carbohydrates definition configuration sugar classification chemical reactions polysaccharides gums...

CARBOHYDRATECARBOHYDRATESSDEFINITIONCONFIGURATIONSUGAR CLASSIFICATIONCHEMICAL REACTIONSPOLYSACCHARIDESGUMS

1

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

2

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

3

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

4

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

5

Monosaccharide Monosaccharide classificationclassification1. The number of carbons (3-9)

◦ triose, tetrose, pentose, hexose….

1

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)

7

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

8

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

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 The most common sugar ring formsforms

Pyranose◦ Six-member rings◦ More thermodynamically

favorable◦ Most common

Furanose◦ Five-member rings◦ More kinetically

favorable

11

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?

12

Other important Other important monosaccharidesmonosaccharides

13

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

14

DisaccharidesDisaccharidesClassified by many as the smallest

oligosaccharides Formed by a condensation reaction between 2

monosaccharide units forming a glycosidic bondMost common:

◦ Sucrose◦ Lactose◦ Maltose

15

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

16

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

17

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

18

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

20

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

21

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

22

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 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


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

25

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

26


= +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


~37%

0.0026%

<<1%

~63%

28

Chemical reactionsChemical reactionsHYDROLYSIS (Disaccharides and

beyond…)

Low pH and high temperature favor reaction Usually stable at alkaline conditions

Starch and Sucrose

29

Chemical reactionsChemical reactions

REDUCTION

Reducing sugarsMonosaccharides

◦ Glucose◦ Fructose◦ All others

Di and oligosaccharides s◦ Maltose◦ Lactose

Non-reducingMonosaccharides

◦ NoneDi and

oligosaccharides◦ Sucrose◦ Raffinose◦ Stacchyose

30

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

31


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

32

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

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

34


35

- Detrimental to thermally

processed fruit juices

- Indicator of thermal abused products

DEHYDRATION REACTIONS


Both contribute to flavor of baked bread

36

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

37


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

38

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

39


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

40

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

41


MAILLARD BROWNINGThe glycosamine undergoes Amadori

rearrangement to produce a 1-amino-2-keto sugar (1-amino-2-ketose)

Amadori compound

42

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)

43

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

44

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”!

45


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


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


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 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

49


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)

50


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

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


MAILLARD BROWNING

Undesirable consequences of browning (cont.)

Acrylamide formation

52

CarbohydrateAsparagineAcrylamide

carbohydrates definition configuration sugar classification chemical reactions polysaccharides gums...

Documents

sugar units of carbohydrates

monosaccharide classification

oh group

sugar ring formation

aldehyde group glucose

ketone group fructose

importance of carbohydrates

forms enantiomers dsugar