text: Tissue contains (amongst other things) proteins and enymes. Both act in concert to cause biochemical reactions.  When you boil proteins you change their structure and therefore the way they behave in a biochemical reaction.  A good way to imagine this is to think of what happens to egg whute (pure protein) when you cook it. When you boil enzymes they become denatured (unable to perform the function for which they were designed).  When you boiled the tissue you denatured the enzymes and changed the sturcture of the proteins and so whatever biochemical reaction was suposed to happen can't happen.  Notice that i did not need to know what you were trying to do or what the full details of the question were.  My answer is a generic truth about the biochemistry of tissue - it ALWAYS works this way.