Once you’ve peeled your banana, you’d better eat it. If you forget about it and return a few hours later it will have turned an unappetizing brown. Same goes for leaving an unpeeled banana on your counter top for a few days. It might have been bright yellow when you bought it, there’s no way to prevent it from browning. Throwing a banana in your bag seems to speed that up even further, who hasn’t found back their banana at the end of the day, turned completely brown?
Bananas don’t seem to be the only foods this happens to though. Ever made apple pie and saw your freshly peeled and cut apples turn brown? Or that guacamole turned brown, thanks to the avocado. And what about lettuce and mushrooms or pears? Even potatoes can turn brown this way. In all cases this is caused by the same mechanism, one of the most common browning mechanisms in food: enzymatic browning.
So what causes enzymatic browning and, even more important, is there something we can do to prevent a brown banana smoothie?
3 types of browning in food
There are three common pathways food can turn brown, split into two categories. The first is non-enzymatic browning and to this group caramelization and the Maillard reaction belong. Both of these reactions are accelerated by heat (preferably over 100°C, 212°F). These reactions cause the browning of bread, the brown colour of caramel and the brown colour of a steak. Browning of the bananas though is caused by the third common browning mechanism in food: enzymatic browning.
As the name says, enzymatic browning is a browning process which is caused by enzymes. Enzymes occur naturally in nature. There are a lot of different types of enzymes, your body contains a whole variety, and fruits and vegetables contain a wide variety as well. Enzymes are a specific type of proteins. Enzymes function as a catalyst for a lot of chemical reactions. This means that they help chemical reactions along, without actually being used in the reaction themselves. We’ve discussed enzymes in more detail in another post.
The enzyme that catalyzes the enzymatic browning reaction is polyphenol oxidase, often abbreviated with PPO. This enzyme catalyzes the reaction of phenols into polyphenols. These polyphenols are red, brown or even black pigments that colour the brown.
Reaction mechanism of enzymatic browning
The formation of brown pigments consists of several steps, and, as is the case for the non-enzymatic browning mechanisms, not all of them are fully understood or known. The first few steps though have been described pretty extensively in literature.
Step 1: Hydroxylation of phenols
Enzymatic browning starts with monophenols. These are a group of molecules with a so-called phenol group. A phenol group is made up of a benzene ring attached to a hydroxyl group (OH), see image below. The most important parts are the ring and OH-group, other groups can be part of the molecule as well, for instance where the image now shows and R. For the reactions themselves the phenol is more important.
In the first step of enzymatic browning an additional OH-group is added to the phenol group, as a result it becomes a diphenol. This reaction is called a hydroxylation. In order for this reaction to occur oxygen has to be present.
Step 2: Oxidation of diphenols
Again, oxygen is required to perform the next step. In this step an oxidation reaction takes place in which the two OH-groups are transformed into a double bonded oxygen group. The resulting group of molecules are called quinones.
Interestingly, different produce contains different PPO enzymes. Some are very good in catalyzing step 1, whereas others focus on step 2. As a result, the mechanism and especially rate of browning can be very different for different produce.
Step 3: Formation of colours
Quinones are very reactive molecules, they don’t need any further help from enzymes to react further. The quinones will form polyphenols, large structures which will cause the actual discolouration. The pigments are called melanins.
Preventing enzymatic browning
You can prevent or delay enzymatic browning of bananas (and other foods) in several ways. One option is to take away necessary ingredients for the reaction to occur, or to slow down the enzyme activity. Often these measures aren’t even too complicated.
Don’t damage that banana
In an undamaged banana or apple the flesh won’t brown. This is because, as long as the cells are whole, the PPO and the required ‘ingredients’ cannot come into contact with one another. Once you damage the product the cellular structure breaks down. As a result, the necessary components interact and browning starts. This browning reaction actually serves as one of many defense mechanisms of plants. This is why a bruised apple becomes brown on the outside and why an avocado remains perfectly green as long as it isn’t sliced.
That said, continued ripening and spoilage of produce will also break down cellular structures. As a result, even if you don’t squash that banana, it will turn brown over time. The same goes for the avocado and mushrooms for instance.
In order for step 1 and 2 of enzymatic browning to occur oxygen has to be present. If you store the product in an environment without any oxygen, the reaction cannot take place anymore. Oxygen sits all around is in the air, so in order to eliminate oxygen you’d have to use some sort of packaging. It is very common practice to use this when packaging freshly cut lettuce. Cut lettuce is very sensitive to browning, by packing it with none (or very little) oxygen the lettuce won’t turn brown or pink. The same can be done for apple slices. At home you can immerse them in water, preventing oxygen to come to the apple. In industrial applications by eliminating any oxygen from the package.
Since fresh produce will spoil very quickly if it cannot respire anymore, this is not a long term solution. It can work for shorter periods of time, to delay the browning just a little longer.
Change pH (acidity)
The activity of the PPO enzymes varies with the acidity of the environment. Some work better and faster at a lower pH-value (more acidic) whereas others prefer a slightly higher one. Nevertheless, most don’t perform well for very low pH-values. This is why recipes for apple dishes say to sprinkle some lemon juice on your freshly cut apples. It will lower the pH-value slightly, delaying the browning. When making a banana smoothie adding some lemon juice or sour yoghurt will also help to prevent browning.
Enzymes don’t only have an optimum pH-value, they also have an optimum temperature. At this temperature the enzymes work fastest. The optimum temperature for PPO various widely for different produce. For cucumber is might be a lot higher than for apples. Generally speaking, storing produce at a lower temperature will slow down enzymatic browning. However, that won’t work for everything. Bananas for instance as very sensitive to cold. Storing them in the fridge will result in more damage than the normal browning. In most cases it is better to store it at room temperature and let it be, than place them in the fridge.
Enzymes are proteins and that means that high temperatures (generally about 60°C) will break down the enzymes. Once you’ve heated enzymes to their critical temperature they cannot recover anymore. Instead, no further browning will occur. Giving food a quick heat treatment is called blanching. You can use it for freezing vegetables, such as spinach, for instance. Blanching the vegetable stops all enzyme activity (not only browning enzymes) and as a result you can for instance freeze the vegetables without those enzymes interfering any further.
Add an anti-oxidant
Since the second step of the browning reaction is an oxidation reaction, you can also add an anti-oxidant to reverse this second step. That way quinones can’t react into those coloured molecules. A commonly used example of this is ascorbic acid (vitamin C). An additional advantage of the acid is that it also lowers the pH-value, acting on the browning system in two ways.
In order for the enzymes to do their ‘work’, they need to be able to move in order to meet the phenols to react with. If you dry your banana, mushrooms or other fruits, this will be a lot harder. The enzymes don’t have that much water available to move through. Therefore drying slows down browning. Do remember that higher temperatures (especially around 40-50°C) will speed up browning! So take care that while drying to prevent browning, you don’t cause a lot of excessive browning. Also, drying won’t completely stop browning, it will slow it down only.
Preventing browning of bananas
Honestly, the very best way to prevent browning of bananas is to simply eat your banana before it turns brown.
If that’s really not possible, place it in a cool but not cold place. And once you’ve made it into a smoothie or puree, add some acid, store it in the fridge or even freeze it to off set the browning.
Physicochemical properties and function of plant polyphenol oxidase: a review, Ruhiye Yoruk & Maurice R. Marshall, 2003, link (Gives the reaction mechanism.)
Inhibition of polyphenol oxidase and peroxidase activities on fresh-cut apple by simultaneous treatment of ultrasound and ascorbic acid, Ji-Hyun Jang, Kwang-Deog Moon, 2011, link (For ways to prevent browning)
Effect of gamma and electron beam irradiation on the physico-chemical and nutritional properties of mushrooms: A review, Ângela Fernandes, Amilcar L. Antonio, M. Beatriz P.P. Oliveira, Anabela Martins, Isabel C.F.R. Ferreira, 2012, link (A case study on mushrooms.)
Senescent spotting of banana peel is inhibited by modified atmosphere packaging, Rujira Choehoma, Saichol Ketsa, Wouter G. van Doorn, 2004, link (For more info on the formation of brown spots on a banana.)
Effects of Anti-browning Combinations of Ascorbic Acid, Citric Acid, Nitrogen and Carbon Dioxide on the Quality of Banana Smoothies, Siyuan Wang et al., 2013, link (A case study on banana smoothies!)