Fermentation - Yogurt


Yogurt is a natural wonder.  There's a lot of hype these days about the billions of bacteria in it that'll make your belly dance, but "probiotic" isn't new.  Yogurt has always been probiotic: bacteria in the teeming billions are what make yogurt out of mere milk.  Long before refrigeration and pasteurization, lacto-fermentation was commonly used to make food last longer.  Fermenting bacteria produce lactic acid, which gives yogurt its sour flavor and prevents spoilage.  Here, we'll introduce you to the process for making your own yogurt at home.  Not only will you save money and use less packaging, it's super easy and fun to customize your flavorings!

The Basics:  Making yogurt is as simple as mixing a live yogurt culture into milk and incubating it overnight.  It gets thicker and more sour the longer it is allowed to ferment.

Quick Reference
 Batch Size Milk  Yogurt Culture

1 qt.

3 3/4 cups 1/4 cup

1/2 gal. (2 qts.)

7 1/2 cups 1/2 cup
1 gal. (4 qts.) 15 cups 1 cup

Heat milk to 185 F.  Incubate at 110-115 F.

  • 2 pots (one that fits inside the other double-boiler style)
  • thermometer (preferably with a clip for attaching to side of pot)
  • small bowl
  • whisk
  • mason jar or food-grade plastic container to keep yogurt in
  • milk (whole milk for fullest flavor and thickest texture)
  • yogurt culture (reserve from previous batch or use plain store-bought yogurt with "live" or "active" cultures; or, use dried culture powder)
  1. Wash all utensils thoroughly.  
  2. Prepare ice bath in a bowl large enough to fit the pot inside.
  3. Using pots like a double-boiler (smaller pot placed in water in larger pot), heat milk over medium heat to 185 degrees F (85 C).  Stir occasionally to prevent scorching on bottom of pot.  
  4. Remove milk from heat and place pot in ice bath and stir gently to cool milk quickly and evenly.
  5. Remove from ice bath when milk cools to between 110 and 115 degrees F (43 to 46 deg. C).
  6. In small bowl, combine 1/2 cup warm milk and reserved yogurt (if using) and whisk until smooth.  If using culture powder, follow manufacturer's instructions for amount to use and whisk until fully incorporated.
  7. Pour milk-culture mixture back into pot of milk and stir to distribute culture evenly throughout milk. 
  8. Pour cultured milk into jar and incubate for 6 to 12 hours at 110 to 115 degrees F (43 to 46 deg. C).  Thermostat-controlled electric incubators are available, but you likely already own some combination of items that will suffice to keep a jar of milk warm over night.  We recommend placing the jar of milk inside a picnic cooler along with another jar filled with the hot water from the double-boiler.  People have also had success placing the jar of milk in an oven with the light bulb or pilot light on, or wrapping the jar in an electric blanket or heating pad.
  9. Enjoy your yogurt plain or stir in a heaping spoonful of jam (preferably homemade) for a tasty breakfast treat!  Remember to save enough plain yogurt to make your next batch.  
Frequently Asked Questions

How can I make my yogurt thicker?

One of the most common complaints about homemade yogurt is that it's not as thick as store-bought.  That's because it's pure and simple fermented milk.  Commercial yogurts often achieve their consistency with artificial thickeners.  At home, you can whisk milk powder into your milk to create a denser product with less moisture.  Or, remove excess moisture (whey) by straining the finished yogurt through muslin for a couple hours in the refrigerator.  This is how "Greek" style yogurt with its super-rich and creamy texture is created; at the commercial scale, though, whey disposal becomes a problem.  In the home, you can use up whey by baking with it, adding it to soups and sauces, or feeding it to pets.

Can I keep culturing new batches of yogurt indefinitely?

Theoretically, yes.  In practice, most likely not.  Most home yogurt makers find that the quality of their homemade yogurt deteriorates after 4-6 batches.  This is probably due to the fact that our kitchens are not sterile, precision-controlled factories; in fact, they are teeming with free-ranging bacteria and yeasts that inevitably compete with our cultivated microbes.  If you do not wish to purchase commercial yogurt every so often to culture your homemade batches, then it's not a bad idea to keep a couple sachets of dried yogurt culture on hand.  They're cheap and the yogurt they produce can, in turn, be used as a culture just as store-bought yogurt can.

If my milk is already pasteurized, why do I have to heat it?

Two reasons: 1) Although your milk was pasteurized when it left the factory, it may be exposed to potential contaminants any time you open it (especially if you drank out of the carton!).  Scalding the milk prior to making yogurt is just a way to ensure that the yogurt you make will be the very best, untainted by stray microbes.  2) The bacterial cultivars that make the kind of yogurt most of us are used to are "thermophilic," meaning they thrive in hot temperatures, 100 to 115 degrees F.  If you want to experiment with room-temperature yogurts, try the Scandinavian products piima, viili, and filmjolk.  

Additional Resources

http://nourishedkitchen.com/troubleshooting-homemade-yogurt-questions/.  The Nourished Kitchen offers excellent tips for what to do when your homemade yogurt doesn't turn out as you want.   

Katz, Sandor Ellix.  The Art of Fermentation: An In-Depth Exploration of Essential Concepts and Processes From Around the World.  2012.  This is the second title from Katz, a self-described fermentation fetishist.  It's loaded with information on all things fermentation, including a large section on dairy ferments.

http://nourishedkitchen.com/viili-piima-fil-mjolk/.  Information for making room-temperature dairy ferments at home.

Yogurt Supplies