First, let us understand what happens when you dispense whipped cream from the canister. When gas enters a fatty substance (like whipping cream), the fat traps much of the gas, causing the substance to fluff up and coalesce. A whipped-cream canister contains pressurized nitrous oxide. Pressing its button does two things more or less at the same instant: It forces gas into the fatty liquid and it forces the contents out of the canister.
There are several possible grounds, some stronger than others, upon which to base a prohibition of using such a dispenser on Shabbat. After a brief review of the issues involved, we will present what we believe is a practical answer.
Combining two substances so that they form a mixture that is different in texture from each of them is defined as lisha (kneading),[1] and this arguably applies to creating whipped cream. However, trapping a gas inside a liquid (even if in unnoticeably small pockets) so that it turns into foam is quite different from standard lisha. Thus, it would be difficult to forbid this action without indications from classical sources.
The Shemirat Shabbat K’Hilchata[2] prohibits the hand-whipping of cream for a few reasons. One reason he presents relates this process to the scrambling of eggs, which is forbidden because it is usually a step in cooking food.[3] While one can dispute whether this issue applies to whipping cream, the process is totally different when it is done by pressing the button on a canister. Thus, this issue would certainly not be grounds to forbid our case.
Recent poskim discuss creating seltzer/soda on Shabbat, in the process of which a somewhat significant change occurs to a liquid by inserting a gas. One of the objections discussed is uvdin d’chol (weekday-like activities, such as producing things). While this principle might or might not apply to the process of inserting a gas canister and preparing the machine to make soda (or a similar device for making whipped cream), it is difficult to apply it to using a ready-to-use canister, which the average user views simply as a dispenser.
The most serious issue is molid, creating a new entity by changing the physical phase of an object. A baraita[4] forbids crushing ice and snow, and Rashi explains that this is like a melacha in that one creates something new (i.e., a liquid from a solid). Some[5] say that it is similarly prohibited to turn a liquid into a solid (e.g., freeze water to make ice cubes).[6]
The Orchot Shabbat,[7] who permits our case, argues that even those who forbid making ice cubes could permit spraying whipped cream from a canister because the only purpose of the cream in the canister is to be turned into whipped cream. Thus, there is nothing “new” going on. However, one could distinguish in the other direction. Perhaps our case is worse than making ice cubes, as here one actively and directly, with the press of the button, creates the new foamy state, as opposed to putting water in a freezer, which only provides a cold setting for the slow process of freezing to begin.[8] Rav Mordechai Willig told me another reason for leniency: The change from a thick liquid to foam is insufficient to be considered molid.
While one can make the argument that it is forbidden to spray whipped cream from its canister, common practice is to permit it. On the basis of the above halachic discussion, we do not feel that it is necessary to change that practice. (We have not dealt with the problem of using the cream to write words or make likenesses of specific objects.)