129 



The results thus indicated that kojic acid could inhibit the 

 oxidation of phenolic substrates by mushroom, potato, apple, white shrimp, 

 grass prawn, and lobster PPO. Significant inhibitory effects with 

 different types of inhibition mechanisms were observed with these PPO 

 activities when DOPA was used as substrate. DOPA is a dominant phenolic 

 substrate responsible for the formation of melanin (blackening spot) in 

 crustaceans. It seems likely that kojic acid could be potentially used as 

 an inhibitor in the prevention of melanosis in seafood products. 



Effect of Pre-incubation Temperature on PPO Inhibition by Ko.iic Acid 



The pre-incubation temperatures did not affect the inhibitory 

 activity of kojic acid on various PPOs (Table 5). More than 75% 

 inhibition was observed with mushroom tyrosinase and lobster PPO when 0.28 

 mM kojic acid was introduced into the assay mixture containing DL-DOPA and 

 enzyme. However, only the respective 26% and 41-46% of the potato and 

 apple PPO activity were inhibited when 0.56 mM kojic acid was added. 

 These observations were in accordance with earlier findings that kojic 

 acid was less inhibitory on potato and apple PPO than on mushroom and 

 lobster PPO. Although the pre-incubation at 0°C usually gave a higher 

 percentage of inhibition than at higher temperatures, the difference was 

 insignificant (P > 0.05). Thus, kojic acid can be used to inhibit PPO at 

 either refrigeration or ambient temperature. 



Effect of Kojic Acid on the Hvdroxvlation Capability of PPO 



The lag period associated with the hydroxylation of 

 monohydroxyphenol by PPO increased with increasing concentrations (35.2 - 



