Development and use of rapid microbiological methods in food quality assessment.

Adenosine triphosphate (ATP) assay and electrical measuring techniques were assessed for their ability to produce an estimate of total colony count. ATP assay could be used for estimating the microbial flora in foods, provided that non-microbial ATP was removed from the sample first. Linear relationships between log[10] microbial ATP content and log[10] colony count were obtained. Electrical measuring techniques provided an inverse linear relationship between electrical detection time and log[10] colony count. The capacitance element of the impedance signal was found to be superior to the conductance element for the estimation of pure cultures and the flora of two foods, since faster detection times and higher correlations with colony count were obtained. Capacitance detection times at elevated temperatures (35° and 25°C) provided a good estimate of psychrotrophic colony count at 15°C. The use of electrical measuring techniques was also investigated for the rapid detection of salmonellae. Most salmonellae tested could be detected electrically in pure culture by the production of a specific signal from the fermentation of dulcitol or by an inhibition of this signal in the presence of a Salmonella-specific bacteriophage. However, when these two tests were used for artificially contaminated foods, an unacceptably high number of false-negative results was obtained. Therefore, further tests were investigated for their potential in the electrical detection of salmonellae. These were: effect of antiserum, lysine decarboxylation and hydrogen sulphide production. The final section of this thesis was concerned with the use of conventional and rapid microbiological techniques in the analysis of bacterial growth/temperature relationships. A Square Root plot (√r = b(T-T[o])) was found to be preferable to the Arrhenius plot (k = Ae[-muu/RT]) in describing the growth of psychrotrophic food spoilage bacteria at chill temperatures. The Square Root plot was also applicable to mixtures of organisms and to ATP and capacitance detection time data. The relevance of such work to the investigation of food spoilage was discussed