Can any custard be made into a frozen custard?

There are a variety of frozen desserts which are all related. The main difference between ice cream and frozen custard is the amount of eggs used to thicken the base mix.

Philadelphia style ice cream is made from a base mix of milk and/or cream, sugar, and flavorings.

French (or simply plain) ice cream is made from a base mix which is essentially a very thin custard or creme anglaise: milk and/or cream, sugar, and flavorings thickened with egg yolks or whole egg. The base mix is cooked to thicken, and then chilled prior to churning.

Frozen custard is very similar to French style ice cream, but with a greater ratio of eggs or egg yolks to dairy or liquid in the mix.

With this information, you can adapt nearly any custard recipe to be freezable as ice cream or frozen custard.

My survey of frozen custard recipes indicates a ratio of eggs to dairy of about 5 egg yolks to three cups (700 mL) dairy, or 6 whole eggs to 4 cups (1 L) of dairy.

To adapt any custard recipe for freezing, then:

1. Adjust the ratio of egg yolks to dairy to no more than about 5 yolks per 3 cups dairy.

   You may also want to adjust the total yield to be based on no more than 3 cups dairy depending on the capacity of your ice cream maker. Many home ice cream makers have a 1 quart (close to 1 L) capacity, but you need to leave room for the air which will be incorporated as the mix is churned.

   I recommend not using whole eggs for frozen custards, as the yolks facilitate a better texture, and create the rich eggy custard flavor.

2. Frozen desserts are served colder than custards normally are, so the flavors will be more muted. You may need to compensate by increasing the ratio of flavoring ingredients.

   This will be hard to judge until you have made the frozen custard/ice cream at least once.

3. Cook the custard until it thickens on the stove top, per the normal custard method.

4. Chill it rapidly for safety, and then hold it for at least 4 hours (overnight is even better). While I cannot explain the science, empirically, allowing the mix to mature lets the flavors meld and produces a better frozen dessert.

5. Churn it into a frozen dessert according to the instructions of your particular ice cream maker.

Note that you can even convert a custard recipe into a Philadelphia style ice cream by eliminating the eggs completely, and simply creating a dairy/sugar/flavoring mix. These still benefit from overnight maturation before churning.

Caring about the yolks is a special case of caring about the ratios needed for an ice cream. So here a more general answer to extend @SAJ14SAJ's first point.

If you are not using external emulsifiers, try to keep your ice cream in the 10%-15% fat for lean ice creams, and 15%-20% for rich, smooth ice creams (French ice creams usually fall in the second category). The total dry matter (including fat) should be in the 35% - 45% range. So, calculate those figures for your custard. If it is within the range, you are golden. If you are not, you may have texture problems after freezing (which can sometimes be mitigated by additives).

If you are not in the range with a given recipe, it is up to you what you will change - the amount of yolks, or the amount of dairy, or just substitute the dairy type (e.g. using a mixture of milk and cream which gives you the fat amount you need). You can doctor around with the sugar too, if you have to do a major change to the dry matter without changing fat. Remember that sugar is perceived as less sweet when frozen, so adding significant amounts of it to a custard designed to eat warm is unlikely to oversweeten it.

Depending on the type of ice cream you want, there is generally no problem to adding more yolks than @SAJ14SAJ recommends. But you will get not just the smoothness from the lecithine, you will get the coloring and the eggy taste too, which may overpower more delicate flavors. In the end, it is mostly a matter of taste whether you change the yolks or the cream amount when you need more (or less) fat.

I agree that you shouldn't include whites when freezing, they make the texture wrong.

The answer to your question depends where you live. In North America, frozen custard must have egg content, and is not called ice cream. In Europe, ice cream is basically frozen custard, whether it is egg stabilised or with another stabiliser such as cornflour. So, in Europe, any custard will do. Italian ice cream or gelato is basically frozen custard, so that is what you will end up with if you freeze custard. The north Italian tradition is to use an egg custard, while the Sicilians prefer a cornflour custard (as also found in some Scottish shops selling "Italian Ices"). Cream does not have to be added, which means the fat content may be less 5% and cannot be called "ice cream" under EU rules. It’s much healthier and lighter in texture if you make a thin custard using only milk, corn flour (UK Bird's Custard Powder incl flavouring) and sugar - without added cream, emulsifiers or other stabilisers. This means crystallisation may happen after 2 hours. Unfortunately, it's not the modern fashion, but I was brought up on it as a kid in Glasgow and loved it served in a wafer sandwich from a mobile ice cream van with rotating freezing vats.

A useful tip of mine is to add a little whiskey to the ice cream maker if it gets too stiff and the paddle starts to jam. The alcohol acts as an antifreeze. The best source of information I know of is the website of a Swedish guy called Anders http://www.icecreamnation.org/ice-cream-nation/ Check out Sicilian Gelato.