Liquid Nitrogen
Liquid nitrogen boils at −196 °C at atmospheric pressure. Poured over a preparation, it freezes it in a matter of seconds.
Impromptu ice cream is made by pouring liquid nitrogen directly onto a mixture of cream, sugar and flavourings: it freezes instantly, in front of the diners. The ultrafine structure melts before the cold reaches the palate, allowing the full aromatic spectrum to be perceived from start to finish. The same technique makes it possible to produce alcoholic ice creams, which are impossible with conventional freezing due to the low melting point of alcohol.
Liquid nitrogen is also used for cryogrinding: the food is frozen in nitrogen and then ground at low temperature, yielding ultrafine powders of products that cannot be reduced to powder at room temperature — fats, chocolate, aged cheeses.
How it works
The rate of cooling determines the size of ice crystals that form in the preparation. In slow freezing (domestic freezer) crystals grow and rupture cell membranes, altering texture and structure. With liquid nitrogen the cooling is so rapid that crystals remain submicrometre-sized and uniformly distributed, producing creamy, stable textures impossible to achieve by other methods.
Technique described in Il gelato estemporaneo e altre invenzioni gastronomiche (Davide Cassi, Ettore Bocchia — Sperling & Kupfer, 2005) and in La ciencia en los fogones (Ediciones Trea, 2005).