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Effects of pH, Acidulant and Sub-Optimal Temperature on the Growth and Survival of Yersinia enterocolitica.

Little, Christine Louisa. (1992) Effects of pH, Acidulant and Sub-Optimal Temperature on the Growth and Survival of Yersinia enterocolitica. Doctoral thesis, University of Surrey (United Kingdom)..

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Abstract

The effects of pH, acidulant and sub-optimal temperature on the growth and survival of Yersinia enterocolitica were measured. These responses were subsequently modelled with the view to predicting the organism's behaviour in foods which depend on these factors for their shelf-life and safety. The rate of growth of Y. enterocolitica was successfully described by both a square root and a response surface function. The response surface model was more reliable for predicting the growth of this organism under conditions of sub-optimal temperature and pH. In food, the square root model increasingly underestimated growth as temperature decreased below 4°C and would 'fail dangerous' if used for predictive purposes. Treating death simply as negative growth and application of the square root function did not successfully model survival of Y. enterocolitica under conditions of sub-optimal temperature and growth inhibitory pH values for a number of different acidulants. The use of a logistic function and log dose (log time) in the Vitalistic model however accurately described survival across the whole range of conditions. Model predictions were also in excellent agreement with observed survival times in food with a negligible competitive microflora and a similar buffering capacity to the medium used in the development of the model. The presence of a competitive microflora and a food with greater buffering capacity led to the model underestimating the rate of death but 'failing safe' if used for predictive purposes under these circumstances. The intracellular pH (pHi) of Y. enterocolitica cells was measured by 31P nuclear magnetic resonance and by radiolabelled weak acid probes at growth inhibitory pH values with different acidulants. The latter method provided a more reliable determination of pHi. pHi decreased linearly with decreasing medium pH (pHext). The cell cytoplasm was always less acidic than the pHext. There was no difference in pHi with different acidulants (acetic acid and sulphuric acid) but viability was lower in cells acidified with acetic acid. Therefore neither pHext nor pHi were the determining factor causing death in Y. enterocolitica. Inhibition may have been caused by specific effects of the undissociated acid on metabolic or physiological activities.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors : Little, Christine Louisa.
Date : 1992
Additional Information : Thesis (Ph.D.)--University of Surrey (United Kingdom), 1992.
Depositing User : EPrints Services
Date Deposited : 30 Apr 2019 08:08
Last Modified : 20 Aug 2019 15:32
URI: http://epubs.surrey.ac.uk/id/eprint/851373

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