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I want to prepare 500 g yogurt. I have 500 ml skimmed milk.
So, will the resultant yogurt be of 500 g?

If not, then how much skimmed milk do I need to get 500 g yogurt?

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Stephie's answer is really good and she touches upon this, but to keep it a separate issue, 1 ml of DISTILLED WATER = 1 g, other fluids will have more or less mass depending on their density. An extreme example, a ml of lead will weight closer to 1/5th of an ounce (I am guessing but it's close) which is many grams. – Escoce Jan 27 at 14:29
up vote 24 down vote accepted

Making yogurt means letting lactic acid bacteria alter the texture and chemical composition of milk by digesting lactose and producing lactic acid, which in turn interacts with the proteins in the milk, causing the milk to thicken and taste sour.

Unlike in cheese making you are not separating curds and whey, so you are not "losing" significant amounts of substance.

Yes, there might be some fluctuation - an increasing number of lactobacillae and them eating lactose - but on a very small level.

For our general kitchen precision, 500 g milk makes 500 g yogurt.

Side note: 500 ml milk is not exactly 500 g, in fact, it's 510 g according to my sources, but I consider these values precise enough for general cooking purposes. Otherwise, you can't even stir, as minute amounts will remain on spooons or in your pots and pans. This is a kitchen, not a lab.

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While I agree that 510g vs 500g isn't a significant difference (it's 2tsp, or 2%), I think it's silly to compare a difference of 2tsp to the couple of drops that would stick to a spoon when you stir it. Also, note that the increasing number of bacteria doesn't increase the total mass: the new bacteria aren't created out of nothing; rather, they're created from the material already present in the original bacteria population and the milk. – David Richerby Jan 28 at 1:37
    
@David by the time you've stirred it and either left a little stuck to the bowl or scraped it out with a spatula -- to which more will stick -- you've probably lost at least a couple of tsp. – Chris H Jan 28 at 9:00
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@DavidRicherby not to mention the difference between starting the yogurt with a few spoonfuls of yogurt vs. adding the lactobacillae separately as powder or similar. As I said: Kitchen maths. – Stephie Jan 28 at 9:07

Since the yogurt making process takes place around 40C, over the course of the 8 to 10 hours it takes, there will be some condensation on the inside of the lid of the machine - which obviously comes from the milk/yoghurt mix - when preparing 8 glasses of 150 ml each, I estimate the condensation to be 10 ml.

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