The status of eating yeast on Passover has me confused. Yeast is used in wine production which is ok to eat, but I've talked to people that won't eat yeast extract, which is not a leavener. Is there a correct answer or is this a gray area?
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My understanding is it all depends on what the yeast grew. If it grew off of wheat or barley or the like, it's a problem.
As I heard from one OU lecture, an interesting ingredient in today's food production is "torula yeast." Paper makers had all this "pine wood soup" from cooking their shredded pulp, which they couldn't just pour down the drain for environmental reasons. So a type of yeast which loves to eat pine soup became popular, breaking down the waste product much better -- and as it turns out, torula yeast is a nice food ingredient, and all it's grown off is pine wood, so there are virtually no kashrut or allergy issues with it! (May still need Passover supervision, though.)
According to this glossary, Se'or, the somewhat lesser known, but no less prohibited, sibling of Hametz, includes the yeast extract products known as Marmite and Vegemite. Unfortunately, I don't know on what basis they make this assertion. I can think of at least two possible reasons, but I am not sure of either of them.
It depends on the source of the yeast. Yeast itself is NOT chametz (it wasn't even known when chametz was "invented"). Essentially what is not allowed by the laws of chametz is when one of the five grains has been in water for 18 minutes or longer - this counts as a leavening of the grain.
The problem is that many (even most) yeast extracts are made from side-products of the beer making process (this is true, for example, of Marmite from the UK or SA and Vegemite). Beer is made using grain that is soaked and fermented (for much longer than 18 minutes) and as such is chametz, and so is the yeast extract that is derived from it.
It is often difficult to know what the source of the yeast in yeast extract is, which is why most rabbis simply say that it is chametz because it is "safek" - that is, there is doubt as to its source, it may well be chametz, so treat it as chametz. If you have a source of yeast extract where you can find out what the source is, and it is not grain-based, you're fine.