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Once you’ve peeled your banana or apple, you’d better eat it. Leave it unattended for a couple of hours and you’ll get back to an unappetizing brown piece of fruit. Same for pears, avocados, potatoes 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.
Enzymatic browning is what causes this undesirable color change. It’s one of the most common types of browning in food, especially in fresh, uncooked, produce. As the same says, it’s catalyzed by enzymes. And luckily, there is a way to slow it down, and in some cases even prevent it from happening. Temperature, pH and anti-oxidants may all play a role.
- Brown color molecules are formed
- Reaction mechanism of enzymatic browning
- Preventing enzymatic browning
Brown color molecules are formed
In order for those fresh apples or bananas to turn brown, new molecules need to be formed. These brown-colored molecules are formed in a series of chemical reactions, that kick off, as soon as the piece of produce is peeled, cut, or otherwise damaged. These reactions can happen at room temperature, or in the fridge. As a matter of fact, they can’t even take place at elevated temperatures because, as the same says, enzymatic browning depends on the presence of enzymes, and those get deactivated at high temperatures. Even though enzymatic browning is not harmful, nor does it affect flavor, it’s just not a very appetizing appearance.
There are two other very common mechanisms for browning reactions caramelization and the Maillard reaction, also referred to as non-enzymatic browning. However, generally speaking, these are desirable, whereas enzymatic browning is almost always undesirable.
Enzymes are catalytic ‘machines’
Enzymes are a subset of proteins. Each protein is made of a long chain of amino acids that configured in a quite complex 3D configurations. 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. Enzymes serve as catalysts for chemical reactions. 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)
The enzyme responsible for enzymatic browning is polyphenol oxidase (PPO). This enzyme catalyzes a chemical reaction in which phenols are transformed into polyphenols (more on that below). A lot of fruits and vegetables naturally contain both PPO and phenols. They fulfill several crucial roles within the products, but unfortunately, can also result in browning.
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
During enzymatic browning, polyphenol oxidase (PPO) 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. The exact reaction mechanism is not yet fully understood, but we do know the first few steps quite well. After those, it becomes so complex, that chemists still aren’t completely sure what happens when. Luckily, understanding these first few steps gives you enough insights to help you prevent it from happening.
Step 1: Hydroxylation of phenols
Once the enzymes and molecules in the products have been released, monophenols and polyphenol oxidase will find each other. Monophenols are a group of molecules will a similar molecular structure. They contain a benzene ring as well as an OH-group (also called a hydroxy group). The simplest phenol is made up of just that benzene ring and a hydroxy group. More complicated phenols will contain additional groups and atoms.
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 produce has been damaged, oxygen can easily access the damaged area.
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.
Step 3: Formation of colors
Quinones are very reactive molecules, they don’t need any help from enzymes to continue reacting. After a series of subsequent 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 turns brown once these have been formed. This is an irreversible reaction, so there’s nothing you can do to get rid of the brown colors again.
Differences between produce types
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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 types of PPO enzymes, with a different overall PPO activity. Some are very good at catalyzing step 1, whereas others focus on step 2. As a result, the mechanism and 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:
- Remove one of the components required for the reaction to occur, without the ‘substrates’ phenols and oxygen the reaction can’t happen.
- Stop that enzyme from doing its job or slow it down
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. So, as long as you don’t slice, peel, cut, or otherwise damage the product, browning won’t happen. That said, continued ripening and spoilage of produce will break down cellular structures. As a result, even if you don’t damage it on purpose, it may still start turning brown over time.
If you have to cut the produce to some extent, consider doing it as little as possible. For instance, thinly cut apple slices have a huge surface on which enzymatic browning takes place, whereas big chunks of apple have a smaller surface area available.
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 necessarily easy. You have a few options:
- If you’ve sliced a piece of fruit or vegetable, you could place the freshly cut surface head down onto a straight surface. This helps to keep the oxygen away somewhat. It won’t work for a long time, but can sometimes be just enough.
- You can submerge cut fruit and vegetables in water. Water keeps the oxygen away. This is often recommended when cutting and peeling potatoes.
- Use packaging that’s free of oxygen. Since fruits and vegetables do contain to respire after harvest, this is a tricky proposition. In order for them to remain fresh, they need some level of oxygen. Manufacturers may use modified atmosphere packaging, that contains less oxygen, maybe a few percent only. This way, browning is slowed down, without immediately killing the produce.
In order for the reaction to take place all molecules need to be able to move around to find each other. By removing water this becomes a lot harder. As such, drying products may slow down browning. However, it only works once most of the water has been removed and that significantly impacts the texture and eating experience of the food.
Option 2: Slowing down/stopping the enzyme
Enzymes are proteins, and like most proteins, they have a complex 3D structure. The long protein chain is curled up and folded in a very specific way. Even a slight change in its structure may stop an enzyme from working. This is where you can try to slow down, or even stop the enzyme.
Change pH (acidity) – e.g. add lemon juice
The first factor that can influence this configuration is the pH value of the environment. The pH value is a measure of 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, but most don’t perform well at very low pH values (<4).
Lower the temperature for enzyme inactivation
PPO enzymes have an optimum temperature at which it catalyzes the enzymatic browning process fastest. The optimum temperature for PPO varies 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 may damage cells and irreversibly change the texture. This is especially true for fruits such as berries and bananas. Also, even in the freezer enzymatic browning can still take place, be it slowly.
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.
Sulphur dioxide (SO2)
The reason golden or yellow raisins haven’t turned brown? Because of the addition of this ingredient: sulphur dioxide (SO2). It’s not yet completely understood how it works, but it’s clear the addition of this ingredient can completely prevent enzymatic browning by inhibiting the activity of PPO. There exist several other PPO inhibitors though not all of them are actually food safe, so can’t all be used in food!
Add an anti-oxidant
Last, but not least, you can add another ingredient that interferes with the chemical reaction. The second step of enzymatic browning is an oxidation reaction. By adding an anti-oxidant you can reverse this step. That way quinones can’t continue to react and form those brown colored 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. And remember, time is of the essence. If you simply use your produce quickly, it doesn’t even have the time to turn brown!
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!)