Aqueous solutions in sealed containers: internal pressure under autoclaving

ABSTRACT

This article is a technical support to the practice of autoclaving aqueous solutions in sealed containers. Another article of his, Autoclaving aqueous solutions in sealed containers: feasibility and risks, provides a more practical guide on the matter. Two “classical” scientific contributions on the matter are due to Beck and to M. A. Joyce & J. W. Lorenz and referred in the article. In this article, metric units (abs bar or abs mbar) are used for the pressure.  1 abs bar is approximately equivalent to 14.504 psia.

KEYWORDS:

moist-heat sterilization, steam sterilization of aqueous solutions, sealed containers, headspace volume, internal pressure, autoclaving, autoclave, superheated water sterilizers, steam/ air mixture sterilizers

INTRODUCTION

The pressure in the headspace of sealed containers filled with liquid solutions is the sum of:

a) the pressure of the vapor in equilibrium with the liquid of the solution, i.e. vapor pressure of the liquid solution at the actualtemperature;

b) the actual pressure of the gases escaped from the liquid solution;

c) the actual pressure of the gas initially filling the head space.

In the case of liquid mixtures of non-soluble components (i.e. of multiphase liquids), the addendum a) is to be replaced by the sum of all the vapor pressure addenda of the single components of the liquid itself. In spite of their pseudo-homogeneous aspect, emulsions are a typical case of multi-phase liquid: thus, their vapor pressure is the sum of the vapor pressure addenda of the single liquid phases at the actual temperature, regardless to the weight percentage of the single components of the liquid mixture. The volume of headspace depends in turn on the temperature, due to the effect of thermal expansion of the liquid, that tends to reduce the head space, and to the opposite effect of the thermal expansion of the material by which the container is made, that tends to increase the head space.

An important question is: how will the change of the headspace volume affect the total pressure in it under autoclaving?

A) Vapor pressure. The first out of the above pressure addenda depends only the temperature: the vapor pressure of the liquid solution follows the P versus T curve of the vapor and liquid equilibrium of the solution.

B) Escaping of dissolved gases. […]