Difference Between Sour Milk and Spoiled Milk

How spoilage bacteria work in pasteurized vs. unpasteurized milk.

Difference Between Sour Milk and Spoiled Milk

Reading Time: 4 minutes

As a cheesemaking instructor, I’m often asked, “What’s the difference between sour milk and spoiled milk?” This is an excellent question, and the answer is the foundation to making most cheeses.   

The initial pH level of fresh milk is around 6.5, which is slightly acidic. This varies by species, with goat milk being a little less acidic than cow milk. When we make cheese, we add some ingredients and do some things to make the milk more acidic, thus making it sourer (think lemons — very acidic, very sour). This souring process is also known as fermentation. And sour milk eventually curdles and separates, leaving us with curds and whey! 

When this souring process is done on purpose, it’s delicious. But when souring happens accidentally — as is the case with spoiled milk — not so much. Like soured milk, spoiled milk also eventually separates, but in this case, this has happened because of spoilage bacteria instead of good bacteria. The result will smell bad, taste bad, and will possibly give you a sour stomach if you consume too much of it. 

So, how do you intentionally sour milk for cheesemaking? Most of the time, we start by adding culture (good bacteria) to slightly warmed milk. This culture could be in the form of freeze-dried cheesemaking culture, kefir culture, clabbered raw milk, or even cultured buttermilk or yogurt. These bacteria begin to feast on the lactose in the milk and convert the lactose to lactic acid. The more lactic acid that is produced, the lower the pH level and the more acidic, or sour, the milk will taste. 

You can also clabber milk with vinegar, lemon juice, or citric acid at a high temperature, and you will get a very quick, easy cheese like whole milk ricotta, farmer’s cheese, or even paneer. These cheeses are known as direct acidification cheeses because you have added an acid directly to the milk. Most cheeses are made with indirect acidification, where you add bacteria to the milk, and it indirectly creates acid through the conversion of lactose to lactic acid. 

This souring process can be done with pasteurized milk or raw (unpasteurized) milk, but the amount of culture or bacteria needed might differ depending on which you use.  Because pasteurization (heating milk to at least 145 degrees F for 30 minutes or 161 degrees F for 30 seconds) kills bacteria, the milk is more or less a blank slate. That means we have to add bacteria back in to acidify the milk and make cheese. Raw milk has not been heat-treated, so the bacteria have not been killed. You may only need half as much culture, if any, to get to the same pH level because the bacteria are already present.  

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Clabbered milk.

Most of us think of pasteurization as a process to make milk safer, and in some instances, this might be accurate. But in large part, pasteurization is done to make milk last longer. And the higher the temperature, the longer the milk will last. How long does raw milk last at room temperature? Not long! Within 12-24 hours, raw milk will clabber, or begin to thicken and separate, due to the quick buildup of lactic acid caused by all those happy bacteria in there chowing down on the lactose. If you know what you’re doing, this clabbered milk can be used as a starter culture for making cheese. But left for more time, the milk will quickly go from soured or clabbered milk to spoiled milk. 

On the other hand, pasteurized milk has far fewer bacteria, so the process of lactic acid buildup is slower. How long does pasteurized milk last? Well, that depends on the temperature at which the milk was pasteurized. Regular pasteurized milk will generally last about two weeks in the refrigerator. Of course, left at room temperature, pasteurized milk will eventually turn too acidic and spoil. Spoilage bacteria cause this curdling, though not good (culture) bacteria. It will not taste good or make your stomach very happy.  

Ultra-pasteurization is a whole other ballgame. This process involves heating milk to very high temperatures (over 275 degrees F), killing ALL the bacteria — the good, the bad, and the ugly. The higher the temperature, the longer the milk will last (sometimes even without refrigeration). Ultra-pasteurized milk may last a month or more. It sounds nice, but the flavor is altered in this process, and the milk is so damaged from this high heat that it is not good for cheesemaking, even if you add lots of culture. 

So, what can you do with sour milk? You may not want to drink sour or spoiled milk, either because it has an unpleasant smell and taste or because it’s far enough along in the spoilage process that it could make you sick. But unless it is moldy, slimy, or very chunky, you can use it for a host of other things from baked goods to salad dressings to tenderizing meat! Baking is probably my favorite thing to do with slightly off milk. You can substitute soured or slightly spoiled milk in any baked good recipe in place of regular milk, buttermilk, yogurt, or sour cream. The baking process will kill any unwanted bacteria that are present in your milk, and the sour taste will give a little boost of flavor as sour cream or yogurt does, so it’s a safe and tasty way to use up that old milk. If you’re looking for an excellent sour milk recipe for bread or muffins, try the one below: 

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Ingredients for soured milk zucchini bread.

Sour Milk Quick Bread or Muffin Recipe 

  • ½ cup sugar 
  • 1/3 cup vegetable oil                          
  • 2 eggs                                                  
  • ¼ cup sour milk 
  • 1 cup fruit or vegetable pulp (mashed banana, grated carrots, grated zucchini) 
  • 2 cups flour 
  • 1 tsp baking powder 
  • ½ tsp baking soda 
  • ½ tsp salt 
  • 1 tsp cinnamon 


Mix the first five ingredients together in one bowl and sift the rest of the dry ingredients together in another bowl. Slowly add the second bowl ingredients to the first bowl until just blended. Pour into greased 9 x 5 loaf pan and bake at 350 degrees F for 50-60 minutes (40 minutes for a mini loaf). You can also fill paper-lined muffin cups with this mixture and bake for 18-20 minutes or until lightly browned on top (12-15 minutes for mini-muffins). 

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Originally published in the November/December 2021 issue of Goat Journal and regularly vetted for accuracy.

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