Quantifying the hurdle concept by modelling the bacterial growth/no growthinterface

The hurdle concept described eloquently over many years by Professor Leistn er and his colleagues draws attention to the interaction of factors that af fect microbial behaviour in foods. Under some circumstances these effects a re additive. Under others the implication is that synergistic interactions lead to a combined effect of greater magnitude than the sum of constraints applied individually. Predictive modelling studies on the combined effects of temperature and water activity and temperature and pH suggest that the e ffect of these combinations on growth rate is independent. Where the effect of the two factors is interactive rather than independent is at the point where growth ceases - the growth/no growth interface. An interesting and co nsistent observation is that a very sharp cut off occurs between conditions permitting growth and those preventing growth, allowing those combinations of factors to be defined precisely and modelled. Growth/no growth interfac e models quantify the effects of various hurdles on the probability of grow th and define combinations at which the growth rate is zero or the lag time infinite. Increasing the stringency of one or more hurdles at the inter-fa ce by only a small amount will significantly decrease the probability of an organism growing. Understanding physiological processes occurring near the growth/no growth interface and changes induced by moving from one side of the interface to the other may well provide insights that can be exploited in a new generation of food preservation techniques with minimal impact on product quality. (C) 2000 Elsevier Science B.V. All rights reserved.