I have seen several references to the sulfur in a knife's steel being then culprit in food discoloring or the taste of the food after cutting, and other things.
Knife Steel has from .005% to .05% sulfur. That is a very small amount. Onions ,on the other hand, have a very high sulfur content. If there was a problem with sulfur and onions, it would self destruct.
Apples, avocados, and onions discolor after cutting - it a product of oxidation with the air reacting on organic compounds called polyphenols. This can usually be prevented by keeping air away, or by coating with something that will prevent the oxidation. Fruit-fresh will prevent the discoloration as well as lemon juice.
If the fruit or vegetable was cut with glass, it would still discolor. An avocado will discolor as you watch. Bruising of the fruit will cause the polyphenols to be released from the cells, and make the bruise dark.
The commonly quoted tale of "being able to taste the knife" is mostly sales hype. Most of these stories were created by the stainless knife industry to get people to switch from carbon blades. Since there was no real attribute with stainless, and it was generally less sharp and durable ( in the early days), the sales pitch about taste was created.
Now days, the same story is told concerning ceramic blades to say that stainless and carbon knives will make the food taste bad, and ceramic won't.
In a few years, they will claim that ceramic leaves a funny taste compared to laser knives.
Final info:
Some people will "prove" that carbon blades have a taste by licking one or putting it in their mouth. They taste a sour or metallic taste, and say they taste the blade. Stick a nearly dead 9V battery on your tongue and you will taste the same thing...but a lot stronger. What the knife taste is, is the taste of the galvanic action with the electrolyte being saliva. Your tongue is the cathode and the blade is the anode. The 9V battery adds your tongue and saliva to the existing group of cells and your taste buds have electrons pulled off them in the process. The nerves in your taste buds are just electrical sensors. The taste buds and nerves that taste "Metallic taste" are sensitive to the galvanic action of metal ions in and on food....or knife blades. However, there is no measurable amount of steel removed when cutting a vegetable, certainly not enough to taint it enough for the tongue of the average human to detect the ions. When food that is very acidic is cooked in iron pots, or exposed to carbon steel ( or copper) for enough time ( and/or heat is added) enough metallic ions can be transfered to taint the flavor.
