In my previous post I went into the microbiology of yoghurt – which for SEO reasons I’ll use the American spelling “yogurt” for the rest of this post. But to be clear, the correct spelling is “yoghurt”. In this post I will explain how to ferment your own yogurt at home, including a few tricks that you can use to make thicker yogurt, and to maintain a “house culture”. This is probably the easiest fermentation that you can do – far easier than making beer – so I encourage everyone to give it a try.
What You Need
Milk: It would be hard to make yogurt – i.e. fermented milk – without milk! Nearly any milk will do – cow, goat, sheep…whale I suppose. Full-fat, skim, or partially skimmed will all work (although full-fat gives a creamier finish). Homogenized or non-homogenized are both fine – although you’ll need to stir the cream back into yogurt made with non-homogenized milk. Even ultra-heat treated (UHT) milk can be used (this is the stuff you don’t need to refrigerate), although you will need to incubate the milk for much longer – perhaps as long as 24 hours.
Culture: To make yogurt you first need a bacterial culture. An easy way to get this is simply to buy a yogurt with “live cultures” from the grocery store. A few tablespoons per liter (quart) of milk will work perfectly in most cases. Another option is to use a yogurt culture. This is the equivalent of dry yeast in the brewing world – long-lasting dried cultures that produce a consistent product. I’ve been working with Codexing’s LyoPro Y+ culture recently. Once you start making yogurt you have a third option – maintaining your culture through backslopping, which I will discuss later in the article. Most cultures contain (at a minimum) Lactbacillus delbrukeii and Streptococcus thermophilus, which as I described in my earlier post, are the key organisms in yogurt fermentation. Some cultures may include other bacteria including probiotics such as Bifidobacterium. Greek/Balkan cultures may contain Lactobacillus acidophillus, which gives these yogurts their softer character.
Some way to keep it warm: Yogurt needs to ferment between 42C and 46C (107-155 F) for up to 24 hours. While you can maintain this manually using a double-boiler or carefully controlled oven, it is much easier to use a yogurt maker, electric cooking pot (e.g. instapot), or some other form of automatic temperature control to maintain fermentation temperatures. Instapots can largely automate this process, which is great for people making their first yogurt – but you’ll likely find after a few batches that you want more control over the process.
Optional: Homemade yogurt tends to be thinner than the stuff you get at the grocery store. This is because most grocery store yogurts have additional thickness added to them, or are strained. You can do both of these at home, and if you want to go this route you will need one (or more) of:
- Cheese cloth. This is used to strain the whey out of the yogurt, making it thicker. This is how Greek yogurt is traditionally made. If you don’t have cheesecloth, and old white pillowcase can be used instead. Do not use coloured fabrics as the dyes may not be food-safe. Cotton or another natural fiber are best, although synthetic or blended materials can be used.
- Unflavoured gelatin. This is commonly used to thicken yogurt and can easily be used at home.
- Dry milk powder. Adding additional protein to the yogurt can make for a thicker yogurt. Dry (skim) milk powder is typically used for this purpose.
- Other thickeners: while untested by myself, thickening agents such as guar gum (a thickener extraced from guar seeds) and xanthum gum (extracted from certain bacterial fermentations) are food-safe options that are used in commercial yogurt production.
A Few Things to Consider
There are a couple of things to consider before starting:
- What are you going to ferment in. If using an instapot or yogurt maker you typically ferment in the internal pot that is part of the machine. If using some sort of homebrewed system, its common to ferment in mason jars. While I use an instapot for temperature control, I ferment in jars. I find this preferable as the milk doesn’t form a skin during fermentation, and the yogurt doesn’t need to be transferred into a storage container after it is done. My instapot fits up to four 500 ml (1/2 quart) jars perfectly.
- How are you going to scald the milk? Most yogurt makers, and instapots, will do this for you automatically. I prefer to do this manually on a stove, using a thick-bottomed pot. I find this gives me better control over the temperature, its easier to add milk powder or gelatin, and I can pour the hot milk into my jars – thus pasteurizing the (already sanitized) jars.
- How tart do you want your yogurt. Tarter yogurts need longer fermentation, so if you want something quite sour consider fermenting overnight, or even for a full 24 hours.
The Yogurt Making Process
The yogurt making process is, in essence, a 4-step process: 1) Sanitize, 2) Scald the milk, 3) Ferment the milk, 4) chill (and perhaps strain) the resulting yogurt. How to perform each step is described below.
Step 1 – Sanitize: Just like beer brewing, sanitization is critical for yogurt making. No-rinse brewery sanitizers such as starsan and iodophore are excellent options, as is boiling the equipment. Anything that is going to touch the milk needs to be sanitized before use.
Step 2 – Scald the milk: The milk needs to be heated to 82 C (180 F) or hotter. This serves two purposes – it pasteurizes the milk; rendering it safer to consume and preventing off-flavours from contaminating organisms. It also starts the coagulation of the milk proteins, which is required for thickening the yogurt. In some cases the milk is simmered for up to half an hour. This concentrates the milk, producing an even thicker yogurt. Personally, I avoid boiling the milk as it is very easy to burn the milk (which tastes horrible). Instead, if I want to concentrate my milk, I hold it at 92-95C for 20 to 30 minutes. Step-by-step::
- Slowly heat the milk to 85C in a thick-bottomed pot (or using the scaled feature of your yogurt maker/instapot). You want to hold the milk at this temperature for at least a minute.
- Optional: If adding dry milk powder, add it before heating. 75 to 125 ml (1/3 to 1/2 cup) of dry milk powder per liter (quart) of milk works well. Stir until incorporated.
- Optional: If using gelatin, add it once the milk reaches 80C/176F. Use 1 tablespoon per liter/quart. Add the gelatin to the hot milk, let it bloom for 5 minutes, then stir into the milk until dissolved. You need to hold the milk above 80C/176F throughout this step.
- Optional: To concentrate the milk, increase the temperature to 92C (200F), or even to a low simmer, and hold for 20 to 30 minutes.
- Skim off any film that forms.
- If fermenting in mason jars or another separate container, pour the hot milk into the pre-sanitized jar/container.
- Chill to 42C (107F). This can be done my leaving the milk, covered, at room temperature until it naturally cools, or by placing in a sink of cold water.
Note: you can find methods on the internet on how to use raw milk – without scalding – to make yogurt. As I microbiologist I cannot recommend this procedure (nor the consumption of raw milk) and will not be discussing it here.
Step 3 – Add the culture: You want to add the culture as soon as your milk is at 42C, in order to limit the risk of infection. This step is quite easy:
Using store-bought yogurt or a home culture: Around the time you start scalding the milk place ~30 ml (1 oz or 2 tablespoons) of yogurt per liter/quart of milk into a sanitized bowl and allow to warm to room temperature. Stir this into the scalded and cooled milk.
Using dried yogurt cultures: Sprinkle the manufacture’s recommended amount of culture on top of the scalded/cooled milk and let rehydrate for 2 minutes. Then stir the culture into the milk. I use 1/32nd of a teaspoon of LyoPro Y+ per liter.
Step 3.5 – Fermentation: Once the cultures are added, place your fermentation vessel into your warming device. If using jars, place lids on the jars but don’t tighten them as gas can be produced during fermentation. Hold at the desired fermentation temperature (42-46C/107-155F) until done.
Fermentation temperature: While the range of fermentation temperatures is fairly narrow, the effect of temperature on fermentation can be dramatic. The LyoPro Y+ culture I am currently using struggles at temperatures above 43C, while my old culture barely worked unless you were at 45C or warmer. If using a commercial culture, follow the manufacture’s instructions. If using a home culture or store-bought yogurt, 44C (111F) is a good place to start, but explore the full temperature range to see what works best.
Fermentation time: The longer you ferment the more tart, and thicker, your yogurt will be. Different cultures work at different rates, but as a general guideline: 6-8 hours produces a mild yogurt, 8-12 hours a tart yogurt, and more than 12 hours produces a sour yogurt. You can ferment up to 24 hours, although it is unlikely that you will get much additional acidification after the 12 to 16 hour mark. When working with a new culture I simply start tasting the yogurt at the 8 hour time point and pull the yogurt from instapot when it tastes the way I want. You don’t need to do regular tastings once you’ve “dialed in” the timing, so long as you use a consistent amount of starter culture for each batch.
Step 4 – Chilling: Once the yogurt is done fermenting it needs to be chilled. During this time it will continue to thicken and take on its final consistency. Yogurt will often be thin and watery at the end of its fermentation period, but will turn into a thick and luscious yogurt after spending the night in the fridge.
Optional – Straining: Yogurt can be strained to remove some of the whey. This will produce a thicker yogurt, an is a critical step in making Greek-style yogurt. It is also a way to “save” a yogurt fermentation that didn’t set properly. To strain:
- Chill the yogurt for at least 6 hours.
- Line a colander with 2 layers of cheesecloth, or with a single-layer of tightly woven uncoloured fabric.
- Pour the yogurt into the lined colander.
- Gather the excess cloth and knot overtop of the yogurt (or hold in place with a twist-tie or elastic band). If you don’t do this, whey may wick up the cloth and then drip onto your countertop.
- Place into your refrigerator and drain until the desired consistency is reached. Again, your recipe, culture, and desired thickness determine the draining time, but as a general rule for 1 liter (quart) of yogurt (larger volumes will take longer):
- 1-3 hours of draining: This will slightly thicken your yogurt, often giving a texture to milk-only yogurts similar to what you get from yogurt made with milk powder, gelatin, or concentrated milk.
- 3-6 hours of draining: This will give you Greek yogurt – e.g. quite thick, but still a liquid.
- 6-24 hours: This will give you very thick yogurt. The only commercial examples of this I have encountered are some of the “artisanal” Greek yogurts sold at some specialty shops.
- 24-72 hours: At this point you’ve converted your yogurt from a liquid to a solid, and it is now cheese. Served fresh you have labneh – similar in texture to cream cheese, but with a more intense flavour. Traditionally, labneh is lightly salted, rolled into balls, and marinated for a few days in olive oil and herbs. You can also add some additional cultures to make a delicious bloomy-rind cheese…a topic I’ll be covering in a few months in another post.
Maintaining Your Own Yogurt Culture
Like brewers yeast, a yogurt culture can be re-pitched, which in yogurt/cheese making is termend “backslopping”. But, like mixed yeast cultures, every time you backslop a culture the proportion of the bacteria in that culture changes. Generally speaking, Lactobacillus grow better than Streptococcus at typical fermentation temperatures, meaning that over time the Lactobacillus will dominate the culture, rendering the culture ineffective. Therefore, if you want to keep a culture going long-term you need to care for it. There are two general approaches that the home yogurt maker can use: 1) A “perpetual” culture, or 2) A “preserved”culture.
Perpetual Cultures: While most cultures can only be repitched (or “backslopping”, in yogurt/cheese making parlance) a handful of times before they stop making good yogurt, there are some cultures which will maintain the correct balance of organisms indefinitely. Many of these are very old cultures which have been maintained for centuries, and some are even cultures of those rare strains of Lactobacillus which can make yogurt without the assistance of Streptococcus. While rare, these cultures can be found and are typically sold as “heirloom cultures”.
To backslop: scald and cool the milk to fermentation temperature as per usual, and then stir in 3-6% the volume of milk of fermented yogurt. If your starter yogurt is less than 2 weeks old use 3% (e.g. for 1 L of milk add 30 mL/1 oz yogurt), if 1-3 weeks old use 6% (e.g. 60 mL/2 oz of yogurt for 1 L milk). If your yogurt is more than 3 weeks old it is best to make a small batch (a cup or so) the night before, and then use a 3% volume of that to start the yogurt.
Preserved Cultures: Another approach is to preserve a culture at a “young” age, and to use that young culture. Maintained properly, a single culture can produce well over a thousand batches of yogurt. The process is fairly simple:
- Prepare a small batch of yogurt using a band-new commercial culture (or culture from your favourite grocery store brand). Let this ferment at 42C for 12 to 24 hours.
- Line an ice-cube tray with plastic wrap, and spray with starsan. Drain off any excess sanitizer.
- Carefully fill the lined tray with the yogurt, cover with sanitized plastic wrap, and place in your freezer until frozen – generally ~12 hours.
- Pop the cubes out of the tray and place in a plastic bag. Keep in a deep freeze (-18C/0F or colder).
Frozen like this, these starter cultures will last for 3 to 4 years.
To make yogurt:
- Scald and cool your milk as per usual.
- While scalding, thaw 1 cube (~30 ml/1 oz) per litre (quart) of milk in a sanitized bowl
- Once the milk is cooled to fermentation temperature, mix the thawed culture into the milk.
- Ferment as usual.
To continue the culture:
- Yogurt made from a “fresh” cube can be backslopped 5 or 6 times before you need to restart with a freshly thawed cube. Simply backslop as described in the “perpetual culture” section.
- When you no longer wish to backslop, make your next batch with a freshly thawed cube.
- When you get to you last cube, or if your frozen culture is nearing 4 years of age, simply start a small (250 mL/1 cup) batch with a thawed cube and re-freeze as described above. By creating a small culture you keep the number of cell divisions low, meaning that the newly frozen cultures should be “backsloppable” as many times as the first time you froze the yogurt culture.
Using the above approach I maintained a yogurt culture from 1999 to 2016, making yogurt 2-3 times per month. The only reason I stopped is because we accidentally threw out the culture when preparing for our last move.