When we first got our cow, Bella, I had read “The healthy house cow” and really wanted to make raw milk yoghurt because Marja made it seem so easy and healthy, but every time I tried, I just ended up with a mess.
Pateurising the milk involves heating it to nearly boiling (around 80 degC) in order to kill both pathogenic bacteria, and other beneficial bacteria in the milk that will compete with the yoghurt bacteria. It also has the effect of denaturing enzymes that can cause the yoghurt to be runny.
By pasteurising, you're starting with more or less sterile milk, it makes the process of yogurt making easier and more repeatable, in fact, I've never had any problems with pasteurised milk yoghurt not working, however if the temperature gets too high during pasteurisation (i.e. if you get impatient, turn up the heat and turn around to do something else) you end up with solids in the milk, which kind of ruins the texture of the yoghurt (although you can then strain it - more here).
By killing off the other bacteria and the enzymes in the milk, you get a more reliable yoghurt, but you miss out on those beneficial bacteria and enzymes that would have been good to eat too. Hence my repeated attempts at making yoghurt from raw milk. There are some great posts around that explain how to make raw milk yoghurt.
Kitchen Stewardship- a method for making raw milk yoghurt using a "cooler" (or esky or chilli bin, see how I speak so many languages!) and a good discussion in the comments section.
Nourished Kitchen- a similar method, with more options for starters and ways to keep the yoghurt warm, and another great discussion in the comments section.
So you can see why I was hoping to find a way to make yoghurt using raw milk, it is way easier to just put the culture in the milk straight from the cow and let it ferment and make yoghurt, with no fiddly pasteurisation involved, and you get to keep all the beneficial bacteria and enzymes in the original milk.
here's where the milk comes from...
Finally someone ( a friend of Ohio Farmgirl) noticed my plea for help and very kindly emailed me with instructions. He said that I need to maintain a small batch of yoghurt made from pasteurised milk and use that batch to inoculate the raw milk, rather than just using a sprinkle of the frozen yoghurt culture. I already (always!) had a batch of yoghurt made from powdered milk, so I tried adding several tablespoons from that batch to a litre of raw milk fresh from Bella. I let it ferment for 12 hours and the yoghurt was ready.
My previous attempts at making raw milk yoghurt had resulted in curds and whey, because the yoghurt bacteria hadn’t had a chance to multiply before the natural lactic-acid bacteria in the milk had started to grow instead. Apparently this method of inoculating with a yoghurt made from pasteurised (or powdered) milk gives the yoghurt bacteria a better start in the raw milk. The key is to keep the pasteurised yoghurt batch going at the same time, this batch will contain only the yoghurt bacteria, and not all the competing bacteria that may eventually take over if you inoculated with the raw milk yoghurt (although I am of course very tempted to try that too, I’m all for simplicity!).
milk fresh from the cow
I wonder now if another reason why I had so much trouble with raw milk yoghurt was because Bella had been treated with antibiotics and wormer before she arrived at our place. The antibiotics were for mastitis (which has not recurred under our care). I mention this because if you are having trouble with raw milk yoghurt, maybe wait for a while and try again, it could be something else in your cow's system that is causing the problem.
Do you make yoghurt? Why not? Its so easy and there's so many different ways to make it, surely one will work for you! Get yourself a big thermos and give it a go!