Lemon Custard vs. Lemon Curd vs. Lemon Posset: A Trio of Citrus Science

First off, happy Father’s Day to all the dads out there!

Especially my dad, who taught me how to cook and spent hours helping me with science experiments. He’s the reason I fell in love with both cooking and science, and I’m so grateful for all the time, patience, and encouragement he gave me.

Love to all the dads!

Secondly, I’m still riding the lemon 🍋 wave, and if you're new here, go check out the other citrusy deep dives:

• Lemon Squares vs. Lemon Meringue

• Lemonade Science

• Lemon Sugar Science

I can't believe it's mid June! I'm still in denial. My summer baking list? Not even close to done. But the weather finally feels like summer where I live, and that means I'm ready to dive headfirst into all the bright, zingy desserts that scream sunshine.

Lemon custard. Lemon posset. Lemon curd.

While all of these fillings may look similar and have a similar tangy taste, scientifically, they're different. Eggs and heat stabilize one. Another by acid alone. Some require precise emulsions; others set effortlessly in the fridge.

Let's break down the Science behind these lemony icons—and figure out which one's right for your next summer dessert.

Lemon Curd, Pudding, Custard and Posset — What's the Difference?

🍋 Lemon Curd

Key ingredients: Lemon juice, sugar, egg yolks, butter

Thickening agent: Egg yolks (coagulation)

Lemon curd is what I call a stovetop emulsion-meets-coagulation miracle. It starts with a mixture of eggs, sugar, and lemon juice, and cook it gently (emphasis on gently!) over low heat. As the temperature climbs, the egg proteins denature and form a mesh-like structure that thickens the curd. Butter is typically added at the end to emulsify and smooth it out, resulting in a glossy, spreadable finish.

💡 Why low heat matters: If you rush it or overheat, the eggs can curdle (i.e., scramble). That's why many recipes use a double boiler for even heat control.

👩‍🔬 Science highlight: Coagulation starts around 160°F (71°C) for egg yolks. The acidity from the lemon lowers the coagulation threshold, which is why curds are at slightly lower temperatures than other custards.

🍮 Lemon Custard

Key ingredients: Milk or cream, eggs, sugar, lemon zest or juice

Thickening agent: Whole eggs or yolks (baked coagulation)

A baked lemon custard—like what you'd find in a lemon tart or a baked pudding—is thicker and silkier than curd, thanks to the gentle bake in the oven. It usually uses cream or milk, sugar, and eggs (sometimes whole, sometimes just yolks); with the lemon added, you'd use it sparingly—not too much, or it can curdle the dairy before it sets.

🧠 Why baking matters: The slow, even heating allows the proteins to unfold and form a delicate gel structure without scrambling.

🧪 Science tip: Adding lemon juice before baking lowers the pH of the mixture, helping it set more firmly. But too much acid + heat = grainy texture, so balance is key.

🍋 Lemon Pudding

Key ingredients: Milk, sugar, cornstarch, lemon juice or zest, sometimes egg yolks

Thickening agent: Cornstarch (gelatinization), sometimes with added egg yolk for richness

Unlike curd or custard, lemon pudding is thickened with starch—usually cornstarch—on the stovetop. This gives it a silky, jiggly consistency. Some versions add egg yolks for extra body and richness, but it's the gelatinization of starch molecules that's doing most of the thickening.

🌡️ Science moment: Starch begins to thicken around 170°F (77°C) as the granules swell with water and burst, releasing amylose that creates a gel. Lemon juice is added after thickening to prevent breaking the gel or interfering with the thickening process.

⚖️ Texture tip: Puddings are smooth and pourable when warm, but they set as they cool. They tend to be more forgiving than curds or custards—perfect if you're getting into stovetop desserts.

🍶 Lemon Posset

Key ingredients: Heavy cream, lemon juice, sugar

Thickening agent: Acid-induced protein denaturation (no eggs!)

Lemon posset is the simplest of them all—no eggs, no baking, no starches. It thickens purely through acid-induced coagulation. You heat cream and sugar, then stir in fresh lemon juice. The acid causes the milk proteins (mostly casein) to denature and form a soft gel as it chills.

✨ Think of it like panna cotta meets lemon pudding but without gelatin.

👩‍🔬 Science fact: The acid lowers the pH of the cream, which causes the casein micelles to destabilize and aggregate into a gel matrix. Translation? It firms up into a spoonable dessert—no eggs or baking required.

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Lemon Cake Recipe

This lemon cake comes from A Domestic Cookbook by Malinda Russell, the first known cookbook written by an African American woman, published in 1866. Russell, a free-born pastry shop owner from Tennessee, compiled her recipes to support herself and her son during difficult times.

Her lemon cake is light and airy—more delicate than a pound cake—thanks to beaten egg whites and gentle folding. Bright with lemon zest and juice, it’s a simple, elegant recipe that still shines today. Serve it with curd, custard, jam, or even a spoonful of posset.

Ingredients:

• 1 cup butter

• 3 cups powdered sugar

• 5 egg yolks, well beaten

• 1 teaspoon baking soda

• 1 cup milk

• Zest and juice of 1 lemon

• 5 egg whites, beaten to stiff peaks

• 4 cups all-purpose flour

Instructions:

1. Preheat your oven to 350°F (175°C). Grease and flour your cake pans.

2. In a large bowl, cream together the butter and powdered sugar until light and fluffy.

3. Add the beaten egg yolks and mix well.

4. Dissolve the baking soda in the milk and stir into the batter.

5. Add the lemon zest and juice, mixing to combine.

6. Gently fold in the beaten egg whites.

7. Gradually sift in the flour, folding it in until fully incorporated.

8. Pour the batter into the prepared pans and bake until a toothpick inserted in the center comes out clean (about 30–40 minutes, depending on the size of the pans).

9. Let cool in pan 15 min, remove from pan and let cool completely before serving. Serve with a side of lemon jam, curd, or even posset!

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Want to learn more? Have a questions? Reply and let me know—I always love talking about the science behind baking.

Stay Sweet,
Sarah @HalfBatchBaking