While our bodies are designed to be alkaline, cells produce acid as a by-product of their normal activity. The acid waste matter thus produced is reduced to carbon dioxide and water which are excreted harmlessly from the body. When food is consumed and metabolised, however, not all of it is used up. So while a fruit will taste acid and present and acid pH if tested raw, its ash will be alkaline and so will its effect on the body. The small amount of protein in such foods will not change this.
Vegetables behave in the same way.
These are all acid. The body has to rid itself of its acid wastes. This type of acid ash cannot be eliminated through the lungs as carbon dioxide and water in the same way as cellular metabolism. Instead the body has to buffer the ash with alkaline substances in order to neutralise it. Buffering takes place both inside and outside the cell, the majority of the buffering occurring in the blood itself.