Vitamin C (ascorbic acid) oxidation & Preventing vitamin C loss (2023)

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.

Vitamin C (ascorbic acid) oxidation & Preventing vitamin C loss (1)

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.

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.

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Vitamin C (ascorbic acid) oxidation & Preventing vitamin C loss (3)

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.

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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.

Vitamin C (ascorbic acid) oxidation & Preventing vitamin C loss (4)

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 nooxygen 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!

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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. Hencethe 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 instrawberry 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

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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.


How can we prevent the loss of vitamin C? ›

Prevention of Vitamin C Deficiency

Vitamin C deficiency can be prevented by consuming the recommended amounts of fresh fruits and vegetables or by taking the recommended amount of vitamin C in daily supplements.

Is vitamin C still good after oxidation? ›

A serum that is dark brown in color indicates the vitamin C serum has oxidized and will not be as effective and should be discarded.

Does oxidation reduce vitamin C? ›

Oxidation of ascorbic acid in stored orange juice is associated with reduced plasma vitamin C concentrations and elevated lipid peroxides. J Am Diet Assoc.

What happens when ascorbic acid is oxidized? ›

The overall oxidation of ascorbic acid typically involves the transfer of two electrons and two protons to form dehydroascorbic acid (DHA): H2A → DHA + 2e + 2H+.

How do you preserve vitamin C? ›

Vitamin C is lost in cooking water and during long storage times. You can preserve vitamin C by cooking the food in very little water (like steaming or microwaving) and by eating fresh vegetables and fruit as soon as possible.

Is ascorbic acid the same as vitamin C? ›

Available Forms. You can purchase either natural or synthetic vitamin C, also called ascorbic acid, in a variety of forms. Tablets, capsules, and chewables are probably the most popular forms, but vitamin C also comes in powdered crystalline, effervescent, and liquid forms.

What does it mean for vitamin C to be oxidized? ›

Oxidised vitamin C is no longer a powerful antioxidant resulting in free radicals, environmental and daily aggressors which leads to the skin vulnerable to damage with premature signs of ageing such as fine lines and wrinkles becoming very prominent and more noticeable.

How long does it take for vitamin C to oxidize? ›

After contact with light, heat and air, vitamin C eventually oxidizes and loses its potency. You'll know it when your serum changes colour, turning brown. (With L-ascorbic acid, this usually takes about three months after opening the bottle.)

Can I use vitamin C serum if it turns yellow? ›

You don't have to toss a serum if it's only slightly yellow but if it's tipping into orange, it's time to bin it. A dark orange would mean that your Vitamin C has over-oxidised and has lost a huge percentage of its efficacy.

Is ascorbic acid a reducing or oxidizing agent? ›

Ascorbic acid is a good reducing agent and facilitates many metabolic reactions and repair processes.

Is ascorbic acid an oxidising agent? ›

The outstanding chemical property of ascorbic acid (vitamin C) is that it is a reducing agent. The suggestion is obvious that its physiological function may be associated with this property, and, if it is oxidized reversibly, with its behavior in an oxidation- reduction system.

Is ascorbic acid oxidation reversible? ›

It is shown by means of proton magnetic resonance spectroscopy that the enzymatic (by ascorbate oxidase) as well as non-enzymatic (by iodine) oxidation of ascorbic acid is, in principle, reversible despite the hydration and structural changes during the formation of dehydroascorbic acid.

How can the oxidation of ascorbic acid be prevented? ›

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.

What causes degradation of vitamin C? ›

Vitamin C can be destroyed by heat and light. High-heat cooking temperatures or prolonged cook times can break down the vitamin. Because it is water-soluble, the vitamin can also seep into cooking liquid and be lost if the liquids are not eaten.

What happens when ascorbic acid reacts with water? ›

TCC's Ascorbic Acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions.

How do you keep ascorbic acid stable? ›

Refrigerate liquid ascorbic acid to preserve it for longer.

The coolest storage spots are usually at the bottom and right next to the freezer. Light and oxygen exposure can still cause ascorbic acid to degrade in the refrigerator.

How do you store vitamin C long term? ›

Vitamins are best stored in their original packaging under cool, dry conditions. It is important to protect vitamins from light, heat, oxygen, and humidity. I like to store my vitamins in a plastic shoe box inside of a file cabinet in our dark basement storeroom.

Do I need to refrigerate ascorbic acid? ›

There are also ingredients like free vitamin C (Ascorbylic Acid or Ascorbic Acid) which should be kept refrigerated because they are very unstable. This active form of vitamin C should also be consumed quickly, otherwise it simply loses its effectiveness.

Which form of vitamin C is best absorbed? ›

Form: Liposomal Vitamin C

This form is the best absorbed and is great for those with digestive problems because it can be absorbed without fully going through the digestive system. Liposomal vitamin C is often made with sunflower lecithin which can help the liver detoxify the body from toxins.

What is the best form of vitamin C to take? ›

Time-release vitamin C is often the preferred choice since vitamin C has better bioavailability when taken in smaller doses throughout the day. A time-release formula aims to solve this problem without taking multiple tablets, by releasing the vitamin C slowly throughout the day.

Is ascorbic acid better than vitamin C? ›

Yes, ascorbic acid is the purest form of vitamin C, but other vitamin C forms contain other supplements. Vitamin C comes in numerous forms, and every one of them has an alternate use and spot in skincare items.

Why is ascorbic acid a strong reducing agent? ›

In fact, all known physiological and biochemical functions of ascorbate are due to its action as an electron donor. The ability to donate one or two electrons makes AscH an excellent reducing agent and antioxidant. Ascorbate readily undergoes pH-dependent autoxidation producing hydrogen peroxide (H2O2).

How do you know if your vitamin C is good? ›

A thoughtful brand will include label instructions on how to store their serums. Many serums are yellow, but if your product takes on a brown or dark orange hue, it's time to toss because it's gone bad. If your serum starts out clear and turns yellow, that's also a sign it's oxidizing and will be less effective.

How do you know if vitamin C is working? ›

Also, keep in mind that it can take two to four weeks of daily usage to see results, so keep a close watch on your skin. If it starts looking brighter, that's a sure sign your vitamin C products are working.

What should you not mix with vitamin C? ›

It's best to completely avoid using vitamin C and AHAs/BHAs, such as glycolic acid and salicylic acid. Due to the chemical exfoliation these acids deliver teaming it with vitamin C can be too much for the skin and cause a great deal of irritation, reactions, and severe dryness.

How do I know if my vitamin C has gone bad? ›

A classic telltale sign of expired or oxidised Vitamin C is a change in colour. If you've spied that your once clear or light yellow serum is now looking a little deeper, or has changed shades entirely, you can take this as a pretty clear indication that your product has oxidised.

What happens when vitamin C serum turned brown? ›

Vitamin C has oxidised (reacted with oxygen) and turned into dehydro-L-ascorbic acid and eventually diketogulonic acid. The yellow-brown colour change is a visual sign of chemistry change. It shows that your Vitamin C serum no longer just contains L-ascorbic acid, the skin active form of the vitamin.

Can vitamin C make hyperpigmentation worse? ›

It's brightening

Melanin is the pigment responsible for skin color. By inhibiting melanin production, vitamin C can help fade dark spots and hyperpigmentation. It may also help brighten your skin's appearance.

Can vitamin C serum go in fridge? ›

Store it in a cool, dark, dry place to prevent your serum from oxidising. Avoid sunlight exposure at all costs, and keep the lid completely sealed. If you're super worried about being careless, keep the serum in your refrigerator instead — it's probably for the best.

What color should my vitamin C serum be? ›

When in the vitamin C is in high concentration, the serum will have a yellowish color; but as it oxidizes, it will turn brown/orange. Cosmetic vitamin C preparations that have discolored should be discarded as they have already oxidized and cannot provide skin benefits.

Why is ascorbic acid a good antioxidant? ›

Ascorbic acid has been shown to scavenge free radicals directly in the aqueous phases of cells and the circulatory system. Ascorbic acid has also been proven to protect membrane and other hydrophobic compartments from such damage by regenerating the antioxidant form of vitamin E.

How vitamin C works as an antioxidant? ›

As an antioxidant, vitamin C provides protection against oxidative stress-induced cellular damage by scavenging of reactive oxygen species, vitamin E-dependent neutralization of lipid hydroperoxyl radicals, and by protecting proteins from alkylation by electrophilic lipid peroxidation products.

How may ascorbic acid function as an oxidation-reduction catalyst? ›

It is observed that ascorbic acid promptly undergoes pH-dependent autoxidation and makes hydrogen peroxide. In the presence of catalytic metals, this oxidation is accelerated. The combination of metal and ascorbic acid is a very efficient oxidizing system.

Is ascorbic acid a weak or strong acid? ›

Ascorbic acid is a weak organic acid also known as vitamin C. A student prepares a 0.20 mol/L aqueous solution of ascorbic acid and measures its pH as 2.40.

What is the meaning by oxidation? ›

(OK-sih-DAY-shun) A chemical reaction that takes place when a substance comes into contact with oxygen or another oxidizing substance. Examples of oxidation are rust and the brown color on a cut apple.

Does orange juice lose its vitamin C? ›

Leaving your orange juice uncovered or at room temperature results in almost complete vitamin C loss after two or three days, Kunkel notes.

Does vitamin C oxidize on skin? ›

When you apply a layer of Vitamin C, it is exposed to air and light, leading to oxidation on the surface of your skin. The process will be the same, wherein L-Ascorbic Acid will oxidise and turn into erythrulose and release free radicals on your skin leading to ageing of your skin and DNA damage.

What can cause low vitamin C? ›

Risk factors for Vitamin C deficiency include:
  • Alcoholism.
  • Babies only fed cow's milk.
  • Seniors only consuming tea and toast diet.
  • Poor people who are not able to afford fruits and vegetables.
  • Smokers.
  • Individuals with eating disorders.
  • Type 1 diabetes who have high vitamin C requirements.
12 Oct 2022

What happens if you lack vitamin C? ›

Not eating enough fresh fruits and vegetables can cause the deficiency. People feel tired, weak, and irritable. Severe deficiency, called scurvy, causes bruising, gum and dental problems, dry hair and skin, and anemia. The diagnosis is based on symptoms and sometimes blood tests.

How can scurvy be prevented? ›

To avoid developing scurvy, eat plenty of fresh fruits and vegetables on a regular basis. Maintain a healthy diet to ensure that your body has all the vitamins and nutrients it needs to prevent nutritional diseases like scurvy. You can also take supplements, but you should talk with your doctor first.

What is the food of vitamin C? ›

citrus fruit, such as oranges and orange juice. peppers. strawberries. blackcurrants.

What is the best form of vitamin C? ›

Time-release vitamin C is often the preferred choice since vitamin C has better bioavailability when taken in smaller doses throughout the day. A time-release formula aims to solve this problem without taking multiple tablets, by releasing the vitamin C slowly throughout the day.

What is needed for vitamin C absorption? ›

Because vitamin C is water-soluble, supplements are best absorbed on an empty stomach. It's best taken in the morning, 30 minutes before eating for optimum absorption.

What interferes with absorption of vitamin C? ›

Vitamin C can be destroyed by heat and light. High-heat cooking temperatures or prolonged cook times can break down the vitamin. Because it is water-soluble, the vitamin can also seep into cooking liquid and be lost if the liquids are not eaten.

How do I know if I have vitamin C deficiency? ›

Symptoms include fatigue, depression, and connective tissue defects (eg, gingivitis, petechiae, rash, internal bleeding, impaired wound healing). In infants and children, bone growth may be impaired. Diagnosis is usually clinical.

Who is most at risk for vitamin C deficiency? ›

The most likely people include those with an overall poor diet, with kidney disease who get dialysis, heavy drinkers, and smokers. You'll need an extra 35 milligrams of vitamin C per day to help repair the damage caused by free radicals that form when you smoke.

Does vitamin C affect sleep? ›

The relationship between sleep and Vitamin C

What many do not know is that vitamin C plays a significant role in boosting sleep health. Studies have shown that individuals with greater concentrations of vitamin C have better sleep than those with reduced concentrations.

How does vitamin C prevent scurvy? ›

Scurvy occurs when there is a lack of vitamin C (ascorbic acid). The deficiency leads to symptoms of weakness, anemia, gum disease, and skin problems. This is because vitamin C is necessary to make collagen, an important component in connective tissues.

What is the main cause of scurvy? ›

Scurvy is caused by not having enough vitamin C in your diet for at least 3 months.

What foods prevent scurvy? ›

Citrus fruits, like oranges, limes, and lemons, have traditionally been used to prevent and treat scurvy. Several other fruits and vegetables contain higher doses of vitamin C than citrus fruits. Many prepared foods, like juices and cereals, also contain added vitamin C.

Which fruit has the highest vitamin C? ›

1. Kakadu plums. The Kakadu plum (Terminalia ferdinandiana) is an Australian native superfood containing 100 times more vitamin C than oranges. It has the highest known concentration of vitamin C, containing up to 2,907 mg per 100 grams.

What drinks are high in vitamin C? ›

Vitamin C drinks for strong immunity:
  • Lemonade. Having lemon water daily is a great way to rev up your metabolism and boost immunity along the way. ...
  • Fruit Juice. Pick any fruit with high vitamin C content, juice it and drink it. ...
  • Herbal Tea. ...
  • Milk Shake Or Smoothie. ...
  • Vegetable Juice Or Soup.
4 Jul 2020

Is lemon rich in vitamin C? ›

Lemons are a good source of vitamin C. One lemon provides about 31 mg of vitamin C, which is 51% of the reference daily intake (RDI).


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