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26 October 1998 |
Ref 98/251 |
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Ever wondered why cartons of milk sometimes spoil in the 'fridge before the use-by date?
Scientists at Food Science Australia (FSA) have found a way to help industry to manufacture pasteurised milk with improved storage life.
When milk spoils before its use-by date, even when it has been carefully stored and handled, it is usually because of the presence of a group of cold-loving microbes called psychrotrophs, says Food Science Australia dairy researcher Dr Heather Craven.
"In health terms these bacteria are pretty harmless, but they do pose a problem if they get into the milk because then whole batches can go off - and dairy companies receive complaints from consumers," she explains.
"Even if you keep the milk chilled, they can multiply from a few per millilitre to over a million in the space of just six days. Even one microbe in a carton of milk can cause a spoilage problem."
The problem is that many of the traditional microbiological tests used by dairy plants around the world are too slow and insensitive, or else pick up the wrong organisms. Till Food Science Australia's breakthrough test, there was no easy, rapid and reliable way for a dairy plant to know if a batch of milk was contaminated by spoilage bacteria.
Research by scientists at Food Science Australia over several years has resulted in Psychro-Fast, the world's best test for dairy spoilage organisms - one that is simple, super-sensitive and indicates the presence of typical spoilage organisms the next day.
Dr Craven says the test can be used to monitor the whole dairy production line from pasteuriser to carton, in order to pinpoint the source of any spoilage problem.
The presence of typical cold-loving spoilage bacteria will be detected using this test. Both milk plants and consumers can then be confident the milk will exceed its declared shelf-life, so long as it is kept at 4 degrees or less.
At present, most processors expect fresh milk to keep for about two weeks in the fridge. But if the product can be guaranteed free of the spoilage bugs, milk can potentially last in the fridge for up to three weeks, she says.
Victorian Quality Manager for Dairy Farmers, Mr Trevor McManus, says the Psychro-Fast test offers greatly enhanced sensitivity in detecting spoilage organisms. It has enabled Dairy Farmers to tune up hygiene at each point in the production line, and has become a key performance indicator for production teams.
"It's the next step in quality assurance. It offers a way of providing consumers with milk which has a greater shelf-life and absence of the stale smell and taste they associate with stored milk.
"It's all about the quality image of our product - and you can't put a dollar value on that," Mr McManus says.
Apart from fresh milk, the test can also be used to improve the quality and shelf-life of other dairy products such as butter, cream and cottage cheese.
"Tests normally used by industry to assess milk quality don't necessarily demonstrate that all products are free of spoilage organisms. Psychro-Fast achieves that," Dr Craven says.
"We've designed it in such a way that any dairy plant with a basic lab can perform the test," she says.
Milk is mixed with a selective agent to screen out unimportant microbes and an indicator, then incubated at 30 degrees. If psychrotrophs are present, their numbers will rapidly climb to millions per millilitre - and the indicator turns the milk sample pink. The test can be easily adjusted to count numbers of spoilage organisms.
The test has proven so effective that about half of Australia's dairy factories have already adopted it, and there is growing interest from round the world.
Food Science Australia is Australia's largest food research organisation, and is a joint venture between CSIRO and the Australian Food Industry Science Centre (AFISC).
This research was supported by the Dairy R&D Corporation and the Victorian Department of Natural Resources and Environment.
More information:
Dr Heather Craven, Food Science Australia 03 9742 0112
Lloyd Higginbotham, Food Science Australia 03 9742 0341
Dr Robert Chandler, DRDC 03 9742 0308
(Australia's largest scientific research organisation)
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