Yogurt | St. Lawrence University Sustainability Program


Sunday, October 29, 2017

Having two dairy goats at the farm means a constant supply of milk. More milk than we drink or eat directly. However, as we eat many dairy products, cheese, ice cream, sour and whipped cream, and yogurt. All these products are derivatives of milk, but they vary greatly in the degree of processing. Yogurt seems to be one of the easiest products to make yourself. The process is straightforward. In a first step, the bacteria that lets milk spoil is killed when heated to 180°F. The milk is cooled down to 120°F when the yogurt bacteria culture is introduced. This can be done by simply adding yogurt, which contains the cultured bacteria. Then, the bacteria do the rest of the job, the temperature needs to stay at 120° for 6 hours that bacteria ferment the milk. The lactose, the milk sugar, is transformed into lactic acid. This changes the flavor and the lactic acid acts on the milk protein making the texture firmer.

As straightforward the process seems, there are many ways it can go wrong. The first challenge is the milk we have: goat milk which differs from cow milk. The texture of the yogurt is not firm enough and remains liquid. This can be fixed by straining the yogurt as it is done for Greek yogurt. By straining the yogurt through a butter muslin, the whey drips off and the remaining yogurt gets thicker. This technique has a drawback by straining and thereby, wasting the whey the yield goes down. I figured out that around half of the volume of milk used ends up as yogurt.

Another challenge is to bring the milk to the actual temperature, this is not difficult as long you monitor the milk closely. The challenge comes in when you try avoiding watching the bacteria work for 6 hours. To hold the temperature constant, I used a crockpot. The only problem with that is that the crockpot does not actual controls the temperature. If I put it on the setting “warm” the temperature decreases slowly until the bacteria stop working. If you turn it on low or high, you need again to monitor the milk to avoid overheating which can destroy your effort by killing the bacteria and degrading the protein.

The good news is, there is some room to play with the temperature. Ten degrees to high have not caused any trouble so far. But it can also go wrong; last week I heated the milk too much and accidentally boiled it instead of heating it to 180°. The product appeared more like sour cream than yogurt.  

Despite these challenges, I think it would be sustainable and interesting opportunity to try to make our own dairy products. The yogurt making could be improved and upscaled to reduce the time required to make it.