The most important piece of equipment in yogurt making is a thermometer; though as long as it is accurate the style is not significant. I previously used a IKEA digital meat probe, am currently using a liquid filled candy thermometer and am looking forward to receiving my Supermechanical Range thermometer
The remaining equipment can be improvised upon and altered from my method, based on what equipment you have available.
Milk is just about the only ingredient so the type you choose plays a large role in the final product. The first batch of yogurt you make originates from a store bought, plain natural yogurt, which contains the vital microorganisms Streptococcus thermophilus, Lactobacillus bulgaricus, and Bifidobacterium (aka "good" bacteria). The temperatures used below are chosen because some of the microorganisms reproduce at 45-47C and some at 37-42C so stirring the 51C milk into a room temperature bowl and starter will mean it begins at the upper temperature band and cools through the lower band allowing the starter to culture the milk with all the bacteria types. It is better to use a milk with a higher fat content -- I am currently using milk from Jersey cows which is high in fat and has the added benefit of being unhomogenized, though for the longest time I used normal full-fat / whole milk from the supermarket. Apparently raw milk makes the best yogurt though that is a bit difficult to get hold of so I haven’t tried it.
The milk is boiled to stop or slow the reproduction of bacteria that is not wanted, and for the first batch the extended simmer has the added benefit of deoxygenating the milk (which Lactobacillus prefers) and reducing the water content which makes for a thicker and creamier yogurt -- in Middle Eastern cultures where yogurt originates from the milk is often reduced in volume by about 30%.
1. Heat milk to a gentle simmer (~95C) and maintain at this temperature for a couple of minutes, or longer if using a store bought starter or for creamier yogurt
2. While milk is heating prepare the bowls. Separate a large and a small nesting bowl with an insulating material such as a folded tablecloth and place starter in the smaller bowl to warm from fridge temperature slightly. For the first batch use a few tablespoons of yogurt, when using your own yogurt (saved from previous batch) less will suffice
3. Once milk has simmered for required time remove from the heat and allow to cool to ~51C. I normally set a timer as I sometimes forget it -- with my saucepan and the volume of milk that I use it normally takes about 20-25 minutes.
4. Splash a bit of the milk into the starter (approximately the same volume as starter) and stir thoroughly. Add the rest of the milk in small splashes initially then pour, stirring continuously
5. Cover with cling film and some sort of lid -- I use one of my daughter's plastic plates as the plastic helps to insulate the yogurt and prevent it from cooling too quickly
6. Place the bowls in a switched off microwave (I normally unplug it!), as the construction of the microwave also helps insulate the mix
7. Leave overnight -- around 12 hours and then refrigerate
Yogurt comprises of living cultures like a sourdough bread starter so don't be dismayed if you have a couple of failures, though this method is fairly reliable. I have been making it this way about once a week for the last couple of years and it has only failed once or twice, which I think I can attribute to my old thermometer breaking down and giving wildly inaccurate readings!
I generally eat the yogurt plain but I have made it into frozen yogurt, and my wife likes it with berries and honey. Store bought yogurts have a lot of sugar and flavorings added so you may prefer it with some kind of sweetener.
When you have eaten some the structure of the yogurt will have been altered and it will separate slightly into curds and whey. Some commercial yogurt makers prevent this by stirring milk powder into their yogurt and this can be done at home apparently.
Alternatively you can pour off the whey, though I generally just drink it!
A further variation is to strain the yogurt with cheese cloth and the whey will drip off -- this will leave you with a Greek style yogurt (as I am bound to say by the protected geographical origin of that name -- unless you are in Greece!).
Apparently you can make a very rich dessert variation by using single cream in place of milk and sprinkling it with Demerara (raw) sugar, though I haven't tried this yet. Let me know how you get on in the comments section!
[Photo of Turkish cacık, made with yoghurt and cucumber, from Wikimedia Commons.]