When milk finds a pulse: The science of pongal, with Swetha Sivakumar

As part of the Pongal festivities, each person then adds a handful of raw rice and moong dal to the bubbling milk. The children then disperse and the adults take over. An hour later, two versions of pongal are ready to be eaten. The savoury Ven Pongal (ven is Tamil for white), flavoured with pepper, cumin, curry leaves, ginger, green chillies and cashew, is served with sambar and chutney. The dessert, Chakkara Pongal (chakkara is Tamil for jaggery), is an aromatic dish flavoured with jaggery and elaichi, topped with ghee-roasted cashews and raisins.

While the festive ritual is special, both versions are also eaten for breakfast through the year. In some cases, such as when sweet pongal is served in temples and at weddings, moong dal is avoided, to increase the shelf life of the dish. (A dal-heavy pongal, if not eaten immediately, will congeal into a gluggy cake). In the savoury version, the ratio of moong dal to rice may vary in a range from 30% to 75%, with less moong used to keep costs down. Higher moong ratios are healthier, packed with more protein, so when cooking for the family, more is preferred.

While these recipes may seem simple, they aren’t. There’s plenty that can go wrong, and a lot of science involved in getting it just right. Take a look at what pongal tells us about how rice and pulses can best be stirred together.

Always add the sugar at the end, when making a grain-based dessert. Sugar loves water, and there’s so much jaggery in pongal (and, similarly, so much jaggery or sugar in dishes such as kheer), that all the liquid in the dish is drawn to the jaggery molecules. If the grain in the dish isn’t already cooked, you can be sure it won’t cook properly now. Because a) there isn’t enough water for it to cook well in. And b) as the water is drawn towards the jaggery, any “free water” inside the partially cooked rice and dal is pulled to the surface of each grain. This gives the grains a crunchy texture, even with prolonged cooking – the opposite of what one looks for in a creamy dessert.

Pick the rice carefully. Pongal is a harvest festival, but that’s not the only reason the rice used to celebrate it is freshly harvested. Here’s why the age of the grain matters. Starches in rice occur in two forms: amylose (straight chain) and amylopectin (branched chain). Basmati rice, which is popular in north India, has high amylose content, and aged versions of this rice are prized because when it cooks, the grains stay separate. Rice varieties such as ponni or sona masuri, popular in the south, have less amylose and more amylopectin. These varieties tend to be stickier because the branched-chain molecules bond more easily with each other. Freshly harvested grain is stickier still, all the better for a soft, fluffy pongal.

A pressure cooker changes things, but is not a deal-breaker: When cooking rice with pulses, an open boil works better, from a physics standpoint. Simmering at a steady 100 degrees Celsius, with constant bubbling and stirring, the starches locked inside the grains leach out slowly, creating a more consistent, saucier texture. In a pressure cooker, in temperatures of 121 degrees Celsius, the grains cook faster and remain relatively more separate. Does this mean one ought to make pongal in a pot? Certainly not. It’s a minor difference that comes at great cost of time and labour. Pressure-cooked pongal is delicious.

Pongal gets gluggy when cold, but that’s fixable. As any pot of rice and lentils cooks, the swollen grains, grain fragments and the starch that has leached from them form a tangled web with the water in the dish. This creates the creamy texture. As the hot dish cools, the starches bond more closely, pushing the water out of the dish (this is why old dals have a pool of water on top). This process is called retrogradation. It may make a dish seem unsalvageable, but it’s not. Starch molecules congeal through hydrogen-hydrogen bonds, which are weak and easily broken. Just reheat the dish to 70 degrees Celsius or more, in a microwave or even on a stovetop, and watch as the hydrogen bonds break and the water is absorbed back into the dish.

I hope these tips are helpful to you whether you next make pongal, khichdi or kheer. Meanwhile, as we celebrate the harvest festival today, I wish you a bountiful year full of love and laughter.

(To reach Swetha Sivakumar with questions or feedback, email [email protected])