Soap and Kashering Utensils: Answers to Responses

Since it was claimed here three weeks ago the fact that nowadays utensils do not absorb and emit tastes is due to the invention of soap, and involves halakhic implications – I received numerous comments * This claim does not contradict common halachic practice, since our Sages already permitted utensils that a non-Jew actually checked and said have no taste * Publication of the claim also does not contradict common halachic custom, and consultation with the public in general refines study in the Beit Midrash * A letter from a chemist explains: The significant change in the use of soap which made the removal of fats much more effective, occurred less than a century ago

Summary of the Previous Column on the Subject

In a column published in the Torah portion of ‘Matot’, I briefly explained the issue of kashering utensils, while addressing the central question that has arisen in the last generation:  from the words of the Talmud and poskim, it appears that metal vessels absorb and emit tastes, while in practice, we do not discern the emission of tastes from metal utensils. As a result, some poskim claimed that today, metal utensils are exempt from kashering, but in my article, I explained that the obligation of kashering utensils does not depend on the sense of taste, but nevertheless, it is forbidden to use a treif (non-kosher) utensil that was not kashered according to the way it was used – if it was used with something cold, it is kashered by washing it in cold water; if it was used for cooking, it’s kashered by hagalah (immersion in boiling water); if it was used with fire, such as a skewer, it’s kashered by libun (with a direct flame).

However, since we know for sure that after cleaning metal utensils, they do not emit tastes, be’di’avad (ex post facto), if one accidentally cooked soup in a treif pot, even though the pot must be kashered, the soup is kosher. In addition, in all disputes regarding nat-bar-nat (a secondary taste) and charif (a pungent food), in metal utensils, pots and knives, the opinion of the lenient poskim should be followed, since, in actuality, metal utensils do not emit noticeable taste.

On the issue of kashering utensils, I received dozens of questions, objections, and scholarly responses which can be divided into three areas: 1) on the halakha itself; 2) on raising the complex issue – which should be decided by rabbis – before the general public; 3) on the interpretation of actual fact. I will endeavor to answer the questions.


Many poskim are of the opinion that even if treif utensils do not emit taste, if they are b’nei yomo (utensils that have not been used for hot food for more than 24 hours), the food cooked in them is prohibited, as the halakha is explained in the Shulchan Aruch (Y. D., 93: 1). Also, nat-bar-nat should be taken into account, i.e., Ashkenazim poskim hold that if one cooked a parve food in a ben yomo baseri utensil, it is forbidden to eat it with dairy, because of the taste absorbed into the parve food (R’ema, 95:2).

However, it seems more likely that these halachot were said in cases where taste cannot be determined, whereas when we know there is no taste – a treif utensil does not prohibit food cooked in it, and a basari utensil does not turn a parve food into being basari. The proof for this is that our Sages relied on a k’feila, i.e. an expert, non-Jew, who, if he said that the cooked food did not have a treif taste, it is permitted to be eaten (Chulin 97a, b). Although, due to various concerns, we do not rely on a k’feila (Re’ma, 98:1), however, all this is in specific circumstances, but when it is clear there is no taste – even R’ma agrees there is absolutely no prohibition (Teshuvot Re’ma 54; Maharsham 3: 377). To reinforce this, a few years ago in classes I gave, I asked the residents of the community Har Bracha if anyone could taste the flavor of food cooked previously in the same pot. Hundreds of people checked this for several years, and no one who had normally cleaned the pot discerned tastes absorbed in them. There were, however, some people who reported traces of tastes, but it turned out it was because they had not cleaned the pot well. Scientific studies also confirm this fact.

This halakha stands by itself, even if we do not know how to explain the differences between utensils in the days of our Sages and the Rishonim, and those of today.

Nevertheless, if be’zadon (maliciously), someone cooked in a utensil that required kashering, even though it does not emit taste, the cooked food is prohibited to the person who cooked it and for those it was cooked for, as many Rishonim and Achronim determined. Although there are lenient poskim, in my humble opinion, it seems proper to be machmir (stringent) in this, in order to reinforce the obligation of kashering utensils.

Presenting the Issue Publicly

Some people argued that it is wrong to present a serious issue that should be debated among the rabbis in a popular newspaper, especially when it comes to conclusions that seem somewhat different from the norm (both for chumra (stringency), in the chiddush that utensils are obligated to be kashered even if they do not emit taste, including glassware, and for kula (leniency), in the case of nat-bar-nat). Others argued that this should have been discussed by talmedei chachamim first.

There are many answers to these arguments, and I will mention one. Torah study has two parts: one – exactness in the methods of the Rishonim and Achronim, which is an issue for talmedei chachamim, and the other – general conjectures and informed explanations from the natural and social sciences. Thanks to presenting the issue to the general public, which is made up of both talmedei chachamim and people knowledgeable in various fields, the Beit Midrash is broadened, and learning becomes tremendously enhanced. In practice, it turns out that I involve the public in many issues, and with the responses of talmedei chachamim, scientists, and the opinions of various people, I reexamine things together with talmedei chachamim from the yeshiva, and consequently, the issue is clarified thoroughly. Below, I will cite an informed letter I received thanks to sharing the issue with the public.

Actual Fact

The question regarding actual fact is difficult: It is clear that our Sages, as well as the Rishonim and Achronim, related to metal utensils as absorbing and emitting tastes, and there is absolutely no doubt that this was the case. On the other hand, from what we now know, metal and glassware do not absorb and emit tastes, because taste molecules are much larger than the spaces between the particles from which the metals are composed. There are lamdanim (Torah students) who are comfortable living with such questions, but in the eyes of ordinary people, such a question requires Torah scholars to investigate and clarify what our Sages said, and the halakha according to the reality they spoke about.


Some explained that metals today are composed of different materials and therefore do not absorb tastes, while the metals used in the days of our Sages were cruder and taste particles seeped into them. Others explained that indeed, even in the past it was impossible for taste particles to penetrate metal, but in the context of the metal industry in the past, casting was not uniform and complete, and therefore particles of taste penetrated into spaces left in the metal. By the way, this explanation sounds a little more logical, but still not sufficient, because our Sages said that the absorbed tastes can be as strong-tasting as the volume of the side of the utensil. Therefore, I was inclined to explain that most of the tastes came from what was stuck to the utensil, and the main change that occurred in recent generations is that only about two hundred years ago, the method of industrial detergent production, which is the basic substance for soap, was discovered in France. Gradually, its use became widespread until about a century ago, liquid soap began to be manufactured in Germany, which, within decades of being used, became widespread throughout the world.

Concerning this explanation, others justifiably questioned: why then, only in the last fifty years did we begin to discern that utensils do not emit tastes, and not two hundred years ago? The answer to this, along with an informed explanation, I fortunately received from Yitzchak Yaffin from Kedumim, a chemist who for years worked as manager of a laboratory for cleaning products, who, together with his wife Yehudit, a chemistry teacher, attempted explaining the issue in a relatively simple language.

A Letter from Yitzchak Yaffin

“Thank you very much, Rabbi, for your article on the laws of bli’at kelim. Rabbi, with your permission, I have comments regarding terminology and scientific accuracy. They do not concern the practical and halakhic conclusions.”

“The essence of the argument: Soap has been known for thousands of years as a substance having the characteristics of cleansing and removing fats, but not on a sufficient level. It is true that in 1790 Leblanc discovered an industrial way of producing a substitute for ash, which led to a considerable increase in the use of soap and enabled its mass industrial production, but the efficiency of fat removal was still unsatisfactory. The significant breakthrough occurred in the 1930’s, with the development of much more effective synthetic detergents. It seems to me, Rabbi, you were alluding to this development from about a century ago, and not necessarily the invention of soap, which is not new.”

“In detail: The difficulty in removing fats stems from a well-known phenomenon – water does not mix with oil. However, there are substances that enable the mixture of water and oil. These substances are called “surfactants”, or surface active agents. The secret of the action of these substances lies in the structure of their molecules: The molecules of these substances have two parts – one similar to water, and capable of combining with water, and the other similar to oil, and capable of combining with oil. Thus, these molecules are capable of combining water with oil, and washing it away (some call these substances “detergents”, but the term ‘surfactants’ is scientifically more accurate).”

“Already thousands of years ago, it was known that when oil is cooked with ash, soap is produced, which is a substance capable of cleaning and removing fats. Today, we also know the reason: When oil is cooked with ash, a new molecule containing residual oil is created. This fatty residue is the part of the molecule which is capable of combining with oil. This molecule also has a water-like part, capable of combining with water, and consequently, the soap acts as a surfactant, although, as mentioned, not so effectively.”

“Modern chemistry has developed ways of synthesizing artificial surfactants. These substances allow a much stronger connection between oil and water, which can produce much more effective detergents. Noteworthy, is the first synthetic substance, Alkyl Benzene Sulfonate, which is still used today as a major ingredient in liquid dishwashing soap and other detergents (in the 1960’s it was slightly modified for environmental reasons).

Summary of the Explanation for the Time Being

The change that has occurred in relation to utensils is first and foremost the ease with which we can now clean dishes, because today, soap is of high quality and inexpensive, and every house has running water, and as a result, all utensils that are cleaned normally, do not emit tastes. In the past, however, even when people attempted to clean utensils, since soap was extremely expensive and ineffective, and additionally, houses had no running water, a thin layer of residue of foods almost always remained stuck to the utensils whose taste was highly concentrated, given their liquids had evaporated. The other change which is also significant is that in the past the surface of metal was generally rough making it harder to clean, whereas today, the metal industry has greatly improved, and the surface of metal is as smooth as glass and easier to clean. And certainly, there are no crevices left unfilled in the casting.

This article appears in the ‘Besheva’ newspaper and was translated from Hebrew.

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