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Knife Skills Illustrated: A User's Manual suggests that knives can be cleaned with hot running water alone. "With sufficiently hot water, soap should not be necessary." Is this safe? What if I am only cutting vegetables? What if I am also cutting raw meat?

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Why does the book want to avoid soap? I mean, yes, in theory, with hot enough water and appropriate scrubbing to remove any particulates you could be OK (though I'm not clear that most residential hot water systems are actually hot enough for that), but why not use soap? It helps move things along and get things off the knife. What's the supposed advantage to to not using it? –  Michael Kohne Jul 23 '12 at 0:25
    
The book doesn't go into any detail on why one would not want to use soap. –  Michael Hoffman Jul 23 '12 at 0:31
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Regardless of safety (=killing bacteria), water cannot dissolve fat, while soapy water can. If you don't use soap, your knives will stay greasy, smearing old rancid fat on anything new you cut with them. –  rumtscho Jul 23 '12 at 11:05
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Note that the book does not say "avoid" or "do not use" soap. It says soap "should not be necessary". Hot water can make fats melt or dissolve so they can slide off the knife. Soap, in itself, does not sanitize anything. Something can be clean but not sanitary and sanitary but not clean. –  Rob Jan 11 '13 at 14:21
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up vote 10 down vote accepted

In order to "Sanitize" any surface you must wash that surface (in this case, knife blades) with water at no less than 190°F (~88°C) according to the (U.S.A) National Sanitation Foundation (NSF). Earlier this year the Conference for Food Protection recommended reducing that standard to 160°F (~71°C) as an "energy saving measure" citing a study at Ohio University that claims that it is equally effective. Additionally they recommended that the temperature could be reduced to 120°F (~49°C) with 'chemical additives' (soap).

So the question is "How hot is your hot running water?" According to "Wikipedia" (a dubious source at times, but I will accept it for this point) suggests that most home water heaters produce water at 104°F (~40°C) to 120°F (~49°C). This would suggest that to sanitize a knife blade something more than 'hot running water' is necessary. In fact since water over 131°F (~55°C) puts you at risk for scalding (Wikipedia, Ibid) and you certainly don't want a water heater putting out water that hot, I would have to disagree with your source and conclude that in household use soap is required to sanitize your knife blade.

note: in order to qualify my answer to the question of "safe", I have elevated the standard from "clean" to "sanitized", certainly you may 'clean' a knife with running water but that level of clean is not what I would consider 'safe'.

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You could, of course, always heat the water in a pot. –  BobMcGee Jul 23 '12 at 2:51
    
Yes, Bob, you could. But the QUESTION was about "hot running water" ;) –  Cos Callis Jul 23 '12 at 3:12
    
FYI, According to this new article mysundaynews.com/2011/06/02/…, the US DOE is recommending 120F at the tap. –  Michael Kohne Jul 23 '12 at 12:24
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Even if it is actually at that recommended temp, 120°F is insufficient to sanitize without additives. –  Cos Callis Jul 23 '12 at 12:27
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Somehow I believe that begins to stray from OP's intentions. –  Cos Callis Jul 23 '12 at 14:18
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