I make my own yogurt at home regularly. It is easy, and takes about an hours work spread out over a 16 hour time span.
This what you'll need:
• 6 large size yogurt containers with lids (Or glass mason jars if you really want to be healthy!)
• A large soup pot
• A kitchen thermometer- Candy thermometers are best since they handle the required temperature range.
• A freshly bought yogurt (high fat, with very little ingredients listed ex: milk and live cultures
• 3 liters of milk
• A kitchen sink less than half full of icy cold water
• A large shallow (3 inches deep), flat bottomed roasting pan. You will sit the 6 yogurt containers in this.
1. Sterilize your yogurt containers and lids. Boil water in a kettle. Use that water to pour into each yogurt container and onto each lid. Protect your hands! The containers get hot! Do not put these plastic containers IN boiling water this is too much heat. Just pour some boiling water in each container, say 1/4 full and swish(carefully!!) or rotate it around till you are sure each part of the interior surface has been touched once by this hot water. Discard the water, or use for the next container while hot. Clean all 6 containers this way. Note! I am using the least expensive possible way to do this. You could use glass mason jars and sterilize them like you would for jellies and such. For me Plastic yogurt containers are already available. Chose your preferred containers. I acknowledge, plastic may (especially with the boiling water) leach some toxins. Glass is healthier. For the plastic, I use a very light touch with the boiling water, and I am sterilizing for less than a minute each.
NOTE: Plastic gets very very hot and soft if you leave the hot water in too long. Do this quickly and with oven mits or waterproof/ heat retardant protective gloves.
Let the containers and lids dry upside-down, so no water is left by the time you need them.
2. Prepare your kitchen sink to be an 'ice bath'. Fill it half full with very cold water from the tap, and if you have ice already made, empty three or four ice trays worth of ice cubes in the sink as well.
3. Take your fresh store bought yogurt out of the fridge if it was in the fridge, and measure out 3 heaping tablespoons of yogurt, put this aside to reach room temperature.
4. Now things will start happening quickly! So you need to focus on temperatures from here on..
Pour all three litres of milk into your soup pot. Put the heat on medium. The milk needs to be heated uniformly to about 175F (76-82C) Use a kitchen thermometer for this to get it exactly right ( I use a candy thermometer). You have to stand around and stir every minute or so.
NOTE:You could have been multitasking during this time to sterilize and prepare ice bath..but #1 priority is stir the milk!!
Heating the milk starts out slow, but once you are near the temperatures needed you must stir constantly.
The heating to this exact temperature is to kill other unwanted bacteria at that temperature (Do not go higher!) and so that the milk undergoes a transformation getting it ready for culturing.
Remember keep stirring!! Scalded milk or too much heat near the bottom of the pot makes the yogurt taste bad and you will have non-uniform heating: Not all of the unwanted bacteria will die in the areas near the surface.
Also be sure not to confuse Farenheit with celcius. The right temperature is really the most important thing at this stage!
5. Once the milk hits 175 degrees Fahrenheit, turn the burner off and keep stirring to try and hold it at this temperature for 3-5 minutes. Stir very often now, and stay right by the pot. You only need to stir to prevent any of the milk from scorching at the bottom of your pot and keep the temperature uniform. If the temperature is still climbing and the milk is getting close to 185 F take it off the burner and right into the ice bath but keep stirring!
After 5 minutes at this temperature, take the pot to the kitchen sink and immerse the bottom of the pot in the ice bath. Remember I said only fill the sink half full? Well when you put the hot pot in the ice bath you need it to sit there without risk of any water overflowing in.
If there is too much water in the sink for this, remove some. You cannot just hold it there with both hands, you need to stir too.
Now once the hot pot of milk is sitting in the colder ice water, you have to keep stirring very regularly. The milk now has to uniformly cool to 115 F, no lower and no higher than this (give or take a few degrees)
Once it reaches 115F you take the pot out of the ice bath and just sit it on the counter. Use a whisk to stir in those 3 tablespoons of fresh yogurt you had on the counter, adding it to the pot of warm milk. Do not whisk vigorously, just get the yogurt spread into the rest of the pot. Live cultures do not like too much shock and shaking.
6. Pour the milk mixture into your dried yogurt containers.
7. Boil another kettle of water
8. Preheat your oven to its lowest possible setting then turn the oven off, leaving only the oven light on inside. You want the inside of the oven to be about 100 degrees F. So if your lowest setting is 170 F, then let it cool down for about 10 minutes before the next step.
9. While waiting for your oven to reach a reasonable temperature. Pour the boiling water in step 8 above around the bottoms of the yogurt containers to keep the heat around 100F. The water level should be about 1 inch deep around the yogurt containers. Leave the hot water there. Put your candy thermometer in the hot water (not in the yogurt) surrounding the containers in your roasting pan.
Now put the roasting pan with everything in it into the cooled oven.
NOTE: the oven here must be OFF, with the oven light ON, and the temperature should be warm about 100F.
Close the oven now, and do not move anything, or touch or check on it for at least 2 hours.
From here on, your new mission is to make sure the candy thermometer keeps reading about 100F (no lower than 80F). The idea is that if the water around the yogurt is at 100, so should the yogurt containers.
Check on it in two hours and gently put the plastic yogurt lids on now without moving or taking the containers out. **If you were using glass the lids could have been put on 2 hours ago. If the oven temperature really was 100F you could have put the lids on right away, but I have melted a few lids before, and couldn't use them anymore so I want you to be careful!!
If the temperature of the surrounding water is below 80-90F, boil more water and pour it gently around the bottoms again.
Do not open the oven door more than once if you can help it. It is very important NOT to move anything..even if you are pouring in hot water, do NOT move the tray at all. Be quick while the oven door is open. The oven is a pretty good insulator, so you should not have to refresh the hot water more than once.
You should be able to leave everything there for another 6 or 8 hours slowly cooling and culturing. Its OK now, if the temperature drops a bit. Just leave it there and come back in 6 hours. If your house is really cold, you may have to refresh the heat, but a normal room temperature home with insulated oven will keep the temperature at the right levels for as long as the yogurt needs it this way.
Final Step 10: After 8 hours take everything out and put in the fridge, try to be gentle, don't move too much and again. Do not touch for another 8 hours even while in the fridge, the yogurt is still culturing and firming up.
When you are finally ready you may notice some yellowy liquid on the top. This is fine it is whey. You can choose to discard it or mix it back into the yogurt and eat it
This yogurt will last about 2 weeks. If you made too much you can use it in recipes, cooking, or make yogurt Popsicles.
Next time though make half a batch as yogurt does not freeze that well unless for cooking.
You can even further thicken the yogurt to make a sour cream or tzaziki consistency by straining it through a coffee filter and strainer for a few days while in the fridge.
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