Dairy Chemistry And Biochemistry

128 DAIRY CHEMISTRY AND BIOCHEMISTRY

'O,, light, M"', lipoxygenase, ionizing radiation

(prooxidants)

1

CH,-------CH,-CH=CH-CH-CH=CH-CH-CH~

~ [FA tar a d]

302

RH

CH3-----CH,CH=CH-CHCH=CH-CH2- ----

I

0

I

0'

~ [Fi peroxide]

RH [unsaturated FA]

ROOH

RCH,-CH=CHCHCH=CH-CH, + R [FA radical]

I

0

I

0

I

H __

[FA hydroperoxide]

~ERMINATION~ M"' /

/

Unsaturated aldehvdes

and ketones (off-flavours)

(FA peroxide)

RH (unsaturated FA)

Primary and secondary

alcohols (off-flavours) ROOH + R' 4 R-R

etc.

Figure 3.33 Autooxidation of fatty acids. AH, antioxidant; M"', polyvalent metal (e.g. Fez',
CUZ+).

concentrated in the fat globule membrane in juxtaposition with several
pro-oxidants and are, therefore, particularly sensitive to oxidation.
The initiation reaction is catalysed by singlet oxygen (lo2, produced by
ionizing radiation and other factors), polyvalent metal ions that can
undergo a monovalent oxidation/reduction reaction (M"" + M"), especial-
ly copper (the metal may be free or organically bound, for example, xanthine
oxidase, peroxidase, catalase or cytochromes), or light, especially in the