Dairy Chemistry And Biochemistry

VITAMINS IN MILK AND DAIRY PRODUCTS^289

stability of B,, in UHT milk. Losses during storage at 7°C are minimal for
up to 6 months but at room temperature (the normal storage conditions for
UHT milk), losses can be significant after only a few weeks. Oxygen levels
in UHT milk do not appear to influence the stability of B12.

6.4 Ascorbic acid (vitamin C)

Ascorbic acid (6.35) is a carbohydrate which can be synthesized from
D-glucose or D-galactose by most species with the exception of primates,
guinea-pigs, an Indian fruit bat and certain birds. Ascorbate can be oxidized
reversibly to dehydroascorbate (6.36) in the presence of transition metal
ions, heat, light or mildly alkaline conditions without loss of vitamin
activity. Dehydroascorbate can be oxidized irreversibly to 2,3-diketogulonic
acid (6.37) with loss of activity. 2,3-Diketogulonic acid can be broken down
to oxalic and L-threonic acids and ultimately to brown pigments.

CH,OH

I

H-$-OH

6.36

Ascorbic acid

CHzOH

H-C-OH I

0 H Lr

Dehydroascorbic acid

CH20H

H-C-OH

I

I

H-yH ,COOH

c-c

00

II II

6.37

2,3-Diketogulonic acid