Effect of Preservative Concentration, Temperature and Size of Inoculum

EFFECT OF PRESERVATIVE CONCENTRATION, TEMPERATURE AND SIZE OF INOCULUM

Changes in the efficacy of preservatives vary exponentially with changes in concentration. The effect of changes in concentration (concentration exponent, η) varies with the type of agent. For example, halving the concentration of phenol (η = 6) gives a 64-fold (2⁶) reduction in killing activity, whereas a similar dilution for chlorhexidine (η = 2) reduces the activity by only fourfold (2²). Changes in preservative activity are also seen with changes in product temperature, according to the temperature coefficient, Q₁₀. Thus, a reduction in temperature from 30 °C to 20 °C could result in a significantly reduced rate of kill for Escherichia coli, fivefold in the case of phenol (Q₁₀ = 5) and 45-fold in the case of ethanol (Q₁₀ = 45). If both temperature and concentration vary concurrently, the situation is more complex; however, it has been suggested that if a 0.1% chlorocresol (η = 6, Q₁₀ = 5) solution completely killed a suspension of E. coli at 30 °C in 10 minutes, it would require around 90 minutes to achieve a similar effect if stored at 20 °C and if slight overheating during production had resulted in a 10% loss in the chlorocresol concentration (other factors remaining constant).

Preservative molecules are used up as they inactivate microorganisms and as they interact non-specifically with significant quantities of contaminant ‘dirt’ introduced during use. This will result in a progressive and exponential decline in the efficiency of the remaining preservative. Preservative ‘capacity’ is a term used to describe the cumulative level of contamination that a preserved formulation can tolerate before becoming so depleted as to become ineffective. This will vary with preservative type and complexity of formulation.