The Talmud tells us that if one cut a cucumber with a knife, and the knife had residue of meat, dairy, or something not kosher, all we need to is clean off the cucumber where we cut it, and we are good to go. However, if we cut "tznon" (usually means radish, but may also mean other pungent vegetables), because of the sharpness of the tznon, plus the pressure of the knife, the tznon becomes meat, dairy, or non kosher, depending on the status of the knife. Here, the Ashkenazi codifiers take what I consider a huge flight of fancy. Even if the knife was perfectly clean, the food cold, and even if the knife hadn't been used in over twenty four hours (which normally is considered as imparting a bad taste, if any), the combination of pungency and pressure resurrects the original tastes absorbed in the knife. Hence, this combination is even more powerful than heat in bringing out flavors. On a practical level, this means that if I cut an onion with a knife that was perfectly clean, but had sometime in the past been used for meat, that flavor has now reemerged, and renders the onion "meat", which, if cooked with dairy, would render the combination non kosher.Similarly, if I had a sandwich that contained chili peppers (one of my favorite foods), and I borrowed my non Jewish friend's knife to cut my sandwich, that pork chop he cut into last January would now render my chili peppers, and hence my sandwich, non kosher. In the last three hundred years, some have gone further. Since we are not certain of the exact identity of "tznon", we must consider any acidy food as "sharp", with some applying this rule even to an apple! The practical result of this idea, is that most Ashkenazim maintain a separate set of knives and cooking utensils for pareve, as your onion, radish, or hot pepper can easily be rendered "meat" or "milk".Sepharadic authorities look upon this with amazement. How does an old, now vile flavor in a knife get resurrected as a good flavor by something sharp?We look in vain in Talmudic sources for this idea. So what was the Talmud telling us, exactly? We know from other Talmudic passages, that knives were generally kept for long periods between cleanings. In fact, in the law of "eruv tavshilin", food put aside before a Yom Tov to permit cooking on Yom Tov for Shabbat, we may rely on the congealed fat on our kitchen knives. Since eruv tavshilin require at least an olive's bulk of food, that means that on an average kitchen knife there is that much grease! Therefore, the Talmud is actually saying that a greasy knife, if cutting ordinary, non pungent food, needs nothing more than rinsing off the grease from the food. If pungent, we must see the grease as having been absorbed into the outer layer of the food. No way can a cold, clean utensil have its flavor extracted by something sharp. If it is clean, and hasn't been used in over a day, even with hot food, it would do no harm, although we don't do this in the first instance. An interesting aside: the Talmud tells us that if a knife has become non kosher, or meat or dairy when we don't want it to be these things, it may be plunged ten times into hard ground instead of kashering, since this did not occur under conditions of heat. This is probably the origin of the common, mistaken folk belief that kashering is done by burying the utensil. That about sums it up for kashrut. One side issue is that in the Land of Israel. there are many additional laws regarding produce. These laws mostly do not apply outside the Land, unless eating imported Israeli produce. A few do actually apply everywhere, but the consensus of opinion is that this would only be a concern if we knew that a problem existed, rather than just fearing a possibility. These need be of no concern to the consumer. I hope this series has been helpful!