Smoke on the water: Swetha Sivakumar on the science of steaming
What makes this such a powerful yet gentle way to cook? The lid comes off the steamer, in this week’s Sound Bites column.
Imagine setting an oven to 100 degrees Celsius, in order to bake a loaf. The heat would barely alter the dough. Yet the same temperature in a steamer is enough to not just cook food, but also destroy any bacteria present. Despite this, we consider this a relatively gentle form of cooking, and we place delicate food in delicate leaf packets in our steamers.
Why this paradox? Let’s take a look at the science of steaming.
Once water reaches boiling point, it must break its intermolecular bonds in order to change states from liquid to gaseous. It takes more energy to change states in this way, than it takes for water to simply boil. (It takes 2,260 joules of energy to vapourise 1 gm of water at boiling point; for instance. Compared to 272 joules of energy to simply bring 1 gm of room-temperature water to the boil).
Now, when the liquid breaks up into a gas, the water vapour carries that extra energy within it. It circulates inside a steamer, charged with this power. And when it touches something cooler, discharges that heat energy, and begins to cool.
This is the process of condensation, when the water molecules coalesce, release their excessive joules, and return to liquid state.
This is where the steamer’s hyper-efficiency kicks in. There is now heat enveloping the food from the vapour around it; heat coming from the water in the container below; and a cycle of high temperatures forming that permeates any food with heat energy.
The leaves and paper that we wrap the food in soften the onslaught of that heat, and let the delicate fish or idlis or dumplings cook more gradually. As a bonus, the leaves release aromatic compounds, giving the food a unique and pleasant fragrance and flavour. Steam picks up these aromatic molecules too, and disperses them, enhancing the overall sensory experience of the eventual meal. (This is why a steamer is often brought to the table still closed.)
In the absence of flavourful leaves, one way to make the process gentler is to take the lid off the steamer, or keep the water in the base at 80 degrees Celsius, instead of at a rolling boil. (Water will still evaporate, with sustained heat, over time.)
We don’t often use it in this way, but steaming is also an excellent way to cook vegetables. It’s quicker than bringing a large pot of water to the boil. It retains many more nutrients than most other methods of cooking.
(Steam sterilisation is also highly effective in killing a wide range of pathogens, including bacteria, viruses and fungi. The high temperature can denature proteins and disrupt the cell membranes of microorganisms, making them non-viable. Plus, steam penetrates porous surfaces better than dry heat does).
It’s a good way to cook leavened foods such as idlis and dhoklas. The injection of heat energy helps these foods puff up and turn airy and porous.
Temperatures do not rise enough for any browning to occur in a steamer. But, on the flip side, one can make soft, fluffy foods in this manner, without the oodles of fat that would otherwise be needed to lock in their moisture. Stuffed, starch-heavy items such as modaks and momos retain the juiciness of their stuffings too.
Traces of steaming practises have been found in ancient India, China and the Mediterranean. Traditional bamboo steamers in fact are still widely used. With their many layers, these allow multiple tiers of food to be steamed at once.
In modern times, the steaming stick, or nozzle, has emerged and caught on too. Coffee shops use these to froth up lattes and cappuccinos. The steam heats the milk with a sudden burst of bubbles that help the whey proteins in the milk unfold, to create a layer of foam on top. The foam adds a velvety texture to the drink, while insulating the liquid beneath, keeping it hotter for longer.
Who knows what this marvellous ingredient might be used for next?
(To reach Swetha Sivakumar with questions or feedback, email email@example.com)