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Homogenized milk has already had its fat molecules broken down before it is sent to the store. If your milk is straight from the cow, you know it hasn't been homogenized yet. Check the label on your store-bought milk and make sure it says "non-homogenized" on it. You can buy non-homogenized milk at many natural grocery stores or farmers markets. Choose a container that you can make airtight. Mason jars, tupperware containers, or even cups with plastic wrap on top will work. Pour as much milk as you want to skim into your container. You can purchase mason jars in bulk for a low price at most grocery stores. Use a clear container to make it easier to see the separation line between the milk and the fat. Keep your milk in its container inside of your fridge. The fat will naturally rise to the top as your milk sits undisturbed. Be sure not to shake the milk or move it around as it is sitting. Cold milk takes longer to separate, but leaving milk out at room temperature could cause it to spoil. Once your milk has separated, you will be able to see a layer of cream, or fat, floating at the top of your container. The cream will be a slightly lighter color, and might have small bubbles in it. Once you identify the cream line, you'll know how much cream to skim off the top. Carefully scrape the layer of cream off the top of the milk with a spoon. You can save the cream to use in a recipe or rinse it down the drain. Be careful not to mix the cream back into the milk. You can keep your skim milk in the container that you were using, or transfer it to a separate one. Be sure to store your skim milk at a cold temperature in the fridge. Try putting skim milk into recipes as a healthier alternative to whole milk.
Make sure your milk is not homogenized. Pour the milk into a clear, airtight container, like a jar with a lid. Let the milk sit in the fridge for 24 hours. Look for the "cream line" in your milk. Open the container and scoop the cream off with a spoon. Store your skim milk in the fridge and use it within 7 days.