This instructable produces fresh yogurt for your eating (and educational) satisfaction from everyday cow's milk and an active culture of L. Acidophilus.
Step 1: Collect Your Gear
What you'll need to make about three cups of yogurt:
2 Tbsp dry milk
3 cups milk (2% or skim)
2-3 Tbsp unpasturized plain yogurt
Vanilla yogurt can be used instead of plain yogurt too. The added yogurt must contain a live culture; pasturized yogurt has been heated to kill both pathogenic and beneficial bacteria. If fruit yogurts are used as the source of this culture, the bacteria usually aren't as healthy.
The smallest portion of yogurt I can find comes in 6 oz. and milk comes in half gallons. I personally like this yogurt, though the missus won't eat it--god bless her--so I'm scaling up the quantities accordingly:
1/3 cup dry milk
1/2 gallon 2% milk
6 oz. unpasturized plain yogurt
Those are more appealing quantities anyway.
Step 2: Scald and Cool Milk
The first step is to combine dry milk and milk in the medium saucepan.
Now scald the milk on the stovetop; scalding is as near to a boil as possible without actually boiling. Scalding kills any undesirable bacteria to allow greener pastures for the L. Acidophilus culture we'll add later.
Allow milk in saucepan to cool. The temperature can still be hot, but definately not scalding. If you need an exact temperature, 100F will work. Divide the milk into your clean containers. I'm using five pint jars fresh out of a pressure cooker to ensure nothing will be growing but the L. Adcidophilus culture. Don't sneeze in your milk either.
Step 3: Add Culture
Now add the unpasturized plain yogurt to each container. Err on the side of caution and allow the milk to cool. If the temperature is too high, the added culture will die and your yogurt will never be.
Step 4: Incubate
The last step is the incubation. Fill your cooler with a couple inches of hot but not scalding water. Insert the milk-containing containers. Close the cooler and let sit from anywhere between three and six hours--depending on the activity of the culture. If your cooler allows too much heat transfer, you may add more hot water at any time. The ideal temperature range for the L. Acidophilus is between 40C and 45C (104F - 113F).
I filled my cooler with the hottest possible tap water and the internal cooler temperature leveled out at about 105F for the next hour.
Step 5: Scrutinize Then Enjoy!
When the yogurt is largely solid (some liquid whey may remain at the top--this is okay), open up a jar and take a whiff: if it smells sweet, you've succeeded; if it smells like, well, spoilt milk, you've failed and should dispose of the containers contents. While there's no serious problem from eating spoilt milk, most people find it disagreeable. You'll be able to tell the difference, but for those wanting to make sure, the final product should have a pH of around 4 while the pH of plain spoilt milk will be higher--around 5--milk itself has a pH around 6.5. Refrigerate the success. Enjoy your cold yogurt with granola or fruit.