Review – Enzymatic Browning

Why do we add a squeeze of lemon juice to freshly sliced fruit salad? And what happens when we age tea leaves? Both answers have their roots in the same phenomenon – enzymatic browning.

Food turns brown for several reasons, two of which we have already covered. The Maillard reaction is a non-enzymatic browning process that requires proteins (or amino acids) and sugars. Caramelisation is a second non-enzymatic browning process that is unique to foods that contain certain sugars only. Enzymatic browning is so named because it involves a set of chemical reactions that are assisted by enzymes present in the food: mainly plants, fruits, and vegetables such as apples, bananas, potatoes, and tea. This particular type of enzyme, polyphenoloxidase, comes into contact with a class of chemicals called polyphenols when the tissue of the food is damaged by bruising or  excessive ripening, and the cells storing them are ruptured. Polyphenols are diverse in structure and origin, and about the only real similarity between the members of this large chemical class is a frustrating propensity for unpronounceable names. Leucodelphinidin is a tongue twister of a molecule that exists in the fruit of bananas. Ellagitannins are absolute giants that can be found in red raspberries. As the polyphenols mix together with the enzyme, the polyphenoloxidase uses oxygen present to transform the polyphenols into new products, each of which is larger and more complex than the next. These weighty structures give the brown colour that makes sliced apples that have sat too long on a counter lose their appeal, or tells us it is time to whip up a sweet loaf of banana bread. It may seem unpleasant to watch a half-eaten nectarine turn a despondent brown, but what is a nuisance for us is a boon for the fruit. Polyphenols are large enough to slow down the spread of toxins released by invasive organisms that would otherwise infect its tissues. Some of the polyphenols even have antimicrobial properties that are part of an important step in the plant’s defense response.

Although it would be easy to condemn polyphenoloxidase for its heavy hand in browning freshly sliced potatoes, we would be overlooking its wonderful utility in providing the planet with a multibillion dollar economy that millions and millions of people worldwide profit from – the tea industry. Up to 40% of the compounds in tea are polyphenols, some of the simplest of which are called catechins. When tea is aged (a process that is incorrectly called fermentation) the polyphenoloxidase uses oxygen in the air to convert these bitter tasting, astringent molecules into larger theaflavins. The theaflavins are darker coloured, and very importantly, less bitter! Theaflavins may in turn be transformed into thearubigens, which are darker and less bitter again. Picture in your head a cup of freshly brewed green tea versus a black that has been aged much longer. The green tea will be a pale yellow, bordering on an oolong orange at best. An earl grey will be a dark red or brown. The taste is equally different, with younger teas having a bitter, less developed flavour, while older teas will have mellowed as the bitter catechins in the tea are consumed and larger polyphenols are constructed.

So it isn’t all bad for enzymatic browning. Which brings us back to one of the original questions – why do we add lemon juice to fruit salad and jerusalem artichokes? There are three simple ways to stop polyphenoloxidase in its tracks. One is to remove the oxygen it needs to process polyphenols, as when you toss freshly chopped potatoes into a bowl of water, an environment containing much less oxygen than air. A second way is to create acidic, and unfriendly, conditions for the enzyme itself. When exposed to acids such as vinegar (acetic acid) or lemon juice (citric acid), the entire structure of the enzyme is altered, inducing an effect called denaturation. This is a point of no return for the enzyme, and it will be unable to perform its essential function, rendering it an inert protein and arresting the browning of the plant. Finally, a blast of high temperatures will also denature the enzyme, which is why a brief blanching of green vegetables is often recommended in order to preserve the bright, desirable colour. One final word on blanching, though – if you want your bok choy to remain green, do not blanch under acidic conditions. The chlorophyll is quickly degraded and the bright hue you fought so hard to preserve will disappear.

It often feels like a new article linking the benefits of tea to protection against heart disease, Parkinson’s, or even Alzheimer’s hits the papers every week, but here is one you probably haven’t heard of. Research at Copenhagen University has shown that green tea catechins can be a source of natural antioxidants that prevent the Maillard reaction from taking place, reducing the amount of toxic, undesirable by-products that are formed during the manufacture of certain highly processed foods. Healthy genmaicha flavoured Doritos? Yet another tick on a rather long list.

Enzymatic browning - tea

* Thanks to Colin Ray, Head of Section at Copenhagen University for a portion of this article.

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