I recently got an iSi whipped cream container that charges with nitrous cannisters. I don't really understand how the thing works, which I think would help my use of the system. How does this actually whip the cream upon dispensing? What does shaking do that will make a solid foam / mousse? Why does over shaking turn it in to a solid in the cannister?
The cream whipper relies on gas expansion to work.
When you make whipped cream by beating, you beat fine air bubbles into the cream. The cream traps air and becomes essentially a matrix that holds those bubbles--a foam.
Your gas-charged whipper does the same thing in a totally different way.
When you charge the whipper with gas, there's high gas pressure inside with the cream. The cream will actually absorb the nitrous oxide you put in. Because of the pressure, the gas absorbed can be thought of as really really really small bubbles within the cream. So you have a matrix of gas and cream, but because the bubbles are so small, it's essentially just cream.
Chilled liquids more easily absorb gases at high pressure, which is why it's good to use cold cream and keep the whole unit in the fridge. A limited amount of agitation (shaking) exposes more cream to the gas, improving absorption.
When you release the cream from the device, the absorbed gas expands rapidly. The bubbles get bigger, and your cream to bubble ratio becomes more like the foam that we know as whipped cream. It's really exactly the same thing, only with nitrous oxide instead of plain ol' boring air inside the bubbles.
Why nitrous oxide? As I understand it, it's because it's the cheapest non-toxic, odorless and tasteless gas you can get. Carbon dioxide would almost be a good choice, but unfortunately it's bitter. Not a good match for cream.
Finally, why is shaking too much a bad thing? That one I don't know for sure, but I know what happens when you over-whip cream with the mixer. You make butter. Perhaps the gas or high pressure encourages this conversion, or maybe you're just churning it that much when you over-shake. Either way, I'm sure you've essentially made butter when you shook it too much.
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The principle is actually very simple to understand if you take the liquid out of the equation, and imagine that you're just charging the dispenser by itself, empty.
If you remember your high school science, you should remember that:
A whipped cream charger is a sealed container holding a sizable amount of highly pressurized gas (nitrous oxide). When you screw one into the dispenser, it punctures the charger, allowing the pressurized gas to expand and enter the dispenser. Since the dispenser has much higher capacity (volume) than the charger, basic thermodynamics dictates that most of the gas will end up in the dispenser.
When you beat eggs, cream, or anything similar, you are gradually incorporating air into the mixture. bikeboy's explanation of what it means to incorporate air is a fairly good one. The difference with a cream whipper is that instead of gradually incorporating air into the liquid, you are rapidly forcing the nitrous oxide into it. Because the entire apparatus is completely sealed, when you shake it up, the gas has no place to go except into the liquid.
That's really all there is to it. You're cramming a certain quantity of a liquid into a container with a large amount of gas and forcing the two to mix. They will still separate over time, because the container is not completely full (and the gas would rather occupy the empty space at the top), but shaking it quickly re-incorporates the ingredients.
There's a reason why a cream 'charger' uses nitrous oxide... it's the same reason nitrous oxide is good as an anaesthetic. This gas dissolves very well at the fat/water interface (cream is minute droplets of fat in milk)... unlike CO2 or butane or any other cheap propellant gasses.
So don't try to use a CO2 cannister in your cream charger, it won't work!