杏吧原创

WHY does cream go rancid more quickly than butter? Researchers in Norwich
think they have the answer, and it comes down to the structure of the food, not
its chemical composition鈥攁 finding that could help to rid some processed
foods of chemical preservatives.

Cream and butter contain pretty much the same substances, so why cream should
sour much more quickly has been a mystery. Both are emulsions鈥攖iny
globules of one liquid evenly distributed throughout another. The difference
lies in what鈥檚 in the globules and what鈥檚 in the surrounding liquid, says Tim
Brocklehurst of the Institute of Food Research, 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. Brocklehurst says that the
bacteria which make the food go rancid鈥攕uch asPseudomonas
鈥攑refer to live in the watery regions of the mixture, which contain the
milk proteins and lactose sugar on which they feed. 鈥淭his means that in cream,
the bacteria are free to grow throughout the mixture,鈥� he says.

When the situation is reversed, the water-dwelling bacteria are locked away
in compartments buried deep in the sea of fat. Imprisoned in this way,
individual colonies cannot spread and rapidly run out of nutrients. They also
slowly poison themselves with their waste products, such as acetic and lactic
acids. 鈥淚n butter, you get a self-limiting system which stops the bacteria
growing,鈥� says Brocklehurst.

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