114 

 Cysteine and other sulfhydryl- related compounds have been studied 

 for their inhibitory effect on enzymatic browning (Lerner, 1953). Roston 

 (1960) proposed that cysteine inhibits enzymatic blackening caused by the 

 oxidation of L-tyrosine and DL-DOPA by forming addition products with 

 quinones. Muneta and Walradt (1968) studied potato PPO and found that the 

 concentration of cysteine required for comparable inhibition was 5-fold 

 higher with cysteine than for sulfite. These authors also noted that 

 cysteine did not inhibit chlorogenic acid oxidation; and the oxidation of 

 tyrosine to DORA is more sensitive to cysteine inhibition than the 

 oxidation of DOPA to dopaquinone. Both Muneta and Walradt (1968) and 

 Henze (1956) proposed that cysteine inhibited enzymatic browning by 

 combining with the quinone. The sulfiting agents have been postulated 

 toreact with the enzyme or the quinones to exert the inhibitory effect 

 (Joslyn and Ponting, 1951; Joslyn and Braverman, 1954). Although sulfite 

 agents can effectively inhibit enzymatic browning, they affect the 

 nutritive value either by preserving ascorbic acid or destroying thiamine 

 (Mapson and Wager, 1961; Markakis and Embs, 1966). 



Effect of Ko.iic Acid on Apple PPO Activity ; • 



Oxidation rates of 4-methyl catechol and chlorogenic acid by apple 

 PPO decreased as kojic acid concentration increased. Only 50% inhibition 

 occurred for kojic acid at 200 iiq/ml (Figure 25). Kojic acid inhibition 

 of apple PPO was similar to potato PPO (Figure 24), but was far less 

 pronounced than for mushroom PPO (Figure 23). Harel et al . (1965) 

 demonstrated that 2,3-naphthalenediol at 5 mM and N-vinyl-2-pyrrolidone at 

 2.5% respectively inhibited 64 and 60% PPO prepared from apple 



