What Makes A Food

The Chinuch in mitzvah #160 explains the reason for the need for hechsher for foods to be suspectable to accept tumah because the food is only considered complete after liquid has been applied to it.  It is clear that he holds the hechsher gives the good the shem food.  This is the opinion of Rashi as well who says that it’s not called a food until water has fallen upon it.  This would seem to be supported by thew Gemorah in Chullin 118a that says fruits that haven’t had water put on them are like incomplete vessels.  Another support is from the Gemorah in Kiddushin 59 that compares the water on the fruits to the competion of vessels.  The Toras Kohanim has a ריבוי  to include that besides tumah from a שרץ, a food that has been huchshar can also be susceptible to tumah it is  in the airspace of an earthenware vessel.  Why do we need a ריבוי  if the point of hechsher is that it is considered a food, what difference does it make what the form of tumah is?  It would seem from here that it is a din in the tumah, the Torah gives a law in tumah that it takes effect only after the food has become wet.  This fits with the reason of the Ramban that tamei things will stick to a wet food (Bad Kodesh volume 4 #60 and on the possuk.)

The Rambam Tumas Ochlin (12:1) holds that the fruits become huchshar latumah only if the water fell on them and the owner wanted it to get wet.  The Kesef Mishne cites the Rashba holds it helps if the one that put the water on the fruits wanted it.  However, the Rambam holds that doesn’t suffice.  The Kesef Mishne explains that someone else can’t cause tumah to what isn’t there’s.  Rav Shmuel Rozovski (Shiurai Bavva Metzia siman 19) adds explanation based upon the previous sources that the point of hechsher is to complete the shem food.  If so, only the owner can complete the shem food just like only the owner can make his utensil complete to be susceptible to tumah.
See more about this here.