We were in college when we did our laboratory on making apple juice and concentrate. We are aware that apples turn brown fast when exposed to air. The solution for this problem is to dip the sliced apples to water with citric acid, calamansi or lemon juice. Acid destroys the enzyme responsible for enzymatic browning. We did the trick and was very effective but the apple still turned brown after passing through grinder.
We also noticed that all apple juices and concentrates on supermarket and grocery shelves have brownish color. The two commodities should be white or crystal clear, apple flesh color is white.
Reason for brown discoloration:
1) Dipping apples in citric acid solution really prevents enzymatic browning. Crushed apple have increased surface area and requires strong acid solution to stop the browning reaction which is not applicable because it will mask the fruit taste.
2) Heating is another way to prevent browning reaction. However, the short period of grinding before heating is enough for browning reaction to occur. Boiling of apples before processing is not done to avoid loss of essential nutrients.
3) There is another type of browning reaction, the Maillard reaction. It takes place between a-amino groups and reducing sugars. Considered as the most important cause of browning in apple juice and concentrate.
4) Caramelization of sugars and and ascorbic acid degradation may also contribute in browning of the end product.