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.
And incidentally, this is how beer and sodas are often carbonated. Whether you let yeast make the CO2 or you just pump it in, the beverage absorbs CO2 as the pressure of gas increases in the closed container (bottle, keg, whatever). When you open the vessel or pour a mug from the tap, the gas expands, forming larger bubbles that rise to the surface and make a nice foamy head on top. Jan 18, 2011 at 19:27
Of note is the fact that there are very similar devices that operate on CO2; they're called siphons.– AaronutJan 18, 2011 at 19:31
1Also, it's surprisingly difficult to make butter by shaking too much. Shaking too little is the more common problem, because the instruction manuals for these things are hyper-cautious. I'm not even entirely convinced that it's possible to make butter with a single charge; it's never happened to me with any amount of shaking.– AaronutJan 18, 2011 at 19:32
@aaronut: Good point on soda siphons. I don't know from how hard it is to make butter within a cream whipper, though, as I don't actually own one. I just know how they work. Jan 18, 2011 at 19:39
I used to do demonstrations at some historic houses, and when we'd make butter with the kids, we'd just put some cream in a jar, put a lid on it, and let the kids shake it in turns. It actually took quite a bit of shaking to make butter - 15 minutes or more, depending on how energetic the kids were - but the cream was not particularly useful as whipped cream long before that.– MartiJan 18, 2011 at 19:53
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:
- Gases, unlike liquids, are highly compressible; and
- A gas expands to fill its container.
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.