Enzymatic Browning of Apples
INTRODUCTION
Apples contain an enzyme called polyphenol oxidase (Phenolase). Phenolase has catecholase and cresolse activity, also containing small traces of copper. When apples are cut, phenolase is released from the cells and is exposed to oxygen.
When phenolase reacts with oxygen, it catalyses one step of the biochemical conversion of phenolic compounds. The presence of oxygen hyrdroxylates the colourless phenols, which are subsequently oxidised to coloured quinones known as melanin. (Vlasta Pilizota and Drago Subaric, 1997). The ‘rust-like’ reaction only occurs on the surface of the cut apples, as those cells have been broken and the phenolase as well as other enzymes within the cells have been released. When the enzymes are exposed to oxygen the result is a brown pigmentation forming on the surface of the apple when it has been cut. This reaction is known as enzymatic browning.
SCHEMATIC DIAGRAM OF ENZYMATIC BROWNING (fig.1)
Phenoloxidase catalysing the reaction.
First step of conversion.
Greater enzymatic browning occurs at an optimal environment for the enzyme. A temperature of approximately 37.5 degrees Celsius is known to be the optimal temperature for enzyme conversion. Figure 2 shows that temperatures below this optimal temperature have a lower amount of activity, increasing as it becomes closer to the optimal temperature. Temperatures above the optimum cause vibration within the enzyme, causing it to unfold and a denaturing of enzyme; reducing the amount of browning. Additionally, a slightly acidic to neutral pH level between 5.0 and 7.0 is known to increase the enzymatic browning, this has a similar trend to that shown in fig. 2. (http://www.rsc.org/Education/Teachers/Resources/cfb/enzymes.htm).
Enzymatic browning in fruits and vegetable is optimised to add colour and flavour to raisins, prunes, cocoa, tea and coffee. However discolouration on fruits such as apples and bananas is usually an undesirable side-effect of slicing
Apples contain an enzyme called polyphenol oxidase (Phenolase). Phenolase has catecholase and cresolse activity, also containing small traces of copper. When apples are cut, phenolase is released from the cells and is exposed to oxygen.
When phenolase reacts with oxygen, it catalyses one step of the biochemical conversion of phenolic compounds. The presence of oxygen hyrdroxylates the colourless phenols, which are subsequently oxidised to coloured quinones known as melanin. (Vlasta Pilizota and Drago Subaric, 1997). The ‘rust-like’ reaction only occurs on the surface of the cut apples, as those cells have been broken and the phenolase as well as other enzymes within the cells have been released. When the enzymes are exposed to oxygen the result is a brown pigmentation forming on the surface of the apple when it has been cut. This reaction is known as enzymatic browning.
SCHEMATIC DIAGRAM OF ENZYMATIC BROWNING (fig.1)
Phenoloxidase catalysing the reaction.
First step of conversion.
Greater enzymatic browning occurs at an optimal environment for the enzyme. A temperature of approximately 37.5 degrees Celsius is known to be the optimal temperature for enzyme conversion. Figure 2 shows that temperatures below this optimal temperature have a lower amount of activity, increasing as it becomes closer to the optimal temperature. Temperatures above the optimum cause vibration within the enzyme, causing it to unfold and a denaturing of enzyme; reducing the amount of browning. Additionally, a slightly acidic to neutral pH level between 5.0 and 7.0 is known to increase the enzymatic browning, this has a similar trend to that shown in fig. 2. (http://www.rsc.org/Education/Teachers/Resources/cfb/enzymes.htm).
Enzymatic browning in fruits and vegetable is optimised to add colour and flavour to raisins, prunes, cocoa, tea and coffee. However discolouration on fruits such as apples and bananas is usually an undesirable side-effect of slicing