The Torah refers to the holiday of Pesach as Chag HaMaztos – the Holiday of Matzah (unleavened bread). Matzah is defined as bread made from one of the five grains (wheat, rye, oats, barley, and spelt) that has not been allowed to ferment. When any of these grains has fermented it is called Chometz (leavened bread) and the Torah prohibits us from eating, owning or deriving any benefit from it during Pesach. The Torah actually refers to two distinct forms of leavened grain – Chometz and Se’or. The world “leaven” actually means “risen”, and Chometz refers to bread that has been leavened. Se’or refers to a leavening agent – essentially a sour dough starter for those familiar with bread making.

Leavening is the process by which gasses are created and trapped in dough so that it rises, and these gasses can be created by either chemical or biological reactions. Chemical leavens, such as baking soda (sodium bicarbonate), work by reacting with acids present in the dough to produce carbon dioxide and are typically used in crackers and cakes. Yeast, on the other hand, is a biological leaven. The yeast cells eat sugar in the dough and secrete an enzyme called zymase (zymose is the Greek word for yeast). When baking bread, the zymase converts the rest of the sugar into carbon dioxide (and a small amount of alcohol) – a process called fermentation – which in turn causes the bread to rise.

Yeast cells are abundant in nature, and by merely leaving dough or a sugary solution open to the air, yeast from the air will grow and react with it. However, in order to ensure a good concentration of yeast, bakers long ago realized that if they allowed dough to become very leavened and rich in yeast cells, a small amount of this heavily leavened dough would be able to leaven other batches of dough both quickly and consistently. This strong, yeasty dough is the type of leaven called Se’or, and it is this type of leaven that is prohibited on Pesach. Indeed, the Talmud (Beiza 7b), in discussing the reason why the Torah needed to list both Chometz and Se’or, notes that Se’or is Chimutzo Kasheh (strongly fermented), and has the power to ferment other dough. Yeast itself, however, is neither Chometz nor Se’or and, if it is not grown on chametz, may permitted on Pesach. Halachic authorities (O.C. 442:5, see Mishna Berura) note, however, that if the yeasts were once grown on Chometz they would remain prohibited on Pesach even if subsequently grown on non-Chometz material

We can see that yeast per se is not Chometz from the production of wine. When a sugary substance is fermented in the absence of oxygen (anaerobically), the fermentation tends to create much more alcohol than in the leavening of bread, and it is this process by which all alcoholic drinks are produced. When squeezing grapes, the yeasts found naturally on the surface of the fruit react with the juice, converting the grape sugar into alcohol and producing wine. [Modern wine makers, desiring to make consistent products, often destroy these natural yeasts and then add domesticated wine varieties which may be grown in problematic ways – thus making hashgacha l’Pesach important]. We certainly drink wine on Pesach (four cups of it at the Seder, no less), so it is clear that the yeasts used to produce such wine are Kasher L’Pesach. Such yeasts can also be used to ferment other sugary substances – and produce Passover-approved alcoholic drinks. For example, if honey is fermented with yeasts the result is mead, which has the potential to be Kasher l’Pesach. Alcoholic fermentations typically produce concentrations of 15%, and spirits of higher proof are produced by concentrating the alcohol through distillation.

Beer poses an interesting question when it comes to Pesach. The scientific name for the species of yeast used in alcoholic fermentation is Saccharomyces cerevisiae, or sugar eating fungus. Cerevisiae is the Latin word for brewer, but some beer making may not involve Chometz at all. The Talmud (Pesachim 107a) records four different types of beer, which are (as explained by the Rashbam): Shechar (date beer), Pirzuma (barley beer), T’ainy (fig beer), and Asni (berry beer). The Gemorah states that barley beer was considered superior to date beer and, indeed, barley beer is the predominant type imbibed in the Western world. Barley beer is certainly Chometz (Pesachim 3:1), since the barley (one of the five major grains) is soaked in water to malt (allow the barley germ to produce enzymes that convert the barley starch into sugar), after which the sugar is fermented by yeast to create alcohol. Fruit beer, however, is nothing more than fruit wine, where the natural sugar in the fruit is fermented with yeast to produce alcohol – which has the potential (again depending upon the origin of the yeast) of being Kasher L’Pesach. Indeed, it seems that in Bavel (Babylonia) this was indeed the case. The Talmud (Pesachim 8a) notes that in Bavel, one was required to make a Bedikas Chometz (search for leavened bread) in a beer cellar, just as one is so required in a wine cellar, lest one had inadvertently left a sandwich between the barrels. It seems that the concern for Chometz was restricted to the sandwich – and not the beer!

Regardless of its base material, however, beer made from fermented sugar would tend to be sweet, not the bitter, astringent beer that is preferred in many cultures. To address this challenge, beer makers have historically added a variety of flavorings to their creations, such as wild rosemary, coriander, ginger, anise seed, juniper berries, and wood bark. The most popular additive, however, is the flower from a vine called hops, which serves to give both a bitter bite to the beer as well as to help preserve it. It also has mild sedative properties, and thus complements the effects of the alcohol. The Talmud (Moed Katan 12b) notes that “Keshusa” is used for beer. Rashi translates Keshusa as “Humlin”, which is in turn translated by the Metargem as “hopfen” – hops. [The word humlin actually comes from the Roman description of the wild vine that grew “like a wolf among sheep”, and gave it the name Lupus salictarius (“the good wolf”), from which hops took its modern botanical name Humulus lupulus.] Today, the hops flower is added directly into the beer vats, or an extract of the rosin of the hops (rich in alpha acids) is used as a flavoring. From a Pesach perspective, hops poses no Chometz concerns, so there should be no difficulty in producing a Kasher L’Pesach beer provided the source of the sugar (and of course the yeast) are acceptable. Indeed, one brewery has decided to make a “Honey Beer” that is Kasher L’Pesach!

The Meforshim explain that the risen dough of Chometz symbolizes laziness and conceit, and on Pesach we at only Matzah to symbolize a higher level of Keudsha and holiness. During the forty years that Bnei Yisroel were in the desert, Hashem sustained them with Mon. Hashem told them (Devarim 29:5) that “You have not eaten bread, and wine and beer you have not drunk, in order that you should know that I am the L-rd thy G-d”. The Ksav Sofer explains that these three leavened foods have the ability to detract from one’s level Kedusha and during the time in the desert Hashem replaced them with the Mon that was imbued with spirituality. Today, we do not merit the direct sustenance of the Mon, but it is nevertheless incumbent upon us to take all of the rich food that we do have – bread, wine, and beer – and ensure that we use them to recognize the blessings that Hashem has given us.