Vitamin C (ascorbic acid) oxidation & orange juice

Updated: 16-Oct-2016

A few centuries ago, seamen died by the dozens on ships travelling from Europe to newly explored territories. Trips in these boats used to take weeks, if not months. Reason for a lot of these deaths: a lack of vitamin C (chemical name: ascorbic acid). The disease? Scurvy.

Unfortunately, the disease, and its solution, had to be discovered several times, being forgotten in between, before being properly preventable. And still, scurvy isn’t completely gone, there are still plenty of people with the disease. Vitamin C is very important for a human body.

However, vitamin C isn’t a very stable component. The so-called oxidation of ascorbic acid leads to vitamin C losses easily.

Vitamin C is ascorbic acid

As was quickly mentioned above, the chemical name for vitamin C is ascorbic acid, it’s chemical formula is C6H8O6. As the name says, ascorbic acid is an acid. This means it can release protons (H+) and lower the pH of a water solution. By lowering the pH of a solution, it becomes more sour. Once ascorbic acid has given away/released its proton, the name changes to ascorbate. This reaction is reversible though, when conditions permit the proton can sit back on the ascorbate.

If you’re unfamiliar with the concept of pH, acids and bases, have a look at my dedicated post on the topic.

Ascorbic acid / vitamin C
The structural formula of vitamin C, or ascorbic acid.

Oxidation of ascorbic acid

Vitamin C can release a proton to become ascorbate, as discussed above. But besides that, it can also be oxidized. An oxidation reaction is a so-called redox reaction. During a redox reaction electrons are exchanged between molecules/ions.

As discussed in food chemistry basics, all atoms have electrons ‘floating’ around them. In redox reactions it’s these electrons that are exchanged.

Vitamin C has the ability to reduce other molecules (and is thus oxidized itself). This means that it can donate two electrons to another component. This can come in handy in a lot of different situations and plays an important role in the human body, hence the scurvy if there’s no vitamin C.

Along with the ‘donation’ of two electrons (e), ascorbic acid also releases two protons. This will result in a new stable molecule: dehydroascorbic acid. The reaction is shown below.

Ascorbic acid oxidation
The oxidation of ascorbic acid (vitamin C) into dehydroascorbic acid. Two protons and two electrons are formed.

This reaction is an equilibrium reaction, which means it can go back and forth. However, dehydroascorbic acid is very unstable and can react further. These reactions are not reversible. If that happens, the vitamin C is lost.

Influencing ascorbic acid oxidation

Generally, food producers (and consumers) do not want vitamin C to get lost before we’ve been able to consume it and have it do its work in our bodies. Therefore, food scientists have investigated how to control the rate of the oxidation of vitamin C as well as the degradation of the dehydroascorbic acid. Five influential parameters have been found:

  • Oxygen
  • Metal ions
  • pH of the juice
  • Light
  • Temperature

1. Oxygen and ascorbic acid oxidation

The oxidation of ascorbic acid can be slowed down/prevented if there is no oxidizer present. In other words, a molecule has to be present that will receive the electrons. Oxygen is known to be a very good oxidizer. Actually, the name ‘oxidizer’ is derived from oxygen since oxygen was one of the first oxidizers to be discovered.

When left open to the air, which contains oxygen, ascorbic acid is prone to oxidation. Hence, if no oxygen is present, less oxidation of ascorbic acid will take place. There might be other oxidizers, but cancelling out this important one will have a significant impact.

Producers solve this problem by deaerating the juice. By deaerating the juice, most of the oxygen will leave the juice. After deaeration, as little air as possible will be left in the bottle. Therefore, they are often filled to a high level.

At home it is important to store orange juice in a closed bottle if not using it all directly. This will limit the amount of oxygen entering the juice and oxidizing the ascorbic acid.

2. Metal ions and reduction reactions

Metal ions are also good in participating in oxidation and its opposite, reduction reactions. In the case of ascorbic acid, metal ions can catalyse the reaction. This means that they don’t actually get used during the reaction, but can serve as temporary storage place for electrons.

If there is any practical advice resulting from this it’s that it’s best not to store orange juice in a copper pot. The copper ions will speed up the reaction greatly.

3. pH greatly influences oxidation

For a lot of reactions, the pH influences the reaction speed. For reactions that involve an acidic or alkaline component, that is especially important.

In the oxidation scheme of ascoribc acid you can see that it releases both protons (H+) and electrons (e).  But, if the ascorbic acid sits in a very acidic environment there will be a lot of protons around it. This could slow down the reaction. It has been found that the reaction takes place fastest at a pH of 4 (The nutrition handbook for food processors).

At a pH higher (thus more alkaline) or lower (thus more acidic), the reaction will proceed more slowly. Since the pH of an orange is around 3-4, orange juice is actually quite a nice place for this reaction to occur.

As long as the orange is whole, no reaction will take place though, it’s protected from oxygen and light and all cells are still in tact. But once you start squeezing and blending, the party starts!

4. Light and vitamin C oxidation

It is known that light influences the rate of vitamin C oxidation, as is the case for olive oil oxidation. Hence the green bottles for olive oil and the often non-transparant packs of long term storage orange juice.

The exact influence of light is complex though. The exact mechanism isn’t known to me yet. Overall advice though is to keep the orange juice in a dark place.

5. Temperature & reaction rate

Temperature determines the movement of molecules in food. The higher the temperature, the more atoms and molecules will move around. This makes it more likely for molecules to meet one another and thus react. Therefore, a higher temperature often results in a higher reaction rate. This is also the case for vitamin C oxidation, as has been investigated by researchers in strawberry juice.

Maillard reaction of vitamin C

Once the vitamin C has been oxidized it can react further into a lot of different pathways. One of which is the Maillard reaction. The Maillard reaction causes browning of a lot of food products through a very complex series of reactions.

Besides the Maillard reaction there are a lot of different ways dehydroascorbic acid can react further. But for now, we won’t discuss those into great detail, there would be too many. Also, trying to prevent oxidation of ascorbic acid is probably a better way to go than preventing these other reactions. Dehydroascorbic acid is simply less stable and most likely harder to control.

 

Lesson learned? If you do not want to get scurvy, please do not store your orange juice in a copper pan, in a hot place, with a lot of oxygen in great sunlight. Just squeeze your oranges and drink the same day. Do think about whether you want to squeeze or blend

Sources

  • A complete article on the role of ascorbic acid in the body, as well as its history.
  • Book on nutrition for food processors for more information on the degradation of vitamin C.
  • A book on changes occuring during processing of foods.
  • If your interested in health effects of ascorbic acid, read this.

Still here? Happen to be a chemist? It’s a pretty easy lab test to determine the quantity of vitamin C in orange juice. I’d be interested to see how fast it actually does degrade?!

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