Yogurt isn't complicated
Before we get to fixing broken homemade yogurt, however, here's a brief primer. Yogurt is simply milk that's been fermented using particular types of bacteria that create lactic acid. You can get this right bacteria using commercial yogurt as a starter, or by buying it in powder packets, like yeast.
Either way, once released into the wilds of your fresh milk, this lactic acid-producing bacteria multiplies, consuming the milk's natural sugar for food and adding, among other flavors, sourness. Meantime, the acid produced curdles the milk, creating the distinctive firmness and thick texture we cherish and expect. Curdling happens when milk proteins (casein) begin to unravel into strands and then tangle up, creating a mesh that traps water and fat (hence, thickness). In the process, some water also gets squeezed out of the curd, and that's why you often see whey separating out from yogurt.
For extra firmness
Along with casein, you can also enlist another milk protein to thicken your yogurt. Whey protein goes unscathed in the lactic acid bath that curdles casein, but by scalding your milk before fermentation at 185° for a period of 30 minutes, the whey protein gets cooked, unravelling it and reinforcing the final texture. You may have heard that scalding milk is necessary to kill stray bacteria or to deactivate enzymes that can interfere with fermentation, but in commercially available milk that's already pasteurized, these reasons no longer apply (if you're using raw milk, by all means scald for protection as well as texture!)
Making the yogurt
Once your milk is scalded, reduce the temperate to 70° by placing your pan into an ice bath (be sure you don't get any water or ice into the milk). Take temperature measurements! If you add bacteria to overly warm milk, it will die. And if your milk is too cold, you may be adding hours to the final fermentation time. Once the milk hits room temp, put it into a clean, airtight container, add the starter, blend it in fully, and find a warm spot where the whole thing can ferment undisturbed.
To achieve an even temperature throughout the process, I use the Yogourmet yogurt maker. I have friends who use the oven light or electric blankets for temperature control, and those methods can work, but I found the minimal investment in this simple piece of equipment to be worth it in terms of consistency and mess control.
After the milk has fully fermented (I leave mine overnight and then some, about 12-14 hours) I then take two extra, quick steps that add firmness and creaminess that really make this yogurt worth the work. If you've already gone to all the trouble of scalding and cooling your milk, this part is nothing!First, I place pour the warm yogurt into a chinois and let it drain until about two cups of whey liquid separate out. Next, I take the strained yogurt and subject it to an immersion blender. This breaks up the curds and creates a smooth, runny texture that will appear ruined. Don't worry, once it has a chance settle and cool down, the texture will snap back. While still warm, I pour the now-runny liquid into glass containers and refrigerate it. After about six hours, I've got perfectly smooth and nicely thickened homemade yogurt that's somewhere between regular yogurt and greek style.
The payoff on making your own yogurt, as well as being fun, is multifold: the cost is about one third to one half of what it costs to buy, it tastes better, and you can be sure you're getting all of the probiotic cultures you want without any unwanted additives. After you make it a couple of times yourself, buying a plastic tub at the store won't seem quite as convenient or worth the money.
