January, this time of the year where the sun is usually hibernating (at least in the Northern hemisphere, and even more in the Netherlands), while some “winter” viruses are having the time of their life, being spread among people in public transports and other closed environments. SARS-CoV-2 virus (the one responsible for you-know-what-if-you-have-not-lived-in-a-cave-between-2020-and-2022), Influenza A and B viruses (the ones mostly responsible for the “real” flu), or rhinoviruses (the most prevalent viruses responsible for the common cold): you can choose (or catch them all, like Pokémons).
It is with one of them (and a snotty nose) that I was sitting on my couch on a rainy day, wrapped like a sushi in my blanket and reading magazines to get a glance on the 2023 trends (in fashion, health, beauty, lifestyle, science, and any of the marketing BS I love to read to get an extra dose of inspiration for the blog). I bumped into an advertisement for two new tea varieties sold by the brand Zonnatura (sorry Zonnatura people; today you have been selected to be the target of my annoyance). What is special about these two varieties added to their collection of beverages? They contain “100% natural vitamin C”1, which – according to their claims – is supposed to “activate [my] natural energy in the body”1 and “has a positive impact on the immune system”1. Both claims have a small asterisk which refers to the following text (written in font size 4 in orange color on a yellow background, to make it easier to read I guess): “use as part of a balanced and varied diet and a healthy lifestyle”1.
(1Disclaimer: the advertisement was in Dutch; this is my translation of their Dutch claims into English)
A tea that contains natural vitamin C – how great is that; isn’t vitamin C supposed to be good to encourage and speed up your recovery when you have a cold?
Shall we start with the basics before answering this question?
The chemical name of vitamin C is ascorbic acid (also called L-ascorbic acid if you want to go deep into the real chemistry, but we keep the explanation of what this “L” means for another post).
It looks like that:
It is a substance that is found in many fruits (especially citrus) and vegetables, as well as dietary supplements and skin products. Vitamin C is what we call an essential vitamin: humans are not capable of producing vitamin C and must obtain it from dietary sources.
The importance of vitamin C for our health was discovered during the 18th century already, when citrus fruits were given to U.S. Navy sailors during long journeys to prevent scurvy, a disease that develops in presence of vitamin C-deficiency, which causes collagen and several enzymes to function poorly. Since then, a lot of research has been performed to investigate the effects of vitamin C on our health.
When it comes to the role of vitamin C in the prevention, duration and severity of common cold (which can be caused by over 200 different viruses!), there has been a lot of controversy in the literature. Some studies showed that vitamin C supplementation (≥ 200 mg/day) had not effect on the incidence of common cold in the population, but a consistent (despite modest) effect on reducing the duration and severity of the cold symptoms at an individual level . These effects were more apparent at much higher doses, i.e., up to 6-8 g/day . Other studies did not show significant impact on the symptoms associated with the common cold. This discrepancy may be explained by many factors, including the level of physical activity of the study participants, their baseline levels of vitamin C in blood, and the dose administered [1,2].
Besides the common cold, diseases that seem to benefit the most (with more or less evidence) from a supplementation of vitamin C are COVID-19, sepsis, metabolic syndrome and atherosclerosis, neuro-degenerative disorders, and cancer [3-6].
With this in mind, let’s go back to our Zonnatura tea, and take the tea product labelled “IMMUNE”1 as an example. This product contains green tea (69%), rosehip (note: no proportion indicated), dried acerola juice (10%), natural black currant flavor (5%), matcha (2%) and black currant (1%) . The label also indicates the amount of vitamin C per 100 g, which is 8 mg.
Wow, 8 mg for 100 g! That sounds great! Wait – is that actually a lot?
In order to answer this question, we first need to know what amount of vitamin C we should consume every day to stay healthy. The recommended values of vitamin C intake per day vary depending on the countries. In average, a daily intake of 90-110 mg/day for adult men and 75-95 mg/day for adult women, respectively, is recommended, according to the European and U.S. Food Safety authorities [8,9]. Higher values are recommended during pregnancy and lactation, and for smokers.
The next step is to figure out what this “per 100 g” means. Does it mean “per 100 g of tea powder”? Or does it mean “per 100 g of tea prepared in water”? This makes a big difference, considering that a tea bag contains 1.8 g of tea, which would be equivalent to 0.14 mg of vitamin C in one tea bag if the “per 100 g” would mean “per 100 g of tea powder”. This would then be a ridiculous amount, as you can imagine.
The answer to this question should be easy to find, but actually requires further investigations, as it is not indicated on the Zonnatura page. I found an answer on the website of one of the supermarkets where this product can be purchased, which states “8 mg of vitamin C per 100 milliliters” . The same page also provides information on how to prepare this beverage, namely, adding 150 mL of water at 100 °C and let infuse for 2 min . In theory, we can thus assume that drinking a 150-mL cup of this product prepared using one tea bag would bring us 12 mg of vitamin C.
Usually when you say “in theory”, it means that this is not exactly what happens “in practice”.
You start to know me well!
This is exactly why it is difficult to answer the question “are 12 mg of vitamin C a lot?”. Based on the daily requirements that we discussed earlier, this would mean that a cup of this tea would bring 13-16% and 11-13% of the daily recommended amount of vitamin C in women and men, respectively. Quite something with a simple cup of tea, you may think.
But there is a caveat to this simple calculation.
Ascorbic acid is not a very stable compound. It is prone to instability, which means that it can degrade into other compounds that are less interesting and do not have the same activity than ascorbic acid. This degradation process is complex, as it is strongly dependent on many conditions.
Vitamin C, unstable? So which conditions are impacting the stability of ascorbic acid? For some reasons, I have the feeling that your answer will contain the word “temperature”.
Exactly. The stability of ascorbic acid in an aqueous solution decreases when this solution is exposed to light, heat, oxygen, alkaline pH (i.e., basic conditions), and some metal ions [11,12]. To make it even more complex, this degradation also depends on the initial concentration of ascorbic acid in this solution, and the time that the solution is exposed to these factors (alone or in combination).
Heat? So this means that the vitamin C in my tea will be totally destroyed when I pour the water at 100 °C, as recommended? Or will I still see some benefits with a proportion of ascorbic acid left?
That is exactly the question I asked myself when reading the Zonnatura advertisement and the reason for writing this post. From past courses I followed, my mind had stored the following information: “vitamin C is sensitive to heat, so cooking food or adding boiling water to it will degrade (part of) the vitamin C present in a solution”. But how much? And at which temperature would this process start to occur? What is the influence of infusion time? Is this dependent on the food matrix itself, or can we draw some general conclusions for all? Apparently still a lot of questions for Dr. Iza – it was time for an update on my knowledge on vitamin C!
I went into the rabbit hole of the literature on vitamin C to try to answer these questions. I assumed that this information would be easy to find, but boy I was wrong – the degradation process of ascorbic acid under heat is indeed very complex. Now I start to understand the challenges of processed fruit juices makers who have to find the best balance between safety of their products (which typically requires high-temperature pasteurization) and stability of ascorbic acid in these products.
Maybe we can start with the easiest – what about the stability of vitamin C in hot beverages?
An interesting example comes from a study published in 2017, investigating the effect of brewing conditions (i.e., infusion time and temperature) on the properties of rosehip beverage, including the content in ascorbic acid . Considering that rosehip is one of the ingredients of the Zonnatura tea, it is interesting to look how the vitamin C content in rosehip is impacted by brewing. The authors used a very sophisticated approach (called “response surface methodology”) that allowed them to define which brewing conditions led to the highest content of vitamin C in the tea beverage, using 2 g of rosehip. They found that the highest concentrations were obtained with an infusion time of 6-8 min at a temperature of 84-86 °C . Interestingly, higher temperatures generally led to a higher content of vitamin C in the beverage, explained by an easier diffusion of vitamin C, which is soluble in water, in the hot beverage. However, this was strongly impacted by the infusion time. A shorter time (<4 min) led to less vitamin C extracted from the rosehip, while degradation of vitamin C was observed at longer infusion times (>6 min).
In case this was not clear, this Figure will either help you understanding it better, or confuse you even more.
What is also important to mention is that the content in ascorbic acid they measured ranged from 2.92 mg/100 mL to 3.29 mg/100 mL. In other words, this means that the worst brewing conditions studied led to a content of vitamin C ca. 11% lower compared with the best conditions.
11% – that is not really consequent, isn’t it? So this study showed us that for rosehip beverages, both the infusion time and temperature have an effect on the content of vitamin C, where a balance needs to be found between sufficient time to extract vitamin C, but not too long to avoid having degradation of vitamin C. Correct?
Yes; and that in the conditions studied – which closely mimic what would happen in our kitchen when preparing tea (70-90 °C, 2-10 min infusion time) – the potential loss in vitamin C, 11%, is relatively low.
That’s great news! But what about solid food, such as vegetables? We often read/hear in media that many vegetables (such as broccoli or cauliflowers) and potatoes have a high content in vitamin C and are therefore good for our health. But these are food items that we usually cook before eating, so what happens to their content in vitamin C?
Let’s first focus on potatoes, as it is indeed needed to cook potatoes before eating them.
**Reminder: raw potato is toxic – don’t eat them raw, please**
Depending on the cultivars, genotype, geographical location and environment where the potato tubers have been cultivated, the vitamin C content ranges between ca. 10-30 mg/100 g of fresh weight [14-16]. Before even reaching our pan to be cooked, our potato will also be submitted to different post-harvesting conditions (e.g., storage for a couple of weeks at 4 °C in the fridge or the basement), which already influence their content in vitamin C  .
A group of researchers investigated two decades ago already the effects of different cooking methods, namely, baking in the oven, boiling, braising, frying, microwaving, sautéing and pressure-cooking, on the content of vitamin C in different potato tubers varieties .
Do you mean that these researchers were cooking potatoes in their lab using all these different approaches for the sake of science?
Who said science was boring?
(not me, obviously – otherwise this blog wouldn’t exist)
You’re totally right. What did the authors find?
They reported the following losses, in percent, of ascorbic acid content in the potatoes after cooking compared to the raw potatoes:
- boiling in water, 77-88%;
- boiling in water containing salt, 61-79%;
- frying in oil, 55-79%;
- sautéing, 61-67%;
- pressure-cooking in water, 56-59%;
- braising, 50-63%;
- baking, 33-51%; and
- microwaving, 21-33%.
In other words, a more important degradation of vitamin C was observed during boiling and frying potatoes, but less during braising, sautéing and pressure-cooking. Both baking and microwaving had the lowest impact on its stability. Interestingly, they also observed a difference in the vitamin C stability depending if salt was added to the boiling water.
This data nicely shows that the content of vitamin C really depends on a complex combination of parameters, including the cooking temperature, time, pressure, presence/absence of water, and presence/absence of other substances (such as salt).
This makes it very complex to know what amount of vitamin C we actually ingest when we consume potatoes, then… And what about vegetables?
Among the articles I found that investigated the fate of ascorbic acid in vegetables, most of them studied broccoli. This did not surprise me, as broccoli is often sold as one of the biggest sources of vitamin C among vegetables. But again, broccoli is most of the time consumed cooked and not raw (even though raw broccoli is not toxic, this time you can go for it).
Similar to potatoes, the content of vitamin C in Brassica oleracea var. italica (that’s the artist’s name of broccoli) is very dependent on pre- and post-harvesting factors, such as crop varieties, growth conditions, harvesting conditions, and storage . In general, the literature agrees that the average content of vitamin C in raw broccoli is ca. 50-100 mg/100 mg of fresh weight [17,18]. This is indeed quite a solid amount of vitamin C content for a vegetable.
One of the articles studying the stability of vitamin C evaluated the effect of different cooking methods, namely, blanching (1 min), boiling (5 min), microwaving (2 min) and steaming (10 min) on the content of ascorbic acid in broccoli (and other vegetables) . The outcome of their study is that both steaming and microwaving seemed to conserve the content of vitamin C in broccoli, while blanching led to ca. 12% loss of ascorbic acid, and boiling destroyed almost half of it. Boiling and blanching led to a degradation of vitamin C in all vegetables studied (due to leaching in water); microwaving was the technique with the least impact on vitamin C content. For steaming, it was really vegetable-dependent.
Oh, so microwaving should be the gold standard technique to avoid degradation of vitamin C in broccoli, then? Do we have a winner?
I wish we would, but a study published in 2007 showed that the degradation of vitamin C in broccoli and its release in the cooking water was ca. 20-40% if the initial amount, and very dependent on the power, cooking time, and volume of cooking water used during microwaving process .
Ooooofffff…. So we don’t have a winner yet.
Another study investigated the thermal stability (using 15-min blanching as cooking technique) of crushed broccoli . Why crushed? Because treatments such as peeling, crushing or cutting can favor the enzymatic degradation of vitamin C by increasing the contact between ascorbic acid and the enzyme that is responsible for its oxidation. But guess what – this enzyme (called AAO) can also be deactivated by heat! The authors found the following:
- At a blanching temperature between 30-60 °C, ascorbic acid was almost totally degraded enzymatically in the crushed broccoli.
- At a blanching temperature between 70-90 °C, the content of ascorbic acid was conserved (no degradation).
We can draw the conclusion that at lower temperatures, the degradation occurred because of activation of the enzyme AAO that degrades ascorbic acid, while at higher temperatures, the enzyme was inactivated and the content of ascorbic acid was not altered. Interestingly, the authors also found that the activity of this enzyme was 7.5 higher in the broccoli florets compared with the stalks .
I’m getting a headache with all these aspects that I need to take into account to know if my broccoli still has vitamin C in it after having been cooked, depending on the cooking technique used, the cooking time and temperature, the volume of water, the part of the broccoli I want to eat, the way I stored my broccoli in my fridge and for how long, and how I cut it. Why is that so complicated?
The information found in the literature confirms that the degradation of vitamin C is a very complex process and that it is simply impossible for a lambda user, even well-informed, to guess how much vitamin C they actually consume when eating processed or unprocessed vegetables and fruits.
And this, despite the claims present on food packaging?
Yes, and that is exactly the take-home message of this post: when it comes to vitamin C, the information on the packaging should really (read: REALLY) be taken with caution. The content of vitamin C in raw material at the moment it has been measured (if it has been measured) is very likely not equal to the content of vitamin C you actually consume.
This advice is also valid for any post you may read on social media or fitness/nutrition apps touting the benefits of some kind of (super)food with high contents in vitamin C. There are no doubts about the benefits of vitamin C, yes, but no one can tell how much will actually enter your body.
Stay alert, as always!
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