While the acid you will use in this method may curdle the milk on its own when used in large enough quantities, heat dramatically accelerates the process, making the milk curdle faster and more noticeably. [1] X Research source This is especially important if you plan to separate the solid curds from the liquid whey, as you would do when making cheese. Note that you can also curdle milk using nothing but heat, as outlined in another method of this article. Doing so will produce smaller curds, though, so this method is recommended if you need larger curds.
Milk contains a protein called casein. Casein groupings are usually spread throughout milk evenly, but when milk becomes acidic, the negative charge keeping the casein groupings separate is neutralized. As a result, the casein proteins clump together, causing the milk to become grainy and curdle. [2] X Research source Lemon juice is generally the preferred acid of choice, followed by vinegar. Both are more acidic than orange juice or other common kitchen acids. The more acid you add, the larger your curds will be and the faster they will form. For smaller “grains” of curds, use a smaller amount of acid.
If the milk has not curdled sufficiently enough for your recipe, you can let it stand longer or return it to the stove and heat it for a longer period of time.
Depending on how watery the curdled milk is, you might need to let it drain for several hours to a day before all the whey separates from the curds. If you do not need to strain the curdled milk, you can use it as is.
Note that high-fat dairy products, like cream, can be boiled with little to no problem. As a result, you may find that low-fat milk takes very little time to boil and curdle while whole milk will take longer. Milk will not begin to curdle until it reaches a temperature of 180 degrees Fahrenheit (82 degrees Celsius). [4] X Research source To maximize and speed up the curdling effect, let the temperature climb even higher. You can monitor the temperature using a clip-on instant food thermometer. Stir the milk occasionally but not frequently. Stirring will spread the heat throughout the liquid, but it will also cause the milk to take longer to reach a boil. Leave the saucepan uncovered.
If you would like the milk to curdle more, you can let it stand for a longer period of time or return it to the heat and continue boiling it until larger curds form.
Understand that using heat only without using acid will produce softer, less-formed curds. This method is best used if you need grainy or soured milk rather than actual curds. If you do not need to strain the curdled milk, you can skip this step and use it as is.
Soy milk curdles more easily than standard whole dairy milk, but if you add an acid to the soy milk without heating it first, the resulting curds will be smaller and less firm. Moreover, they will also take longer to form. If you only need the soy milk to sour or become grainy and do not need actual curds, you can skip the heating portion of this method, though.
Lemon juice is the recommended acid of choice for curdling soy milk. On average, you will need about 1 Tbsp (15 ml) of lemon juice for every 1 cup (250 ml) of soy milk. [5] X Research source Note that adding more acid will cause more prominent curds to form while adding less acid will cause smaller curds or grains to form.
If you heated the soy milk before adding the vinegar, you should notice curds beginning to form. If the curds have not reached the desired size or consistency yet, you can let the milk sit longer or return it to the heat for a few minutes.
Note that you may need to let the whey drain for several hours to a day, depending on how runny your curdled milk is when done. If you do not need to separate the curds from the whey, you can use the curdled soy milk without straining it.
Cornstarch is generally preferred over flour. You will need to add about 1 tsp (5 ml) of cornstarch or flour for every 1/2 cup (125 ml) milk to ensure that the milk will not curdle in the presence of an acidic ingredient or high heat. [6] X Research source For best results, whisk cornstarch into the milk while the milk is still cold. Heat the milk up and add your other ingredients afterward.
Dairy milk and soy milk should not be cooked above a temperature of 180 degrees Fahrenheit (82 degrees Celsius) if you do not want the milk to curdle. [7] X Research source Monitor the temperature using a clip-on instant food thermometer. Attach the thermometer to the side of the pan. Make sure that the bulb touches the milk but not the bottom of the pan, since the temperature of the metal at the bottom of the pan will be hotter than the temperature of the milk.
In the case of coffee, it is also a good idea to let the coffee cool slightly before adding it to the soy milk. Doing so will make it even less likely that the coffee will curdle the milk. Note that, while coffee is acidic, it is less acidic than vinegar or lemon juice. As a result, cool to lukewarm coffee is not likely to cause dairy milk or soy milk to curdle. While dairy milk is less likely to curdle when poured into coffee, if you do have problems with dairy milk curdling in your coffee, you can use this same practice with dairy milk.
