Double digest: Why do some foods refuse to break down?

We don’t even notice them in our raita and chaat, but plant fibres are impossibly, helpfully hardy. How do they work? Swetha Sivakumar takes on the insolubles.

Fibre sounds healthy. Nutritious. Boring, to be honest. Doctors tell us to eat more of it. Humans have a very simple definition of it. Fibres are essentially plant parts that resist human stomach enzymes and digestion. Unlike other macronutrients, such as carbohydrates, proteins and fats, these refuse to be broken down. Having escaped digestion, they move into the large intestine, where they may be partially or fully fermented (depending on the fibre, and the state of the individual’s gut health). Here, they contribute greatly to gut health.

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What makes plant fibres so impossibly, helpfully hardy? It all boils down to cellulose, which, incidentally, is the most abundant biomass on Earth.

No amount of chewing, grinding pounding or boiling can break it down. It doesn’t react to most acids. It only reacts, in fact, to temperatures higher than 320 degrees Celsius.

It is cellulose that gives tree trunks, branches, stems, leaves and grass their hardiness, providing the very structure that allows plants to grow tall. And it has a fascinating origin story — perhaps best explained through the analogy of twins in 1970s Bollywood.

A common theme in these movies was twins separated at birth (Seeta Aur Geeta, Ram Aur Shyam). Invariably, they ended up being polar opposites. If one was brave, the other was timid; if one was a cop, the other was a criminal.

Cellulose and starch are a bit like that. They are both made from the same building block: glucose. But because of one key difference in how their molecules are bonded, they end up barely alike at all.

Starch is connected by alpha-1,4-glycosidic bonds that can be broken down by human stomach enzymes and digested. Cellulose is connected by beta-1,4-glycosidic bonds, which are among the toughest to break in the plant world.

Look more closely, and one sees an incredible detail. If there’s anything tougher than cellulose, it’s lignin, another type of indigestible fibre, and the substance that holds cellulose fibres together.

Lignin resists digestion more than any other naturally occurring polymer. Even cellulose can be acted upon to ferment in the gut, for instance; lignin cannot.

Lignins evolved to protect plants from insect and pest attacks by making them resistant to puncture. The only organisms capable of breaking down lignins are certain fungi (such as white rot). This stoic polymer also has a superpower: It is aromatic. One of the most popular compounds achieved by depolymerising lignin is vanillin (the base of synthetic vanilla). This is also why old books emit a mild vanilla scent; it’s the lignins in the pulp, breaking down over time. But we digress.

Let’s return to the crock pot.

In the kitchen, cellulose and lignins can be a force of good or bad, depending on the recipe. When baking, for instance, they interfere with gluten formation. Take bread. The starch in the flour gelatinises, the proteins brown and become delicious. But the insoluble fibres (which include most bran) do nothing. They just get in the way, and they retain water. Which is why wholewheat breads taste dense and do not rise as well as refined-wheat ones.

On the plus side, these fibres provide the turgidity and crunch we desperately crave in vegetables. Without cellulose, all our sabzis would be gooey puddles. It is responsible for the crunch that onions add to chaat, the bite of well-cooked cabbage, the chewiness of cucumbers in raita.

Processed-food companies understand this, and often collect the insoluble fibres generated when ingredients such as grains, dals and fruit are refined, for use in other products. Orange peel and pulp are added to biscuits, to provide fibre and micronutrients. Because of their texture and capacity to retain water, defatted rice bran and oat bran are added to plant-based meats and ground-meat products, where they improve juiciness and mouthfeel, and serve as a low-cost way of improving the nutritional profile.

I haven’t even got to soluble fibres yet. Let’s take a look at those next week. Meanwhile, get your insoluble fibre where you can. You don’t even notice it in raita, do you? And your gut will thank you for the effort.

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