Dairy Chemistry And Biochemistry

442 DAIRY CHEMISTRY AND BIOCHEMISTRY

CHPOH

I

H-$-OH

Ascorbic acid

Rcduction Oxidation

11

CH2OH

I

H-C-OH

0 H k2

Dehydroascorbic acid

11 Hzo

CH20H

I

H-C-OH

OH

CHzOH

%O * H-c-OH I

I

H-C-OH ,COOH

00

‘c-c

II II

2, SDiketogulonic acid

Hydrated hemiketal form

Figure 11.2 Chemical structures of ascorbic acid and its derivatives.

due mainly to the denaturation of b-lactoglobulin (and the consequent
exposure of -SH groups) and loss of 0,. Compounds formed by the
Maillard reaction between lactose and proteins can also influence the E, of
heated milk, particularly dried milk products.
Fermentation of lactose during the growth of micro-organisms in milk
has a major effect on its redox potential. The decrease in the E, of milk
caused by the growth of lactic acid bacteria is shown in Figure 11.3. A rapid
decrease in Eh occurs after the available 0, has been consumed by the
bacteria. Therefore, the redox potential of cheese and fermented milk
products is negative. Reduction of redox indicators (e.g. resazurin or