The math behind the quantity of yogurt starter and the volume of yogurt it makes

Bacillus Bulgaricus starters are great at making excellent products. They are easy to use and the yogurt always turns out great.

If this is the first time you are using yogurt starter (or maybe the first time thinking about it), it might be somewhat confusing understanding how the starter works.

Occasionally, we get get asked about that, so we decided to write a quick explanation about it.


If the 1L pack contains 1 gram of starter, how come the 50L pack only contains 10 grams of starter? It surely should contain 50 grams, right?

And while this sounds like a solid argument, everyone who is familiar with how fermentation works would know that it works a bit differently than that.

But let me explain what’s going on.

To start with, if you go ahead and make a simple algebraic calculation, then you would quickly come to the same conclusion as the one above — if 1 gram makes 1 liter of yogurt, then 10 grams should make 10 liters of yogurt, right?

Well, it does, but in fact, in real life, it makes much more than that.

The confusion comes from the assumption that the quantity of starter required to make yogurt is a linear function to the quantity of yogurt that it makes. While actually it is much closer to an exponential function.

The yogurt starter quantity to volume of yogurt it makes is not a linear but an exponential function

But why is that?

The simple answer is that you are dealing with live organisms and the way they procreate and function.

Think about it this way — when a bacterium multiplies and then each of the new bacteria multiply again, then you would have an exponential growth, and not linear. A similar thing is happening when you introduce your starter into milk.

Let me elaborate just a bit further for the more inquisitive readers 🙂

It is the bacteria that converts milk into yogurt during fermentation. When you introduce the starter into the milk, the bacteria in it comes to live. Then the streptococcus thermophilus bacteria goes into action first and prepares the perfect environment for lactobacillus bulgaricus, which starts multiplying and fermenting the milk into yogurt (or kefir, Skyr, Viili, etc.)

Ultimately, it’s the numbers in the colonies of bacteria that drive the fermentation and determines the volume of milk that could be converted into yogurt. There are a certain numbers of freeze dried bacteria in your starter pack and when they come to live, they start chomping on the sugars in the milk and start procreating, i.e. multiplying. The more access to food the bacteria have, the larger in numbers their colonies will grow. And they will keep eating and multiplying and growing until there is food for all, i.e. until they have eaten up all sugars in the milk, while fermenting the milk in the process.

Theoretically, you can use 1 gr of starter to ferment 50 liters of milk, however that will take number of days and keeping milk warm for a number of days will result in nothing but spoiled milk (so it’s near impossible to do so in practice).

By suggesting different starter amounts for different volumes of milk, we are simply trying to optimize the amount of starter you need in order to ferment a certain volume of milk in a certain amount of time.

For example using 1 gram of starter will turn 1 liter of milk into a great tasting yogurt in 6-8 hrs. And using 10 grams of starter will turn 50 liters of milk into the same great tasting yogurt in the same 6-8 hrs.

So to sum up, what those assigned grams to volumes mean, is that this is the amount of starter, which can turn that amount of milk to yogurt to a certain level of pH (e.g. 5.4pH) in a certain reasonable amount of time (e.g. 6-8 hrs, and that time varies for the different strains).

Indeed, it takes a gram of starter to make a liter of yogurt, it takes about 10 grams of starter to make 50 liters of yogurt, and it takes a mere 400 grams of starter to make 10 metric tons of yogurt.

Now, that is a lot of yogurt!


But then why are the packs marked for the full volume they make? Don’t you think that’s confusing?

No, not really.

The packs are packaged and labeled with the grams they contain and the volume those grams make when used entirely in one go. We clearly explain this on each product page and on the product information pages too.

When you think about it, there is no way for us to know how you are going to use your pack.

Yes, the packs are intended for single use but they could also be split and their contents could be used multiple times (our customers share they do that because gram for gram, the larger packs come off more economical). When you split the contents of the pack and use less starter amount to make smaller volumes, keep in mind that you are also reducing the total amount of yogurt that you can make from the pack, because splitting the packs does not change how much starter you need for the different volumes of milk (check this page to see the gram to Liters (gallons) chart and to find out how much starter to use for different volumes of milk).

So instead of guessing how you might be using your pack, we provide you with all the necessary information about how the starter works, how to use it, how much to use, etc. so you can get the pack sizes that works best for you.


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