Download Document

CARBOHYDRATES (or Glucides, from Greek glucos, sweet)
• They are the most abundant and spread organic compounds on earth.
• They have a central role in the metabolism of plants and animals.
• Their biosynthesis in green plants, i.e. photosynthesis, starting from CO2 and H2O, in
presence of light, is the existence base of all the other organisms.
• They are the basic constituents of many foods representing a large portion of the total
intake of nutrients in human diet (40-75% of the daily energy intake).
• Also non digestible carbohydrates are important in a balanced daily diet.
• Carbs have other important functions in foods (beyond the energetic):
-they act as sweetening;
-as gel- and paste- forming;
-as thickening;
-as stabilizers;
-they are forerunners of aromatic substances and colorants which are formed in the food
during production and subsequent processes (also cooking).
1
Carbohydrates can be defined as polyhydroxy aldehydes, ketones, alcohols, acids,
their simple derivatives and their polymers having acetal type linkages.
They may be classified according to their degree of polymerization and may be divided
into three principal groups, namely sugars, oligosaccharides and polysaccharides.
Cn(H2O)n
Carbs chemical formula:
This is a simplification, there are other molecules having different formula but reacting as
carbs, thus belonging to the same category (e.g. deoxysugars, aminosugars, sugars with
carboxyl moiety).
CLASSIFICATION
Class (DP*)
Sub-Group
Some components
Monosaccharides
Glucose, galactose, fructose
Disaccharides
Sucrose, lactose
Polyols
Sorbitol, mannitol
Malto-oligosaccharides
Maltodextrins
Other oligosaccharides
Raffinose, stachyose, fructo-oligosaccharides
Starch
Amylose, amylopectin, modified starchs
Non-starch polysaccharides
Cellulose, Hemicelluloses, Pectins, β Glucans, Fructans, Gums, Mucilages2
Sugars (1-2)
Oligosaccharides (2-9)
Polysaccharides (>9)
DP * = Degree of polymerization
•
Monosaccharides: polyhydroxy-aldehydes or -ketones with a linear carbon chain (3 to 8 carbon
atoms); e.g.: glucose, fructose and galactose;
glucose
fructose
galactose
• Oligosaccharides: formally derived from the condensation of monosaccharides, with H2O elimination;
disaccharides: sucrose, maltose, lactose; trisaccharides: raffinose; tetrasaccharides: stachyose;
raffinose
lactose
• Polysaccharides: polymers with high MW,
having different characteristics with respect to
other carbs; often insoluble in H2O, they are
not sweet and they are somewhat inert; e.g.:
starch, cellulose, pectins.
cellulose
(portion)
3
Total sugar in
various foods
Food
Total sugar (%)
Vaccine milk
4,8
Human milk
7,2
Cheese
0,1-0,9
Yoghurt
7,8
Fruit yoghurt
15,7
Ice cream
22,2
Apples
11,8
Banana
20,9
Grape
15,4
Oranges
8,5
Honey
75
Jam
Chocolate
50-70
60
Beer
1,5-2,3
Wine
0-5
Dessert wine
5-15
4
Sugars distribution in fruit and vegetables
Glucose
Tomatoes
Mais
Fructose
Soy
Pineapple
Sucrose
Orange juice
Onions
Grapes
Maltose
Figs
Carrots
Galactose
Cabbages
Bananas
Stachyose,
Raffinose,
Verbascose
Apples
0%
20% 40% 60% 80% 100%
5
MONOSACCHARIDES
NOMENCLATURE
The suffix –ose indicates the presence of the carbonyl moiety in an hydroxylated carbon chain.
ALDOSES: polyhydroxyaldehydes
KETOSES: polyhydroxyketons deriving
deriving formally from glyceraldehyde by
formally from dihydroxyacetone by the
the addition of –CH-OH units (they can
addition of –CH-OH units (they can be
be triose, tetrose…).
triulose, tetrulose,… ); the position of
carbonyl is specified by a numeric prefix
H
(usually is in position 2).
O
H
OH
CH2OH
CH2OH
suffix: -ose
O
suffix: -ulose
CH2OH
If two carbonyl moieties are present:
-the molecule can be a dialdose (two aldehydes groups);
-or osulose (one aldehyde and one ketone group);
-or diulose (two ketone groups).
When –OH is substituted by –H, the molecule is a deoxysugar, when –OH is substituted
by –NH2, the molecule is an aminodeoxysugar.
6
CYCLIZATION
All the monosaccharides starting from tetroses and 2-pentuloses cyclize to five and six
membered lactols (furanoses from furan, and pyranoses, from pyran) respectively by
intramolecular hemiacetal (or hemiketal) formation.
With the exception of erythrose, monosaccharides crystallize in cyclic forms; in solution there
is an equilibrium between the open chain and the cyclic forms, the second being
predominant.
7
CONFIGURATION
ALDOSES
Glyceraldehyde has a chiral center, thus it exists as a pair of enantiomers, D and L forms.
From D-glyceraldehyde it is possible to obtain a mixture of D-erythrose and D-threose,
while from L-glyceraldehyde a mixture of L-erythrose and L-threose is obtained.
CN
CN
O
CHO
H
OH
HCN
H
OH
HO
H
H
OH
H
OH
CH2OH
D-glyceraldehyde
1) H
+
CH2OH
O
OH
+
H
H
OH
OH
+
OH
H
OH
OH
H
2) NaHg/NaBH4
CH2OH
D-erythrose
D-threose
8
Through the cyanhydrin reaction, starting from D-glyceraldehyde two D-tetroses are
obtained and from each of them two D-pentoses, and so forth; thus from Dglyceraldehyde 8 hexoses belonging to the same D-series can be obtained.
CHO
H
OH
D-glyceraldehyde (D-glycero-)
CH2OH
CHO
CHO
D-erythrose (D-erythro-)
H
OH
HO
H
OH
H
OH
CH2OH
CH2OH
CHO
H
H
OH
HO
H
OH
H
OH
HO
H
OH
H
OH
H
CH2OH
CHO
CHO
CHO
CHO
CHO
HO
H
OH
H
OH
HO
H
OH
H
OH
H
OH
H
OH
HO
H
OH
H
OH
H
OH
H
OH
H
D-allose
(D-allo-)
HO
H
H
HO
H
OH
H
CH2OH
CHO
OH
HO
H
H
OH
HO
H
HO
H
H
OH
H
CH2OH
CH2OH
H
OH
CH2OH
D-altrose D-glucose D-mannose D-gulose
(D-altro-) (D-gluco-) (D-manno-) (D-gulo-)
OH
D-lyxose
(D-lyxo-)
CHO
OH
CH2OH
OH
D-xylose
(D-xylo-)
H
CH2OH
H
CHO
CH2OH
D-arabinose
(D-arabino-)
CHO
H
H
CH2OH
D-ribose
(D-ribo-)
D-threose (D-threo-)
H
HO
H
H
CHO
H
CHO
OH
HO
H
OH
HO
H
HO
H
H
HO
H
HO
H
OH
CH2OH
D-idose
(D-ido-)
H
OH
CH2OH
D-galactose
(D-galacto-)
H
OH
CH2OH
D-talose
(D-talo-)9
An important aldoses degradation reaction takes place via disulfone formation
starting from the dithioacetal.
CH(SEt)2
CHO
EtSH, H
Epimers:
+
CH(SO2Et)2
-
OH
RCOOOH
CHOH
CHOH
CHOH
R
R
R
CH2(SO2Et)2
+
HC
O
R
two molecules differing only for the configuration of a chiral center, e.g. D-
glucose and D-mannose.
Occurence of aldoses
10
KETOSES
Name, structure
Where is it found?
hexulose
D-fructose
vegetables, honey
D-psicose
Eptulose, octulose,
nonulose
D-manno-2-heptulose
D-glycero-D-manno-2octulose
D-erythro-L-gluco-2nonulose
residues of fermented
molasses
avocado
"
"
L series??
11
SYSTEMATIC NOMENCLATURE
ALDOSE
If the number of C atoms is <=6, traditional name can be used, otherwise the molecule
portion adjacent to the carbonyl is assigned the maximum possible prefix and the remaining
portion of the molecule, (if constituted at least by 2 C atoms), another prefix is assigned and
this is named first; then the name is written depending on the total of C atoms.
Examples:
CHO
CHO
H
OH
HO
H
H
OH
H
OH
H
HO
H
H
OH
H
OH
OH
H
OH
H
OH
H
OH
H
OH
HO
H
HO
H
H
CH2OH
D-gluco-
H
HO
H
OH
CH2OH
D-glucose or
D-gluco-hexose
HO
L-manno-
H
OH
D-glycero-
D-glycero-L-mannoheptose
KETOSE
If it contains less than 4 chiral carbons, traditional name can be used, otherwise the groups
12
adjacent to the carbonyl must be considered:
The longest of the two portions adjacent to carbonyl is named first:
HO
HO
OH
H
OH
HO
H
HO
H
L-glycero-
H
H
H
HO
H
H
OH
H
OH
D-arabino
D-threoD-lyxoD-threo-L-glycero3-hexulose
or D-arabino-2-hexulose
or
D-lixo-3-hexulose
When sugar cyclizes to lactol, a new chiral center is formed, thus two diastereomers
(anomers) are formed, named α and β anomers.
Fructose
glucose
α-D-glucopyranose
β-D-glucopyranose
13
[α]20D=
+110°
°
°
[α]20D= +19°
64%
36%
At equilibrium:
°
[α]20D= +52,53°
All the monosaccharides can
exist in solution in five forms:
<1%
Cyclic forms are much favoured with respect to open chain forms.
Generally, the favourite cyclic form, more stable, is the pyranosic one.
14
PHYSICAL PROPERTIES
1. OPTICAL ROTATION AND MUTAROTATION
Non racemic chiral compounds deviate polarized light by an angle α proportional to their
concentration in the solution.
[α ]λ
t
100 ∗ α
=
l ∗c
α = deviation angle at T °C;
l = polarimetric tube length (dm);
c = grams of optically active substance in 100 ml of solution;
[α
α]tλ = specific rotation constant;
λ = selected wave length (generally sodium D-line light);
t = temperature at which the measurement is done (usually 20-25°C).
Obviously specific rotation for two anomers is different (and even for furanose
and pyranose forms of a same sugar); thus a solution of a pure isomer freshly
prepared has a rotation angle varying during time till it reaches a constant value
(at equilibrium among the various forms).
MUTAROTATION
PHENOMENON
15
2. Higroscopicity and solubility
• The amount of water kept by the sugars depends on the sugar structure, the isomers
present and the sugar purity.
• Solubility of mono- and oligosaccharides in water is good.
• Anomers can have very different solubility (e.g. α and β lactose).
• Monosaccharides have low solubility in ethanol and they are insoluble in organic
solvent such as benzene, ethyl ether, chloroform.
16
SENSORY PROPERTIES
• Mono-, oligosaccharides and their alcohols are sweet (few exceptions);
• Main sweeteners: sucrose, glucose, fructose, invert sugar (glucose and fructose),
lactose and alcohols (sorbitol, mannitol, xylitol).
• Sugars differ in the quality of sweetness and taste intensity.
• As oligosaccharides dimension increases, their sweetness power decreases.
• The taste intensity can be quantificated by determining the minimum level of
concentration at which the sweet taste is still detected or referring to a reference
solution (usually sucrose).
Sugar
Limit of
detection %
Fructose
Glucose
Lactose
Maltose
Sucrose
0.24
1.17
2.60
1.36
0.36
Sugar
Sucrose
D-Glucose
D-Galactose
D-Fructose
Invert sugar
Maltose
Lactose
Relative
sweetness
100
69
63
114
95
46
39
Sugar
D-Mannitol
D-Mannose
Raffinose
D-Ramnose
D-Sorbitol
Xylitol
D-Xylose
Relative
sweetness
69
59
22
33
51
102
67
17
• The minimum value depends on the affinity between the substance’s structure
and the chemoreceptor sites for sweetness.
• Further parameters influencing the quality and intensity of sweetness are: pH,
temperature, presence of other compounds.
Relative sweetness
Temperature dependance
of relative sweetness of
some sugars
• There is also a relation between sugar content
and volatiles compounds.
fructose
glucose
galactose
maltose
Temperature (°
°C)
• Also the color of the solution can influence the
organoleptic evaluation.
• Composition and concentration of sweetener
must be carefully evaluated in each food
formulation to give an optimal sensory result.
Need of an AH (H donor) B (H acceptor) X
(hydrophobic site) system in a substance in
order to give sweet taste.
D-glucopyranose
18
REACTIVITY
1) REDUCTION to ALCOHOLS
• NaBH4
• electrolysis
• catalytic hydrogenation
Alcohol name: in the sugar name –ulose or –ose is substituted with –itol.
Xylitol (pentose), sorbitol (naturally found in many fruits), D-mannitol are used in diet
formulations, to decrease water activity, as softeners, etc. They afford 2,4 Kcal/g.
2) a. OXIDATION to ALDONIC ACIDS
β-D-glucopyranose
NAME: ALDOSE
ALDONIC ACID
19
2) b. OXIDATION to ALDARIC ACIDS
Stronger conditions (e.g. HNO3) allow oxidation of both the terminal carbons of
aldose:
It can form mono or dilactones
NAME: ALDOSE
ALDARIC ACID (dicarboxylic acid)
2) c. OXIDATION to URONIC ACIDS
To oxidize saturated terminal carbon only, the carbonyl moiety of the aldose
must be protected; then, after deprotection, the uronic acid is obtained.
20
Uronic acids are widespread in nature, forming polysaccharides (e.g. pectines) having
industrial applications as gel-forming.
3) REACTION in BASIC and/or ACIDIC MEDIA
• Monosaccharides are stable in a pH range of 3-7 (if compounds with amino
groups are not present).
• At low pH enolization followed by H2O loss, predominates.
• At very high pH, enolization followed by chain fragmentation, predominates.
21
3) a. REACTION in STRONGLY ACIDIC MEDIUM
• Disaccharides and oligosaccharides are formed (intermolecular glycosidic
bond).
• When the monosaccharide conformation is proper a glycosidic intramolecular
bond can be formed.
• Warming in acidic medium, enolization, dehydratation, formation of
substituted furans and pyrans take place:
H
HC
H
HO
HC
O
HC
OH
OH
H
O
HO
H
- H2O
HC
O
O
O
H
O
H
- H2O
H
CHO
OHC OH
H
- H2O
O
O
H
OH
H
OH
H
OH
H
H
OH
H
OH
H
OH
H
CH2OH
CH2OH
CH2OH
Where is the
mistake???
OH
CH2OH
CH2OH
CH2OH
HMF (Hydroxy
Methyl Furfural)
22
3) b. REACIONS in STRONGLY BASIC MEDIUM
• In strongly basic medium aldose and ketose enolise quickly, thus fructose, mannose and
glucose equilibrate by the formation of the shared 1,2-enediol.
• In presence of O2 or other oxidants (Cu2+) the double bond C,C breaks forming
carboxylic acids.
The method is applied to the quali- quantitative determination of reducing sugars.
• Anyway also other transformations can take place leading to formation of several volatile
compounds.
Some of the volatile
compounds formed
warming up
fructose syrup at
pH 8-10 for 3 h.
•
Acetic acid
•
Hydroxyacetone
•
Hydroxybutanone
•
Furfurylalcohol
•
5-Methyl-2-furfurylalcohol
•
γ-butyrolactone
•
Various cyclopentenolones
Cyclopentenolones are
typical compounds with
“caramel like” aroma
O
HO
H3C
23
NON ENZYMATIC BROWNING
Sugars contribute to the organoleptic characteristics of a food not only by their presence, but
also by the products of their degradation.
Caramelization (neutral or basic/acidic catalysis, high temperature, sugars)
Maillard reaction (neutral or basic/acidic catalysis, high temperature, sugars, amino groups)
NR
O
OH
HO
glucide
RNH2
OH
HO
Schiff base
NHR
NR
O
OH -H2O
OH
O
-NH2R
O
O
HOH2C
H
CH2
HO
HMF
enaminol
NHR
Amadori compound
O
HO
Browning is due to the polymerization of
many molecules having low MW.
Polymers formed (melanoidins) have
structures as the following:
N
R
N
R
N
R
X
X
X=O, NR
24
Aroma compounds are formed: lactones, furanons, pyranons, aldehydes, etc.
O
CH3
H3C
O
HO
CH3
CH3
Sotolon (typical aroma of
brown sugar).
OH
HO
O
O
O
CH3
O
Negative aspects:
• Milk browning;
• Loss of essential aminoacids;
• Formation of potentially carcinogenic compounds (heterocyclic amines);
• Formation of aroma not always good (acrolein, piruvic aldehyde, glyoxal, etc.).
Positive aspects:
Formation of desirable aroma (toasting of coffee, cooking of food, etc.);
Formation of colour compounds (cooking of bread, of meat);
Formation of antioxidant compounds that protect the food against oxidation.
25