Three or four times a week, I go into a local coffee shop and order a double espresso. It costs about £1.70, although I'm not absolutely sure, and the place is usually packed. Yet I suspect that some of them head straight from Caffè Nero or Starbucks to a supermarket boasting about rock-bottom prices, the kind of place that sells anaemic chickens which have never seen the daylight in their brief existence.

It's a testament to the power of advertising: supermarkets have taught the public to value price above everything, while café culture has managed to achieve exactly the opposite. Few people seem to wonder why the milk that goes into their cappuccino or flat white is so ridiculously cheap on the supermarket shelf, where it's often treated as a loss leader – the National Farmers' Union (NFU) accuses Asda and Morrisons of "waging war with each other on milk prices". Now there's a crisis facing dairy farmers, with several going out of business each week, and the NFU says some farners are being paid less per litre of milk than it costs to produce.

Dairy farmers need 30p per litre to cover their costs. The price that many of them get from milk processors was cut in June and another cut is scheduled in August, making the price of a litre as low as 25p. Last week, farmers blockaded plants owned by one of the big three milk processors, Arla, and more protests are planned. They've got the support of chefs Jamie Oliver and Hugh Fearnley-Whittingstall, who point out that milk is cheaper than mineral water. Actually, a pint of semi-skimmed costs less than a chunky Kit Kat, suggesting that something has gone seriously awry with consumer expectations.

The way food is produced and marketed in this country has created an obesity epidemic, factory farming, and an endless struggle by farmers to make a living. We've been hearing for years about big supermarket chains using their power to impose unreasonable terms on suppliers, suggesting that the milk of human kindness is in short supply in the food industry. The NFU says the latest cuts are driving down the price of milk to a point where it's no longer sustainable; it wants shoppers to support farmers by shopping at stores such as Waitrose, Sainsbury's, Marks and Spencer and Tesco, which give farmers a better deal.

Four years ago, a study by economists at Oxford University upheld "the common belief that supermarkets currently hold the bargaining power". Farmers do all the hard work but are the weakest point in the supply chain. I've thought for ages that supermarkets should be obliged to display information not just about where their produce comes from, but if they adhere to fair terms of trade.

The revolution in the nation's coffee drinking shows that people do change their habits. And consumers have power, if they can be persuaded to use it. Anyone who's happy to pay £2 for a cup of coffee shouldn't think twice about paying a fair price for essentials, whether it's a chicken or a pint of milk.

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