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I was reading the small print on a carton of milk recently. It claimed to have ''no added permeate'' and because it was ''less processed'' it claimed it had ''more goodness''.
I'd had a general notion that in those huge dairy factories our milk comes from, the milk was deconstructed and put back together again during processing, but this notion of adding permeate puzzled me, so I asked Pat Silcock, senior research fellow and manager of the product development research centre of the department of food science at University of Otago.
Permeate is produced when milk or whey (a byproduct of cheesemaking) is ultra filtered to remove protein concentrates, which are sold separately. It contains lactose (milk sugar) and vitamins and minerals, he said.
''It has riboflavin in it, so it's a yellowy-green and because it has none of the casein in it, it's clear. Casein is the main milk protein. It's what makes the cheese curdle.''
Why add it back to milk? It's all about standardisation, he explains.
''The composition of milk changes through the seasons, depending on the quality of the feed, the breed and the stage of lactation of the cows.''
In New Zealand, cows calve around the same time, unlike in the US where cows are made to calve at different times, so our milk varies in composition more, he said.
''We can get some quite large changes in composition and this can pose some processing problems. It's not so bad for production of pasteurised milk, but for producing spray-dried milk at different stages of the year, you can get problems with the dryer, problems with the evaporator.''
It was important to have a consistent composition of milk going through the spray-drying equipment as it helped control the process, produced a more consistent product and smoothed out the cost of the raw materials, he said.
''Essentially, the cost of the milk is related to the protein content, so if you add permeate it decreases the protein content and so makes it cheaper. But it doesn't change the protein content by that much.''
The FSANZ (Food Standards Australia New Zealand) regulations allow processors to adjust the components of milk, such as lactose, protein, fat or vitamins and minerals, by adding or removing those components to produce a standardised product. It also regulates the minimum fat and protein in milk for retail sale ( 3.2% fat for regular, 0.15% for skim milk and 3% protein for both).
When the tanker arrives at the factory the milk composition is analysed, the cream taken off and some added back to achieve the fat content for the milk they are making; blue, light blue or green top. Permeate is often added to standardise the protein content, then they homogenise the milk, pasteurise it, and fill the containers, Mr Silcock explains.
Homogenisation is when the size of the fat globules is reduced by forcing them under pressure through an orifice. The smaller fat globules stay suspended in the milk, and don't rise to the top as cream. That is why non-homogenised milk, such as ''farmhouse'' (silver top) or artisanal milk, like Holy Cow milk sold at the Otago farmers market, still has the cream on top.
Pasteurisation was introduced in the 19th century primarily to prevent the spread of tuberculosis and other illness as raw milk was a high risk product, he said.
When milk is pasteurised it is held at 72degC for about 13 seconds. It kills pathogens and results in some change to the protein structure but it's relatively minor change compared to the risk of not pasteurising, he said.
Pasteurisation also inactivated some of the enzymes present in milk, which helped stability. Depending on the time of year, pasteurised milk could last for 20 to 30 days as long as it was kept properly chilled.
''There are different points where it can have some temperature increase, and the most common place is between the consumer buying it and getting it home, or taking it out to make a cup of coffee and discovering it still sitting on the bench two hours later,'' he said.
''In pasteurised milk the pathogens, the food-poisoning organisms, have been killed. You can get post-pasteurisation contamination but in general what grows is spoilage organisms. Spoilage organisms make the product smell bad, they make curds form, you put it on your coffee and it goes lumpy. Generally that's just going to annoy you that you've wasted your coffee but it won't make you sick.''
He is not a fan of raw milk as any food poisoning organisms that might be present would still be active. With internationalisation, some bacteria had become more virulent than before. For example, some strains of E. coli caused food-borne illness now, but 50 years ago that wasn't a problem, he said.
''Some people say my grandparents drank raw milk and never had a problem but it was collected at a point in time when these organisms weren't a problem.''