Why does enzymatic browning happen

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.

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. The quinones will form polyphenols, large structures which will cause the actual discolouration. The pigments are called melanins.

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. 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. That said, continued ripening and spoilage of produce will also break down cellular structures.

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. It is very common practice to use this when packaging freshly cut lettuce. The same can be done for apple slices.

At home you can immerse them in water, preventing oxygen to come to the apple. Structure 3 : Formation of melanins from tyrosine Source. Polyphenoloxidase catalyses two basic reactions: hydroxylation and oxidation. Both reactions utilize molecular oxygen air as a co-substrate. The reaction is not only dependent on the presence of air, but also on the pH acidity. The control of browning is one of the most important issues in thefood industry, as colour is a significant attribute of food which influences consumer decision and brown foods especially fruits are seen as spoiled.

Several methods can be applied to avoid enzymatic browning, based on inactivating the enzyme heat or by removing essential components most often oxygen from the product. Blanching is a short heat treatment to destroy or inactivate enzymes before freezing of products mainly vegetables. Enzyme activity may discolour or toughen vegetables during freezing, which results in quality loss. Blanching brightens the colour, softens the texture, but has little effect on nutrient content or flavour as it is a relatively short process.

Table 2 below gives an indication of the temperature needed to inactivate some important enzymes. Steam or boiling water blanching is a type of heat treatment for controlling enzymatic browning in canned or frozen fruits and vegetables. It is scalding the vegetables or food in water or steam for a short period of time. The steam blanching is 1. Microwave blanching may not be effective, since research shows that some enzymes may not be inactivated. This could result in off-flavours and loss of texture and colour.

Refrigeration and chilling are used to prevent spoilage of vegetables and fruits during distribution and retailing. Chilling is applied often for broccoli, berries, spinach, peas, bananas, mangoes, avocados, tomatoes. Therefore the temperature should be well controlled.

Like refrigeration, freezing inhibits, but not inactivates the enzyme. After thawing, the enzyme activity will resume.

The enzyme activity is pH dependent. Lowering of the pH to 4. During home-preparation of vegetables or fruits lemon juice or vinegar is often sprinkled on the fruit to prevent browning.

You can see this for yourself if you observe a banana for several days or weeks. Would you buy a brown banana? Enzymatic browning is one of the largest causes of quality loss in fruits and vegetables—even though it does not make the food harmful to eat. So what exactly happens during enzymatic browning? The process occurs over several steps.

The enzyme responsible for the browning is called polyphenol oxidase or PPO. In the presence of oxygen the PPO enzyme changes substances known as phenolic compounds through a process of oxidation into different compounds called quinones.

The quinones then react with other compounds to form melanin. Melanin is the same dark brown pigment that colors hair, skin and the irises of our eyes.

It also turns fruit and vegetables brown. This reaction, however, usually does not happen within fresh fruits and vegetables because the PPO and the phenolic compounds are separated in produce plant cells. The enzymatic browning process is only triggered when PPO, phenolic compounds and oxygen come in contact with one another. This is exactly what happens when a fruit is cut, falls or is knocked around too much. When fruit tissue is damaged because of heat, cold, age or mechanical stress, its cells break open and the phenolic compounds and the enzyme are released and mix with oxygen in the air.

As a result the damaged tissue turns brown almost immediately. In this activity you will see this browning process for yourself and investigate how this reaction can be avoided.

So grab some fruit and get started! Observations and Results Were you able to change the color of your banana? Most likely, yes! You probably didn't observe a big difference in the banana right after putting it into the boiled water, but within the next 30 seconds and after taking it out of the water it should have turned pretty dark. You should have noticed that the color change only happened where the banana was submerged in the hot water. This is because the boiling water caused heat stress to the cells in the outer layers of the banana peel and destroyed them.

As the cells broke open they released PPO and phenolic compounds, which then reacted with the oxygen of the air to form melanin. Only the peel should have been affected by enzymatic browning as the inner part of the banana was protected by the peel. If you put a banana in the fridge, the whole banana should have turned brown. As the banana is a tropical fruit, it is evolved for warm temperatures, which is why the banana cells get damaged in the cold.

The longer a fruit is exposed the less vitamins it will have. Have three containers ready, one containing water, one containing sugar and water say 5g sugar, 50ml water , and one containing a little lemon juice. Leave one sample of each fruit or vegetable on a plate and quickly place one sample into the water, one into the sugar solution, and one in the lemon juice.

You should see browning in the samples left on the plate. Compare this with the other containers. What conditions prevented browning most? Think of food preparation and cooking where similar conditions are used. Why do chefs often tear rather than cut up lettuce leaves? See our video exploring the effect of different variables on enzymic browning.