Enzymatic browning in apple
Food chemistry KLI011
Many fruits and vegetables contain polyphenol oxidases (PPOs), which are a group copper-proteins. PPO is located in the plastids of plant cells, whilst the phenolic compounds are located in the vacuoles. When the tissue is damaged by cutting the fruit, both the plastids and vacuoles are broken, so that PPO can come into contact with the phenolic compounds. When the cut or damaged surfaces of these fruits and vegetables are exposed, they turn brown due to the oxidation and hydroxylation of polyphenols. PPOs catalyze the oxidation of colorless polyphenols to brown-reddish quinones. (Fig. 1) The quinones, which are highly reactive, are converted by non-enzymatic…show more content… (Haard & Simpson, 2000)
In this research different inhibitors are added to apple parts of different species (Royal, Golden, Alice, Granny) to determine the effect of the inhibitors on the browning of the apple. There is also extra substrate (chlorogenic acid) added to enhance the effect of browning. The inhibitors that are used are ascorbic acid, citric acid, sodium bisulphite and sodium chloride and combinations of these inhibitors to determine which inhibitor prevents browning best. These inhibitors have all different mechanisms of action.
Ascorbic acid acts as an antioxidant and reduces the produced quinones to colorless dihydroxyphenols, so that the browning reactions cannot take place. (Pizzocaro et. al., 1993) So ascorbic acid acts as a reducing agent (Fig. 1) In addition, ascorbic acid decreases the pH, so that PPO activity is reduced, because the optimum pH of PPO catalysis is between pH 5 and 7.5. (Queiroz et. al., 2008)
As previously mentioned, PPO needs copper ions to be active. Citric acid is a chelating agent, which can bind the copper ions of PPO, so that enzymatic activity of PPO is reduced. Citric acid is like ascorbic acid an acidifying agent and decreases the pH, reducing the PPO activity. (Ioannou & Ghoul,…show more content… The PPO becomes inactivated due to the temperature increase. The oxidative activity of PPO varies according to temperature. When temperature is increased, the activity reaches a plateau and the optimal activity. When the temperature is further increased, the activity of PPO drops again. (Ioannou & Ghoul, 2013) Blanching inactivates also lipase and peroxidase. The inactivation of peroxidase is used as an indicator of sufficient blanching, since peroxidase is the most heat resistant and can also facilitate enzymatic browning. Hydrogen peroxide formed by the PPO catalyzed reactions is needed for peroxidase to oxidize phenolic compounds. In this experiment the PPO activity and the peroxidase (PO) activity is tested, in heat treated apple extract (Royal, Golden, Alice and Granny). The apple extract is heated at different temperatures, so that the temperature can be determined when PPO and PO become