Carbonyl formation discoloration

Reiney (12) attributed this to conjugated carbonyl products. Mathe-son and Boyer (10), who also observed carbonyl formation, reported that the yellow discoloration occurs only in the presence of air or oxygen and is primarily a surface phenomenon. 

Figure 17. Relationship between discoloration and carbonyl formation...

Figures 18 and 19 show that inhibition of discoloration by these additives under Fade-ometer exposure was also accompanied by inhibition of carbonyl formation.

Figure 20 illustrates that a similar reduction in the rate of carbonyl formation also occurs under fluorescent blacklamp exposure. These results suggest that the antioxidant acts as a free radical inhibitor, reducing the over-all rate of photooxidation. The similarity in the relationship between the change in absorbance at 365 m/i, and carbonyl formation observed in the presence of the additives also lends further support to the previous conclusion that under exposures of this type, discoloration is caused by carbonyl products. 

In contrast to the above, Figure 21 shows that the antioxidant did not inhibit the rate of either discoloration or carbonyl formation under mercury-arc exposure. Grassie and Weir (5) also reported that under 2537-A. irradiation the antioxidant, 2,6-di-tert-butyl-p-cresol, had no effect on the rate of polystyrene photooxidation. It may be concluded,... 

In addition to the above two ketones, additional carbonyl compounds have been suggested to explain the discoloration and absorption spectra of irradiated polystyrene. Reiney (12) and more recently Zapol skii (13) postulated that further photooxidation results in the formation of cr-diketones. 

Hie initial autoxidation process of ASA monoanion with oxygen molecule in the absence of metals is considered to proceed via the formation of oxygen adduct of ASA monoanion. After this initial autoxidation process, degradations of DHA and DKG take place and reactions of these carbonyl compounds with amino compounds also occur to give intensive discoloration. Thus, autoxidation of ASA is considered to play an important role as a trigger reaction of discoloration, resulting in deterioration of various foods. 

---