Download CARBOHYDRATES - Food Science & Human Nutrition

CARBOHYDRATES
DEFINITION
CONFIGURATION
SUGAR CLASSIFICATION
CHEMICAL REACTIONS
POLYSACCHARIDES
GUMS
1
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
2
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
3
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
◦ 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
4
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
◦ >10 monosaccharide units
5
Monosaccharide classification
1. The number of carbons (3-9)
◦ triose, tetrose, pentose, hexose….
1
2
3
4
5
6
Fischer projection of monosaccharides
6
Monosaccharide classification
2. Configuration
(simplest of all sugars)
◦ 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)
7
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
8
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
9
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
10
The most common sugar ring forms

Pyranose
◦ Six-member rings
◦ More thermodynamically
favorable
◦ Most common

Furanose
◦ Five-member rings
◦ More kinetically favorable
11
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?
12
Other important monosaccharides
13
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
14
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
15
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
-1-2
between the anomeric
carbons of Glu and Fru
This renders the anomeric
carbons non-reactive and the The bond can be broken by hydrolysis
sugar is therefore called a
- Enzyme (fructosidase invertase)
NON-REDUCING sugar
- Acid/heat
Product called invert sugar
16
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
17
Lactose


Galactose and glucose
The only sugar found in milk
-1-4
◦ 4.8% in cows
◦ 6.7% in humans
◦ The primary carbohydrate source
for developing mammals
◦ Stimulates uptake and retention of
calcium

Food products
Reducing end
◦ Milk
◦ Unfermented dairy products
Cleaved by lactase (enzyme)
◦ Fermented dairy products
 Contain less lactose
 Lactose converted to lactic acid
18
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 >> 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
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
20
Tri- and tetrasaccharides
Galactosylsucroses
 Raffinose (3) and Stachyose (4)
◦ Found primarily in legumes
Gal
◦ 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
Glu
Gal
Fru
Glu
Gal
Fru
◦ How do we minimize this problem?
21
Some properties of mono and oligosaccharides
RELATIVE SWEETNESS
SUGAR
RELATIVE SWEETNESS
D-FRUCTOSE
SUCROSE
-D-GLUCOSE
-D-GLUCOSE
-D-GALACTOSE
-D-GALACTOSE
-D-MANNOSE
-D-MANNOSE
-D-LACTOSE
-D-LACTOSE
-D-MALTOSE
175
100
40-79
<40
27
--59
BITTER
16-38
48
46-52
SUGAR
RELATIVE SWEETNESS
RAFFINOSE
STACHYOSE
XYLITOL
SORBITOL
GALACTITOL
MALTITOL
LACTITOL
23
--90
63
58
68
35
22
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
23
Some properties of mono and oligosaccharides
RELATIVE SWEETNESS

Artificial sweeteners
◦ Much sweeter than natural sugars






Cyclamate
Aspartame
Acesulfame K
Saccharin
Sucralose
– 30 times sweeter
– 200
– 200
– 300
– 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 oligosaccharides
WATER ADSORPTION AND AW CONTROL
SUGAR
D-GLUCOSE
D-FRUCTOSE
SUCROSE
MALTOSE (HYDRATE)
MALTOSE (ANHYDROUS)
LACTOSE (HYDRATE)
LACTOSE (ANHYDROUS)

WATER ADSORPTION
0.07
0.28
0.04
5.05
0.80
5.05
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
25
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
26
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
27
Chemical reactions
MUTAROTATION
~37%
<<1%
0.0026%
~63%
28
Chemical reactions
HYDROLYSIS (Disaccharides and beyond…)
Low pH and high temperature favor reaction
Usually stable at alkaline conditions
Starch and Sucrose
29
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
30
Chemical reactions
REDUCTION

Hydrogenation to the double bond between the oxygen and
the carbon group of an aldose or ketose
oxidation
H+
What about
fructose?
reduction
31
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 1,2Endiol
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
32
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
33
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
34
Chemical reactions
DEHYDRATION REACTIONS
H C O
Detrimental to
thermally
 processed fruit
juices
- Indicator of
thermal abused
products
-
H C OH
D - Glucose
H C OH
H
H C OH
HO C H HOH
1,2-Endiol
H C O
C O
H C H
3-Deoxy-Dglucosulose
CHO
HOH
O
HOH
Furfural
CHO
HOH2C
O
CHO
HOH2C
O
OH
5-Hydroxymethyl
furfural
35
Chemical reactions
DEHYDRATION REACTIONS
H2
C OH
H2
H
C O
C OH
C O
HO C H
HOH
HO C H
D-Fructose
H C H
C OH
CH3
Both contribute to
flavor of baked
bread
HOH
C O
O C
C OH
O C
C OH
1-Deoxy-D-Erythro
2,3-hexodiulose
C OH
H C
CH2OH
HOH
HO
H3CO
HO
HO
O
CH2
H3CO
HOH
O
O
Isomaltol
HO
Maltol
H3C
O
36
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
37
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
38
Chemical reactions
MAILLARD BROWNING

Not well defined and not all pathways known

However, the following must be there for Maillard
browning to occur:
1)
2)
3)

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
39
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

40
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
O
H
HO
R
OH
H2N
H
H
OH
H
OH
HO
+
NH
R1
H
R1
- H2O
OH
D glucose
-
H
CH 2OH
N
O
OH
R1
NH
OH OH
D glucosylamine
R1
R
41
Chemical reactions
MAILLARD BROWNING
 The glycosamine undergoes Amadori rearrangement to
produce a 1-amino-2-keto sugar (1-amino-2-ketose)
Amadori
compound
42
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 (>5)
Favored by low pH (<5)
43
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
44
Chemical reactions
MAILLARD 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”!
45
Chemical reactions
MAILLARD 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
46
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
47
Chemical reactions
MAILLARD 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
48
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-dlactone

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
49
Chemical reactions
MAILLARD BROWNING
Undesirable consequences of browning
1)
Aesthetically and sensorially undesirable
◦
Dark colors, strong odors and flavors
Formation of mutagenic compounds
2)
◦
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
3)
◦
◦
Loss of essential amino acids
Primarily lysine; may be critical in lysine limited foods (cereals, grain
products)
50
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
51
Chemical reactions
OH
MAILLARD BROWNING
H
Undesirable consequences of
browning (cont.)
 Acrylamide formation
O
O H
H
OH
H
HO
H2N
+
O
OH
H
OH
NH2
OH
OH
H
Carbohydrate
Asparagine
Acrylamide
COOH
OH
OH
H
H
OH
NH2
O
N
HO
NH2
+
H2C
O
H2O
NH3
Glucose
52