Why the laddoo is a crystal ball: Swetha Sivakumar, on the science of mithai
Is your favourite the kaju katli, burfi or the halwa? Whatever your answer, there is surprising science - precision, chemistry and physics - holding it together
There’s an odd idea — and Diwali always makes me think of it — that mithai is easy to make. “With that much ghee and sugar, how could it not taste amazing,” we joke. But it takes expertise to make even the simplest of these sweets. Look at them from a scientific and structural viewpoint, and one develops a whole new appreciation for their complexity.
Take the kaju katli and coconut burfi. Both contain healthy ingredients (whole nuts, seeds, shredded coconut) with no reason to stick together. The sugar isn’t just flavouring, then; it is the glue that binds them. It is also the preservative that keeps them fresh; the jolt of flavour that makes them irresistible. And it gives the mix a shape so cohesive, one can jostle about with a box of these all day and not have them so much as crack under the pressure.
Now, ordinary syrup could not do this. It takes just the right amount of heat to create the kind of cohesiveness that these mithais rely on. How does heat act on sugar to make it a stronger binding agent? Well, the heat transfers energy to the sugar molecules, which makes it easier for them to break free of their rather strong polar bonds.
This is why, using heat, one can dissolve more and more sugar in a small amount of water. At about 40 degrees Celsius, for example, 100 gm of water can dissolve 238 gm of sugar. Heat the water to 100 degrees Celsius, and one can now dissolve almost double the amount: 485 gm of sugar.
When the temperature gets to about 118 to 130 degrees Celsius (called the soft/hard ball stage), the syrup is only about 10% water. This is the point at which a syrup can be used to shape a ball or slab. It is still liquid enough to mix ingredients into, but as it cools, the original bonds reinforce themselves as the sugar molecules reach out to each other. Because of the high concentration of sugar molecules in the syrup, crystallisation occurs, and a mix of this kind will crystallise in more or less any shape it is given.
It will take a hard bite to break this ball apart now. But the reward is a burst of flavour from the sweetened nuts and seeds, and a delicious, chewy texture too.
Sugar is just one of the tricks used in the vast array of mithais made in India. Other binders include proteins (powdered dal, chenna) and starches (corn starch, wheat starch, etc). These still need crystallised sugar syrup to help with the binding, but can do with far less of it.
My favourite of the protein-based mithais is the moong dal laddoo. Here, once the ingredients are cooked and combined, it is compression force that gets the proteins to bond. Before the temperature of the mix drops, pressing it into a ball or slab causes the proteins to stick together. This reduces the burden on sugar as a binder. Thus, protein-based mithais can contain as little as one-third the sugar of a regular laddoo or katli, and still hold their shape.
Perhaps the most interesting, from a scientific standpoint, are the mithais where pure starch provides the structural support. These include the Tirunelveli and Karachi halwas. Here, grains such as corn, ragi or wheat are soaked and ground, and the milk extracted from them. This milk is essentially refined starch, with most non-soluble particles such as fibres eliminated from it.
Add water, slivers of nuts, and flavouring agents such as cinnamon or cardamom and the result is a translucent gel that is beautiful to look at, chewy and soft, but can still stand firm. This is because the starch solution, when diluted and heated, absorbs water to form a mesh of long-chain molecules that trap liquid (and flavours). As the mix cools, the bonds between the starch molecules become more stable, and set firmly enough to be cut up into blocks.
Whatever your favourites, take a moment today, as you choose between burfis, laddoos and halwas, to pause and appreciate the molecular forces at work… then pop one in your mouth quickly before the box passes you by. Happy Diwali!
(To reach Swetha Sivakumar with questions or feedback, email email@example.com)