The other day, having some rhubarb left and not wanting to make a rhubarb pie, cake or crumble (>90% of rhubarb recipes online seem to be for one of those!), I decided to try a rhubarb ice cream. I had no idea how it would turn but, to my surprise, it make a very almost creamy ice cream! This, despite the fact that the ice cream didn’t contain any cream or fat for that matter.
Intrigued, I decided to dig into the science of rhubarb. What’s at play here and what else don’t I know about rhubarb that would surprise me?
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 there 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.
What is rhubarb?
So what is rhubarb? Some might think it’s a fruit, since you will often find recipes that use rhubarb in a very similar way as you would berries. However, rhubarb is a vegetable since the stalks don’t serve any role with regards to the spread of seeds of the plants.
Rhubarb grows in long stalks straight from the ground with large green leaves at the end. The leaves are inedible, more on that later whereas you can eat the stalks. The stalks vary in color from a bright pink/red, to red/greenish. The stalks need to be quite sturdy since they’re holding up the (sometimes enormous) leaves. Their role and structure probably remind you most of celery stalks. However, rhubarb is more closely related to buckwheat than it is to celery!
There are a lot of rhubarb varieties that all belong to the Rheum genus. Most of the species though are only used for medicinal purposes by humans. Only some specific species are used and suitable for culinary purposes. Where these originated from still isn’t very clear.
Leaves of rhubarb are poisonous
We humans only eat the stalks of the rhubarb (so we’ll focus on these here) and for good reason, the leaves of rhubarb are actually poisonous to humans. They’re not that poisonous that a single bite will kill you, but it’s best to avoid them altogether.
What it is that makes them poisonous is not fully understood. We know that the leaves contain quite a lot of oxalate (which is the salt of oxalic acid). This in itself isn’t poisonous when eaten in moderation, spinach also contains quite a bit of oxalate, as do the rhubarb stems. So researchers think there’s another component in the leaves that makes it poisonous, but that still remains a bit of mystery for now!
The structure of rhubarb
As we discussed in another post, the role of a vegetable in the plant determines the structure and resulting texture of the vegetable. A leafy vegetable needs to capture light, but doesn’t have to be strong. A stalk on the other hand needs to be strong enough to hold up that leaf, though not as strong as a tree trunk has to be. The cell structure that gives this strength to the rhubarb is called collenchyma. Collenchyma tissue is made up of mostly cellulose and pectin.
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.
Cooking rhubarb and its texture
Even though fresh rhubarb is firm strong stalk, it falls apart quite quickly and easily upon cooking. The pectins release from the cells and help thicken the overall mixture of cooked rhubarb, in a similar way it does for pumpkin! It is probably also what gives a rhubarb ice cream it’s creamy texture.
Even though the rhubarb softens very easily, the fibers of the rhubarb, that collenchyma, will remain visible, they don’t fully dissolve or disappear. If you do want to get rid of them an immersion blender will be able to break them all up. It’s what we use when making rhubarb ice cream!
Science of rhubarb color
See the bright pink color of that rhubarb jam on the photo above? Even though it’s been cooked for several minutes, the color 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 color?
Greens are chlorophyll
The green color comes from chlorophyll which is what literally makes most of the planet green! Chlorophyll is the energy factory of a plant. Chlorophyll absorbs the light the plant needs to produce glucose from carbon dioxide and water. Just about any plant runs on this energy factory.
Reds are anthocyanins
The red/pink color 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 color. The exact composition of molecules will differ between these vegetables. In the case of rhubarb, the main anthocyanins are cyanidin‐3‐glucoside & cyanidin‐3‐rutinoside.
The color of anthocyanins depends on its environment and most of them are very much influenced by the pH-value of the surrounding environment. A red cabbage is brighter red when cooked in acid and a similar effect, but less strongly, seems to apply to rhubarb.
Anthocyanins are less sensitive to heat. In the case of rhubarb, research has shown that the amount of color molecules increases during baking or stewing of the rhubarb. Likely because the color molecules are released from the rhubarb structure. Cooking for too long though will reduce color again.
Science of rhubarb flavor
Rhubarb has a reputation of being very tart and acidic. When measuring the pH-value, a measure for acidity, rhubarb will come out at just above 3. For reference, neutral is 7 whereas a lemon can be as low as 2 and an apple, depending on its variety, may also be around 3. The components that make rhubarb acidic are oxalic and malic acid.
Whether you find rhubarb too acidic depends on your personal preference of course. I find most rhubarb perfectly pleasant to eat without needing a lot of sugar. That said, sugar can ‘hide’ some of these tarter flavors. It’s why rhubarb is often cooked with sugar and used in sweet applications such as tarts and cakes.
- 350g rhubarb stalks (cleaned, without the leaves)
- 35g brown sugar
- 30g granulated sugar
- 35g glucose syrup
- 50g water
- 1 star anise
- few drops of vanilla extract
- Cut the rhubarb stalks into shorter pieces, the length isn't too crucial as long as they can easily fit within the pot and can be stirred around.
- Add the rhubarb, sugars, water. and star anise to a pot and bring to a gentle boil. Keep on a slow simmer for another 10-15 minutes, until the rhubarb has softened completely. You want it smelling faintly like anise.
- Cool the mixture down in the fridge.
- Remove the star anise from the mixture and use an immersion blender to create a homogeneous mix of your rhubarb, breaking up all the fibers.
- Make into an ice cream using the manufacturer's instructions of your ice cream machine.
- After churning, freeze for at least 4 hours, to allow the ice cream to harden out completely.
- Enjoy! It's great with a few chocolate sprinkles.
Health benefits of rhubarb
When researching rhubarb we had trouble finding any decent sources on the flavor and texture of rhubarb. However, there is a lot of information available on the potential health benefits of rhubarb in various applications. For instance, rhubarb has traditionally been used in Chinese medicine for its health benefits. Analysis of rhubarb has shown that it contains various molecules which could be beneficial for human health. Other positive effects were seen with regard to antimicrobial properties. If you’re interested in these, we suggest doing some more research, seeing as how we’re not health experts.
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!
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