Naked Science Forum
General Science => General Science => Topic started by: tweener on 09/03/2004 21:56:10
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What causes cream to change to butter when it is agitated?
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John - The Eternal Pessimist.
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Well, cream is an emulsion of milkfats and proteins suspended in water. Any emulsion will break up into separate layers if you slap it around long enough. I'm guessing this is what happens with cream.
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How is cheese made then? and what process did it went through?
We've learnt it in Food Technology in school but the process to me at that time was quite complicated with the adding of things and the producing of other things and adding things to that thing...See confused already. (There is something to do with curd)
Tom
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Cheese is the product of a number of reactions involving the fermentation of milk by bacteria (nice!) The process is basically the same as with yoghurt except an enzyme called rennet is added.
This acts as a coagulent (thickener) on the milk and turns it into cheese.
Very often Carbon Dioxide is given off from the respiration of the bacteria. This is what makes the holes in cheese that you sometimes find!
Hope that was of some use to you!
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The type of bacteria and their metabolic byproducts (as well as what enzymes you add) determine what kind of cheese you get. For instance, swiss cheese is made with a process that uses a propionic acid secreting bacteria. This is one of the things that gives the cheese its unique "bite" and flavor. (propionic acid is chemically similar to acetic acid, aka vinegar...in fact it's only one CH2 group longer)
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Jay,
You talk about the emulsion breaking up. I thought it was some sort of coagulation process. Don't know.
Cheese is very interesting. I've made yogurt but never tackled cheese. I've thought about making some cheese many times. Yummmm.
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John - The Eternal Pessimist.
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Breaking up an emulsion is coagulation, of sorts. You have a bunch of lipids suspended in water (they are insoluble as you know) as very small "balls" of lipid. This is an emulsion...insoluble particles suspended in a solution. When the solution containing lipid particles, which have an attraction for each other to some degree, are agitated to a large enough degree that they overcome the repulsive forces of the charged water molecules surrounding them, they start sticking together, making bigger and bigger lipid particles. Eventually, it all condenses, one layer of lipids and one layer of water. This is what is meant by breaking up an emulsion...you cause the suspended insoluble material to clump together, or coagulate.
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Ok, that makes sense. Thanks!
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John - The Eternal Pessimist.
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So what happens when it becomes whipped cream before it becomes butter?
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I've made butter while trying to make whipped cream, but never the other way around. I think you don't have the sugar in butter that makes it possible to make whipped cream. But if you put in the sugar and don't whip hard enough, you get very sweet butter.
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John - The Eternal Pessimist.
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change in density of different layers or compounds cause the cream to float on the milk.
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An overview of cheese making
To educate myself I thought I would research cheese making and write up a summary :
The starting material in cheese making is milk. Because the composition of milk varies between species, cheeses made from the milks of different animals have different characteristics, textures and flavours.
Milk is an emulsion comprising dissolved proteins (albumen), colloidal proteins (casein), carbohydrates (the chief sugar in milk is lactose), and suspended fats. When milk goes off (sours) it becomes acidic due to the action of bacteria metabolising the lactose to lactic acid (the same stuff that makes your muscles ache when you exercise). The bacteria reponsible for this effect were originally identified by Pasteur in the mid 1800's. He christened them Bacterium lactis although they are now known as Lactococcus lactis. Thankfully, the adicifying action of these bugs also prevents other, harmful bacteria from multiplying.
The drop in pH brought about by the action of Lactococcus causes the proteins to denature and the casein begins to form an insoluble precipitate - that's why off milk goes all flaky. This is the starting point for cheese making.
At this point rennet (which contains the enzyme rennin or chymosin) is added to coagulate the milk. Rennin is found naturally in the stomachs of milk-consuming animals. Its role in nature is to make milk 'chunky' as soon as it hits the stomach in order to slow down its passage through the GI tract and hence increase the time available for the intestines to absorb the nutrients.
Following the addition of rennin the milk forms big blobs of 'curd' containing milk proteins, fats, ions (chiefly calcium), and small amounts of sugar. Most of the lactose remains in the watery layer left over, called the whey. With gentle heating and the lactic acid produced by the bacteria, the slabs of curd slowly shrink to the size of small rice grains and are allowed to sink to the bottom. The whey is then drained off and the remaining solids fuse together to form 'firm curd'.
In the past whey was dumped, as a waste product, but then people realised its economic potential and began drying it out to crystallise the lactose which they sold to drug companies.
After the whey is discarded the firm curd is salted (for flavouring and to help preserve the cheese), moulded, and packed for maturing.
That's cheese making in a nutshell. Obviously there are additional techniques which can be used to make different types of cheeses but the basic procedure remains the same. For instance, 'soft' cheeses like Brie, or Camembert, owe their pleasant texture to the proteolytic enzymes secreted by the white mould which grows around the outside of the cheese. These work their way in from the outside, breaking down the protein fibres in the cheese to make it soft. Similarly, blue cheese is produced by inoculating the starter cultures with a blue mould which is then 'helped' during the maturation process by spiking the cheese which lets in air and encourages the mould to spread. As the mould digests the cheese consitutents it imparts flavour in the form of its metabolites.
All of these processes take time, which is why cheese needs to 'mature' for maximum enjoyment.
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When I was making a pavlova a week ago, my cream kept turning to butter before it became thick enough to be called whipped cream. Tell you what, it was bloody annoying. Spent 15 € on cream. Finally, though, I managed to get thick whipped cream ! I was so relieved.
I have a big bowlful of white/cream coloured butter, it doesn't have any taste, what are you mean to do with it to make it taste nice? I guess I could just use it in recipes that call for butter?
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Can someone help me? I want to start making Keifer. I ordered starter. Is that the same thing as keifer grains? I want to ferment vegetables. Where do I get the whey? Can I get it when I make keifer? Or do I need to order some?