Britons now spend more than £52bn on food every year - and more than 90% of that money is spent on processed food. But the canning, freezing and dehydrating techniques used to process food destroy most of its flavour. Since the end of the second world war, a vast industry has arisen to make processed food taste good.

During the past two decades the flavour industry's role in food production has become so influential that many children now like man-made flavours more than the real thing. As marketing to children has become more and more important to processed food companies and fast food chains, flavourists have increased their efforts to discover what children like. The flavour companies constantly run "taste tests" for kids - focus groups in which new products are piloted.

Fresh fruit and vegetables often have complicated, unpredictable flavours that combine bitterness with sweetness. When flavourists create additives for adult foods, they try to imitate nature as closely as possible. When flavourists create additives for kids' foods, they usually get rid of the bitterness and increase the sweetness. Children's flavours are often twice as sweet as those made for adults.

"Children's expectation of a strawberry is completely different," says one flavourist. "They want something that is strong and that has something like bubblegum notes."

The phrase "artificial strawberry flavour" offers little hint of the scientific wizardry that can make a highly processed food taste like a strawberry. For example, if you wanted to make a strawberry milkshake at home, here's all you'd need: ice cream, strawberries, sugar and a touch of vanilla.

Now take a look at the ingredients you might find in a fast-food strawberry milkshake: milkfat and nonfat milk, sugar, sweet whey, high-fructose corn syrup, guar gum, monoglycerides and diglycerides, cellulose gum, sodium phosphate, carrageenan, citric acid, E129 and artificial strawberry flavour.

And what does that "artificial strawberry flavour" contain?

Just these few yummy chemicals: amyl acetate, amyl butyrate, amyl valerate, anethol, anisyl formate, benzyl acetate, benzyl isobutyrate, butyric acid, cinnamyl isobutyrate, cinnamyl valerate, cognac essential oil, diacetyl, dipropyl ketone, ethyl butyrate, ethyl cinnamate, ethyl heptanoate, ethyl heptylate, ethyl lactate, ethyl methylphenylglycidate, ethyl nitrate, ethyl propionate, ethyl valerate, heliotropin, hydroxyphrenyl- 2-butanone (10% solution in alcohol), ionone, isobutyl anthranilate, isobutyl butyrate, lemon essential oil, maltol, 4-methylacetophenone, methyl anthranilate, methyl benzoate, methyl cinnamate, methyl heptine carbonate, methyl naphthyl ketone, methyl salicylate, mint essential oil, neroli essential oil, nerolin, neryl isobutyrate, orris butter, phenethyl alcohol, rose, rum ether, undecalactone, vanillin and solvent.

The chicken nuggets and hamburgers at fast food restaurants are usually the least profitable things on the menu. Selling French fries is profitable - and selling soft drinks is incredibly profitable. "We at McDonald's are thankful," a top executive once said, "that people like drinks with their sandwiches." Today, McDonald's sells more Coca-Cola than anyone else in the world.

The fast food chains buy Coca-Cola syrup for about 53p a litre. They add the syrup to bubbly water and serve it in a paper cup. A medium Coke that sells for 75p contains about 5p worth of syrup. Buying a large Coke for 85p instead, as the worker behind the counter always suggests, will add another 2p worth of syrup - and another 8p in pure profit.

Thanks in large part to the marketing efforts of the fast food chains, Americans now drink about twice the amount of soft drinks as they did 30 years ago. In 1975, the typical American drank about 120 litres of soft drinks a year. Today, the typical American drinks about 240 litres of soft drinks a year. That's well over 500 340ml cans of soft drink, per person, every year.

Even toddlers are now drinking soft drinks. About 20% of American children between the ages of one and two drink soft drinks every day.

· Read an exclusive extract of Eric Schlosser's new book, Chew on This, on how the fast-food industry exploits the very young.