The way some dairies separate fat from whole milk may be wrecking the process supposed to kill harmful bacteria, say scientists.

Researchers from the UK's Public Health Laboratory Service (PHLS) found two small dairies where the pasteurisation process had been severely undermined by the addition of equipment to make skimmed milk.

Their findings could send a wave of anxiety through the milk production industry - the government's own Food Standards Agency has admitted it was not aware of the potential problem.

The research, reported in New Scientist magazine, suggests it may only be smaller dairies that are at risk.

The demand for skimmed and semi-skimmed milk has only emerged in the past two decades in the UK, as people became more aware of the benefits of a low-cholesterol diet.

But many smaller dairies are still using machinery which pre-dates that era.

These have been modified so the milk can be separated following the pasteurisation process.

But these modifications may be compromising the safety process, which requires milk be heated to approximately 72 degrees Celsius for at least 15 seconds.

Too cool

The PHLS team found that, at the dairies they checked, the modified machines were passing the milk too quickly through the heating system, holding it at the correct temperature for too few seconds.

Anita Rampling, one of the researchers, told New Scientist: "They weren't designed to separate the milk during the pasteurisation process.

"The flow rate is speeded up, so the milk is not held at high temperature for long enough."

The team concluded: "This has significant public health implications and may account for sporadic cases of illness."

The main culprit for milk-borne illnesses is the salmonella bacterium.

The researchers were not able to pin any particular outbreak to these dairies - there have been seven outbreaks of salmonella nationwide linked to milk since 1992.

In fit people, salmonella causes food poisoning symptoms with no lasting effects - although the illness is far more dangerous in the elderly, very young and medically vulnerable.

The biggest ever outbreak of salmonella, in Chicago in 1984, involved 18,000 confirmed cases, and was traced back to skimmed milk.