A steamy affair: Swetha Sivakumar on the science of pressure-cooking - Hindustan Times

A steamy affair: Swetha Sivakumar on the science of pressure-cooking

BySwetha Sivakumar
May 25, 2024 02:51 PM IST

Air pressure acts as an invisible force on food, no matter what the utensil. See how it works, on pulses, milk, juices, and popcorn.

How would food cook in the crushing atmosphere of Venus?

A 1950s print advertisement for Ekcomatic pressure cookers. (HT Archives) PREMIUM
A 1950s print advertisement for Ekcomatic pressure cookers. (HT Archives)

Think about that, and it helps one begin to understand how different foods cook here on Earth, because air pressure acts as an invisible force, no matter what the planet… or utensil.

Let’s start with the pressure cooker. At sea level on Earth, atmospheric pressure stands at 14.7 psi (pound-force per sq inch; pressure caused, incidentally, by gravity pulling atmospheric gasses toward Earth’s surface; which is partly what keeps our atmosphere intact).

How does any of this affect food?

Well, at sea level, water boils at 100 degrees Celsius. In a pressure cooker, the pressure rises to about 30 psi, as the air is compressed. This raises the boiling point of water to about 121 degrees Celsius, as the increased pressure prevents the liquid from turning into vapour and escaping. (Whenever the pressure rises above 30 psi, the excess is released in short bursts, by allowing a little vapour to escape through the whistle.)

This enhanced activity helps foods cook faster, in the presence of far less water, for two reasons. First, the available water isn’t evaporating. Second, that superheated water, its molecules bouncing about under the pressure, is being diffused into the rice or dal at a far higher rate than it would be without the pressure.

This is why dal cooks in a mere 15 minutes, in about two cups of water, in a pressure cooker; against about 45 minutes and five cups of water in the absence of added pressure.

Interestingly, while most people will say that a pressure cooker saves time, it isn’t really time that one saves — because getting that superheated food to cool takes quite a while. What one does save on is water, and fuel. And if the device feels like a time-saver, it’s because it offers immense convenience. The person doing the cooking doesn’t have to stand by the pot and tend to it.

Pressure is used in numerous ways, when it comes to food.

Take homogenised milk. This is simply milk that has been treated so that it will not develop those globules of fat that are so irksome to many.

In the natural order of things, milk does contain tiny fat globules. They tend to clump together and rise to the top, particularly when the liquid is heated, because the fat is less dense than the milk.

During homogenisation, milk is pushed through a tiny hole under intense pressure, to break the fat globules into bits that are essentially so tiny, they blend into the milk seamlessly, and would take much more heat and time to coalesce again.

This makes milk look whiter, feel creamier, and makes it more stable when mixed into hot beverages.

Changes in air pressure can have visually dramatic results as well. Think of the vintage popcorn maker, that canon-shaped metal device into which grain is poured, and from which masses of puffed rice or popped corn later emerge.

A 1966 Hawkins pressure cooker ad that promises liberation from long hours of ‘old-style cooking’. (HT Archives)
A 1966 Hawkins pressure cooker ad that promises liberation from long hours of ‘old-style cooking’. (HT Archives)

To achieve this, the grains are first roasted, then placed in the puffing gun along with some oil. Heat is then applied, causing all the moisture in the grains to expand into steam. With nowhere to go, this steam causes immense pressure to build within the device (which is why it is made of such thick metal). When the machine is opened, the sudden drop in pressure causes the steam in each grain to suddenly expand, making each grain puff or “pop”, and so the masses of popcorn pour out, in what seems like a magic trick.

Something else pressure is extremely good at is keeping foods safe from microbes. Bacterial spores, for instance, are notoriously difficult to kill. High temperatures, chemicals, a sudden freeze… none of these perturbs them. But add pressure along with high temperature, and the spores wilt. This combination is used to render canned items sterile before they are sealed.

The heat isn’t always essential. Lately, food companies have begun using a non-thermal food-preservation technique called high-pressure processing or HPP, in which pressures of up to 87,000 psi are applied, to kill almost all microbes. This technique helps preserve raw fruit and vegetable juices.

All in all, it feels like the word pressure gets a bad rap these days; it is equated too often with stress. So I take joy in pointing out that, in the context of food science, the phrase “now that’s a lot of pressure” can be really good news.

(To reach Swetha Sivakumar with questions or feedback, email upgrademyfood@gmail.com)

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