sliced ice berg lettuce turned red

What Is Enzymatic Browning and How Can You Prevent It?

Once you’ve peeled your banana, you’d better eat it. Leave it unattended for a couple of hours and you’ll get back to an unappetizing brown banana. Same for apples, pears, avocados, and plenty of other fruits (and some more vegetables). They’re fine with their peel on, but one peeled/sliced, they turn brown in a matter of minutes or hours.

They all suffer from enzymatic browning, one of the most common types of browning in food, especially in fresh, uncooked, produce. So what is enzymatic browning? And, more importantly, is there anything we can do to prevent it from happening?

We recently had a LIVE event in which we discussed and demonstrated this concept! You get free access to the recording if you’re a Food Chemistry Basics student. (Read more here).

Three types of browning

When food changes color, there’s bound to be some sort of chemical reaction going on. Brown molecules that weren’t there before, have been formed.

There are three common ways this can happen. The first two are non-enzymatic browning reactions: caramelization and the Maillard reaction. They are what make sugar, bread or a piece of steak turn brown. They are accelerated by intense heat, and pretty slow (or even non-existent at room temperature).

Browning of our fresh produce on the other hand isn’t accelerated by intense heat. What’s more, it doesn’t occur at high (e.g. 100°C/212°F) temperatures! This browning mechanism instead relies on the presence of active enzymes and is appropriately named: enzymatic browning.

Enzymes

As the name says, enzymatic browning is a browning process that needs enzymes. Enzymes are a specific type of protein and are generally quite complex in structure. Enzymes naturally occur all around us. Our body is full of them and so are most living organisms, they are crucial for us to function normally.

In living organisms, chemical reactions happen all the time. Enzymes serve as catalysts for these reactions. This means that they help reactions take place and speed them up, without taking part in them themselves. Without enzymes, a lot of crucial reactions in this world wouldn’t take place.

Polyphenol Oxidase (PPO)

One of the many enzymes that are part of a lot of produce is polyphenol oxidase (PPO). This enzyme catalyzes a chemical reaction in which phenols are transformed into polyphenols (more on that below). In produce, these enzymes have several useful functions. An unfortunate side-effect is that this enzyme also causes sliced/peeled apples, bananas, pears, and more to turn brown!

Damaging cells releases enzymes

In a whole, undamaged apple or banana, enzymatic browning won’t happen. It only starts when you cut, peel, or otherwise damage a piece of produce. By damaging the cells of the products, the contents of those cells are released. They are no longer protected by a cell. As a result, molecules and enzymes can find each other and start to react!

Reaction mechanism of enzymatic browning

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So what happens during that reaction? Overall, polyphenol oxidase transforms small molecules (phenols) into large molecules (polyphenols). It does so through a set of chemical reactions. In reality, a lot of different reactions occur simultaneously, and the exact reaction mechanism is not yet fully understood.

We do know the first few steps quite well, after that, it becomes so complex, that chemists still aren’t completely sure what happens when. Luckily, understanding these first few steps gives you enough insights into what happens to help you prevent it from happening.

phenol
Basic phenol structure.

Step 1: Hydroxylation of phenols

Once the enzymes and molecules in the produce have been released, monophenols and polyphenol oxidase will find each other.

Monophenols are molecules that contain a benzene ring as well as an OH-group (also called a hydroxy group). A benzene ring is a ring of 6 carbon atoms that are connected with 3 alternating double bonds

The simplest phenol is made up of just that benzene ring and hydroxy group, but it can also contain other atoms attached to that ring.

So once the monophenol and PPO have found each other the enzyme catalyzes a reaction to transform the monophenol into a diphenol. This reaction is called a hydroxylation reaction and can only happen if oxygen is present! The air surrounding us is full of oxygen though, so once the produce has been damaged, oxygen can easily access the damaged area.

ppo enzymatic browning step 1 hydroxylation
Step 1: Hydroxylation of a monophenol into a diphenol.

Step 2: Oxidation of diphenols

Next up, an oxidation reaction occurs, again catalyzed by PPO. In this step, two OH-groups are transformed into two double-bonded oxygens (see below). The resulting molecules are called quinones. Again, oxygen is required for the reaction to occur.

ppo enzymatic browning step 2 oxidation
Step 2 of the enzymatic browning catalyzed by PPO, the oxidation of a diphenol into a quinone.

Step 3: Formation of colours

Quinones are very reactive molecules, they don’t need any help from enzymes to react further. After a series of reactions, the quinones will have formed polyphenols. These polyphenols are large molecules, which have a red, brown, or even black color. Your fruit or vegetables will turn brown once these have been formed.

peeled banana, starting browning
A freshly peeled banana, it has started browning within minutes.

Differences between produce types

Even though a lot of fruits and vegetables turn brown after they’ve been cut or peeled, they each do so slightly differently. Different fruits en veggies contain different PPO enzymes. Some are very good at 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 products even though the underlying mechanism is the same.

Preventing enzymatic browning

To prevent enzymatic browning you have a few options:

  1. Take away one of the molecules (PPO, phenols or oxygen) that is required for the reaction to occur
  2. Stop that enzyme from doing its job (or slow it down)

We’ll start by discussing a few methods that focus on ensuring the ‘ingredients’ required for the reaction don’t meet each other. Next, we’ll look at stopping or slowing down enzymes.

Option 1: Prevent reactants from ‘meeting’

Do not damage

In an undamaged piece of fruit or vegetable, browning won’t occur. As long as the cells are whole, the PPO enzyme, phenols and oxygen are separated from one another and as such can’t react. It’s only when cells are damaged that they can find one another and react, turning the product brown.

So, as long as you don’t slice, peel, cut or otherwise damage the produce, browning won’t happen.

That said, continued ripening and spoilage of produce will also break down cellular structures. As a result, even if you don’t damage it on purpose, it may still turn over time.

Eliminate oxygen

In order for steps 1 and 2 to happen the presence of oxygen is crucial. As such, if you store the product in an environment without any oxygen, enzymatic browning won’t take place.

Since oxygen is part of the air that surrounds us, that isn’t easy. If you have a straight surface, you can place the product onto a flat surface to reduce the amount of oxygen coming in. Alternatively, you can submerge it in water. Water replaces air and so keeps the oxygen away.

Manufacturers may use packaging technologies to prevent oxygen from coming into contact with their freshly cut produce. For instance, they can inject a package with nitrogen to take out most of the oxygen. However, since produce will spoil more quickly when there’s no oxygen at all, it’s a delicate balance of keeping the produce alive vs. maintaining its color.

Fresh fruits and vegetables respire even after harvest, it’s essential for staying fresh and vibrant. They need oxygen for respiration.

sliced ice berg lettuce turned red
Sliced iceberg lettuce is very prone to turning color when exposed to oxygen. Instead of turning brown, it will often turn red. It’s the same mechanism though, caused by polyphenols forming. It does greatly reduce the shelf life of cut lettuce, consumers will no longer find it appealing. This is why manufacturers package iceberg lettuce with very little oxygen in its package.
Remove water

In order for the reaction to take place all molecules need to be able to move around to find each other. By removing water, so drying your product, this can become a lot harder and browning can be slowed down.

Keep in mind though that the product needs to be very dry for this to work. As a matter of fact, the reaction might even speed up when only some water is removed since the remaining liquid will only be more concentrated!

Option 2: Slowing down/stopping the enzyme

In order to understand how you can influence enzymes, keep in mind that enzymes are proteins. They are a long chain of amino acids that has curled itself up in a very specific configuration to be able to catalyze reactions. If a protein is forced to ‘curl up’ in even only a slightly different way it might not be able to do its job anymore.

Change pH (acidity)

The first factor that can influence this configuration is the pH-value of its environment. The pH-value is a measure for the acidity of a liquid. A low value (<7) indicates an acidic environment, a high value (>7) indicates an alkaline environment.

Different types of PPO enzymes prefer environments with a different pH-value, their so called optimum value. This may depend on the pH-value of the product the enzyme is a part of. Most don’t perform well at very low pH-values (<4) though. 

This is why apple pie recipes often call for squeezing some lemon juice on freshly cut apples. The lemon juice is quite acidic and lowers the pH-value. As a result, it can slow down (or even stop) browning.

slowing down enzymatic browning of apple
One apple, cut into quarters and each treated in a different way. Photo was taken a few hours after treatment. Top: reference, nothing was done. Bottom, from left to right: 1) the right side of this piece was placed on a plate to protect it from oxygen, you can see it’s slightly less brown than the right side. 2) the right side was dipped into lemon juice, it’s clearly lighter in color than the reference. 3) This whole pice was dipped into a citric acid solution. It has not browned at all! The acid did its job.
Lower the temperature

Enzymes don’t just have an optimum pH-value at which they work fastest. They also have an optimum temperature at which is catalyzes fastest.

The optimum temperature for PPO various widely for different types of produce. For cucumber it might be a lot higher than for apples for instance. Generally speaking though, storing produce at a lower temperature (e.g. in the fridge) slows down enzymatic browning.

Keep in mind though that you might be causing other unintended side effects since not all types of produce can actually withstand low temperatures! Stay clear of freezing as well. Freezing will damage cells and most fresh produce will change because of freezing! Also, even at freezing enzymatic browning can still take place, be it slowly.

2 week old frozen bananas
These banana slices have been frozen for two weeks. They have barely browned. However, their texture has suffered so they’re not great to eat as is, best to use them for a dish calling for mashed bananas.
Increase the temperature

Enzymes are proteins and their 3D structure is crucial for their functionality. Like most proteins, the 3D structure of PPO is quite sensitive to heat. At too high temperatures the protein will denature, causing the 3D structure to unravel. This denaturation process is permanent, it won’t be able to recover once cooled down.

As such, heat can be used to deactivate PPO. Keep in mind that not all fruits and vegetables will be able to withstand a heat treatment, but for those that can, it’s a great solution.

This methodology is used very commonly when freezing vegetables. These are blanched before freezing. Blanching consists of a quick heat treatment, meant to stop any enzymatic activity. This doesn’t just stop PPO, but other enzymes as well, ensuring a longer shelf life once frozen.

Add an anti-oxidant

Last, but not least, you can add another ingredient that interfers with the chemical reaction. The second step of enzymatic browning an oxidation reaction. By adding an anti-oxidant you can reverse this 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!

That should give you plenty of tools to slow down or prevent enzymatic browning from occurring. Let us know in the comments which method(s) you’re planning to use!

Sources

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!)

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6 Comments

  1. Hi! I’m a student who’s going to take a chemistry course starting this fall! I feel very excited yet apprehensive.. My past performance in a chem class was sub-par.. I believe it may have been lack of eagerness on my part and my inability to ‘get’ the material- that is, since everything was so small and with my knowledge in chemistry was so lacking, I didn’t have any examples (or rather meaningful analogies) or materials to compare what I was learning with (this is where your website comes into play)!

    Asides from scanning this “Enzymatic browning & lots about Bananas” article, I’ve just read your post on leavening agents. Everything looks promising, and I’m grateful you’d take the time to write these articles, explaining the role of chemistry in cooking. I can now apply my knowledge in chemistry and connect it with everyday life! Thank you!

    • Hi Wispy,

      I am so glad to hear it’s been useful! Chemistry is really interesting, but I agree with you, it greatly helps if you can actually see how can be applied. If you have any requests in the future let me know, I’m always looking for new ideas. Also, if you’re interested, in a few days a new food science basics course I made starts. It is fully focussed on explaining science in food. It’s free and it’s fun (at least I find so), it might be perfect for you to check out now as well, since there is a big section on food chemistry. You can find it here: https://foodcrumbles.teachable.com/courses/food-science-basics.

  2. Does this only apply to peeled bananas? Or does this affect the peel of an unopened banana too?

    • Hi Amy, great question!
      Yes, it also applies to the peel of an unopened banana, especially if it’s been damaged somewhere. That said though, the formation of a lot of brown spots on the banana can also be caused by slightly different mechanisms (although enzymes will play a role).

    • Hi Kidumetsi,

      Once you’ve heated food, there’s no longer enzymatic browning! The heat will have broken down the enzymes. If there’s no heating involved, enzymatic browning can’t really be fully stopped, but you can slow it down by sprinkling some acid on the food (e.g. some lemon juice).

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