but typically occurs within 1 to 15 days at 50 u00b0 c in another embodiment of the method of the present invention the tissue may be heat treated in a surfactant solution e g tween 80 with or without ethanol and or formaldehyde or in a physiologic solution e g saline or a balanced salt solution at a temperature between about 37 u00b0 c and 60 u00b0 c preferably about 45 u00b0 c for about one hour to six months preferably about one to 15 days and then heat treated in a glutaraldehyde solution as described above prior to concurrently with or after the tissue treatment step the tissue may be cryopreserved or otherwise preserved i e by fixation b an example of a method for manufacturing a fixed heterologous heart valve bioprosthesis having mitigated propensity for post implantation calcification fig2 is a flow diagram of a specific process for manufacturing a bioprosthetic device such as a stented or stentless porcine heart valve or bovine pericardial heart valve of the type referred to herein the following is a description of the exemplary process shown in fig2 initially an aqueous solution of 1 8 by weight glutaraldehyde is prepared in a clean inert vessel e g a vessel made of stainless steel plastic or borosilicate glass and such solution is then buffered to the ph of a approximately 7 4 by adding phosphate buffered saline solution the glutaraldehyde in the solution is then preheated such preheating of the glutaraldehyde is accomplished by heating of the solution to about 70 u00b0 c u00b1 5 u00b0 c and maintaining such temperature until the ph of the solution falls to approximately 6 0 at this point the color of the solution can be colorless to golden or brown the fall of the.