Articles

By Rabbi Dovid Cohen, Administrative Rabbinic Coordinator

Q. Can I use the same knife sharpener for meat and dairy knives?  How about giving my knives to a sharpening service?

A. Sharpening a knife essentially involves intensely rubbing the knife against a rough surface or rubbing the surface against the knife.  Any food residue left on the knife will transfer to the sharpener (and vice versa), and, therefore, if you want to use the same sharpener for meat and dairy knives, you must make sure that all knives – meat and dairy – are completely clean before you do so.  That means that if you sometimes sharpen a knife while you’re in the middle of using it, such as while you’re cutting a piece of meat, then that sharpener cannot be used for both meat and dairy knives.  But if you would only sharpen a clean knife, you can use the same sharpener for meat, dairy, and pareve knives.

The friction of sharpening creates heat, and we know that heat causes ta’am (absorbed taste) to transfer, even if there is no physical residue.  If so, even if the knives are perfectly clean, why aren’t we concerned that meat and milk ta’am will pass between the knife and sharpener?  The answer is that there is a principle that – even in the presence of heat – absorbed ta’am does not transfer between utensils unless there is a liquid medium to help with that transfer (ain habaluah yotzeh m’kli l’kli bli roteiv).  Sharpening a knife is a perfect example of this: there is no liquid between the knife and sharpener, and, therefore, absorbed ta’am cannot transfer between them.  [Sharpening qualifies as “duchkah d’sakinah”, but that is only a factor in the presence of a davar charif, which is impossible if the knife is clean.]

When sharpening is done by a professional sharpening service, the knife will always be clean before they start.  In most cases, this is because the customers send clean knives in for sharpening.  If, however, you send them a dirty knife, the service will wash it before they sharpen it, which is an obvious kashrus concern, since they also wash non-kosher knives.  Therefore, you must (a) be sure that your knives are always perfectly clean before you send them to be sharpened, and (b) make sure that the service does not wash all knives even if they are clean.  [Some professional sharpening machinery will have a continuous flow of cold water spraying on the sharpener which means that there is a “liquid medium”.  However, in those cases, the water will cool the knife and sharpener, so that it is not considered “hot” and ta’am cannot transfer.]

A more significant concern is that some sharpening services – especially ones that work with restaurants or “company” stores (e.g., Cutco) – will replace knives that get ruined (or misplaced) during sharpening.  Thus, the kosher consumer or restaurant should mark their knives so that when they come back from the sharpener, they can confirm that these are the kosher knives which were sent in for sharpening.

This article first appeared in the Let’s Talk Kashrus column, Yated Ne’eman, July 19, 2024.