carbohydrates definition configuration sugar classification chemical reactions polysaccharides gums...
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CARBOHYDRATECARBOHYDRATESSDEFINITIONCONFIGURATIONSUGAR CLASSIFICATIONCHEMICAL REACTIONSPOLYSACCHARIDESGUMS
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Importance of Importance of carbohydratescarbohydrates
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 tasteWe 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
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Importance of Importance of carbohydratescarbohydratesWe use them as fat substitutes
◦ Modifies starches & celluloses, and gumsWe 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
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Definition of a Definition of a carbohydratecarbohydrateThe word originates from “carbon” and
“hydrate” or “hydrates of carbon”Cx(H2O)y
The empirical formula showed equal numbers of carbons and water◦ X=6 and Y=6 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
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Classification of Classification of carbohydratescarbohydrates
Monosaccharides◦ The simplest of the CHO forms◦ Building blocks of other higher
carbohydratesDisaccharides
◦ Two monosaccharide unitsOligosaccharides
◦ 2-10 monosaccharide unitsPolysaccharides
◦ >10 monosaccharide units
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Monosaccharide Monosaccharide classificationclassification1. The number of carbons (3-9)
◦ triose, tetrose, pentose, hexose….
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5
4
3
2
6
Fischer projection of monosaccharides6
Monosaccharide Monosaccharide classificationclassification2. 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)
(simplest of all sugars)
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Monosaccharide Monosaccharide classificationclassification3. Type of carbonyl group
◦ ALDOSE = Aldehyde group Glucose, galactose and mannose most common in
foods
◦ KETOSE = Ketone group Fructose most important
isomers
Aldehyde
Ketone
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Sugar ring formationSugar ring formationMost sugar units of carbohydrates in nature
(and thus foods) have ring structuresFormed 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
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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
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The most common sugar ring The most common sugar ring formsforms
Pyranose◦ Six-member rings◦ More thermodynamically
favorable◦ Most common
Furanose◦ Five-member rings◦ More kinetically
favorable
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The more correct representation of the The more correct representation of the ring formring formThe 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?
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Other important Other important monosaccharidesmonosaccharides
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Sugar alcoholsSugar 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
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DisaccharidesDisaccharidesClassified by many as the smallest
oligosaccharides Formed by a condensation reaction between 2
monosaccharide units forming a glycosidic bondMost common:
◦ Sucrose◦ Lactose◦ Maltose
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Sucrose (table sugar)Sucrose (table sugar) 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
The bond can be broken by hydrolysis- Enzyme (fructosidase invertase)- Acid/heatProduct called invert sugar
-1-2
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Note that Fructosehas been flipped
and that it is in the-position
MaltoseMaltose 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
Reducing end
-1-4
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LactoseLactose 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
Reducing end
Cleaved by lactase (enzyme)
-1-4
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Lactose Lactose Problems with lactose in foodsA) 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 >> 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 19
LactoseLactose
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
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Tri- and tetrasaccharidesTri- 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
Gal
Glu
Gal
Glu
Fru
Fru
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Some properties of mono and Some properties of mono and oligosaccharidesoligosaccharides
RELATIVE SWEETNESS
SUGAR RELATIVE SWEETNESS SUGAR RELATIVE SWEETNESS
D-FRUCTOSE 175 RAFFINOSE 23SUCROSE 100 STACHYOSE ----D-GLUCOSE 40-79 XYLITOL 90
-D-GLUCOSE <40 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
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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
Some properties of mono and Some properties of mono and oligosaccharidesoligosaccharides
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Some properties of mono and Some properties of mono and oligosaccharidesoligosaccharidesRELATIVE 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 (γ) 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 24
Some properties of mono and Some properties of mono and oligosaccharidesoligosaccharides
WATER ADSORPTION AND AW CONTROL
SUGAR WATER ADSORPTION
D-GLUCOSE 0.07D-FRUCTOSE 0.28SUCROSE 0.04MALTOSE (HYDRATE) 5.05MALTOSE (ANHYDROUS) 0.80LACTOSE (HYDRATE) 5.05LACTOSE (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
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Chemical reactionsChemical reactionsMUTAROTATION
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
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Chemical reactionsChemical reactionsMUTAROTATION
= +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
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Chemical reactionsChemical reactionsMUTAROTATION
~37%
0.0026%
<<1%
~63%
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Chemical reactionsChemical reactionsHYDROLYSIS (Disaccharides and
beyond…)
Low pH and high temperature favor reaction Usually stable at alkaline conditions
Starch and Sucrose
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Chemical reactionsChemical reactions
REDUCTION
Reducing sugarsMonosaccharides
◦ Glucose◦ Fructose◦ All others
Di and oligosaccharides s◦ Maltose◦ Lactose
Non-reducingMonosaccharides
◦ NoneDi and
oligosaccharides◦ Sucrose◦ Raffinose◦ Stacchyose
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Chemical reactionsChemical reactionsREDUCTIONHydrogenation to the double bond between the
oxygen and the carbon group of an aldose or ketose
H+
What about fructose?
oxidation
reduction
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Chemical reactionsChemical reactions
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
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ENOLIZATION/ISOMERIZATION
Chemical reactionsChemical reactions
Lactulose used in infant nutrition as a bifidus factor - promotes friendly bacteria in breast milk
Not hydrolyzed by digestion - strong laxative - prevents constipation
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ENOLIZATION/ISOMERIZATION
Chemical reactionsChemical reactionsDEHYDRATIONFavored at acid pHOccurs when you heat sugar solids or syrups
with a dilute acid solutionLeads to dehydration of sugars with the b-
elimination of waterLeads to furan end productsHEXOSE - 3 H2O + HMF
(Hydroxymethyl furfural)◦ Flowery odor, bitter/astringent flavor
PENTOSE - 3 H2O + Furfural
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Chemical reactionsChemical reactions
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- Detrimental to thermally
processed fruit juices
- Indicator of thermal abused products
DEHYDRATION REACTIONS
Chemical reactionsChemical reactions
Both contribute to flavor of baked bread
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DEHYDRATION REACTIONS
Chemical reactionsChemical reactionsDEHYDRATION REACTIONS
CARMELIZATIONBrown pigment & caramel aromaFormed by melting sugar or syrups in acid or alkaline catalystsDehydration, degradation and polymerization
1
2
3
4
5PIGMENT
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Chemical reactionsChemical reactions
MAILLARD BROWNINGBrowning in foods happen via:
1) Oxidative reactions2) 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
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Chemical reactionsChemical reactionsMAILLARD BROWNING Not well defined and not all pathways
known However, the following must be there for
Maillard browning to occur:1) A compound with an amino group (typically an
amino acid or protein – most commonly lysine)2) A reducing sugar (most commonly glucose)3) Water
Can follow the reaction by observing color formation (420 or 490 nm in a spectrophotometer) or by following CO2 production
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Chemical reactionsChemical reactions
MAILLARD BROWNINGGeneral 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 foodsUndesirable Attributes Off-flavor Texture - unintentional in products such as dried
milk and mashed potatoes
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OH
O
H OH
OH H
H OH
H OH
D-glucose
+
NH2
R1
OH NH
R1
R
H
- H 2 O
N
R1
R
H
O
CH2OH
OH
OH
OH
NH R1
D-glucosylamine
Chemical reactionsChemical reactionsMAILLARD BROWNINGGeneral stagesFirst 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
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Chemical reactionsChemical reactions
MAILLARD BROWNINGThe glycosamine undergoes Amadori
rearrangement to produce a 1-amino-2-keto sugar (1-amino-2-ketose)
Amadori compound
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MAILLARD BROWNING
Degradation of Amadori compound2 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
dehydrationReductones
- Strong odor/flavor- Can also form melanoidins
Favored by low pH (<5)
Favored by less acid pH (>5)
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Chemical reactionsChemical reactionsMAILLARD BROWNINGStrecker 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
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Chemical reactionsChemical reactionsMAILLARD BROWNING
Examples of volatiles that form via Maillard browning 50:50 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”!
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Chemical reactionsChemical reactionsMAILLARD BROWNING
Examples of volatiles that form via Maillard browning (cont.)
Aroma compounds can vary with temperature◦ Valine at 100°C rye bread aroma◦ Valine at 180°C chocolate aroma◦ Proline at 100°C burnt protein◦ Proline at 180°C pleasant bakery aroma◦ Histidine at 100°C no aroma◦ Histidine at 180°C cornbread, buttery, burnt sugar aroma
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Chemical reactionsChemical 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
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Chemical reactionsChemical reactionsMAILLARD BROWNING
Factors which affect browning (cont.)◦ Temperature
Higher temperatures catalyzes Linear up to 90°C then more rapid increase
◦ Carbohydrate structure Pentoses (most reactive) > Hexoses > Disaccharides >
Oligosaccharides > 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
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Chemical Chemical reactionsreactions
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--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
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Chemical reactionsChemical reactionsMAILLARD BROWNING
Undesirable consequences of browning1) Aesthetically and sensorially undesirable
◦ Dark colors, strong odors and flavors
2) 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
3) Leads to anti-nutritional effects◦ Loss of essential amino acids◦ Primarily lysine; may be critical in lysine limited foods
(cereals, grain products)
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Chemical Chemical reactionsreactions
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
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CH2
NH2
O
OH2NH3
+
O
OH
HH
H
OH
OH
H OH
H
OH
+
O
O
NH2
NH2
OH
H
H
OH
OH
H OH
N
COOHNH2
O
OH
OH
Glucose
Chemical Chemical reactionsreactions
MAILLARD BROWNING
Undesirable consequences of browning (cont.)
Acrylamide formation
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CarbohydrateAsparagineAcrylamide