Rhubarb – Science of a fibrous vegetable

One of the best things of writing this blog is the questions we get from you. So far they vary from black salt to collagen drinks to olive oil oxidation to rhubarb. It’s always interesting to try and figure out the answers to your questions. In some cases they’re relatively easy, I know the answer already, or I know where to find it. In other cases some more extensive internet and book searches are required to find an answer. In most cases, I end up with an answer that is mostly helpful.

That is, except for the case of the rhubarb question where someone asked how frying rhubarb makes it taste sweet, without having to add sugar. I was assuming their might be a chemical reaction of some sorts (tannins breaking down? or starch being converted into sugar?), but I was disappointed to learn that I couldn’t find any proof for my theories. All I could give as an answer were my theories, rhubarb simply isn’t that heavily studied it seems.

That, I thought, is a perfect reason to get some more rhubarb information out there, because, despite its short season, it’s a great and pretty special vegetable to prepare and study.

What is rhubarb?

Rhubarb is a vegetable, although you will often see it used in ways that fruits are, in pies and desserts. It’s scientific name is Rheum rhabarbarum and it belongs to the family Polygonaceae which makes it related to buckwheat even though it looks more like celery stalks.

Rhubarb grows in long stalks straight from the ground with large green leafs at the end. The stalks vary in colour from a pink/red to red/greenish colours. They are quite firm since they have to hold on to those leaves, which can become pretty heavy due to their size.

Leaves of rhubarb are poisonous

The leafs of rhubarb are green, thanks to chlorophyll. They are poisonous for humans, not in a sense that one singly bite will make you sick but you’d better avoid them all together. What makes them poisonous is nevertheless not fully understood. Leaves contain quite a lot of oxalate (which is the salt of oxalic acid). This in itself isn’t poisonous (spinach also contains quite a bit of oxalate, as do the rhubarb stems). Nevertheless, when you eat too much of it, it is.

Researchers think there’s another component in the leaves as well though that makes it poisonous, but it hasn’t been investigated that extensively yet unfortunately. All in all, just don’t eat the leaves. In the next few sections we’ll stick with the stems.

Science of rhubarb texture

The rhubarb stalk serves as the stem for the rhubarb plant. As we discussed before, the role of a vegetable while it’s still part of the plant determine the structure and resulting texture of the vegetables. This is why rhubarb is quite a fibrous vegetable. At the same time, since the stem is still growing vertically as well as thickening sideways the structure isn’t yet as rigid as say a trunk of a tree. The name for this unique type of cells structure is collenchyma and quite apparent in rhubarb.

The stem also needs to transport all water and nutrients up into the plant leaf. If you cut your rhubarb in smaller pieces and have a look at a cross section you can see those transportation channels of the plant.

If you’ve sauteed a piece of celery stalk you will know that it takes quite a while for it to become soft and it never really falls apart into all its separate fibers. Rhubarb on the other hand cooks in a matter of a few minutes when you put it on the stove and falls apart completely. To make it into a jam all you have to do is to stir it slightly vigorously. Somehow, the fibers in the two seemingly similar textures are kept together by a different set of molecules.

I couldn’t find any research or evidence of this but I’m suspecting that the material keeping the fibers together in the rhubarb is a lot more water soluble than the celery. Also, since the rhubarb is quite acidic, more on that later on, the acidity might help the breakdown.

stack of pancakes with rhubarb jam

Science of rhubarb colour

See the bright pink colour of that rhubarb jam on the photo above? Even though it’s been cooked for several minutes, the colour of the rhubarb puree was possibly even brighter than that of the original rhubarb which also had some flecks of green in it. So what gives the rhubarb its nice colour?

As we mentioned in the introduction rhubarb has red/pink and green hues to it. Fruits and vegetables are great sources of a variety of colours. The green colour comes from the chlorophyll which is so common in green vegetables. Chlorophyll is the energy factory of a plant, the green chlorophyll absorbs the light the plant needs to produce glucose from carbon dioxide and water.

Reds are anthocyanins

The red/pink colour of the rhubarb comes from a group of molecules called anthocyanins. These are the same group of molecules that give red cabbage their bright red/purple colour. As you can well see when cooking red cabbage, an acidic environment makes the anthocyanins even brighter. Since rhubarb is quite acidic by itself, it makes for a beautiful colour.

Whereas colours tend to be very vulnerable to breaking down during cooking, that seems to be less the case for rhubarb. Research has actually shown that the amount of colour molecules actually increases during baking or stewing of the rhubarb, probably because it is released from the rhubarb. Of course, if you bake it for long enough though, the colour will disappear again. Natural colours just aren’t that stable, as we noticed when studying red colourants in food.

There are a lot of different anthocyanins. Researchers tend to like analyzing which molecules exactly are in a food. Using a technique called chromatography they can analyze the different molecules present. In the case of rhubarb they were able to determine that the main anthocyanins present are cyanidin‐3‐glucoside & cyanidin‐3‐rutinoside.

Science of rhubarb flavour

Rhubarb has a reputation of being very tart and acidic. Opinions differ here of course, I find a lot of rhubarb perfectly pleasant to eat without a lot if any sugar when it’s been cooked down to a soft texture.

That said, it definitely is acidic, with a pH-value just above 3. This is thanks to the presence of relatively high concentrations of oxalic and malic acid in the rhubarb.

What not to use rhubarb in

Pancakes are a great way to experiment with food, different ratios, new ingredients, all give a slightly different result. I’ve incorporated berries, apples, bananas, nuts and various grains in my pancakes. Rhubarb seemed like a nice next experiment. I shortly cooked the rhubarb, just until it started falling apart, cooled it down and mixed that through the pancake batter I was preparing. It gave the batter a nice funky colour, but the final pancake turned grayish (something to do with baking soda?) and it tasted downright awful. I don’t say this easily and tend to eat all my experiments even if they’re somewhat failed. However, this one was thrown out it had such a weird off flavour, I’m not even really sure where it came from.

The next batch of pancakes turned out great, no rhubarb, just a little extra water to compensate the moisture loss. This time, the rhubarb jam just sat on top and that worked wonderfully well!

Health benefits of rhubarb

Despite there being only a few researchers who looked into the flavour and texture of rhubarb, the potential health benefits and potential to extend shelf life of other products are clearly more popular to investigate. Rhubarb has traditionally been used in Chinese medicine for its health benefits. Analysis of rhubarb has indeed shown that there are various molecules in there which could be beneficial for human health. Other positive effects were seen with regards to antimicrobial properties, it can kill certain micro organisms.

But that’s not all, researchers have also looked at rhubarb for preventing browning of freshly cut fruits. The oxalic acid is what seems to help prevent discolouration of fruits caused by enzymatic browning. It makes rhubarb a nicely varied vegetable, which you may also see as a fruit!

Sources

Chronicle flask, Rhubarb rhubarb rhubarb, 2013 link; article on the poisonous nature of rhubarb and oxalates

Urszula Kosikowska, Helena D. Smolarz, Anna Malm, Antimicrobial activity and total content of polyphenols of Rheum L. species growing in Poland, 2010, link

Mauseth, J.D., Plants & people, p.53-57, link; on cellular structures in plants

McDougal, G.J., Dobson, P., Jordan-Mahy, N., Effect of cooking regimes on rhubarb polyphenols, 2010, Food Chemistry 119, link

Pussa, T., Raudsepp, P., Kuzina, K., Raal, A., Polyphenolic composition of roots and petioles of Rheum rhaponticum, 2008, link

Son, S.M., Moon, K.D, Lee, C.Y., Rhubarb juice as natural anti-browning agent, 2000, Journal of Food Science, link

Wrolstad, R.E., Heatherbell, D.A., Anthocyanin Pigments of Rhubarb, Canada Red, 1968, Technical paper No. 2377, Oregon Agricultural Experiment Station, link

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