This post is a bit different from what you'll generally find on this blog. It's my response to a discussion on Beth Terry's Show us your plastic trash! challenge.
I make yoghurt roughly once a week. I like it and it's an easy way to get the extra calcium I need (I have to eat a lot of calcium to prevent my bones from 'demineralising' unduly from underuse). It also provides one of my twice-daily protein snacks: unusually for a Westerner, I have to work moderately hard to get sufficient protein in my diet as I eat so few calories. I particularly enjoy a bowl of thick creamy yoghurt topped with a swirl of manuka honey and a sprinkling of milk masala :-)
Over the years I've realised that a lot of people have trouble keeping their yoghurt culture alive batch after batch. I've been able to use the same culture for around five years now and I believe that the key to doing so is being really careful to keep other bugs1 out of your yoghurt. In other words, make sure that you use:
- Yoghurt starter that has no other bugs in it other than the actual yoghurt bacteria;
- Milk that has no bugs in it at all;
- A container for the yoghurt to grow in that has no bugs in it at all.
It is also important to keep your yoghurt sufficiently warm for sufficiently long, i.e. at 40-45°C/110-120°F for around 5-14 hours.2 The longer your milk/starter mix stays warm, the thicker the yoghurt will become until it eventually separates out into curds and whey. I have always kept mine warm in a wide-mouthed thermos flask such as you might use for soup or stew. Other people use a jar placed somewhere well-insulated such as inside a chilly bin or the microwave, or placed somewhere with very gentle heating such as in a crockpot set to 'low' or an oven with only the light on.
So, back to the three things you need to be careful of in order to keep unwanted bugs out of your yoghurt.
1. A starter with no other bugs in it.
When your yoghurt has just been made (or before you first open the container, for bought yoghurt) it only has yoghurt bugs in it. You need to take out any yoghurt you want to use as a starter at this point, i.e. before you transfer your yoghurt to another container, eat any of it or in any way disturb it.
You also need to remove your starter with a clean spoon that hasn't been used for anything else since it was last washed and hasn't been lying around on the bench picking up who knows what.
Lastly you have to make sure any yoghurt you set aside to use as a starter doesn't come into contact with any other bugs. I presume that in the 'old days' they did this by putting the starter straight into a clean container and adding more milk to it right away. That was back when yoghurt was mostly seen as a way of keeping milk edible for a few days when they didn't have refrigeration.
I have a perfectly good fridge and absolutely no desire to make yoghurt every day ;-) so I freeze my starter instead. I spoon yoghurt straight from the container it was made in into a clean icecube tray and freeze it. Whenever I want to make yoghurt I just take a cube of frozen yoghurt out of the freezer and add it straight to my warm milk (there's no need to thaw it out before using it). When I'm down to my second-to-last icecube I freeze some more starter culture from the yoghurt made with that icecube. That way I'm not caught short if I overheat or contaminate the yoghurt made from my last icecube. Ideally you use at least 20mL of yoghurt to make a litre (or quart) of yoghurt so you may need more than one icecube per batch depending on the size of your container and the size of your icecubes. A little too much starter is much better than not enough.
If you don't want to freeze your starter then you'll need to be very careful to keep it uncontaminated until you're ready to use it. I've only very occasionally done this so I don't have much experience here. In general you need to spoon the first of each batch of yoghurt into a very clean container with a tight lid and store that out of the way in a cool part of your fridge until you make the next batch. You shouldn't leave yoghurt culture in the fridge for much more than a week before using it as various bugs will gradually find their way into it no matter how careful you are.
2. Milk that has no bugs in it at all
I expect that in the 'old days' yoghurt was made immediately after milking so bacteria in the milk wasn't really a problem. They would barely even have had to warm the milk. These days, however, most of us use milk that's a little less fresh than that and has at least a few bacteria lurking in it. These would multiply rapidly in the warm conditions we use to make yoghurt so the milk must first be heated almost to boiling (above 85°C/185°F) and kept that hot for at least 10 minutes to destroy them. Any bacteria likely to be found in your milk are now dead and you can add your starter as soon as the milk has cooled down enough (it needs to be no more than 45°C/120°F: if you can keep your little finger in it for at least 10 seconds then it should be fine).
These days my various physical restrictions make this too hard so I use milk powder instead because it has many fewer bacteria than fresh milk. I simply dissolve the powder in cold water, heat it to 45°C/120°F and add my starter.
In the past I have also used a previously unopened carton of UHT milk instead of standard 'fresh' milk. The UHT process has already killed all the bugs in the milk (that's why it keeps so long) so you only need to heat it to 45°C/120°F and add your starter.
3. A container that has no bugs in it at all
You must grow your yoghurt in a really clean container with a decent lid. If your container is hard to get really clean with normal washing then just fill it with boiling water and set the lid on top while you prepare your milk (don't screw the lid down as a vacuum will form inside the container and the lid will be almost impossible to remove!). The boiling water will also preheat your container and help your milk to stay at the right temperature a bit longer than it otherwise would, which is especially useful if you're using a lid with a less than perfect seal. When your milk is warm simply tip out the boiling water and pour in the milk and your yoghurt starter.
Yummy, thick yoghurt
One final thought: if you want your yoghurt to be as thick as commercial yoghurt then you'll probably need to thicken it a bit no matter how long you keep it warm. Commercial yoghurt is practically always thickened, either by adding thickeners such as gelatine or by concentrating it. I always go for concentrating, either by concentrating the milk before making the yoghurt or by concentrating the yoghurt itself.
If you make your yoghurt with milk powder then you can simply make concentrated milk by using more milk powder than it says on the packet. I like to use nearly double-strength milk, i.e. 1 1/2 cups of milk powder per litre instead of the recommended 4/5 of a cup. Yoghurt made this way comes out quite solid but if you give the container of yoghurt a bit of a shake after it's set then it transforms into lovely creamy yoghurt. I did indeed find this out by dropping a container of yoghurt one day! I expect you could also make nice thick yoghurt with unsweetened condensed or evaporated milk but I've never tried this myself.
If you make your yoghurt with actual milk and want a thicker yoghurt you will need to concentrate the finished yoghurt itself, i.e. drain off some of the yellowy whey. I find the following method works well:
1. put a seive over a bowl and cover it with a fine-mesh cloth (I use cheesecloth or an ironed hankie);
2. divide your batch of yoghurt into two roughly equal portions;
3. pour/spoon one portion of your yoghurt onto the cloth on the seive;
4. put the other portion of your yoghurt into the container you want to keep your yoghurt in;
5. put both lots of yoghurt into the fridge overnight;
6. the next day, using a fork, blend the stiff white 'yoghurt cheese' left on the cloth into the unseparated yoghurt you put into your container to thicken it (you can also use a blender or food processor for this).
When you do this, don't throw away the whey from the bowl! It makes a great substitute for milk in any baking and acid-loving plants also appreciate a drink of it from time to time. I like a drink of it, too :-) In Switzerland, carbonated, sweetened, thinned-down whey is a very popular soft drink! Who'd have thunk it? I was quite sure I'd misunderstood something when someone explained to me what 'Rivella' was made of. I came to like it so much that I've even made a few (unsuccessful) attempts to concoct something similar now I'm back in New Zealand.
That's all I can think of, but I'm happy to answer any questions you may have.
Happy yoghurt making!
a. won't survive normal dishwashing with soap or detergent;
b. generally won't survive being dried out and certainly can't multiply in a dry place;
c. need warmth, food and water in order to grow and multiply;
d. are very good at finding what they need, even if it's inside a closed container;
e. can live no more than a few minutes at temperatures hotter than 80°C/175°F;
f. multiply more slowly the colder they are (i.e. slowly in the fridge and barely at all in the freezer).
2When I make yoghurt in a pre-heated thermos flask I find it's reasonably thick after about 5-6 hours and even better if it's left overnight. At the moment I'm making my yoghurt in an Easiyo thermos and leaving it for 18 hours to set. Yoghurt will always take a bit longer to form in an Easiyo thermos than a regular thermos. An Easiyo thermos system consists of a container that you fill with cold milk and a reservoir that you fill with boiling water. The yoghurt culture only starts to grow once the heat from the water has warmed the milk to at least 40°C/110°F and this takes a bit of time.