Ice cream is one of the most delicious things known to man. But how many of us actually know the secrets behind how it works? How is air able to be held within ice cream? Is cream necessary in ice cream? What really is ice cream? You have questions. I have the answers.
Ice Cream Nerd
I am what you call an ice cream nerd. The science behind making ice cream is really fascinating to me, so I’ve spent a lot of time learning as much as I can about it. At the risk of getting to technical, I want to start with the nerdy stuff then transition into what that means for ice cream makers.
Ice cream is a mixture of 50% air, 30% ice, 15% flavor matrix, and 5% fat (typically). This means that there is serious chemistry and physics at work. It all starts with the proteins and fats in the milk and cream. As ice cream churns, air is constantly being incorporated into the custard base, but the things that hold the air are the proteins and fats. It’s very similar to how meringues (egg white proteins) and whipped cream (fats in cream) operates. That said there are many key differences in between these three foods, but I won’t get too into that.
In addition to this, every molecule that you add to the ice cream has a distinct effect on how the ice cream will freeze. While researching the physics and chemistry behind ice cream, I learned that ice cream is at its best when it has a freezing point depression of about 2.5F (-1.4C). This is pretty arbitrary, but I find that it works well. When solutes (sugar, salt, alcohols, etc.) are added to the solvent (milk, cream, water) the freezing point of the solvent is lowered. This is caused by the effects of the solutes at the molecular level. The solutes get in-between ice crystals and stop them from congregating, effectively reducing the temperature at which freezing occurs.
Back to Reality
So what does all this mean? For starters it means that for optimal texture, you need plenty of milk proteins to stabilize the ice cream. I recommend that at least half of the custard base be milk based and the other half be cream based. Also if you are adding something high in fat, like browned butter, to the custard then you need to cut the cream a little and replace it with milk.
In terms of freezing point depression, the main solutes (ingredients) that lower the freezing point are sugar, salt, and alcohol. Sugar has the least dramatic effect on freezing depression. However, sugar based ice creams also melt much more slowly. If the freezing point depression is too low, then the ice cream will be very resistant to freezing. You’ll have small puddles of unfrozen liquid in your ice cream. Salt is also a force to be reckoned with in terms of freezing point depression. In fact it is the most dramatic of all of the solutes because of its ionic properties. Salt breaks down into sodium and chlorine meaning it has at least twice the power of sugar (it actually has more than 10 times the ability). The trade-off is that salt based ice creams melt the fastest.
I know that you don’t want to use the Van’t Hoff Equation every time you make ice cream, so I’ll give you some of my ratios. My go to amount of sugar for a batch of ice cream is ¾ cup with 1/8 tsp of salt. If I’m adding alcohol, I add ½ cup of sugar with 1 ½ tbs of 40% alcohol by volume. The alcohol should be added after the cooking step because alcohol has a low boiling point (173˚F). There are ways to add ridiculously large amounts of alcohol to ice cream, but it requires gelatin.
The Coolest Flavors?
One of the reasons that ice cream is so cool is the flavor options. You can literally make any flavor of ice cream you like as long as you remember these rules:
- If you are changing the custard base, you must be aware of the solute concentrations. That is, if you add more sugar to the ice cream you’ll have to eliminate the salt. Or if you add alcohol to the base, then you have to reduce the amount of sugar.
- If you add extracts or alcohol to the custard base, you must add it after the custard is finished cooking, or else the alcohol or flavor will evaporate.
- Cocoa powder or chocolate should be added to the base before tempering with eggs.
- And finally, if you want to add chunks of cookies or chocolate to the ice cream as a topping, add it after the ice cream has churned.
Now you have all the tools that you need in your tool box to make some delicious ice cream. Go crazy!