How do you like your air?: Swetha Sivakumar on the vital, invisible ingredient

In food, air usually takes the form of foam, which is the dispersion of gas bubbles in a liquid. A bubble wall is essentially an extremely thin layer of that liquid; the thin layer supports a volume of air inside that can be about a thousand times larger.

Bubbles can play a key role in the texture and mouthfeel of a food item. This is especially true of beverages, ice-creams and sweets.

In the first, bubbles must form, survive and fizz for a cold, aerated drink to hit the spot. The sound and feel of the fizz — the burst of flavour and aroma that they provide — contribute significantly to the sensory perception of the drink (just as the crunch adds to the perception of flavour in popcorn).

This is why beer, colas and cappuccino taste so disappointing after their froth, fizz and foam have gone “flat” (and why popcorn seems inedible when it’s soggy).

In eatables, air can create pockets that later soak up flavour — as with appams (and stew), or warm bread (and butter).

Here, the bubbles must be trapped by immobilising them in order to keep them from bursting. This is done by converting the liquid batter around them into a solid, through the use of steamers or ovens. The high heat solidifies the starches and proteins, creating an airy solid such as an idli, bread or cake.

Another way to immobilise foam is rapid cooling, as in industrial ice-cream recipes. Here, air is folded into the mix, then the temperature dropped rapidly (from ambient levels to about -18 degrees Celsius, in a few minutes). This causes the fats in the cream to turn from liquids to solids, trapping the air. This trapped air is what gives the eventual treat its fluffy mouthfeel and scoopable texture.

By contrast, kulfi and popsicles do not have air whipped into them, which is why, while delicious, they are neither soft nor scoopable.

Air is also added to sweets, to make them chewy. This is done quite simply, by pulling and stretching treats such as taffy when warm, to fold in air just as the sugar crystals start to form. (It helps that sugar responds to temperature in exactly the same way, every time; get the heat right, and it really is an exact science). The tiny air pockets occupy spaces between the sugar crystals, adding a delicious chewiness to sweets such as taffy and soan papdi.

Now, bubbles are not always a force for good in food. In the sous vide (French for “under vacuum”) method of cooking, air is removed from the equation entirely, so that the ingredient (usually meat) can heat and cook evenly. In this method, foods are vacuum-sealed into pouches, then immersed in heating water.

In soups and stews too, bubbles are best avoided. They have an unsavoury association, possibly because bubbles in food typically indicate decomposition. (Here, of course, the bubbles come from the blending of the ingredients, but that does nothing to help the brain get past the popping obstacles).

Even in breads, which depend on the presence of air, bubbles can be tricky. After a batch of dough has finished proofing, a good baker will knead it roughly again, just to redistribute the air, so that it doesn’t concentrate in one area, causing the loaf to collapse.

Because bubbles will only play along up to a point. After that, they will do what they must: Pop!

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

Box: How is air folded into baked goods?

There are three ways. The first and most traditional involves adding yeast to flour. The yeast digests the sugars in the flour, producing bubbles of carbon dioxide and alcohol. This is the most time-consuming of the three methods, however; since one must wait for the yeast to complete its cycle of breaking down the flour. The baker then kneads the dough to help its gluten molecules bond and build an even network around the now-trapped air. This process is called leavening, and the result is a relatively chewy bread.

In the second method, baking powder or a combination of baking soda and an acid generate gasses to provide the pockets of air. This approach is quick, and works well when aiming to make runny batters for dishes such as cakes or dhokla. A runny batter cannot hold on to its air for long, though. Which is why it must be mixed just before being placed in an oven or steamer. (Unless one uses double-acting baking powder, which generate some bubbles before the batter goes into the oven, and some while it’s there!)

The third way to incorporate air when baking is by hand. Whip egg whites into the mix and they trap the air in a rather magical way. You see, egg whites start out liquid, and with just the application of the whisking motion, turn into a foamy semi-solid. Amid this process, as the stretched-out protein molecules bond more strongly, they trap the air within their folds. When baked, this airy web becomes a solid structure, in treats such as macaroons and angel food cake.