Lemons by themselves aren't something you'd pick up to eat as a snack, but they have an excellent ability to enhance flavors and start beneficial cooking and baking reactions. That's why lemons show up in everything from salad dressings to desserts. But heating lemon juice sometimes results in a brownish liquid that spoils food's appearance and causes unpleasant results. The tricks to avoiding unfortunate lemon reactions are technique, time and temperature.
Phenols and Enzymes React
Lemons contain organic compounds -- phenols and enzymes -- that react easily with oxygen. The heat of cooking and baking speeds up that reaction. Otherwise, clear lemon juice turns brown when it reacts, which can spoil a food's look. Heating lemon juice also can speed up its acidic reaction on other ingredients in a recipe, especially dairy products, causing them to curdle.
Add Lemon Juice Last
Lemon curd is a good example of how lemon juice reacts when it's exposed to heat. Traditionally, the ingredients for this dessert were combined together in the pan while heating. The result was that some of the eggs in the mixture overcooked before the curd thickened properly, leaving flecks that had to be strained out. One method for avoiding this reaction is to cream the recipe's butter, sugar and eggs together with a mixer or blender before mixing in the lemon juice thoroughly. Once all the lemon curd's ingredients are blended, it can be heated gently, stirring constantly, until the mixture thickens without turning brown or curdling the eggs.
Preserve the Leavening
Incorporating lemon juice last to reduce reactions can be used in baked goods such as cakes. Most recipes calling for lemon juice use it as the acid to react with a leavening agent, often baking soda, to create the carbon dioxide bubbles that make the baked good rise. Adding lemon juice last both cuts down on the juice's oxidation and helps preserve the leavening agent's reaction so the baked goods will rise better.
Cook Slowly, Moderately
Temperature and time also offer ways to avoid lemon reactions when cooking. Stovetop recipes using lemon juice typically should be cooked slowly, at moderate temperatures and stirred constantly to ensure the ingredients stay well combined. This technique keeps lemon juice less exposed to air that causes its compounds to react, and helps the entire dish to cook more evenly. It's essential for stovetop recipes to cook lemon juice in a non-reactive pan, such as stainless steel, enamel or anodized aluminum. Unlined copper or plain aluminum pans react with lemon juice's acid, resulting in discoloration and a metallic taste in the finished dish.
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