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 on board.
They wouldn’t necessarily run out of sufficient calories, they did run out of those oh so important fruits and vegetables. Reason for a lot of these deaths therefore wasn’t a lack of carbohydrates, proteins, etc. No, it was a lack of vitamin C (ascorbic acid). You’ve likely heard of the disease before: 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 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. It does so (as does any acid) by increasing the concentration of proton (H+) in the water.
The chemical formula of vitamin C is C6H8O6. In the structural formula below you can see how the atoms in the molecule are connected. Notice the ring within the structure. This ring is actually very important for the activity and properties of vitamin C. Such a ring is common in a lot of other special food molecules and is especially good at holding onto electrons.
Ascorbic acid and ascorbate
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 actually 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.
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.
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:
- Metal ions
- pH of the juice
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 reaction of ascorbic acid that we discussed above 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…
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.