Saturday 10 December 2016

Why are dairy products such a big part of summer?

Alan Kelly

Published 18/06/2015 | 02:30

A cow is a very efficient machine for turning grass into milk, extracting from it the raw materials it needs to produce the complex mix of proteins, fats and lactose (the milk sugar) that we know as milk.
A cow is a very efficient machine for turning grass into milk, extracting from it the raw materials it needs to produce the complex mix of proteins, fats and lactose (the milk sugar) that we know as milk.

Where does milk come from? That's easy - cows. Well, what do cows make it from?

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Simple questions in science often have complicated or surprising answers; in the case of the question above, part of the answer is 'grass'.

A cow is a very efficient machine for turning grass into milk, extracting from it the raw materials it needs to produce the complex mix of proteins, fats and lactose (the milk sugar) that we know as milk.

This relationship between grass and milk, in fact, almost defines the Irish dairy industry. In Ireland, we know that grass grows best between February and October, and so perhaps it is not surprising that we produce most milk in this period also. Indeed, some cheese factories in Ireland have so little milk during the grass-poor winter months (about a tenth as much as in peak summer) that they don't make any products.

This situation is very unusual though, as other countries don't rely as much on grass as their main feed for cows, and so produce milk evenly year-round.

In the next few years even more milk will be produced during the summer, as EU-wide limits on how much milk could be produced (called Milk Quotas) have been removed since 1 April, 2015.

Our dependence on grass does give Irish dairy products a unique advantage though, with our products having a richer flavour and, in particular, colour than those from other countries. A molecule called beta-carotene derived from grass, for example, lends Irish butter a golden hue unmatched elsewhere.

So, in summer, we have lots of milk being produced, very high milk quality (including highest levels of protein in particular), and lots of good-tasting cheese being made, as well, of course, as one of the products we associate most with summer is a dairy product, ice cream.

Ice cream is a great example of how a food product we are very familiar with hides a surprising amount of complicated science, and perfectly illustrates some of the science of summer.

If you put milk in a freezer, what you get is clearly not ice cream, but much colder, harder and less smooth. What is the difference? This depends on three main components: air, fat and water.

Firstly, ice cream is a foam, and is whipped during manufacture to incorporate a large amount of air (accounting for up to half the volume of ice cream), to give a light airy structure, which gives the softness.

Secondly, ice cream is also an emulsion, in which fat globules from the milk are whipped to give a firm, creamy structure.

Finally, there is the ice, or frozen water, dispersed in tiny crystals. Ice cream includes ingredients called stabilisers (usually polysaccharides or gums) which trap much of the water present, ensuring that only very small ice crystals can form, rather than a solid icy block, while dissolved added sugar gets progressively more concentrated and harder to freeze as some of the water present coverts to ice.

So, in a shop when we ask for a whipped cone, we set in motion the transformation of a mix of milk, cream, stabilisers and sugars (along with flavours and colours), which involves reducing the temperature to around five degrees below freezing very quickly while pumping the mix and air through the barrel of a machine called a scraped-surface heat exchanger.

In this, blades break up the ice crystals as they form and disperses the air into tiny bubbles until a very thick product consisting of a sugar solution containing tiny ice crystals and air bubbles exudes out, neither exactly solid nor exactly liquid, yet able to form a perfect cone on top of its wafery base.

And there it is: ice cream, a world of physics and chemistry, in a delicious summer treat - the peak achievement of the summer reign of dairy products in the Irish food landscape.

Professor Alan Kelly, School of Food and Nutritional Sciences, University College Cork

Irish Independent

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