This calculation represents the pressure stress supported mechanically by packaging sides during thermal treatment, according to the formula:
Delta P = Pressure in packaging – Pressure of autoclave
Delta P = Internal Pressure – External Pressure
<strong>Delta P variations are the result of several parameters:</strong>
• external pressure variation
• temperature variation, resulting in a variation of internal pressure by expansion, vaporization (in the heating phase) or condensation/shrinkage (in the cooling phase).
<strong>Notes on delta P behaviour:</strong>
• For rigid packaging (jar or small diameter can), a reduction in the quantity of gas initially enclosed in the packaging will increase the final value of delta P in the event of heating.
• For semi-rigid or flexible packaging (large diameter can or plastic packaging), the reduction in the quantity of gas initially enclosed in the packaging will reduce the final value of delta P in the event of heating.
• NB: for flexible packaging (plastic packaging), delta P loses its relevance. In effect, the heated packaging would have to be very significantly distorted to perceive a pressure variation. In general, the packaging is denatured before showing a significant variation in delta P. The usable indicator is then delta D (packaging distortion).