or how fast they can grow. Perhaps a better way to visualise it is as a
protective wall; each adverse condition contributes one or more layers of
bricks to the wall. The overall height of the wall will determine which
organisms are able to climb it and how fast they will grow once they have
done so. Scientific description of this multi-factorial technique for pre-
serving foods may be relatively recent but the concept has been applied
empirically since antiquity in numerous traditional products such as
cheese, cured meats, smoked fish and fruit preserves, all of which rely on
a number of contributing factors for their stability and safety.
When confronted with this situation there are three basic approaches
to predicting the fate of particular organisms. The first is to seek an
expert judgement, based on the individual expertise of a food microbi-
ologist and their interpretation of the published literature. While this can
be useful qualitatively, it rarely provides reliable quantitative data.
To its credit, the food industry has generally not placed too great a
reliance on this sort of approach but has resorted to the challenge trial.
In this, the organism of concern is inoculated into the food material and
its fate followed through simulated conditions of processing, storage,
distribution, temperature abuse, or whatever is required. Though it
provides reliable data, the challenge trial is expensive, time consuming
and labour intensive to perform properly. It also has extremely limited
predictive value since its predictions hold only for the precise set of
conditions tested. Any change in formulation or conditions of processing
or storage will invalidate the predictions and necessitate a fresh challenge
trial under the new set of conditions.
The third and increasingly popular approach is the use of mathemat-
ical models. A model is simply an object or concept that is used to
Figure 3.14 The hurdle effect
Chapter 3 53