Words by Simon Difford
Foams can be applied to the surface of a cocktail, the aroma and flavour of which usually contrast with that of the drink beneath, so adding complexity. These foams are made by a chemical reaction between protein and nitrous oxide (N2O). The most convenient way of dispensing foam is from cream-whippers (also known as cream siphons, whipping siphons or just siphons).
Before continuing to read about foams and certainly before making a foam please see our page on nitrous oxide (N2O) and its associated risks and laws.
You should not only be aware of the potential dangers of misusing nitrous oxide, but also associated dangers of the high pressures inside a charged cream whipper. In June 2017, Rebecca Burger, a prominent French lifestyle blogger tragically died when a whipped cream exploded and hit her in the chest. Be sure to buy a quality cream whipper and cartridges, preferably made by iSi. be sure to read and abide by the manufacturer's instructions.
How to make culinary foams
The secret behind the creation of foams is a reaction between protein and nitrous oxide (N2O). This causes protein strings to unravel to form structures which hold the gas and harness the flavour of other ingredients in the foam. Thus to make a foam you need protein and this is usually in the form of egg white but protein substitutes such as gelatine or agar-agar can also be used.
You can make a foam just using water, egg white and nitrous oxide but it will have little taste and an eggy dog breath-live aroma. Instead of water use a flavoursome liquid such as cold tea, fruit juice, champagne or pretty much any other liquid provided it is not oily and the alcohol content is kept under 20% and ideally under 10% alc./vol.
A little sweetness in the form of sugar syrup, flavoured sugar syrup, honey or agave syrup will amplify the flavour of your foam.
Then add your protein such as egg white or a substitute such as gelatine, agar-agar, Hi-foamer or gellan gum. Protein substitutes have the advantage of not having the dog breath notes associated with egg-white so may be preferable when ingredients with very subtle flavours are being used.
Powdered egg white (albumin) is actually preferable to fresh egg white as it delivers concentrated protein into your solution without the large percentage of water in fresh egg white. However, I favour fresh or ideally pasteurised egg white, around one medium egg (30ml/1oz) white per 200ml liquid, as it reliably produces mousse-like foam without the hassle associated with the alternatives. Even when using egg white, some bartenders advocate the addition of gelatine (half a leaf per 500ml) due to it producing more mousse-like, longer lasting foams.
Both the ingredients and the charged siphon should be stored in a refrigerator as the colder the foam, the thicker it will be when discharged and the longer it will last on the surface of the drink. If stored in this manner then foams will last for around a week.
How to use a cream-whipper
1. Read the instructions supplied with your cream whipper.
2. Combine your ingredients. Strain the liquid to remove fruit juice pulp and other solids and clumps that could cause clogging upon discharge.
3. Pour your liquid into the siphon base.
4. Do not overfill the siphon. Refer to the whipper's instructions and only fill to half of capacity. Overfilling prohibits ability for gas to dissolve in liquid and expansion of liquid could lead to explosions.
5. Tightly screw the gasket-lined threaded head of the siphon onto the base.
6. Place a nitrous oxide (N2O) cartridge into the charger sleeve and then screw this onto the threaded receptor on the siphon's head. As you screw this charger sleeve into place so a hollow pin in the centre of the receptor will puncture the cartridge and you will hear the nitrous oxide flow into the siphon under pressure. One 8g (0.28oz) cartridge whips up to ½ litre / 1 US pint) of liquid.
7. Vigorously shake the cream-whipper six times to help the nitrous oxide dissolves into the liquid but beware that over shaking is detrimental.
8. Place cream-whipper in a refrigerator for at least 40 minutes prior to use to chill the contents and allow the protein to react with the gas. Do not put in freezer.
9. To dispense your foam, invert the cream-whipper over your drink and squeeze the discharge lever. This forces the liquid under pressure from the dissolved gas out of the whipper through the narrow nozzle. As it leaves the nozzle it expands forming a foam.
10. If the liquid fails to foam after one charge, briefly shake the canister and try again. If this fails (depending on the size of your whipper and the manufacturer's instructions) then try charging with a second nitrous oxide cartridge, shake again and leave for 5 minutes before retrying. If foam still fails to dispense then your recipe probably needs adjustment. DO NOT charge the whipper with nitrous oxide more than twice.
10. Store charged cream-whipper in refrigerator between uses. (When not in use store clean, empty whipper open rather than sealed.)
11. Never unscrew the whipper's head before all of the gas has been discharged. Invert the whipper over a sink and squeeze the discharge lever to empty contents. Alternatively, hold the whipper upright over sink with one hand cupped above but not directly over the discharge nozzle to catch any remaining liquid and squeeze the discharge lever with your other hand to allow all of the gas to escape before opening the whipper.