Heat process adds weeks to the shelf life of milk

The shelf life of milk can be extended by several weeks by rapidly heating and cooling it, US researchers have shown.

Bruce Applegate, associate professor in the Department of Food Science at Purdue University, and collaborators found that increasing the temperature of milk by 10°C for less than a second eliminates more than 99% of the bacteria left behind after pasteurisation.

“It’s an add-on to pasteurisation, but it can add shelf life of up to five, six or seven weeks to cold milk,” Applegate said.

In the study, the low-temperature, short-time (LTST) method sprayed tiny droplets of pasteurised milk, which was inoculated with Lactobacillus and Pseudomonas bacteria, through a heated, pressurised chamber, rapidly raising and lowering their temperatures about 10°C but still below the 70°C threshold needed for pasteurisation. The treatment lowered bacterial levels below detection limits and extended shelf life to up to 63 days.

“With the treatment, you’re taking out almost everything,” Applegate said. “Whatever does survive is at such a low level that it takes much longer for it to multiply to a point at which it damages the quality of the milk.”

Sensory tests compared pasteurised milk with milk that had been pasteurised and run through the LTST process. Panellists did not detect differences in colour, aroma, taste or aftertaste between the products.

Phillip Myer, an assistant professor of animal science at the University of Tennessee and a co-author of the paper, published in the journal SpringerPlus, said the process uses the heat already necessary for pasteurisation to rapidly heat milk droplets.

“The process significantly reduces the amount of bacteria present, and it doesn’t add any extra energy to the system,” Myer said. He said the promise of the technology is that it could reduce waste and allow milk to reach distant locations where transport times using only pasteurisation would mean that milk would have a short shelf life upon arrival.

Applegate said the process could be tested without pasteurisation to determine if it could stand alone as a treatment for eliminating harmful bacteria from milk.