Who hasn’t eaten a banana in their life? It’s one of the most convenient fruits to snack on. However, you do have to be careful with them. I’ve personally experienced semi-squashed brown bananas :-(. It’s a shame, even though you can still eat the banana, it just isn’t that good anymore.
So time to dig into some science to help us prevent these brown bananas. Why do they turn brown? How do they turn brown and how can we prevent that from happening?
Ripening of bananas
Bananas are green when they are harvested from the tree and thus not yet ripe. There’s a good reason this is done, they still have to make a long journey by boat to the countries where they’ll be eaten. Bananas are very well suited for this since they are climacteric fruits. This means that they can ripen after they’ve been harvested. Ripening is started by a chemical called ethylene (see, food chemistry!, have a look at my post on chemical formulas in case the formula below is hocus pocus to you).
In special ripening warehouses ethylene is released when the fruits have to ripen. This ethylene will initiate the ripening process and as a result the bananas themselves will also start producing ethylene. During ripening starch is broken down to sugars. That’s why a ripe banana is softer and sweeter than a green, unripe one. Some chemistry, starch is actually a long chain of glucose (thus sugar) molecules.
Bananas change colour during ripening because chlorophyll, which gives bananas their green colour, is broken down during the process. Since this molecule is broken down the other colours in bananas show up, which are carotenoids, more specifically, the xanthophylls. These are yellow colours and form one of the two main carotenoids.
Enzymatic browning of bananas
So, once the bananas turn from green to yellow, they also start having this tendency to turn brown, especially when you put them in your bag with other stuff, ready to become squashed. Although there are several reasons bananas can turn brown, the most important one is enzymatic browning. Those who have been reading more food chemistry posts on my blog, will recognize the ‘enzyme’ part in the word, to those who haven’t, have a look at my post on enzyme basics!
Enzymatic browning is browning of a product which is catalyzed by enzymes (remember? enzymes catalyze reactions, so they speed them up, they don’t actually get used). The enzyme at play here is polyphenol oxidase (often abbreviated as PPO). This enzyme catalyzes reactions with polyphenols. Polyphenols are a big group of molecules which all contain the same structural groups, which are shown below. It’s a phenyl group with an alcohol group (-OH) attached to it.
PPO catalyzes the reaction of polyphenols into two other structures, as shown above. In order to catalyze these reactions oxygen should be present as well! The quinones which are formed in such a reaction are very reactive and they will react on into brown and black pigments. These reactions are very complex, just as is the case for caramelization!
After the catalysis of these first two reactions however, the role of PPO is over. The reactions will take place by itself. However, without this PPO those first reactions will not occur, they will require to much energy to be initiated.
Preventing enzymatic browning
Enzymatic browning of bananas can be prevented or delayed in several ways. If you’ve read my previous post on enzymes you might already come up with several ways to do this!
The first and easiest way is to cool your bananas. When you place them in your fridge (<7°C), the enzyme will stop working. However, it will not be deactivated, once the temperature rises again, the browning can start again. There’s is one problem with this solution. Bananas don’t like the cold! The quality of a banana deteriorates when you store it in your fridge for too long.
Oxygen is required for PPO to do its job. By damaging cells, oxygen can enter them and PPO might stat catalyzing reactions, initiationg browning. This is way a freshly peeled banana may still be yellow, whereas one that has been peeled some time ago will have turned brown! Keeping out the oxygen and preventing damage to your banana is a good way to prevent browning.
There are several other ways to prevent PPO from working, however, most are not very suitable if you’d still like to enjoy your fresh banana. Freezing is an option for instance, as is sprinkling over some lemon juice (an acid). Whereas that works well for freshly sliced apples, it’s not a bi success for bananas. Drying your banana will stop the reaction as well, since PPO needs enough water to function. High pressures can also stop PPO from working, but I don’t think you have a high pressure system at home, do you? Blanching your banana will break down the enzyme, but again, would you want to blanch your banana? I expect not…
So, the best way to prevent enzymatic browning in your banana is to simply eat your banana before it turns brown. It’s jummiest that way anyhow. Hope you enjoyed this post again. This is the last post of this month’s special on food chemistry. Do come back for more sciency food posts and leave behind a question related to food and science, I’d love to dig and find the answer!