"God, why do I give interviews to the Guardian?" Jamie Oliver groans. "They always try to dissect you, and I don't really think about stuff in the way that you're asking me these questions. It's far more natural, and less . . . less strategic than you seem to think." Looking thoroughly fed up, he mutters under his breath, "I just know what this article's going to be like." It's only halfway through the interview, and I fear he may get up and walk out.

Well I am mortified. You would have to be mad, in my view, not to admire Oliver, whose evolution from mildly grating Essex geezer to heroic public campaigner has elevated him to secular sainthood. From his restaurant Fifteen, training troubled youngsters to be chefs, to his campaigns for decent school dinners and basic cooking skills, he has been a national inspiration. My mistake had been to assume that the popularity of these campaigns must have given him invincible confidence – and to infer from their success that he must have the mindset of a political strategist. On both counts, it turns out I'm completely wrong, for instead he is a scattergun of raw emotions, chief of which appear to be beleaguered frustration, and acute sensitivity to any perceived criticism.

Part of this may well be down to the fact that he is exhausted. When we meet, his fourth child is only two weeks old, and Oliver's speech patterns bear the unmistakable mark of sleep deprivation, with sentences abandoned halfway through, and thoughts colliding into one another. But he also seems shattered by his campaigning work, as if unaware of quite how much respect – even adoration – it commands. He describes his new TV series, a straightforward cooking show called Jamie's 30 Minute Meals, as "an antidote", and "heaven", after all his struggles to change the way we eat.

"I hate making TV documentaries," he says with feeling. "Because it takes quite a lot of energy to know that you're going to get your arse kicked and people will hate you, or fight you, for large proportions of time. You know when I did School Dinners I got so much abuse for a year and a half, and the people who were getting in the way of some of the biggest progress . . . you know, once the show was broadcast, all of a sudden it was 'authentic'. But until then it was just lots of – well, people hate change. So I don't particularly enjoy doing the stuff I'm most proud of."

The health secretary Andrew Lansley recently accused him of "lecturing" people about what to eat. I ask if he thinks the remark was a one-off, or suspects it may be indicative of the new government's hostility to anything that smacks of the nanny state.

"It's very clear that there's a lot of double standards going on. Should there be a 30mph speed limit? Of course there bloody should. And certainly with kids and school food, kids need to be nannied for sure. So give them a bloody good meal at school."

But I wonder if he ever worries that he might lay himself open to the charge of lecturing by saying things, as he did in School Dinners, like: "If you're giving your young children fizzy drinks you're an arsehole, you're a tosser. If you give them bags of crisps you're an idiot. If you aren't cooking them a hot meal, sort it out."

"But that," he says indignantly, "came after a sequence where some obviously quite thick woman was having a pop at me about taking her kid's sticky toffee pudding away, and we gave them fruit and yoghurt instead. And while she was having a go at me she had the gall to have a baby in her arms feeding it Coke in a bottle, so she's obviously thick as shit, you know; she was clearly thick as shit, because you don't feed babies Coke in a teated bottle, and anyone who does is categorically wrong.

"I challenge you," he adds angrily, "to go to any school and open 50 lunchboxes, and I guarantee you there will be one or two cans of Red Bull, there'll be cold McDonald's and jam sandwiches with several cakes." Then he pauses, as if reminding himself to be diplomatic.

"Look, Lansley wrote an apology letter. For me it's all in the past, I've got nothing against him. But it's easy to say you can't tell people what to do, when clearly it's the only way forward. If you're going to be civically responsible for feeding children once or even twice a day for 190 days of the year from age four to 18, you'd best know what you're talking about, cos we're half responsible for all their nutritional intake. You know that government advertising campaign, Change4Life, cost £20m on billboards? I could have built over 100 Ministries of Food in towns all over the country for that. The public doesn't need to know that we're in a fucking state, that we need five a day. What it needs is skin on skin, it needs beacons locally where you can find out stuff for free, and have lessons. It's the only way forward, and it won't blossom through cuts."

Jamie's Ministries of Food have been established in four cities now, where the public are taught basic cooking skills in a bid to wean them off processed food and ready meals. But the future of the original Ministry in Rotherham is now in doubt, threatened by cuts, and Oliver is incredulous.

"The reason why I'm so passionate about the Ministry of Food is that we're fully booked, and if we had another staff member we could put another third on the numbers. We do about 8,000 people a year from one little cheap £130,000 setup grant from Rotherham council. We're fully booked, we're busy. It works. But they're all looking at me now for money, and the thing is I don't have it. I haven't got dough sitting in banks for me or for anything else."

Some readers may think come off it, you're really rich, I suggest. His wealth is routinely reported to be anywhere between £25m and £45m; could he not write a cheque for £130,000?

"I can't. I can't. I just can't, it's as simple as this. I've got my businesses that I look after, I don't have venture capitalists swoop in and pay for everything. Basically everything I've got funds my restaurants, the vulnerabilities are all mine. I've got 18 months of wages for my staff in the bank, but I'm not spending their money."

Oliver has recently extended his campaign to America, where he made a series called Jamie's American Food Revolution, set in the nation's most obese town. It won him an Emmy, and has been recommissioned, but far from celebrating, Oliver is still recovering. "The town didn't react very well to me being there, and there was one fellow on the radio who did a lot of shit-stirring that caused basically six weeks of aggro for me. No one really wanted to get involved or help, they thought we wanted to make them look stupid." With hindsight, I ask innocently, does he feel he made any tactical errors?

"No," he shoots back, "it was brilliant. You know, change is very hard – structures, organisations, businesses, people, anyone really. And if you're shining a light on one of the most unhealthy places in the world, it has to be a car crash, there's no pretty way. I knew what I was flying over there for, I knew it would be horrible, but I hadn't done horrible without my family. When you have shit days you need to be able to go and hug your kids, do you know what I mean? I didn't have that, and it was hard, really hard."

And yet, I say, it's these documentaries which make us love him. "But there's still lots of people who don't like me," he counters straight away. "You can tell that if you go on any blog. I annoy lots of people. You know people often don't like the good guys, and I try to be a good guy, I'm consistent. You know, I've been consistent in my direction, the beliefs that I have. And people hate that."

It's on the subject of consistency that he takes offence. Oliver has always been passionate about his family life – "Family first, always, always," as he says. He and his wife Jools were childhood sweethearts, and family life with their three daughters and now a son too provide the narrative backdrop to many of his programmes and his books.

"Jools is a militant Gina Ford and it bloody works," he volunteers. "What people don't understand is that sticking to Gina Ford is a job, it's not easy, it's not casual, it doesn't suit you. I see some of my friends who have a very casual approach – you must never judge," he adds quickly, "but when their kids are going to bed at two in the morning they look knackered. Gina's basically just a structured routine, and it's quite a lot to remember, but it does work. It's like communism," he grins. "If everything's done then it does work."

Authenticity is what makes Oliver's television so compelling, and part of this has involved filming his personal life in intimate detail. I want to ask him about how he navigates the balance between authenticity and family privacy, but as soon as the words are out of my mouth he begins to look annoyed, anticipating an attack. "Now you're going to ask about family, compromising, why have you put your family on the telly and all that sort of stuff," he says grumpily. I only make matters worse by asking him about his contract to promote Sainsbury's, which is now in its 10th year and earns him a reported £1.2m a year, but has laid him open to the charge of hypocrisy by critics who point out that the supermarket sells the very processed meals he tells us not to eat. Personally I don't see a problem, so my question is purely one of strategy: as a political campaigner is the money worth it, when it allows critics to claim it undermines his message? But Oliver thinks I'm attacking him again.

"I think journalists try to get more upset about Sainsbury's than the public, cos the public shop in supermarkets. And that's the truth of it. Sainsbury's does feed the country. It's nothing I apologise for," he snaps.

I think Oliver is sick and tired of people trying to define him – is he a chef, a businessman, a social entrepreneur or a TV star? In his mind the distinctions are irrelevant; he talks instead about following passions, creating cultures, and prefers to employ people who "get it"; his righthand woman "isn't qualified to do her job at all", but just pestered him in the street for a job, and "she knows what I care about, and the things that are true to me". His ambition appears to be literally infinite – even if, he sighs, his motivation doesn't always slot into a neat category.

"No one understands me. No one. My wife doesn't even understand me in terms of what I want to do. Everyone thinks everything's about money. You think I'm going to America to make money? That is probably the worst financial use of my time in the world, going to America next year, cos there's no money in TV, and they don't buy books. I don't want to break America, I don't want to move there, I'll be there for three months next year but I don't want to be making that show, I want Americans to be making that fucking show. I'm not pleased I got the Emmy cos I got the Emmy; I'm pleased because it will get other people to make these shows, and get the public active, and get McDonald's to start doing some other shit instead of the shit they are doing.

"I have a fairly low regard for money to be honest, it doesn't really add that much to a lot of the things that give me pleasure in life. However, if you have an idea, and you've got it, you can do it. If you haven't got money and you've got a great idea, it's hard to get it done. So for me I want to get in a position where I can do stuff myself. I want to be able to go into Essex and say: 'I want all your schools.' I want to set up a company that would be not for profit; I want to set up a company that would be like the government used to be, where we train dinner ladies militantly, where we'd fit the kitchens out and deliver on budget. But it's not just Essex, you see, it's trying to create things that can be rolled out elsewhere. But it all comes down to money."

In the end, Oliver believes that change will only come through public pressure. "Although they don't know it, the public is still king. So what I try and do is shit-stir. In America, what hasn't happened yet is the public haven't really told business what they want. For instance, McDonald's America and McDonald's UK are totally different. You've got one public that's fairly well informed, which is here, so you know you've got organic milk, 100% free-range eggs; they do a huge amount of salads, they've done a huge amount of inward thinking in the last five years. So although they've been the enemy for many years, you've got to take your hat off and say well done, and carry on. America hasn't even done that, they've done nothing in comparison. The only difference is the public ask for more."

So if he had to choose only one element of his empire – the cookery shows, the restaurants, the books, Fifteen, or the campaigns – which one is closest to his heart?

"I'd love to be elitist, cos that's where my heart is – I'm a food geek. But it's fuck-all use to anyone, absolutely no use to anyone, it doesn't change anything really. I really want to get school food sorted, and it ain't going to get sorted by the government. It needs investment, entrepreneurialism, expert management – and it's not going to happen, cos they'll never put their hands in their pockets or be there long enough to change anything."

Jamie's 30-Minute Meals is on Channel 4 every weekday from today at 5.30pm. The book of the series is published by Michael Joseph