Cream and butter contain pretty much the same substances， so why cream should sour much faster has been a mystery. Both are emulsions tiny globules (小球体) of one liquid evenly distributed throughout another. The difference lies in what s in the globules and what s in the surrounding liquid， says Brocklehurst， who led the investigation.

　　In cream， fatty globules drift about in a sea of water. In butter， globules of a watery solution are locked away in a sea of fat. The bacteria which make the food go bad prefer to live in the watery regions of the mixture. This means that in cream， the bacteria are free to grow throughout the mixture， he says.

　　When the situation is reversed， the bacteria are locked away in compartments (隔仓室) buried deep in the sea of fat. Trapped in this way， individual colonies cannot spread and rapidly run out of nutrients (养料)。 They also slowly poison themselves with their waste products. In butter， you get a self-limiting system which stops the bacteria growing， says Brocklehurst.

　　The researchers are already working with food companies keen to see if their products can be made resistant to bacterial attack through alterations to the food s structure. Brocklehurst believes it will be possible to make the emulsions used in salad cream， for instance， more like that in butter. The key will be to do this while keeping the salad cream liquid and not turning it into a solid lump.

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