulence is not socially acceptable in some
    societies really?
  • Possibly inhibited by phenolic compounds
  • How do we minimize this problem?

Gal
Glu
Fru
Gal
Glu
Gal
Fru
22
Some properties of mono and oligosaccharides
  • RELATIVE SWEETNESS
  • SUGAR RELATIVE SWEETNESS SUGAR
    RELATIVE SWEETNESS
  • D-FRUCTOSE 175 RAFFINOSE 23
  • SUCROSE 100 STACHYOSE ---
  • ?-D-GLUCOSE 40-79 XYLITOL 90
  • ?-D-GLUCOSE lt40 SORBITOL 63
  • ?-D-GALACTOSE 27 GALACTITOL 58
  • ?-D-GALACTOSE --- MALTITOL 68
  • ?-D-MANNOSE 59 LACTITOL 35
  • ?-D-MANNOSE BITTER
  • ?-D-LACTOSE 16-38
  • ?-D-LACTOSE 48
  • ?-D-MALTOSE 46-52

23
Some properties of mono and oligosaccharides
  • RELATIVE SWEETNESS
  • Sweetness of molecules is explained in part by
    the AH-B theory
  • Level of sweetness depends on how strongly
    certain receptors in our tongue interact with
    molecules
  • Depends on
  • Type of chemical groups
  • Spatial arrangement
  • Polarity
  • Distance between groups
  • Electron density
  • Hydrogen and hydrophobic bonding

24
Some properties of mono and oligosaccharides
  • RELATIVE SWEETNESS
  • Artificial sweeteners
  • Much sweeter than natural sugars
  • Cyclamate 30 times sweeter
  • Aspartame 200
  • Acesulfame K 200
  • Saccharin 300
  • Sucralose 600
  • Problem ? they are all very bitter
  • Another bond (?) is apparently needed for good
    sweetness (lipophilic interaction)
  • Reason why artificial sweeteners taste bitter
  • Sucralose, derived from sucrose, is believed to
    give the most natural sweet taste of them all

25
Some properties of mono and oligosaccharides
  • WATER ADSORPTION AND AW CONTROL
  • SUGAR WATER ADSORPTION
  • D-GLUCOSE 0.07
  • D-FRUCTOSE 0.28
  • SUCROSE 0.04
  • MALTOSE (HYDRATE) 5.05
  • MALTOSE (ANHYDROUS) 0.80
  • LACTOSE (HYDRATE) 5.05
  • LACTOSE (ANHYDROUS) 0.54
  • OH-groups in sugars reason for water-binding and
    solubility
  • e.g. 4-6 per sucrose
  • More H2O binding more reduction in aw as well
    as increased viscosity
  • Water-binding and solubility is temperature
    dependent

26
Chemical reactions
  • MUTAROTATION
  • Process by which various anomeric forms attain an
    equilibrium in solution
  • First established studying spectral properties of
    sugars
  • Rotation of plane polarized light by an
    asymmetric center
  • Rotation varies from sugar to sugar and anomere

27
Chemical reactions
  • MUTAROTATION
  • ? 112
  • ? 18.7
  • Equilibrium 52.7
  • At equilibrium
  • 37 ?
  • 63 ?

For any sugar - the occurrence of mutarotation
implies that a small amount of the straight chain
form must be present
28
Chemical reactions
MUTAROTATION
37
ltlt1
0.0026
63
29
Chemical reactions
  • HYDROLYSIS (Disaccharides and beyond)

Low pH and high temperature favor reaction
Usually stable at alkaline conditions Starch and
Sucrose
30
Chemical reactions
  • REDUCTION
  • Reducing sugars
  • Monosaccharides
  • Glucose
  • Fructose
  • All others
  • Di and oligosaccharides s
  • Maltose
  • Lactose
  • Non-reducing
  • Monosaccharides
  • None
  • Di and oligosaccharides
  • Sucrose
  • Raffinose
  • Stacchyose

31
Chemical reactions
  • REDUCTION
  • Hydrogenation to the double bond between the
    oxygen and the carbon group of an aldose or ketose

oxidation
What about fructose?
H
reduction
32
Chemical reactions
ENOLIZATION/ISOMERIZATION
  • Aldose ketose sugars are enolized in the
    presence of alkali solutions
  • Thus glucose, mannose fructose can be in
    equilibrium with each other through a 1,2-Endiol
  • Therefore, you can get isomerization (transfer of
    1 sugar type to another type) of varying yield
  • Can happen during storage and heating
  • Glucose in dilute alkali after 21 days
  • 66 Glucose
  • 29 Fructose
  • 1 Mannose

33
Chemical reactions
ENOLIZATION/ISOMERIZATION
Lactulose used in infant nutrition as a bifidus
factor - promotes friendly bacteria in breast
milk Not hydrolyzed by digestion - strong
laxative - prevents constipation
34
Chemical reactions
  • DEHYDRATION
  • Favored at acid pH
  • Occurs when you heat sugar solids or syrups with
    a dilute acid solution
  • Leads to dehydration of sugars with the
    b-elimination of water
  • Leads to furan end products
  • HEXOSE ? - 3 H2O HMF (Hydroxymethyl furfural)
  • Flowery odor, bitter/astringent flavor
  • PENTOSE ? - 3 H2O Furfural

35
Chemical reactions
DEHYDRATION REACTIONS
  • Detrimental to thermally
  • processed fruit juices
  • Indicator of thermal abused products

36
Chemical reactions
DEHYDRATION REACTIONS
Both contribute to flavor of baked bread
37
Chemical reactions
DEHYDRATION REACTIONS
1
CARMELIZATION Brown pigment caramel
aroma Formed by melting sugar or syrups in acid
or alkaline catalysts Dehydration, degradation
and polymerization
2
3
4
5
PIGMENT
38
Chemical reactions
  • MAILLARD BROWNING
  • Browning in foods happen via
  • 1) Oxidative reactions
  • 2) Non-oxidative reactions
  • Oxidative reactions involve enzymes and oxygen
  • Polyphenol oxidase ? browning in pears, apples,
    bananas, shrimp etc. (covered later)
  • No carbohydrates directly involved
  • Non-oxidative reactions are non-enzymatic
    browning reactions
  • Maillard browning

39
Chemical reactions
  • MAILLARD BROWNING
  • Not well defined and not all pathways known
  • However, the following must be there for Maillard
    browning to occur
  • A compound with an amino group (typically an
    amino acid or protein most commonly lysine)
  • A reducing sugar (most commonly glucose)
  • Water
  • Can follow the reaction by observing color
    formation (420 or 490 nm in a spectrophotometer)
    or by following CO2 production

40
Chemical reactions
  • MAILLARD BROWNING
  • General effects
  • Flavor, color, odor
  • Decline in protein quality
  • Usually a decline in digestibility as well as
    lysine availability
  • Temperature and aw (0.6 to 0.7) favor the
    reaction
  • Desirable Attributes
  • Color flavor of baked, roasted and dried foods
  • Undesirable Attributes
  • Off-flavor
  • Texture - unintentional in products such as dried
    milk and mashed potatoes

41
Chemical reactions
  • MAILLARD BROWNING
  • General stages
  • First reaction
  • Carbonyl carbon of the reducing sugar is reacted
    to the nitrogen of an amino acid (nucleophilic
    attack electron of the N attack C)
  • A glycosamine (a.k.a. glycosylamine) is formed
  • Reversible reaction
  • Not favorable at low pH

42
Chemical reactions
  • MAILLARD BROWNING
  • The glycosamine undergoes Amadori rearrangement
    to produce a 1-amino-2-keto sugar
    (1-amino-2-ketose)

Amadori compound
43
  • MAILLARD BROWNING
  • Degradation of Amadori compound
  • 2 pathways
  • Melanoidin pigments
  • Brown N-polymers
  • Flavor and color of cola, bread, etc.
  • HMF
  • Astringent bitter flavor
  • Unacceptable
  • Good odor
  • Can form melanoidins
  • Can also form via dehydration
  • Reductones
  • Strong odor/flavor
  • Can also form melanoidins

Favored by less acid pH (gt5)
Favored by low pH (lt5)
44
Chemical reactions
  • MAILLARD BROWNING
  • Strecker degradation
  • Reaction of an amino acid with dicarbonyl
    compounds formed in the Maillard reaction
    sequence
  • The amino acid is converted to an aldehyde
  • Aldehydes formed that contribute to the aroma of
    bread, peanuts, cocoa, maple syrup, chocolate
  • CO2 produced
  • Produces pyrazines
  • Very powerful aroma compounds
  • Corny, nutty, bready, crackery aromas
  • Also produces pyrroles
  • Strong aroma and flavor compounds
  • Favored at high temperature and pressure

45
Chemical reactions
  • MAILLARD BROWNING
  • Examples of volatiles that form via Maillard
    browning
  • 5050 amino acid D-glucose
  • Glycine ? caramel aroma
  • Valine ? rye bread aroma
  • Glutamine ? chocolate
  • Amino acid type matters
  • Sulfur containing a.a. produce different aromas
    than other a.a.
  • Methionine glucose ? potato aroma
  • Cysteine glucose ? meaty aroma
  • Cystine glucose ? burnt turkey skin!

46
Chemical reactions
  • MAILLARD BROWNING
  • Examples of volatiles that form via Maillard
    browning (cont.)
  • Aroma compounds can vary with temperature
  • Valine at 100C ? rye bread aroma
  • Valine at 180C ? chocolate aroma
  • Proline at 100C ? burnt protein
  • Proline at 180C ? pleasant bakery aroma
  • Histidine at 100C ? no aroma
  • Histidine at 180C ? cornbread, buttery, burnt
    sugar aroma

47
Chemical reactions
  • MAILLARD BROWNING
  • Factors which affect browning
  • Water activity
  • Max at aw 0.6-0.7
  • pH
  • Neutral and alkaline pH is favored
  • Acid pH slows down or inhibits browning
  • Amino group on amino acid is protonated and
    glucosamine production prevented
  • Metals
  • Copper and iron catalyze browning
  • Catalyze oxidation/reduction type reactions

48
Chemical reactions
  • MAILLARD BROWNING
  • Factors which affect browning (cont.)
  • Temperature
  • Higher temperatures catalyzes
  • Linear up to 90C then more rapid increase
  • Carbohydrate structure
  • Pentoses (most reactive) gt Hexoses gt
    Disaccharides gt Oligosaccharides gt Sucrose (least
    reactive)
  • Fructose (ketose) is far less reactive than
    glucose (aldose)
  • Concentration of open form
  • Pigment formation is directly proportional to the
    amount of open chain form

49
Chemical reactions
  • MAILLARD BROWNING
  • Inhibition/control of browning
  • Lower pH and T
  • Control aw
  • Use non-reducing sugar
  • Remove substrate
  • E.g. drying of egg whites
  • Add enzyme (D-glucose oxidase) prior to drying to
    oxidize glucose to glucono-d-lactone
  • Use sulfiting agents (most common chemicals used)
  • React with carbonyls to prevent polymerization
    and thus pigment formation
  • Problems
  • Degrade thiamine, riboflavin and oxidize
    methionine
  • Can cause severe allergies

50
Chemical reactions
  • MAILLARD BROWNING
  • Undesirable consequences of browning
  • Aesthetically and sensorially undesirable
  • Dark colors, strong odors and flavors
  • Formation of mutagenic compounds
  • Data shows that some products from the reaction
    of D-glucose or D-fructose with L-lysine or
    L-glutamic acid may demonstrate mutagenicity
  • Leads to anti-nutritional effects
  • Loss of essential amino acids
  • Primarily lysine may be critical in lysine
    limited foods (cereals, grain products)

51
Chemical reactions
  • MAILLARD BROWNING
  • Undesirable consequences of browning (cont.)
  • Due to its highly reactive and basic amino group
    lysine is most susceptible to Maillard browning
    reactions
  • Extent of lysine degradation in milk products

Milk ºC Time Degradation ()
Fresh 100 Few minutes 5
Condensed --- --- 20
Non-fat dry 150 Few minutes 40
Non-fat dry 150 3 hours 80
52
Chemical reactions
  • MAILLARD BROWNING
  • Undesirable consequences of browning (cont.)
  • Acrylamide formation

Carbohydrate Asparagine Acrylamide
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