The pain is usually worse in the evening, when angry, weeping sores start to appear.

Washing does little to alleviate the agony, it just opens old wounds.

And there are plenty of those — six years' worth, mapping her hands like scorch marks. She winces as she prepares dinner for her husband, daughter and two sons.

Chopping onions and chillies is torture. And because she eats with her hands — there are no knives and forks here — the spicy curry can make her cry out in pain.

This is the life of Pushpa Gandhi, 30, in southern India, who supports her family as a cashew nut sheller. She's an unseen face of the industry feeding the UK's voracious appetite for the nuts — we ate 17,000 tonnes in 2016, 35 per cent up on 2012.

Cashew nut processors in India have been left with burns on their hands from the superfood

The rise in veganism is thought to have played a part in our increasing consumption. Cashews are in energy bars, butters and salads as well as vegan alternatives to milk, cheese and creamy puddings.

A good source of protein, magnesium, potassium, iron and zinc, the monounsaturated fats they contain help protect against heart disease.

But there's a catch to cashews. The nuts — nearly all processed in India or Vietnam — are difficult to extract and are therefore shelled by hand. A cashew has two layers of hard shell, between which lie caustic substances — cardol and anacardic acid — that can cause vicious burns.

Burns are a fact of life for up to 500,000 workers in India's cashew industry, nearly all women. They are employed without contracts, with no guarantee of steady income, no pension or holiday pay.

Many don't even get gloves, and if they did, they probably couldn't afford to wear them. Gloves would slow their shelling down, and they are paid by the kilo. When their pain becomes unbearable, they need medicine — and, of course, they must pay for it. So they soothe the acid burns with ash from their fires.

I was horrified when I found out my diet might be funding this misery. I'm a vegan, and the dairy-free 'cheeses' I love typically use cashews. The creamy sauces I love in pasta bakes do, too.

Cashew nut shells have cardol and anacardic acid — that can cause vicious burns. Burns are a fact of life for up to 500,000 workers in India's cashew industry, nearly all women. They are employed without contracts, with no guarantee of steady income, no pension or holiday pay

Cashew nut production in India. Yashoda Arumurugan, 48, shells cashews in a processing unit in the village of Pudhukuppam in the southern state of Tamil Nadu

Pushpa Gandhi, 30, in southern India (pictured right with Femail reporter Emily Clark), supports her family as a cashew nut sheller. She's an unseen face of the industry feeding the UK's voracious appetite for the nuts — we ate 17,000 tonnes in 2016, 35 per cent up on 2012

The youngest girl at the unit: Suganthi Ramalingami, 13. She is taking time off school to shell cashews

But I had no idea about how they were being produced. And so I travelled to the village of Pudhukuppam in the Indian state of Tamil Nadu to meet the nut shellers.

When Pushpa was younger, she wanted to study English at university and become a teacher. Her parents didn't approve, so she married at 18 and started work — first as a farm worker, then here.

Sitting on the ground among heaps of cracked cashew shells, she says her life is over. It's not just the repetitive work that has worn her down. Her face and arms bear similar scars to her hands, caused by the cashew acids.

'It's already starting to burn,' she says, five hours into her day. 'Today when we go home and wash, we will see the boils on our skin. It takes about a week for them to heal. But as the old ones heal, new ones keep coming.'

The charity Traidcraft Exchange blames these conditions on the way European buyers — including UK supermarkets — aggressively push down prices, forcing cashew companies to hire cheap labour.

Follow cashew supply chains back and you will find women and children in unregulated shelling units all over India.

Many don't even get gloves, and if they did, they probably couldn't afford to wear them. Gloves would slow their shelling down, and they are paid by the kilo

The charity Traidcraft Exchange blames these conditions on the way European buyers — including UK supermarkets — aggressively push down prices, forcing cashew companies to hire cheap labour

A tub of cashew shells and cashews ready to be separated at the production plant in India

The youngest working with Pushpa was 13. There are regulated factories where conditions are better — for example, in the adjacent state of Kerala.

But when buyers squeeze suppliers, shelling is outsourced over the border to unregulated units. Pushpa earns just 200 rupees a day, or £2.15. A broker pays her 7p per kilogram of unshelled nuts, and she produces 10kg of shelled cashews a day.

A 200g packet of cashews in Tesco is £3, so had the nuts Pushpa shelled been destined to go there, at this price, just 1.4 per cent of the money we pay would go to her.

The salary is meagre, but enough to survive — when work is available. The women at this unit say there can be breaks of up to two months when crops suffer.

Competition from Vietnam, where the whole industry has been automated, has seen hundreds of Indian factories shut.

The Vietnamese designed and built all their own cashew machinery to cut labour costs, but India just doesn't have this machinery, because most growers haven't had the chance to invest in it.

The youngest working with Pushpa was 13 (pictured). There are regulated factories where conditions are better — for example, in the adjacent state of Kerala

Pushpa earns just 200 rupees a day, or £2.15. A broker pays her 7p per kilogram of unshelled nuts, and she produces 10kg of shelled cashews a day. The salary is meagre, but enough to survive — when work is available

Pushpa's husband is a builder, whose income is also unstable. When they struggle to pay bills they take out a loan with an interest rate of 3 per cent per month. So far, they owe £550.

This puts pressure on workers like Pushpa to shell faster to earn extra cash — meaning accidents are more likely. Pushpa has caught her fingers several times in the blades of the shell cutter.

Uma Jayamurthi, a nurse at the local Cuddalore medical centre, says she has seen several patients in the past year who have chopped off the top of a finger.

Around 40 per cent of patients at the centre have cashew-related injuries. 'The main reason people come here is when the cashew acid goes under their nails and it gets infected,' says Uma.

But she adds they only come when the pain is 'unbearable', because of the cost.

There would be fewer infections if shelling units had basic washing facilities, but many do not. At Pushpa's, there is neither a toilet nor a sink with soap and water.

Co-worker Yashoda, 48, who has asthma, says: 'As children, we questioned why the hell we were born into such poverty. Every day we live our lives without any kind of financial stability and that is always weighing down on me.'

Uma Jayamurthi, a nurse at the local Cuddalore medical centre, says she has seen several patients in the past year who have chopped off the top of a finger. She said: 'The main reason people come here is when the cashew acid goes under their nails and it gets infected'

The hands of Yashoda Arumurugan, 48 as she uses a pebble to crack the cashew shell, instead of a cutting machine

Pushpa Gandhi, 30, a cashew sheller at the unit in Pudhukuppam. She wears an old shirt to cover her arms and sari. It is covered in dark spots where the cashew acid has squirted out from the shells

Pushpa Gandhi has cut the shells in two with a machine and now has to pick out the nuts from a shell. She wears two gloves, or one for better dexterity, and has cut the fingertips off

We play our part in this cycle of despair. So, how can we change it? Boycotting cashews is not a solution — women like Yashoda and Pushpa would lose their jobs.

Fairer pay from supermarkets would be a start — as would a transparent supply chain. The 'country of origin' on Sainsbury's cashews says 'packed in France' — hardly enlightening.

British supermarkets should also better police their supply chains. All the major UK supermarkets, including Sainsbury's, Asda and Tesco, have voluntarily signed up to the Ethical Trading Initiative (ETI).

In order to do so, they had to say nine 'base code' rules — including safe and hygienic working conditions, no child labour, living wages and regular employment — were already in place.

The British Retail Consortium, which represents supermarkets, said they have 'robust safety and welfare standards and support suppliers in meeting these through audits, training and in-depth interviews with workers'.

A spokesman added: 'Retailers are conscious of the problems that exist in parts of India's cashew industry and therefore are careful to work with suppliers who provide decent working conditions.'

A spokesperson for The British Retail Consortium said: 'Retailers are conscious of the problems that exist in parts of India's cashew industry and therefore are careful to work with suppliers who provide decent working conditions'

But Fiona Gooch, of Traidcraft Exchange, says producers 'are under too much [price] pressure' to comply with the agreements.

She says finance experts should cross-check wage slips with a sample of workers, and super-markets should send health and safety teams to inspect factories.

However, factory visits can only do so much as workers cannot always speak freely.

As customers, we can act too. By contacting supermarket head offices by email, phone or letter we can demand that they are meeting their obligations.

So this morning, as you dig into porridge with pomegranate seeds and cashews, or spread cashew butter on toast, think of Pushpa and Yashoda. It's time our major supermarkets did, too.