The simple answer to why apples brown after being sliced is polyphenols and oxygen. Polyphenols in the apple flesh oxidize and turn brown. That’s the shortest Mālus Trivium I have ever written! Well, maybe there is a little bit more to it. As with most things about apples and hard cider, the answer is often slightly more complex. First, what are polyphenols? I thought tannins turned apples, juice, and hard cider brown. They do. However, tannins are formed when polyphenols oxidize.
Polyphenols are a large group of phenolic compounds. Phenolic compounds are known for producing bitter and astringent characteristics in hard cider. They also impart color. For example, phenolic compounds are what ultimately make red wine red. Two of the key polyphenols found in apples and pears are anthocyanins and phenolic acids. Anthocyanins are highly concentrated in the apple peel, which makes sense as they give fruit it’s color. Anthocyanins can also form fruity esters when broken down by enzymes. They can produce some bitterness but are mostly known to contribute astringency, which impacts mouthfeel. They can impact your cider color, especially if you macerate your fruit before pressing or include the apple peel in your primary fermentation as I recommend. However, they are not usually the cause for the browning of apples slices or the amber color of some hard ciders. That is usually from phenolic acids.
Phenolic acids include numerous compounds but some of the commonly referenced are vanillic acid, caffeic acid, and ferulic acid. However, in apples and pears, the most common cause of browning is from chlorogenic and p-coumaroylquinic acids(1). These oxidize readily causing the browning. Like anthocyanins, these phenolic acids are generally concentrated in specific areas. Depending on the fruit variety, the fruit flesh can have higher or lower levels of phenolic acids but, there is normally a concentration around the seeds and core. These acids create the compounds we often refer to as tannins. They readily oxidize and create brown colors on the apple flesh or in hard cider. This is why you might find the core of your apple turning brown while the outer flesh of an apple doesn’t. They also tend to have a bitter taste. They can contribute astringency but are usually associated with bitterness.
So, why do apples turn brown? Well it is the polyphenols and oxygen of course.
(1) V.K. Joshi and associates, Science and Technology of Fruit Wines: An Overview, 2017
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