Aldose:

Ketose:

Acetal:

Hemiacetal:

A carbonyl can add water or alcohol to form a hemiacetal:

The alcohol may be a portion of the carbohydrate molecule and thus, there can be ring formation. For most carbohydrates there exists an equilibrium between the aldehyde or keto form of the hemiacetal. Aldoses and ketoses cam be inter converted at either alkaline or acid pHs. Some near neutrality.

Reactions of sugars
Enolization

Catalyzed by alkali
Endiol formation

Lobry de Bruyn-Albreda van Eckenstein Reaction

Dehydration, thermal degredation
Osone formation
Final products may have intense colors, odors, off-flavors

Osone:

Deoxyosone:

Near neutrality, it is possible to convert an aldehyde to a deoxyosone as follows:

- OH

The deoxyhexosone can be dehydrated by heating to yield more conjugated products:

In dilute alkali, two aldehydes can react as follows:

In alkaline conditions, it is relatively easy to remove a proton that is alpha to a double bond. The resulting ion can undergo a keto-enol shift.

This can now react with another aldehyde to form an addition product:

The product contains and aldehyde and an alcohol and is know as an Aldol. The reaction is called an aldol condensation. Upon heating a molecule of water can be removed to yield:

Caramelization

Sucrose - 200° C

1. 2C₁₂H₂₂O₁₁ = 4H₂0 C₂₄H₃₆O₁₈ Caramelan
2. 3C₁₂H₂₂O₁₁ = 8H₂0 C₃₆H₅₀O₂₅ Caramelen
3. Continued heating yields humin or caramelin

C₁₂₅H₁₈₈O₈₀

Sucrose at 160° C

Sucrose Glucose + Fructose

Caramel colors

Acid fast caramel
Glucose or sucrose + NH₄HSO₃
Ammonium bisulfate caramel
Cola beverages
Brewers caramel

Glucose or sucrose

Ammonium bisulfite caramel + NH4+

Beer, ale

Bakers Caramel

Glucose Brown polymers

Sucrose Brown polymers

Caramel colors contain:



Reaction rate increase as a function of increasing temperature and pH

Rate at pH 8.0 = 10 X rate at pH 5.8

Some pyrolytic products: