Something strange has happened to Joanna Blythman.

She's written a food book that is positive. All things are relative, of course, and we are talking about the woman who, as the daughter of the late great socialist campaigner and Scottish republican songwriter Morris Blythman, is never going to rest on her laurels when it comes to questioning the food we eat.

Yet when I meet with the sometimes excoriating restaurant reviewer of our sister title, the Sunday Herald, and angry author of such seminal titles as 2004's Shopped: The Shocking Power Of The Supermarkets, and 2006's Bad Food Britain, I'm surprised to find her sipping tea and smiling. Her new book is essentially a detailed guide to food in the round, aimed at helping young people living on a diet of Grazia and Heat magazines to choose foods from an ethical, environmental, health and financial perspective – and although there's a good helping of food politics in the mix, the tone is benign rather than belligerent.

For example, she patiently explains why tinned fish in oil is better than brine (less salty), why we need to get out of the habit of eating factory-farmed chicken breast, and why branded breakfast cereals are to be avoided. She says she hopes it's the kind of book mothers might buy for their daughters. Highly detailed, heavily researched and simply presented, it was inspired by her own daughter, who complained that although she knew she wasn't eating well or healthily, buying the right food was "just so complicated". It's packed with Blythman-coined terms such as "the aware diet" and "the progressive food agenda" and its strapline, she jokes, could easily have been: "Cut the crap."

We're sitting at her big kitchen table in Edinburgh, sampling first-flush Darjeeling, (Blythman has given up drinking coffee because it makes her "nervy").

"I think the progressive food agenda is quite positive now, spreading out from the middle-class into a wider demographic and strengthening all the time," she enthuses. "Alternatives to the supermarkets are increasingly credible and not weird or cranky. People are no longer in thrall to them, and they see that they are charging too much for what is basically over-processed rubbish. Look at the rise of the high-street butcher, baker, cheesemonger and fishmonger. We're seeing that the independents can outmanoeuvre the multiples. We have food co-operatives, pop-up shops, bread clubs and a whole new economy of really high-quality food at good prices. It feels the tide is turning, and there are grounds for hope. I'm excited by that."

So the grassroots food revolution in which she has played such a pivotal part has taken root. Yet the campaigner and author, who was born in Springburn in Glasgow, refuses to take the credit. "The recession has made people more interested in cooking and given a huge boost to the homegrown economy," she says. "It's cheaper to cook at home and it's getting people back to an emotional, intuitive response to food. The big weekly food shopping is becoming a thing of the past.

"That said, I do feel Shopped was a helpful contribution to the debate."

Her next book – of which details have yet to be finalised – will likewise be "enabling and positive". However, it won't be a cookbook. The world, she says, doesn't need any more of them. So does this mean the big fight is over? "Not at all," comes the immediate response. "The big issues aren't going away. We need to stop the big GM crops and the capitalist phenomenon of food speculators [financial people who, backed by world banks, have no interest or connection to agriculture or the food sector, yet manipulate the supply and demand of staple crops and create spikes and troughs in prices]. They are bad enough for us, but even worse for the poor people of the world. We're lucky in our government, but unfortunately Scotland is not in control of that. Food issues aren't sovereign to one country."

She believes building food security should be the number one priority for the Scottish Government, pointing out that producing only 58% of the food we eat makes us dependent on global markets. And while she is an admirer of the Scottish Government's food policy – in particular its resistance to GM crops and general "progressive" attitude – she says that, by encouraging the increased production of farmed Scottish salmon for export to emerging markets such as China, it's creating "an environmental catastrophe for Scottish waters".

"Salmon farming has made a sewer out of Scotland's rivers and scaling up production to meet demand from China is even more of an issue. There's no visual eyesore with fish farms, but if people could see the damage under the water level they'd have no truck with it. Good chefs won't touch farmed salmon. The most iconic Scottish fish has been debased."

Blythman considers herself a global food writer who happens to live in Scotland and, although she has a flat in London where she spends more than half her time, her Scottish perspective undoubtedly gives her an advantage.

"Scotland has a particular problem with diet and bad eating in common with places such as Hull, Birmingham, Liverpool ... I don't think the poorest Glasgow diet is any worse than those cities. We have a shared problem with urban working-class roots and past poverty. I was born in a place that's associated with the worst diet in the world, which is a great irony."

For a while she campaigned for Women's Aid and the Scottish Council for Civil Liberties, and continues to be involved in Friends of the Earth, the Fairtrade Foundation, the anti-GM movement and Oxfam.

"My job as a journalist is to give these people a platform. Like my mother, my father was a teacher who believed in what he called the democratic intellect. I guess I have their genes."

Although not a teacher in the strictest sense, she is one of sorts. And, as a restaurant reviewer, her loyalties lie with her readers rather than with the chef. "After all, they're spending their hard-earned cash." For her, Michelin stars are a distraction. "I'm not interested in fine dining. I'm more interested in what people can do to address a democratic clientele. I don't mean to diminish chefs, but I think top-level cooking is not the way to gauge a food culture. It's on how it addresses everyday eating that Scotland should be judged."

What To Eat by Joanna Blythman is published by Fourth Estate, priced £16.99.