YOGURT – Temperature, Time and Culture
Making yogurt is easy once you know what you are doing… until then, it can be quite difficult. There are a lot of conflicting strategies out there, and I had to do a lot of experimenting to sort things out. Here is what works for me… and why.
Sterilize and Denature. Gently heat whole milk to 185°F in a stovetop bain-marie (hot water bath); hold at 185°F a full 30 minutes to optimally denature lactoglobulin (a milk protein). Even if the milk is commercially pasteurized, this step is essential because commercial treatments are quick and intense; they do not denature the protein. To demonstrate the importance of initially heating milk to 185°F for 30 minutes, I brought five batches of whole milk to a range of temperatures (110°F, 145°F, 160°F, 170°F, 185°F). As you can see, 185°F has a huge impact on final thickening potential.
Inoculate. Pour hot milk into 1-pint containers, cap and set at room temperature to cool to 110°F (a safe temperature for introducing desired cultures). These cultures are readily available in the form of plain, whole milk yogurt (I prefer Dannon). No matter the brand, the package should list active bacterial strains of Lactobacillus bulgaricus and Streptococcus thermophilus. Why both? Because they are what make yogurt “yogurt”, and they help each other thrive. From each pint, measure out 3 tablespoons milk and 1 tablespoon freshly opened yogurt; gently stir into a smooth slurry. Add back to the pint, stirring gently to evenly distribute the culture. Cap tightly.
Incubate then Refrigerate. Incubate, undisturbed on a tray, at 110°F (an unused oven with a light on works well) 8-10 hours. As the beneficial bacteria (probiotics) get settled into their warm, comfortable space, they gobble up lactose and release lactic acid, lowering the pH. As acidity increases, the environment becomes unfriendly to potential pathogenic bacteria. Meanwhile, the acidity denatures casein (another milk protein), which promotes final thickening. Remove from incubation and refrigerate, using yogurt from this batch to make the next batch at around day 7, when the probiotics are most vigorous.
So, essentially we use TEMPERATURE and TIME to eliminate initial pathogens and to denature lactoglobulin. We introduce desirable CULTURE, which lowers pH, protecting against potential pathogens and denaturing casein. Ultimately, temperature, time and culture work together to deliver the texture and tang we expect in yogurt.