Eggs (the yolks specifically) are a vital part of ‘French Style’ ice cream as they make it rich, creamy and smooth. Home ice cream making is relatively easy overall but making the egg custard is the hardest part and one where you’ll need to pay attention and know what you’re doing in order to get it right. I’ve stuffed up the egg custard more than a few times in my 100+ batches but I could’ve avoided many of those by doing proper research into the process (which I've now done). Read on to learn all you need to know about how to create the perfect egg custard.
The Role of Eggs in Ice Cream
Some ice cream recipes call for whole eggs or even extra egg whites but when we’re talking about an egg custard we’re primarily talking about the yolks. Egg yolks consist of mainly fat, protein and water as well as a chemical called lecithin. When heated, lecithin acts as both an emulsifier and a stabilizer and makes several important contributions to creating a rich, smooth and great tasting ‘french style’ ice cream.
As you know from my ice cream science article, stabilizers promote smaller ice crystal growth which in turn makes the end product smoother. Stabilization also means the ice cream will melt slower and have a longer shelf life. Emulsifiers help to disperse the fat molecules (from the cream, milk and eggs) within the water molecules which is required to make ice cream. It’s worth noting that eggs are not the best stabilizers or emulsifiers and more effective alternatives are often added in commercial operations but they're handy to serve both roles in homemade recipes.
Science of Egg Custard
The finished egg custard will be noticeably thicker than the mixture of egg yolks you started with. This occurs because, as the mixture heats, the proteins in the yolks start to bind together similar to the process for creating a hard boiled egg where the yolk ends up completely solid. However, in the case of your egg custard, the milk and sugar molecules that are mixed in with the egg yolks get in the way of the proteins and prevent them from fully fusing together. In addition, the minerals dissolved into the mixture cluster around the protein molecules and encourage more bonds to be formed between them while the sugar and milk continue to obscure the process. The proteins will start binding at 65C (150F) and the ideal texture will be reached at 82 - 85C (180 - 185F) at which point a very fine mesh of egg proteins forms, resilient yet yielding to the fork. If you end up with an eggy smell it probably means you’ve gone too far and the smell is resulting from the cysteine and methionine components of the protein breaking down to release sulphur.
Food Safety & Pasteurization
Another concern when using eggs is food safety from bacteria, particularly salmonella. The process of heating the eggs to kill the salmonella bacteria is called ‘pasteurization’ and can be done at the same time as creating your egg custard. Heating to 71C (160F) is enough to kill the salmonella bacteria. Be careful not to taste your egg custard before it’s fully pasteurized or cross contaminate your cooked custard by using the same utensils that were used in the uncooked eggs.
How to Do It
Your ice cream recipe will usually describe the process of making your egg custard and it will look something like this;
- Separate your egg yolks in a bowl and whisk to combine evenly
- Heat the milk, sugar and half the cream in a saucepan until just boiling then remove from heat
- Temper your eggs with the hot mixture by adding just a small amount to start (½ a cup) and stirring. Gradually add more hot mixture, stirring the eggs constantly until you’ve added around ⅓ of the hot mixture and raised the temperature of the eggs
- Add the partially warmed eggs and dairy back to the rest of the hot mixture and return to a low heat
- Heat on low, stirring constantly and making sure to scrape the bottom and sides
- Continue until the mixture thickens and coats the back of a spoon then immediately remove from the heat
- Pour through a fine mesh strainer into the remainder of the cream and mix to combine
It’s important that you follow the steps carefully as a wrong step can lead to overcooking the custard and ruining the flavor and texture. I’ll expand on that a little bit in the quick tips below but first I want to discuss the direction ‘coats the back of a spoon.’ If, like me, you’re not a fully fledged chef but rather just an ice cream enthusiast you might be excused for not realising that this is not a mere description of what to do but a reference to a specific cooking process and test. I remember thinking that the mixture already ‘coats the back of a spoon’ before I’d even started heating it so how could that be the test for knowing that it’s finished?!
This video does a good job of explaining the precise meaning of this phrase and how to use it as the test for knowing your custard is ready.
- Use a low heat and go slowly - you’ll have several things to do at once from watching the temperature and thickness to stirring so it makes things a lot easier if you keep the heat low and go slowly
- Use a digital thermometer - an analogue thermometer won’t react quickly enough so digital is a must. You’ll want it to make sure you’ve passed the pasteurization level (71C / 160F) and let you know when you’re getting close to the ideal custard temperature (82 - 85C / 180 - 185F)
- Don’t skip or rush the egg tempering process - the heated dairy can be hot enough to cook the eggs if you combine the mixtures too quickly
- Scraping the bottom and sides of your saucepan is important as egg yolk can be hiding there and secretly cooking without your knowledge
- The fine mesh strainer removes any bits of egg that have overcooked and clumped together which can negatively affect the flavor and texture
- Cool the mixture rapidly after the custard is ready to prevent the residual heat from continuing the cooking process. Pouring it through the strainer into the cold reserved cream will help to do this
See, that wasn’t that hard was it? Keep these tips and guidelines in mind when creating your next batch of ice cream and graduate to the amazing creaminess that egg custard batches can deliver.