Nonenzymatic Browning

w/ sugarw/o sugar

Two major types of non-enzymatic browning reactions

• Caramelization

• Maillard browning

Louis Camille Maillard (1878-1936)

Reactions during caramelization

• Anomeric equilibration (alpha, beta)• Pyran, furan equilibration• Sucrose inversion

– Hydrolysis• Aldose, ketose interconversions• Inter- and intramolecular condensation• Dehydration• Skeletal fragmentation• Browning

Caramelization

• Flavoring caramelization

Sucrose syrup Inversion, fragmentationbuffer

heat (maltol, etc.)

Caramelization

• Caramel pigments

Glucose Partially Caramelsyrups neutralized Colloids

pH 4

heat

Used to color colas. They are not naturally brown but havethese pigments added to color them.

Clear colas• See the Wikipedia

entry on Pepsi Crystal here http://en.wikipedia.org/wiki/Crystal_Pepsi

• A disaster-see the marketing section of the Wikipedia entry

Maillard reaction (slide 161)A

mad

ori r

earr

ange

men

t

Glycosylamine formation

Glycosylamine Formation

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Amadori rearrangement

Amadori Rearrangement

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Decomposition of the Amadoricompound (slide 162)

Control point

Strecker degradation

Requires dicarbonyls and amino acidsNutritional loss of amino acids (especially L-lysine)Pyrazines -- flavor compounds

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 pairas it is already reacting with the hydrogen ion

-

+

Maillard browning reaction variables• Water

%water

0%, min 100%, min30%, max

Maillard browning reaction variables• Oxygen

– Seems to have little effect

• Metal ions– Cu(I), Cu(II) and Fe(II), Fe(III) speed up the

reaction. Other metals seems to have little effect. Can control the with metal chelators (e.g., EDTA).

Maillard browning reaction variables

Sugar Open chain MelanoidinGlucose 1 1Fructose -- 1.2Galactose 3.4 2.0

Xylose 7.1 5.5Arabinose 7.1 6.7

Ribose 354 Very high

A practical example of the effect of Maillard browning in cookies

Image courtesy of Oreg. St. Univ. (www.orst.edu/food-resource/information/maillard/nancy.html)

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. This technology is used in preserving dried egg whites.

– Ribose oxidase--This enzyme activity occurs in Lactobaccilus pentoaceticum and is used to prevent fish from browning.

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

OH

O

OS

H

OH

m elanoid inp igm ents

R

RO 3

-

stab le--w ill not proceedto form p igm ents

HM F

HSO 3-

Maillard overview (page 169)Control pointPigmentAromas

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

O

NH2

CH3OH

O

NH2

NH2OH

L-alanine L-lysineAlpha amino

Reactive epsilon amino

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.