Peeling back the layers: Swetha Sivakumar on the science of onions
What’s the best way to cook an onion? That depends on what you want from it. It’s a versatile but temperamental vegetable. See what makes it so, in this week’s Sound Bites.
Food historians and archaeologists believe that the common onion (Allium cepa) originated somewhere in Central Asia. Vedic writings mention the vegetable being grown in China 5,000 years ago. Their cultivation spread rapidly across continents, with hints of onion found in archaeological ruins from ancient China, Greece, Egypt and Rome.
These bulbs are hardy. They travel well. They can be stored for up to three months without refrigeration. All these factors led to this becoming a staple in most homes.
The 6th-century Charaka Samhita lists onion as a cure for indigestion and aching joints. The Egyptians viewed its structure as symbolic of eternal life, and thus often buried a few bulbs with dead pharaohs, in their tombs. Onions were a key crop in the diets of Medieval Europe’s peasants. And the city of Chicago was named after the Native American word for the bulb. When the first colonists arrived, the land held vast fields of wild onions.
It is interesting that the word “onion” comes from the Latin “unionem”, which means unity. It is aptly named, considering that it is a base ingredient in so many dishes across cuisines. Indian curries often start with the trio of onion-ginger-garlic, sauteed. The French mirepoix uses onions, celery and carrots (paired with butter). The Spanish sofrito is onion, garlic and tomato, braised in oil.
To satiate the vast appetite for this vegetable, almost 93 million tonnes are produced a year, worldwide, according to the UN’s Food and Agriculture Organization, with the top growers being China, Mali, Japan and South Korea.
The fact that this vegetable makes our eyes water can be traced to the soil it’s grown in. Onions grow best in soils that are rich in sulphur, which it absorbs and transforms into defensive compounds stored in its cells. These cells rupture as we chop, or as pests chomp, and an enzyme in the plant combines with the sulphur compounds to produce an irritating, pungent group of molecules called thiosulfinates and thiosulfonates.
Sweet onions are grown in sulphur-poor soil and are less likely to make one cry. But they have a higher water content (as much as 95%), giving them a much shorter shelf-life of just one month, without refrigeration.
Now for some not-so-great news. Each bulb is rich in flavour, even though it’s about 80% water. But that flavour comes largely from sugars. The average onion (110 gm) contains 45 calories and only 1.9 gm of dietary fiber and 1 gm of protein, compared with, for instance, green peas, which contain 5 gm of fiber and 5 gm of protein per 100 gm.
Onions are a good source of antioxidants, though, with the greatest concentration in the layers near the peel. One bulb also contains 20% of a person’s daily requirement of Vitamin C. This, combined with its long shelf-life, made it very popular among sailors, as a way to prevent scurvy on long voyages in the 1800s.
It is, of course, a supremely versatile vegetable. A cook who knows how to manipulate the sulphur and sugars can elicit a wide variety of flavours from the same bulb. For starters, before cutting it, it helps to chill the onion briefly; the cold deactivates some of the enzymes, leading to less tears.
Next, how you chop it matters. Because the cell walls line up from root to the tip, cutting lengthwise rather than widthwise releases fewer sulphur molecules. Finely chopping or pureeing it, on the other hand, releases a lot of sulphur compounds. However, if you can withstand the tears, you will be rewarded. When heated, these sulphur molecules break down to provide a rich, savoury flavour to a dish.
To get the raw crunch without the pungency, it helps to soak the chopped bulb in water for about 30 minutes. This allows most of the sulphur compounds formed on the cut surfaces to wash off, softening the sharpness.
Let’s say you only want sweetness from the onion. It helps to remember that the sugars are hidden behind the pungency of the sulphur molecules. To tease them out, be patient and slow-cook. The sulphur compounds will break down first. Then, with slow, gentle heat, the sugars break down and, combined with the protein, brown the whole, creating the subtle, sweet flavour of caramelised onion!
It’s just layers and layers of possibility.
(To reach Swetha Sivakumar with questions or feedback, email firstname.lastname@example.org)