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While there are thousands of strains of yeast including many thousands not yet formally identified all yeasts are from the fungi kingdom and are included with other fungi such as mold and mushrooms. The most common forms of yeast for food productions are variations of Saccharomyces and sometimes in brewing Brettanomyces.

Aside for possible concerns regarding Pesach for some types of yeast which are grown on a chometz medium such as barely, why should yeast require a hechsher the rest of the year?

  • Re Pesach, As I understand, yeast is forbidden not b/c it is chametz, but b/c it is "machmetzet" or it is a fermenting agent that makes things chametz. Machmetzet is also prohibited. As for the rest of the year, you raise a good question. A natural product, shouldn't need any hechsher. So, I have to research how consumer yeast is packaged and produced. – DanF Mar 1 '17 at 21:58
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    @DanF "A natural product shouldn't need any hechsher." That's nowhere near true. – Daniel Mar 1 '17 at 21:59
  • @DanF this question isn't about pesach. the only reason I mention that is b/c yeast can be grown on chametz. The yeast I use in making beer is grown on barely and would not be kosher for pesach. I only bring this up to specifically avoid this point and to address the rest of the year – Dude Mar 2 '17 at 0:19
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There are yeasts that are grown using non-kosher nutrients and methods:

Star-K discusses this issue under General Kashrus Issues in their FAQ

Does yeast need kosher certification?

In a word, yes. Yeast is a fungus that has many food applications. Yeast is a fundamental component used in the fermentation of beer, wine and dough. Yeasts are used as flavor enhancers for cheese powders and spice blends. Yeasts are found in nature, and for the most part, natural fungi are kosher. However, commercially produced yeasts are grown and propagated using various media, ingredients and nutrients requiring kosher certification. Natural wine yeasts are found in grapes and would need reliable kosher certification. Autolyzed yeasts are found in beer and would not be kosher for Passover. For these reasons, yeasts require kosher certification.

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  • first of all something that grows on something else isn't the thing itself. for example if a cow ate non kosher food the cow would not become treif nor would a vegatable that is helped to grow using animals blood or a non kosher fish for nutrients. WHy should yeast be any different? – Dude Mar 2 '17 at 5:06
  • "Natural wine yeasts are found in grapes and would need reliable kosher certification." - the prohibition of stam yenom is on drinking wine produced by non Jews not on grapes or on yeast found on grapes so this doesn't make any sense – Dude Mar 2 '17 at 5:07
  • " Autolyzed yeasts are found in beer..." no they aren't. Autolyzed yeast is dead yeast. Dead yeast would not ferment wort into beer. Autolyzed yeast often called "brewers yeast" or "nutritional yeast" found in super markets was once live yeast that was heated until it was killed. It has no more use in beer but has nutritional value and often a cheese like flavor which is good for putting on popcorn – Dude Mar 2 '17 at 5:08
  • Lastly, non flavored beer using the ingredients that follow the Bavarian purity laws known as reinheitsgebot don't require a a hechsher. The ingredients are water, malt, hops, and yeast. These breweries are not using yeast specifically produced with kosher supervision. Why then should yeast require it? – Dude Mar 2 '17 at 5:10
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    @Dude This is the citation from the Star-K. Since I am not an expert in this, you would have to get in touch with them for more details. – sabbahillel Mar 2 '17 at 16:57
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According to the Chicago Rabbinical Council's mobile app:

Yeast requires kosher certification as it is produced via fermentation.

You brought up an interesting point about beer requiring no certification despite containing yeast. I wrote an email to the cRc asking this question, and I got the following reply:

Hi Daniel,

Thank you for contacting the cRc with your Liquor question.

That’s a really good question you’re asking! The answer in short is that yeast is in a funny category. There is a good chance that yeast is always kosher but since it can be produced from non-kosher sources, we recommend that hashgacha be required. If, however, a company used it already (such as a non-certified beer company) the end product would surely be allowed (due to bitul and likelihood of being kosher).

All the best,

Rabbi Akiva Niehaus

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  • which doesn't make any sense given that unflavored beer requires no hechsher given the ingredients of only malt, water, yeast, and hops. – Dude Mar 2 '17 at 0:17
  • also yeast isn't produced via fermentation. It exists naturally on everything now. It can be grown via fermentation as yeast multiplies during the fermentation process which is cause by the yeast but is not how it comes to exists – Dude Mar 2 '17 at 5:11
  • @Dude IINM, the leniency for allowing unflavored beer without a hechsher doesn't apply everywhere. If I remember correctly, the cRc's position is that it only applies to American beer. My guess (and it's just a guess) is that the reason they allow it for American beer is because they have some insider knowledge about the provenance of the yeast used for domestic beers. – Daniel Mar 2 '17 at 14:32
  • This seems unclear. Can you summarize why fermentation, itself, would make something non-kosher, or, rather, why fermentation requires certification. Fresh pressed apple cider (i.e., if I pressed the apples) also ferments. Does that mean that now that it is apple vinegar, it MUST be certified? – DanF Mar 2 '17 at 16:03
  • @Daniel no it isn't only applied to American beers. It applies to all beer that follows the laws established in Germany. Originally no beer required a hechsher even those with fruit in them. The only reason to require one now is b/c not every brewery is using raw ingredients and fruit flavors are sometimes established with syrups and other chemicals – Dude Mar 2 '17 at 20:11

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