Shira Cohen is studying Nutrition at the School of Dietetics and Human Nutrition at McGill University, specializing in Global Nutrition.
The process starts by heating a mixture of some sort of fat, a “triglyceride” in chemical terms, with a strong base such as sodium or potassium hydroxide, generally referred to as “lye.” The result of this “saponification” reaction is soap, which is the sodium or potassium salt of a fatty acid. Glycerol is also produced as a byproduct and the extent to which it is removed affects the texture and appearance of the soap. Any unreacted lye also has to be removed because lye is a potent skin irritant. Soap, whether kosher or not, is made with lye so that any suggestion that kosher soap is unsafe because it is made with lye is unfounded.
The soap molecule has one end that is water-soluble and the other fat-soluble. The latter anchors in any greasy residue while the former is attracted to water, so that rinsing with water pulls the greasy material from a surface. Any animal or vegetable fat can be used to make soap and that choice is relevant when it comes to “kosher soap.” According to Jewish dietary laws, animals have to be butchered is a specific fashion that causes no pain, and all blood has to be removed from the meat. But since soap is not a food, the laws of kashrut really are not applicable. Still, very observant Jews may be concerned about soap residues on dishes and may want to avoid any product that is made from non-kosher animals. When it comes to cleaning efficacy, there is no difference between regular soap and the kosher variety.
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