We have a 2-ingredient recipe (including water) and a whole geeky food science post to make up for how quick the recipe is! In other words: an ode to lentils.
In the image: red lentil tohu
Have you heard of ‘Tohu’?
You might have heard of ‘Chickpea tohu’ specifically.
It is considered to be Burmese “tofu”, assimilated to tofu in Myanmar due to their similar uses in the kitchen, as well as providing a good source of plant based protein.
Technically, the nature of what it is made of and how it is made is very different to tofu. A more precise reference might be polenta.
Similarly to Polenta, chickpea tohu preparation relies on cooking whole chickpea flour (or pulverized chickpeas) with water while constantly stirring until a semi-gelatinous mass forms, which solidifies into a solid gel upon cooling.
Well, it can also be made using red lentils:
In fact, I prefer making it using red lentils, as I prefer their flavour and texture.
The texture is the very definition of comfort food (when reheated it has a scrumptious, gnocchi-like mouth feel), yet, it is light enough that it doesn't leave me with a food-hangover after lunch.
This tohu really shines a light on the mighty lentil’s versatility.
It can be so much more than the basis of dals and soups.
Don’t get me wrong.
I am the first person to swoon over a hearty lentil stew
But if we explore and exploit legumes’ full culinary uses, they can offer us so much more.
The phenomenon at the heart of making this solid tohu gel out of lentils is called: Gelation.
More specifically; Starch gelatinization.
In the image: red lentil and water right at the end of the gelatinization process in the pot, before cooling.
Contrary to popular belief, the term gelatinization does not come from the product gelatin (a protein derivative of bone collagen), but it is related in the sense that the name gelatin was given to a protein after it was found to be able to set water (or liquids) to a semi-solid state resembling ice formation in the sense that it 'solidified' water.
In Latin; Gelatus, meaning stiff or frozen, was the term that helped coin the name for gelatin (it was in close competition with “irreversibly hydrolyzed bone collagen peptides”. Shockingly, the former proved more catchy....)
Many of us will be familiar with terms such as gelato (Italian ice cream), which comes from the same etymological root.
Gelation is the phenomenon of turning a liquid into a gel or a solid by use of polymers. Polymers which can solidify a liquid are collectively known as hydrocolloids.
Gelatinization refers specifically to the reaction that takes place where starches are used to gelate a liquid.
Gelation biochemistry; Starch can thicken a large amount of liquid upon heating:
Source: The culinary institute of America
Starch is made up of glucose molecules arranged into two types of polymers; Amylose and Amylopectin. Amylose consists of linear long molecules, which coil up into long helices when dissolved in water, while amylopectin is short, branched, and more compact. The latter can form double helices with other amylopectin molecules and become highly organized and crystallized,
These starch molecules are found in starch granules within plant cells.
When cold water is added to starch only a small amount is absorbed by the starch granules, not resulting in the thickening of the liquid.
In the image: red lentils pulverized with water added to the pot and stirred whilst heating. Photographed at the beginning of heating.
Upon heating, water molecules move and vibrate with more energy, disrupting the order of the starch granule and causing amylose from the less ordered areas of the granule to leach out, as well as disrupting and displacing intramolecular H bonds between starch molecules in the more crystalline amylopectin molecule. This hydration of amylopectin will lead to further swelling of the granule and place increasing amounts of pressure on the more organized granule areas, compromising their integrity with rising water influx. At this point in the process the liquid will have began to become more viscous due to the swollen starch granules moving less easily through the liquid that is already filled with amylose molecules hindering the free movement of water molecules and swollen starch granules.
In the image: beginning of gelatinization: some viscosity observed.
At around 60-75℃ the granules will finally completely lose their original architecture, absorbing a lot of water to swell and become a starch-water mesh. This temperature range is called 'gelation range'. A final step in the gelatinization of starch occurs as these granules rupture completely, leaking more starch molecules into the surrounding liquid. The long amylose molecules will interact with each other further extending the meshwork which traps water and starch granule fragments.
Essentially this is the biochemical definition of gels: water molecules trapped within a meshwork of long polymers. It hinders their movement and is essentially responsible for the gel holding its shape as opposed to fluid liquid.
In the images: Gelatinized red lentils still in the pot before cooling.
Cooling things down further sets the gel:
As we cool things down, the molecules within it vibrate less and less until the energy found in the newly made bonds between the molecules within the network overcomes the vibrating energy and these interactions hold the molecules in place.
The longer amylose molecules will form the most extensive interactions among themselves as they have larger surface area (being linear and long). This translates into further ‘thickening’ where the mass congeals into a solid gel.
In the image: Gelatinous mass being cooled in a container.
In the image: Cooled and further congealed red lentil tohu can be easily cut and handled
And that is the biochemistry of tohu!
Legume seeds store carbohydrates in the form of starch. Gelatinization of this starch is at play in many dishes you may know; vegan quiches and pies, Italian farinata, vegan omelets and frittatas, etc.
The preparation method used here; stirring while heating; maximizes the gelatinization and water holding capacity of the starch by encouraging the rupturing of the starch molecules due to physical strain on them, and keeping the homogeneity of the mixture during heating, exposing all the starch molecules to water molecules (in contrast to baking where the particles settle and the bottom layers are somewhat less hydrated).
Is it just the starch that is in charge of this phenomenon?
Lentils contain up to 69% starch, stored in the seed cotyledon.
One might ask themselves if any components in the other 31% of the lentil composition play a role in this. It is not an unlikely scenario, since we are aware that lentils are rich in protein (21-31%) which plays a similar role in other preparations (egg white, meat) where it denatures in response to heat, unravels, takes up a larger space and thereby has a solidifying effect. However, it seems this is not the case here.
It is likely however that insoluble fibre in the lentil helps solidify the texture of the tohu a little more and provide further body.
It seems that legume-proteins’ characteristics are different from the protein behaviour we know from animal-based products. Even in the case of tofu coagulation, the reaction is not purely heat-dependent as with animal-derived materials. Soya milk is heated with no visible gelling or solidifying effects upon heating alone. For solidifying/curdling to occur it requires a setting agent, like gypsum or nigari.
Another evidence that the lentil protein itself is not responsible for the effect seen in the tohu can be found in this amazing red lentil tofu recipe (experiment) from Mary’s test kitchen, where the lentil starch was extracted, and the enriched fraction of lentil juice did not react to heat until the addition of gypsum coagulant
Unique features of lentil starch generate characteristic legume based gels:
Pardon the cliche - but not all starches are created equal.
The unique composition of starch being used composition of starch (The ratio between amylose to amylopectin, the length of amylose helices, and the size of the starch granules into which these molecules pack within the starch source) will affect the water holding capacity, strength, flexibility, mouthfeel, response to reheating and other important properties of the final product after gelatinization reactions.
When heating the starch-liquid solution, the degree of viscosity relates primarily to:
- The speed of amylose leakage from the granule
- Size and swelling of the granule which gets caught in the meshwork of the amylose network,
- The shear rate of starch granules which will become fragments, thinning out the liquid a little.
When the mixture cools, the viscosity becomes a function of the composition of the continuous phase:
- How much amylose leached into the liquid. The extensive bonding of the amylose chains packing tightly will also affect the texture of the gel upon reheating, where the stable and high-energy association between them will maintain a solid gel upon reheating.
Lentil starch has a very high amylose content in comparison with other common sources of starch. It therefore congeals into a firm gel, which can be cut and reheated without losing its shape, giving us a tofu-like product.
In the image: Seared cubes of red lentil tohu. This tohu can be dry-seared (no oil), or baked, or air-fried.
Importance of understanding the science behind the food:
It is a great tool for a chef to understand the nature of the starches and anticipate the result they are looking for, in order to manipulate the recipe and modify it to influence the result.
For example; adding salt speeds up the gelatinization by lowering the temperature of gelation. Sugar impedes gelatinization, etc.
Why I love red lentil tohu:
Delicious: Have I mentioned it is delicious?
Nutritionally, these curds are made from whole foods. It may have a lower protein content than tofu, which is the extracted protein phase from the soybean, but on the other hand, it has fibre which a western diet is all too often short of. Generally, it has the nutritional profile of lentils.
Economical; It is cheap.
Versatile; Not only can we create tofu-like cubes, and the pasta sheet that stuck to the bottom of the pot whilst cooking it; it can be spread very quickly into (fragile) pasta-like sheets, blobbed into gnocchi wanna-be’s, made into a lentil scramble, brushed on a silicon sheet straight from the pot to make a new food, with no name, cos heck - not all the food has to resemble something already established! We can make new foods!
You are in control: Fancy a delicate and spongy tohu today? No problem, use less red lentils in the recipe (see recipe and notes), fancy and extremely sturdy preparation that will withstand a stirfry? Just increase the amount of lentils in the recipe (also see notes)
Perhaps you want to add some salt? Spices? Liquid smoke?? (Hello smokey sausage cubes).
Effortless; It is such an easy preparation method, which requires only a pot and a container (and blender if using whole lentils rather than flour). Not only is it simple to make, the washup after is a doddle. If some of the tohu sticks to the pan bottom - soak it in cold water and after about an hour you will get a magic lift-off effect of a thin sheet of lentil tohu (which I totally use like a pasta sheet).
In the image: this tohu sheet attained from the bottom of the pot after an hour's soak.
Lentil tohu - the recipe:
For reproducibility across whole lentils and different coarseness of lentil flours I use weight measurements. It allows me to be precise. Though it is worth mentioning that even so - whole lentils produce a better result for me.
90g red lentils (if you have a strong blender) or lentil flour*
2 cups (470ml / g) water
Soak red lentils in the water for about 1 hour (and up to 8 if using a non-high-speed blender). If using fine lentil flour you can skip the soaking. Just mix and skip to (3).
Transfer all the water and the lentils into a blender. Blend until completely smooth.
Transfer blended lentils into a pot. Using a heatproof silicone spatula, or a wooden spoon, stir the mixture continually (but not frantically) over medium-high heat.
The tohu will begin to thicken in parts. Mix them through until the whole lot has reached a viscous and homogenous texture. This should take approximately 4 minutes. Keep stirring beyond that point for another minute or so (you may notice the colour changes slightly from orange to paler yellow).
Transfer the thickened mass into a plastic or glass container. Allow to cool at room temperature before placing in the fridge.
The tohu is set and ready to eat after about 2 hours in the fridge. It may thicken a tad bit more overnight.
* Texture variation: 90g lentils will yield a squishy gnocchi-like gel. 110g will make for a more hearty gel that has a bit more body and might be a better texture if you are looking for a meat substitute. 130g lentils will really make a stiff gel that is a wonderful substitute for a meaty moiety of a dish. You are the captain - you decide the texture.
Recipe variations: You can add your favourite herbs and spices to the mixture to make flavoured tohu.
In the image: I use 80g of lentils because my automatic cup measurements are smaller
gelation, gelatinization, congealing, starch, amylose, amylopectin