Lemon juice scores a bitter victory over orange juice at the task of thickening condensed milk. Lemon juice's bitterness comes from its high amount of citric acid, an acid comprising nearly eight percent of a lemon's dry weight. Citric acid is the dairy-thickening agent in citrus fruits; it reacts with milk's proteins, swiftly turning them to curd. Even in small doses, lemon juice's high citric acid content makes it an especially potent dairy-thickener.
How Lemon Juice Thickens Condensed Milk
Milk protein particles, called casein micelles, are negatively charged and sheathed in a water soluble anti-clumping substance. This allows the casein micelles to float through the milk without sticking together. When you add acid to the milk and change the milk's pH from alkaline to acidic, the negatively charged casein micelles become neutral, lose their anti-clumping agents and begin coagulating at once. They form three-dimensional structures, trapping fat globules within them and forcing out a certain amount of water.
Making Lemon Curd
Condensed milk contains little water, and its fat and casein are already somewhat concentrated. When the lemon juice reacts with the milk proteins, the condensed milk's low water content and coagulated casein translate into a gelatinous texture. This is why key lime pie recipes with condensed milk typically do not require any cooking in order to "set up" the curd. Refrigerating the already dense, low-moisture curd is enough.
Lemon Juice vs. Orange Juice
Freshly squeezed lemon juice contains an average of 48 grams per liter of citric acid. Freshly squeezed orange juice, on the other hand, contains an average of 9 grams per liter. These concentrations of citric acid matter when you're using citrus juice to thicken condensed milk. You need just enough juice to impart flavor and to thicken the milk, but not so much juice that its flavor predominates.
Fresh Citrus Juice vs. Citrus Juice From Concentrate
If you don't have fresh lemon juice, you can substitute with lemon juice from concentrate in a one-to-one ratio. Lemon juice from concentrate has an average of 34 to 39 grams of citric acid per liter, enough to thicken condensed milk. You can also use a mixture of equal parts lemon juice and orange juice to thicken condensed milk as for an orange-flavored curd.
- National Center for Biotechnology Information: Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice and Commerically Available Fruit Juice Products
- TheKitchn: Food Science: Why Lemon Makes Milk Curdle
- Tallin University of Technology: Rennet Coagulation of Milk
- Real Simple: Easy Key Lime Pie
- Sarasota Magazine: Robin Draper's Authetic Florida's Sour Orange Pie
- Photo Credit ITStock Free/Polka Dot/Getty Images