Cho Reactions

8/11/2019 Cho Reactions

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Carbohydrate Reactions

Important reactions

of carbohydrate molecules

Carbonyl group

(alone)

Oxidation to a

carboxylic acid

group

Reduction to a

hydroxyl group

Cyanohydrin

reaction (and

reaction with other

nucleophiles)

Important reactions (cont.)

Hydroxyl groups Ester formation

Ether formation

Cyclic acetal

Oxidation to

carbonyl

Reduction to deoxy

Replacement with

NH2, SH, or X


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Important reactions (cont.)

Both carbonyl and

hydroxyl groups

Cyclic hemiacetals

(pyranose/furanose) Formation of acetals

(glycosides)

Aldose/ketose

isomerizations

Important reactions

Anomeric hydroxyl

group

Formation of

glycosyl halides

Oxidation to lactones

Carbonyl group oxidation


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Tollens test

2Ag(NH3)2+ + RCHO + 3OH-

2Ag(s) + RCOO- + 4NH3 + 2H2O

A silver

mirror

This is

reduced to

This is

oxidized to

Fehlings test

2Cu(OH)2 + RCHO

RCOOH + Cu2O + H2OA brick red

precipitate

This is

reduced

This is

oxidized

Tollens test Fehlings test


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Variations on the

Fehlings and Tollens tests

Nelson-Somogyi - uses an

arsenomolybdate reagent. You read theoptical density at 500 nm

Benedict - uses copper citrate. This is a

less alkaline medium than the Fehlings

test

Stoichiometric

sugar determination

RCHO + I2 + 3NaOH

RCOO-Na+ + 2NaI + 2H2O

This reaction utilizes oxidation of the aldehyde with

hypoiodite (IO-

) at pH 9.5

Glucose oxidase

O2

H2O2

In alkaline

solution

this exists

as

D-gluconate

D-glucono-

1,5-lactone


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Oxidation of D-glucose

This reaction is used to measure D-

glucose in foods, blood, and otherbiological materials

D-gluconic acid is found naturally (in

small amounts) in fruit juices and honey

Lactones

-lactone

-lactone

Lactone stability

The 1, 5-lactone can be isolated but the 1,

4-lactone is somewhat more stable and

often the only product isolated

Gamma lactone is ~5.8% vs delta lactone

at 4.1% of the acid level


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Glucono-delta-lactone (GDL)

A food acidulant

Primary use is in baking industry where

it reacts slowly with sodium bicarbonate

to produce carbon dioxide

Also GDL prevents discoloration of

refrigerated dough

Produced by oxidation of -D-

glucopyranose

GDL as preservative

Due to its general acidity, GDL will

decrease microbial action in such systems

as

bread dough

fish cakes

surimi

Other uses of GDL

In meat

Reduces the amount of nitrite used in curing

Increases shelf life by inhibiting growth oflactic acid bacteria

In pasta and rice

Improves color and texture


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Other uses of GDL

In canned and frozen vegetables

Improves stability and texture

Can be used as a partial replacement for

vinegar in salad dressings

Can be used as a protein coagulant in the

manufacture of cheese and tofu

Carbonyl group reduction

Glucose to glucitol (sorbitol)

D-glucose D-glucitol

(sorbitol)

reduction

NaBH4,

H2/Pt or Pd,

H2/Raney Ni

Yield 100%


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Sorbitol

Note the suffix -itol

Sorbitol is widely distributed in nature

but in relatively low concentration

Was discovered in the berries of the

mountain ash (Sorbus aucuparia)

Physical form: syrup or crystal

Sorbitol food uses

Functions as a humectant (water

retainer)

Non-cariogenic (S. mutans cannot

metabolize)

About 50% as sweet as sucrose

Largest use (non-food)

Toothpaste (cooling effect; endothermic heat

of solution)

Sorbitol food uses (cont.)

Sugarless gums

Sugarless mints

Sugarless hard candies Sugarless cough drops

A cryoprotectant in surimi

A starting material for sorbitan esters

(emulsifiers)


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Sorbitol and Vitamin C

Acetobacter

suboxydans

Vitamin C synthesis

High yields at each step allow Vitamin C

as a low cost product

L-ascorbic acid functions

Collagen formation

Fatty acid metabolism

Brain function

Drug detoxification

Other Vitamin C functions

Plants

Respiration

Growth Maintenance of CO2 balance

Foods

General antioxidant function, especially in

aqueous systems

Natural occurrence

Rose hips, persimmon, citrus fruits


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Mannitol

Mannitol preparation

Laboratory methods

Hydrogenation of D-mannose

Fermentation

Commercially

Hydrogenolysis of sucrose

Sucrose hydrogenolysis

Sucrose

D-glucose D-fructose

D-glucose,

D-fructose,

D-mannose

Mannitol,

Glucitol

Isomerization Hydrogenation

Hydrogenation


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Mannitol

characteristics and uses

Not a humectant

Crystallizes easily, moderately water

soluble

Candies

Anti-caking agent

Dusting agent

Sweetness: 65% of that of sucrose

Mannitol uses (cont.)

Sugarfree

Chocolates

Mints

Cough drops

Hard and soft candies

Xylitol

hydrogenation

(from hemicellulose

xylans)


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Xylitol characteristics

Has an endothermic heat of solution,

therefore it produces a cooling sensationin the mouth

Sweetness: 70-95% of that of sucrose

Non-cariogenic

Maltitol

(from corn syrup)

Maltitol uses

Reduced calorie candies

Miscellaneous other foods

Also included here are isomaltitol and

hydrogenated starch hydrolyzates


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Erythritol

Non-cariogenic

0.2 kcal/gram

Strong cooling effect in the mouth

RS ~ 0.5

Not hygroscopic

No D or L designation

This is a meso compound

Cyclitols

Only contain hydroxyl groups

But unlike alditols, these are cyclic

Most are 6 membered rings

(polyhydroxycyclohexanes and hexenes)

Myo-Inositol

Myo-inositol

Derivatives such as phosphate esters,

ethers, and deoxy forms also occur

Myo-inositol occurs free in virtually allplants

Isomers also exist

Phytic acid


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Phytic acid

Mostly occurs as phytates, mixed

potassium-magnesium salts

Generally an anti-nutritional factor as it

binds minerals, especially calcium and

doesnt allow them to be absorbed

Recent studies have shown some positive

effects related to cancer, heart disease,

antioxidant activity and contribution to

fecal bulk

Oxidation of

non-anomeric hydroxyl groups

The product of this

type of oxidation of

an aldose is an

aldaric acid


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Periodate oxidation

Periodate oxidation

This reaction can be used to

quantitatively measure the number of

adjacent hydroxyls in a molecule

It is used in the determination of

polysaccharide structure

The reaction is most rapid at pH 3-5

Dinitrogen tetroxide (N2O4)

This reagent is specific for oxidation of

primary alcohol groups

If you start with an aldose, you get analduronic acid


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Dinitrogen tetroxide

Methyl -D-

glucopyranosideMethyl -D-

glucopyranosiduronic

acid

Aldobiouronic acid

When the substrate is a disaccharide, the

product is an aldobiouronic acid (a

disaccharide with a uronic acid at its

non-reducing end)

An aldobiouronic acid

N204

Uronic acid preparation

Laboratory preparation of uronic acids

typically involves the use of oxygen in

aqueous solution with platinum or

palladium catalysts


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Galactose oxidase

Hydrogen peroxide

A non-specific oxidant

Depolymerizes oligo- or polysaccharides

Involves a free radical mechanism

Employs an Fe+2 catalyst

Fe+2 + HO-OH Fe+3 + HO. + OH-


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Esterification

Esters

Reaction of alcohol with acid anhydride

or acid chloride forms an ester

Usually done in the presence of a base

such as triethylamine, pyridine, sodium

acetate, sodium hydroxide, etc. This is

known as the Schotten-Bauman

technique and is done to shift the

equilibrium toward the product ester

Use of acid anhydrides


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Use of acid chlorides

Uses of esters

Esters may be used

In determination of the monosaccharide

composition of polysaccharides

In analysis of the sugar composition of foods

As blocking groups in sugar synthesis

Alditol acetates

Sometimes aldoses are reduced to sugar

alcohols before acetylation

The resulting alditol acetates are Volatile

Thermostable

Ideal for determination by gas

chromatography (GC), where they are

identified by retention time or mass

spectrometry


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Alditol acetates

Esters-General information

Esters occur in nature

Acetates, e.g., xanthan

Succinate half esters

Esters are extensively used as OH

protecting and blocking groups in

carbohydrate synthesis

Easily prepared

Usually crystalline

Easily removed with base

Sugar phosphates


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Sugar phosphates

Sugar phosphates are common

intermediates in sugar metabolism, e.g.,the Embden-Meyerhof pathway

Monophosphate esters are also found

naturally in certain starches, e.g., potato

Modified food starch phosphates (cross-

linked starces) are highly stable to

viscosity breakdown

Starch phosphates

Ethers

Williamson ether synthesis

SN2 mechanism

RO-

Na+

+ RX

ROR + NaX


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Williamson ether synthesis

SN2 reaction video

4-O-Methyl D-glucuronate

This ether is a

constituent of various

hemicelluloses and

gum arabic

Etherification

Etherification of some polysaccharides

Modifies their properties

Makes them more useful Examples (derivatives of cellulose)

Methyl (-CH3)

Carboxymethyl (-CH2COO-Na+)

Hydroxypropyl (-CH2CH(OH)CH3)


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Methylation analysis

Ethers are very useful in methylation

analysis of polysaccharide structuredetermination

With this method it is possible to

determine the nature of the linkages

between the component sugars, e.g., 12,

1 3, 1 4, or 1 6

Methylation analysis

Typically involves

Permethylation

Acid hydrolysis

Acetylation

GC-MS analysis

The hydroxyl groups involved in the

glycosidic linkage are the only ones not

methylated

Internal ethers

(anhydro sugars)

A 1C4ring


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Anhydrosugars

Typically this internal ether is a 3, 6

linkage

Such structures are found in seaweed

polysaccharides such as

agar

furcellaran

-carrageenan

-carrageenan

Non-ionic surfactants

(emulsifiers)

Sorbitol

Cyclic dehydration accompanying

esterification with fatty acids produces

sorbitan esters

The products of these reactions are mixtures

Sorbitan monostearate

A mixture of

C16 and C18 esters of sorbitol

1, 5-anhydro-D-glucitol 1, 4-anhydro-D-glucitol

and 1,4:3,6-dianhydro-D-glucitol

(isosorbide)

This mixture may be modified with

ethylene oxide to produce ethoxylated

sorbitan esters (non-ionic detergents)


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Sorbitan derivatives

The mono-, di-, and

triesters of thesestructures are the

Spans

Polyoxyethylenated

derivatives are the

Polysorbates and

Tweens

Cyclic acetals

The hydroxyl groups on carbohydrates

react with aldehydes or ketones to form

cyclic acetals

Common carbonyl compounds include

acetone and benzaldehyde

Sometimes such acetals occur naturally,

as in xanthan gum

Cyclic acetals

The 4,6-O-(1-carboxyethylidene)--D-mannopyranosyl unit

of the polysaccharide xanthan. The acetal is derived from

pyruvic acid.


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Cyclic acetals

Such acetals are good

blocking groupsbecause they react

with two hydroxyls

at once and are later

easily removable

with dilute acid

Browning reactions

Alternative names

Non-enzymatic

Non-enzymic

Maillard

Maillard browning

This is the common browning of foods on

heating or storage

Requires a reducing sugar and a freeamino group

The colors (and aromas) so produced

may be desirable or undesirable or only

desirable below a certain level

Thus, we need top know how to control

this reaction


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Food sources of browning

Soy sauce

Non-fat dry milk

Bread crust

Used to be a problem in dried egg white

but no longer (use of glucose oxidase)

Aromas

Browning reactions also contribute

flavors in

Milk chocolate

Caramels

Toffees

Fudges

36%

64%

0.022%

Furanoses are present are

very low levels


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Maillard reaction

Decomposition of the

Amadori compound

Control

point

Strecker degradation


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Maillard browning

reaction variables

Temperature Generally, the higher the temperature, the

greater the browning

pH

As pH goes down, so does browning. Due to

protonation of the reactive amino group,

making it unreactive

Under near neutral conditions

..NH2 CH

O

Under acidic conditions

..NH2 CH

OH+

No reaction is possible with the nitrogen

lone pair as it is already reacting with the

hydrogen ion


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A practical example of the

effect of Maillard browning in cookies

Control of browning

Create unfavorable conditions for the

reactions to take place

Water--low or high

pH--lower, decreases browning

Temperature-- the lower the temperature,

the less the browning

Control of browning

Removal of substrates

Glucose oxidase--removes the open chain

form by converting it to gluconic acid. Thistechnology is used in preserving dried egg

whites.

Ribose oxidase--This enzyme activity occurs

inLactobaccilus pentoaceticum and is used to

prevent fish from browning.


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Sulfite browning inhibitors

Principally sulfur dioxide or bisulfite.These sulfur compounds react with HMF

near the end of the decomposition of the

Amadori compound and divert it to a

non-reactive product, that is one that

cannot be converted into melanoidin

pigments

Sulfite inhibitor reaction

Maillard overviewControl pointPigmentAromas


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Why worry about

Maillard browning?

Aesthetics Overly browned foods are not aesthetically

attractive and may not be accepted

Also, due to the pyrazines produced by the

Strecker degradation, these highly browned

foods may have odd, off flavors and also not

be accepted

Why worry about

Maillard browning?

Nutritional effects

Amino acids that participate in the Maillard

reactions are lost from a nutritional point of

view. This may be especially important

where the amino acid is very reactive and in

foods where it is already in very low

concentration. This would be the case for L-

lysine in cereals.

Reason for the

reactivity of L-lysine

Alpha amino

Reactive epsilon amino


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Glucose and lysine

Why worry about

Maillard browning?

Mutagenicity

This is not yet settled in the literature. Some

researchers find mutagenic products in

browned model systems, others do not. But

there is still the possibility that highly

browned food may contain potential

mutagens.

Reactions during caramelization

Anomeric equilibration (alpha, beta)

Pyran, furan equilibration

Sucrose inversion Hydrolysis

Aldose, ketose interconversions

Inter- and intramolecular condensation

Dehydration

Skeletal fragmentation

Browning


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Thermolytic products

Vigorous heating

producesintramolecular

condensation

products such as

levoglucosan

Caramelization

Flavoring caramelization

Sucrose syrup Inversion, fragmentation

buffer

heat(maltol, etc.)

Flavor compounds


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Flavor compounds

Maltol and isomaltol contribute to fresh

baked bread flavor

2H-4-Hydroxy-5-methylfuran-3-one is

characteristic of burnt meat flavors and

is also a flavor and sweetener enhancer

Caramelization

Caramel pigments

Glucose Partially Caramel

syrups neutralized Colloids

pH 4

heat

Caramel pigments

During the reaction, unsaturated ring

compounds are produced which condense

and produce the polymer pigments

(caramel colloids)

Specific catalysts will produce specific

caramel colors, solubilities and acidities


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Pigment uses

Colas

Baked goods

Syrups

Candies

Pet foods

Dry seasonings

Beer and other alcoholic beverages

Pigment structure

Unknown

Contains

hydroxyl groups

carboxylic groups

ketone groups

hydroxylated carbon-carbon double bonds

phenolic groups

Pigment production

The formation rate of the pigments

increases with increasing temperature

and pH

In the absence of buffering salts, large

amounts of humin (C125H188O8), a bitter

substance, is produced

Minimal production of humin is desired


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Acrylamide

A study in 2002 detected small amounts

of acrylamide in foods

The concern is the acrylamide is a known

neurotoxin and carcinogen

Initial reports targeted starchy foods but

it is now known that acrylamide is not

directly formed from starch nor is it

required for its formation

Where found?

Consumption?


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Common factor

High heat processing

Not formed in boiled foods (100oC), e.g.

boiled potatoes

Neurotoxic no observed adverse effect

level was >>>> than what could be achieved

by food consumption

Main concern was carcinogenic potential

Human intake is 1000 times lower than

carcinogenic level in rats

No known case of cancer from eating

acrylamide containing foods

Formation

Formation

L-asparagine is required

The carbons in acrylamide come from

asparagine

Promoting formation

Free asparagine

Reducing sugars

High temps (>130oC, best at 200oC)

Low moisture

pH 5.5-7.0


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Reducing acrylamide formation

Reduce asparagine

Select cultivars low in asparagine or use

asparaginase to remove

Reduce reducing sugars

Select cultivars, avoid cold storage, blanch

Lower temp

But this can decrease brownness

Lower pH

Dip in citric acid, may cause off taste

Reducing acrylamide

formation

Add other amino acids

Provides competition for asparagine, e.g.

glycine or glutamine

Degradation of formed acrylamide

Prolonged heating, e. g., coffee roasting

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