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
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

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
>10 monosaccharide units

6. Monosaccharide classification
1. The number of carbons (3-9)
triose, tetrose, pentose, hexose…. 1 5 4 3 2 6
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) Naturally present
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 -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
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 >> 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 C) Lactose intolerance
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 RAFFINOSE 23
SUCROSE STACHYOSE ---
-D-GLUCOSE XYLITOL 90
-D-GLUCOSE <40 SORBITOL 63
-D-GALACTOSE GALACTITOL 58
-D-GALACTOSE MALTITOL 68
-D-MANNOSE LACTITOL 35
-D-MANNOSE BITTER
-D-LACTOSE
-D-LACTOSE
-D-MALTOSE

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 (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

25. Some properties of mono and oligosaccharides
WATER ADSORPTION AND AW CONTROL
SUGAR WATER ADSORPTION
D-GLUCOSE
D-FRUCTOSE
SUCROSE
MALTOSE (HYDRATE)
MALTOSE (ANHYDROUS)
LACTOSE (HYDRATE)
LACTOSE (ANHYDROUS)
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%
<<1%
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 Non-reducing
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 Detrimental to thermally processed fruit juices
DEHYDRATION REACTIONS H C O D - G l u c o s e 1 , 2 E n d i 3 x y g 5 r m t h f rf a
Detrimental to thermally
processed fruit juices
Indicator of thermal abused products O C H F u r f ur a l

36. Chemical reactions Both contribute to flavor of baked bread
DEHYDRATION REACTIONS H 2 C O D - F r u c t o s e 1 x y E h , 3 d i l I m a M
Both contribute to flavor of baked bread

37. Chemical reactions DEHYDRATION REACTIONS CARMELIZATION1
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. Degradation of Amadori compound 2 pathways Melanoidin pigments
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)

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
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”!

46. 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

47. Chemical reactions MAILLARD BROWNING Factors which affect browning
Water activity
Max at aw 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 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

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 40
3 hours 80

52. Chemical reactions Carbohydrate Asparagine Acrylamide
MAILLARD BROWNING
Undesirable consequences of browning (cont.)
Acrylamide formation
Carbohydrate
Asparagine
Acrylamide