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Vitamin C (ascorbic acid) oxidation & Preventing vitamin C loss

Only a few centuries ago, European (and likely many others) seamen died by the dozens during months long sea voyages to the East. Trips to the East, essential for European traders to get those beloved spices, fabrics and other trading goods, used to take months. And as you can image, they didn’t have the freshest of food supplies. Reason for a lot of these deaths? Not a lack of carbohydrates, proteins, etc. No, it was a lack of vitamin C (ascorbic acid). The disease? Scurvy.

Unfortunately, the disease, and its cure, had to be discovered several times, being forgotten in between, before being properly preventable. And still, scurvy isn’t completely gone from the world, only showing just how important getting enough vitamin C is for our bodies.

To make matters more complicated: our bodies can’t make our own vitamin C and vitamin C is pretty unstable. It gets lost quite easily, by leaving your glass of orange juice out for too long but especially during processing of food. The culprit: oxidation of the vitamin C. Luckily, once you understand how this happens, there are ways to prevent it.

Vitamin C is ascorbic acid

When reading about vitamin C, you might have come across the name ascorbic acid. Ascorbic acid is vitamin C, it’s just the chemical name. As the name says, vitamin C is an acid. This means that if you add ascorbic acid to water it will turn the water slightly sour, the pH-value will drop below 7. All acids do so by losing a proton (H+) into the water.

The chemical formula of vitamin C is C6H8O6. you can see how the atoms in the molecule are connected to one another in the structural formula below. Notice the ring within the structure. This ring is actually very important for the activity and properties of vitamin C. Such a ring is good at holding onto electrons.

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

You can find the formula for the acidic reaction from ascorbic acid below. It is important to know that this reaction is reversible. That is, it can go in two directions. Which way it goes depends on the external conditions. If there are a lot of protons (H+) in the system it is more likely to go from right to left. Vice versa if there are barely any.

C6H8O6 (ascorbic acid) ↔ C6H7O6 (ascorbate) + H+

Once ascorbic acid has given away/released its proton, its name changes to ascorbate(-ion). Ascorbate can form a salt when it connects with positively charges ions such as sodium (Na+) or calcium (Ca2+). You can buy these ascorbate salts from various manufacturers.

In your body as well as in a lot of food systems the most prevalent form of vitamin C is ascorbate, not ascorbic acid.

Oxidation of ascorbic acid

Remember how atoms are built up of a core of protons and neutrons with electrons ‘floating’ around? In most chemical reactions it’s these electrons that are transferred between species.

Ascorbate is not a very stable ion and tends to give away its electrons under the right conditions. We call this an oxidation reaction, ascorbate is oxidized, losing some of its electrons. An oxidation reaction is an example of a redox reaction. During this type of a reaction electrons are transferred from one species to the other. This would look something like (in reality it is a little more complex with other reactions going on involving radicals but they have been left our simplicity):

C6H7O6 (ascorbate) ↔ C6H6O6 (dehydroascorbic acid) + 2 e + H+

Note again that the arrows point in two directions. In other words, the reaction can proceed in both ways. It depends on the conditions again, which of the two happens.

Dehydroascorbic acid

The molecule that is formed because of the oxidation of ascorbate is dehydroascorbic acid. Both ascorbic acid and dehydroascorbic acid are involved in vital functions within the body. These reactions keep on going back and forth. The electrons that are taken on board or given away take part in all sorts of reactions. That ability of donating/taking electrons is one of the major functions of vitamin C in the body.

Since both dehydroascorbic acid & ascorbic acid are relevant for the human body, generally the overall vitamin C content is the sum of these two added up.

Summarizing the overall reaction

If we leave out the in between step of ascorbate (which you will often see being done in textbooks) the overall equilibrium reaction will become:

C6H8O6 (ascorbic acid) ↔ C6H6O6 (dehydroascorbic acid) + 2 e + 2 H+

Overall we can say that: vitamin C has the ability to reduce other molecules (and thus be oxidized itself). This means that it can donate two electrons to another component. The slightly extended versions of the chemical reactions we just described involve several radicals. We won’t go into the details of those, but it does mean that vitamin C is an antioxidant. It inhibits the oxidation of other molecules that shouldn’t be oxidized.

This can come in handy in a lot of different situations and plays a very important role in the human body. Preventing scurvy is just one of the many reasons that vitamin C helps our bodies.

Freshly squeezed orange juice contains plenty of vitamin C. However, if you leave it out too long it may all disapear.

Losing vitamin C

As long as ascorbic acid and dehydroascorbic acid keep on reacting into one another, no vitamin C will be lost. However, dehydroascorbic acid can react further in irreversible reactions. If that happens, you actually lose your vitamin C.

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 de-aerating the juice. By de-aerating the juice, most of the oxygen will leave the juice. After de-aeration, 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

Buettner, G.R., Schafer, F.Q, Ascorbate (vitamin C) and its antioxidant chemistry, link

Domitrovic, R., Vitamin C in disease prevention and therapy, 2006, Biochemia Medica 2006;16(2)89–228, link

HACIŞEVKĐ, Aysun, AN OVERVIEW OF ASCORBIC ACID BIOCHEMISTRY, 2009, J. Fac. Pharm, Ankara 38 (3) 233-255, link ; to learn more about the role of vitamin C in the body

Henry, C.J.K., Chapman, C. The nutrition handbook for food processors, 2002, chapter 10, link

Richardson, T., Finley, J.W., Chemical changes in food processing, 1985, Springer Science, link

Roth, K., and Streller , S., Vitamin C Deficiency – Part 3, 4-March 2014, Chemie in Unserer Zeit/Wiley-VCH, DOI: 10.1002/chemv.201400009

Wikipedia, Antioxidant, link

Wikipedia, Chemistry of ascorbic acid, link

Book on nutrition for food processors for more information on the degradation of vitamin C.

A book on changes occuring during processing of foods.

22 comments

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  • This is the most informative article I have found on the web about vitamin C oxidation. Which makes me now have serious questions about the bottle of liquid vitamin C I have…

    • Hi! I am writing a paper for school on the affect of light on ascorbic acid in orange juice, but am confused about the oxidation piece. How is oxidation bad?

      • Hi Sophie,

        Thanks for your question! Once vitamin C is oxidized it loses its nutritional value for the body which is why you wouldn’t want it to happen. Hope that helps!

    • Just found this article whilst researching Vit C shower heads as I have real problems believing they work, especially as the Vit C comes into direct contact with water. Do you have any info regarding their chlorine reducing capability and the science around the claims?

      • Hi Lena,

        I must immediately admit, I’m not a vitamin C shower head expert, I had never heard of them before your question! If I look at it purely from a nutrition perspective: you have to take the vitamin C in, that is swallow it in, for it to enter your body and do its thing. Whether and how it has an effect on skin, I would not be able to tell you, but I’m sceptical.

        With regards to chlorine neutralization, this article might help (I’m not an expert, the science sounds about right). In the case of these shower heads the filters they supply may also play a role here.

        Hope that helps! I’ll at least stick with eating oranges and other fruits to get my vitamin C 🙂

        • The claims are it neutralises chlorine in the water. If the Vit C comes into contact with the water after the initial use, won’t it oxidise, rendering it pointless to use?

          • Hi Lisa,

            In reacting with the chlorine, the vitamin C already oxidizes into dehydroascorbic acid (the same molecule as we mention above). Therefore, it won’t oxidize anymore later, it’s already ‘done’. Also, the oxidation of vitamin C is reversible as we discussed above. As a result, if you’ve just got water and nothing else, the reaction is reversible and can go back. By reacting with the chlorine the reaction becomes irreversible. Hope that’s clear!

            Also, online I read someone tested these shower heads with chlorine test strips and they seemed to work. I have never tested it, but if you can easily get a hold of the strips it might be worthwhile testing (and returning the head if it doesn’t work!).

  • Hi! I’m not really a chemist, but I’ve just done some vitamin C titrations for a school project and now I’m researching for my writeup. This article is a nice place to get started on factors involved in ascorbic acid oxidation – thanks!

    I investigated vitamin C degradation in lemon juice cooked for various lengths of time. I used juice squeezed that day and compared the concentrations of raw samples to those cooked for 5-30 minutes. My samples were only 30ml, and the 30 minute conditions tended to boil dry and caramelise a little, but vitamin C concentration was still about 80% of raw after 30 minutes cooking! The acidity in lemon juice seems to really help preserve ascorbic acid, so be sure to take some with you on your next long sea voyage!

    (pH4 being the optimum pH for oxidation seems counter-intuitive though and I’ve read somewhere that neutral and basic conditions promote oxidation which is what you’d expect, so I’ll need to look into that further.)

  • During ascorbic titration there is addition of oxalate ammonium. What is the role of this compound ?

    • Hi Kim,

      Thanks for your question! I am not 100% sure why it is added, but I believe it has to do with pH adjustments at the start of the titration (see also)!

    • Hi Surendra,

      Thank you for your question and apologies for the late answer. I have been working on a bigger update of this post to better reflect your answer, but also several of the reaction mechanisms. In short, you don’t necessarily have to prevent the oxidation into dehydroascorbic acid as long as that doesn’t break down. Hopefully the updated article will answer most of your questions!

  • Hey, I’m writing a paper on the effects of oxidation on the concentration of vitamin c within orange juice, note that we did a titration experiment on the effects of temperature on the concentration of vitamin C within the juice, picking different temperature to test.

      • Hi Kylee, that’s a great paper to do!

        What were your observations when you did your titration experiment? Did you see more or less vitamin C in the orange juice? Remember that the main effect of temperature on chemical reactions is that it increases the movement of molecules so generally speeds reactions up. Would that agree with what you’re finding?

        • I recently did the same experiment, but had some unexpected results. The heated OJ had less ascorbic acid than the unheated, but the ascorbic acid levels increased in the heated sample as it cooled. Why would this be? We also mixed an OJ sample with milk. The heated milk/OJ sample had more ascorbic acid than the unheated. Why would this be?

          • Hi Katie,

            Those are some interesting results indeed, would any of the following be possible?
            – Remember that the oxidation of vitamin C to DHAA (dehydroascorbic acid) is a reversible reaction. As long as DHAA does not react further, it can react back into ascorbic acid.
            If your drink contains reducing agents, these might be oxidized before the vitamin C gets oxidized. I’m not 100% sure whether milk contains these and if they are strong enough to protect the ascorbic acid (calcium ions might play a role here), but that would be worthwhile to investigate!
            Did the unheated milk + OJ sample also have more ascorbic acid than the unheated OJ? That would help determine whether the hypothesis above is correct.

            Keep in mind also that vitamin C (ascorbic acid) is very unstable. Did you store and treat the orange juice in all experiments in a similar way as not to loose any vitamin C while it’s standing or being exposed to light?

            For a more advanced method for determining vitamin C content and some more background, you might be interested in this article.

            Hope that helps!

  • How can it be that when i look up dehydroascorbic acid that i get C6H6O6 and not C6H7C6 what i see in this article.

    Above you see:
    C6H7O6– (ascorbate) ↔ C6H7O6 (dehydroascorbic acid) + 2 e– + H+

    Shouldnt it be:
    C6H7O6– (ascorbate) ↔ C6H6O6 (dehydroascorbic acid) + 2 e– + H+

    ??

    • Hi Eline,

      You are completely correct, and thank you for noticing this! That was a typo from our side. It’s now stated correctly in the article and all equations add up properly again.

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