Yolks

I remember when I was only 6 years old and used to prepare the sunday dessert for my family. I cooked standing on a chair because I couldn’t reach the stovetop. We used to go to my grandma’s apartment at the Colonia Roma, a very classic and old part of Mexico City. Grandpa loved my dessert: natillas. 

Natilllas is a dessert from the custard family. It involves milk, yolks, sugar and vanilla.  It is cooked slowly and you may add cornstarch to thicken it faster. It is one of those things that came from Spain many centuries ago and was cooked by the noons in the convents and is still popular. Back then it was flavored with cinnamon instead of vanilla.

As a child, I cooked the yolks and sucrose with a wooden spoon in a big orange pot. Something I don’t do anymore because it can easily turn into a sugar-omelette. But that was the way my mom thought me to do it. Next was adding the milk and bringing it to a boil, then the vanilla extract and, here comes the trick, a package of vanilla atole: a mix of cornstarch, sugar and vanilla flavor to prepare a very popular drink. This helped to add flavor and thicken the mixture. Then stirring, slowly, until the right consistency was achieved: very similar to a vanilla sauce.

My mother helped me to pour my custard to a big blue crystal bowl that had matching little plates, then chill it until it was time to get to grandma’s home. The most difficult part of the preparation was separating the yolks. My mom was very clear that you had to get rid of the chalaze (the yolk cord) and any whites.

Years later, working in a restaurant kitchen I learned another recipe that involved 2 hours of stirring: no starch included. The result was an incredible creamy, very smooth, velvety dessert full of flavor. That one deserves a post of its own.

About the yolks

A large egg weights about 2 ounces, or 60 grams. The shell accounts for 10 to 12% of this weight, the white for about 60%, and the yolk for abut 30%. According to Harold Mc Gee from his book “On Food and Cooking”, The yolk “is about 50% water, 34% lipids (fats and related substances) and 16% protein, with traces of glucose and minerals. Of the lipids, about two thirds are ordinary animal fats; one quarter, phosphorus and fatty acid complexes, including the emulsifier lecithin; and about one twentieth, cholesterol. The yolk contributes most of the cholesterol, about three quarters of the calories, and most of the vitamin A, thiamine, and iron of the whole egg. Its yellow coloration is caused mainly by pigments that are relatives of  carotene, a precursor of vitamin A.“

Why do pastry chefs love eggs? Because they thicken liquids into solids, they “foam” when whipped, and are emulsifiers (stabilize fats and water). You can cream, whip, fold, beat, or blend them with other ingredients to achieve different preparations such as pound cakes, ice creams, souffles, desserts, sauces, etc.

When we cook or bake eggs they “coagulate”. What is to coagulate? Coagulation is when the protein molecules bond together. Proteins are shaped as coils. When they are exposed to heat, salt, or acid, they denature: the coils unwind. When proteins unwind, they bond together.

Whites begin to coagulate at 60˚C (140˚F) and are fully cooked at 65˚C (145˚F). Yolks begin to coagulate at 65˚C (149˚F)and are fully cooked at 70˚C (158˚F). It is a good idea to cook any egg preparation up to 82 to 85˚C (179.6˚-185˚F) to prevent bacteria. Cooking the yolks (eggs) any further, only evaporates water, leaving a less tender product, and developing an eggy flavor. Even further heating, the proteins in the eggs fully coagulate and they curdle. The yolk cords, or chalaze, remain liquid long after the rest of the egg components have set. That is why it is important to get rid of it  either before cooking the yolks or by straining the preparation once it is cooked.

Lecithin is the emulsifier found in egg yolks. It helps to absorb the fat content in the other ingredients. Like any emulsifier, it binds the fat molecules to the other molecules, making them small, and evenly distributed, and the result is a product with a soft mouthfeel. Since lecithin is not in the whites, it is not a good idea to substitute whole eggs or egg yolks in a recipe for only whites. When baking, the effect of lecithin results in a softer crumb, because when the yolks are whipped, they incorporate air, dissolve sugar, and disperse the fat molecules. 

When you combine eggs, milk or cream, and sugar you can produce a creme brulee or a creme caramel if baked, or a vanilla sauce, creme anglais, or natillas if cooked over gentle heat. Starches or gelatin can be added to achieve different textures. All these are custard preparations. 

Sugar or acids can start the coagulation process or “burn” the yolks. When combining them, be careful to do it at the last minute, just before cooking, and constantly stirring, to prevent small granules from forming.

Recipe:Natillas

Milk                   700gr/23.33oz

Vanilla bean   1

Yolks                80gr/2.820z

Sugar               110gr/3.88oz

Cornstarch      15gr/0.529oz

Butter               15gr/0.529oz

*If you cannot find a vanilla bean, 1 teaspoon vanilla extract is a suitable substitution.

• Split the vanilla bean in half using a paring knife. Scratch the seeds into the milk.
  
• Bring the milk, the vanilla bean seeds and the rest of the bean to a boil. If using vanilla extract, add it at the end.
  
• Combine the yolks, sugar, together. Whisk them until a light pale yellow is achieved. Whisk in the cornstarch.
  
• Add the hot milk to the yolk mixture, little by little, whisking constantly (temper). Return to the stove.
  
• Cook this slowly, stirring with a wooden spoon continuously, over low heat. When stirring, run the spoon along the sides and bottom of the saucepan to prevent the yolks from curdling. Cook until the custard covers the back of a spoon.
  
• Strain, add the butter (vanilla extract, if used) and chill, covered with a plastic wrap in contact.
  
• Serve with powdered cinnamon.