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Van Mierlo J.,BioTeC Chemical and Biochemical Process Technology and Control | Valdramidis V.P.,University of Malta
Food Research International | Year: 2015

The microbial growth morphology, as a consequence of the food structure, has been acknowledged to play a key role on the growth behavior. While in liquid media planktonic growth is observed, in a solid(like) environment cells are immobilized and forced to grow as (surface) colonies. This immobilization could affect the cell physiology and metabolism. Apart from the growth morphology, other intrinsic factors influence microbial growth. Osmotic stress and acidic stress have an impact on microbial growth. Moreover, the combination of intrinsic factors could result in a synergetic inhibitory effect on the growth behavior (hurdle technology). In this paper, the growth dynamics of Salmonella Typhimurium and Listeria monocytogenes under stressing conditions are studied for two different growth morphologies, i.e., (i) planktonic cells and (ii) surface colonies. Microorganisms are grown in petri dishes, incubated at 20. °C under static conditions. To create the solid(like) environment, 5% (w/v) gelatin is added. Stressing growth conditions are created by adding salt [0-8% (w/v)] and adapting the pH [5.5-7.0]. Cell density is determined via the viable plate count technique. While the studied pH-range has a negligible effect, the addition of salt significantly reduces growth. The growth morphology, resulting from the intrinsic food (model) structure, affects the microbial growth dynamics under static incubation conditions. In contrast to literature, surface colonies have higher or similar maximum specific growth rates than planktonic cells under most of the selected experimental conditions. For example, for S. Typhimurium at pH. 6.5 and 2% (w/v) NaCl, planktonic cells have a maximum specific growth rate of 0.435. 1/h while μmax for surface colonies has a value of 0.464. 1/h. For L. monocytogenes at pH. 6.0 and 0% (w/v) NaCl, μmax, planktonic cells is 0.375. 1/h and μmax, surface colonies is 0.424. 1/h. This is due to limited oxygen availability as a result of the experimental protocol implemented, leading to lower μmax values for planktonic cells as opposed to μmax values for shaken cultures in other studies appearing in literature. This indicates that under static incubation, the effect of the microstructure is often negligible as compared to the oxygen availability. However, for the most stressing experimental conditions, the combination of high salt concentrations and a solid(like) growth environment inhibits growth, and the order μmax, planktonic cells ≥. μmax, surface colonies is respected. This is for instance illustrated in the case of S. Typhimurium at pH. 5.5 and 6% (w/v) NaCl: μmax, planktonic cells reaches a value of 0.183. 1/h while μmax, surface colonies is only 0.142. 1/h. Also for L. monocytogenes at pH. 6.0 and 8% (w/v) NaCl, μmax, planktonic cells (0.145. 1/h) is higher than μmax, surface colonies (0.113. 1/h). This study indicates the relevance of intrinsic factors on the growth dynamics and addresses the importance to include growth morphology, determined by the intrinsic food structure, in predictive models. © 2015 Elsevier Ltd. Source


Smet C.,CPMF Flemish Cluster Predictive Microbiology in Foods | Noriega E.,CPMF Flemish Cluster Predictive Microbiology in Foods | Van Mierlo J.,BioTeC Chemical and Biochemical Process Technology and Control | Valdramidis V.P.,University of Malta | Van Impe J.F.,CPMF Flemish Cluster Predictive Microbiology in Foods
Food Research International | Year: 2015

The microbial growth morphology, as a consequence of the food structure, has been acknowledged to play a key role on the growth behavior. While in liquid media planktonic growth is observed, in a solid(like) environment cells are immobilized and forced to grow as (surface) colonies. This immobilization could affect the cell physiology and metabolism. Apart from the growth morphology, other intrinsic factors influence microbial growth. Osmotic stress and acidic stress have an impact on microbial growth. Moreover, the combination of intrinsic factors could result in a synergetic inhibitory effect on the growth behavior (hurdle technology). In this paper, the growth dynamics of Salmonella Typhimurium and Listeria monocytogenes under stressing conditions are studied for two different growth morphologies, i.e., (i) planktonic cells and (ii) surface colonies. Microorganisms are grown in petri dishes, incubated at 20 °C under static conditions. To create the solid(like) environment, 5% (w/v) gelatin is added. Stressing growth conditions are created by adding salt [0-8% (w/v)] and adapting the pH [5.5-7.0]. Cell density is determined via the viable plate count technique. While the studied pH-range has a negligible effect, the addition of salt significantly reduces growth. The growth morphology, resulting from the intrinsic food (model) structure, affects the microbial growth dynamics under static incubation conditions. In contrast to literature, surface colonies have higher or similar maximum specific growth rates than planktonic cells under most of the selected experimental conditions. For example, for S. Typhimurium at pH. 6.5 and 2% (w/v) NaCl, planktonic cells have a maximum specific growth rate of 0.435. 1/h while μmax for surface colonies has a value of 0.464 1/h. For L monocytogenes at pH 6.0 and 0% (w/v) NaCl, μmax, planktonic cells is 0.375 1/h and μmax, surface colonies is 0.424 1/h. This is due to limited oxygen availability as a result of the experimental protocol implemented, leading to lower μmax values for planktonic cells as opposed to μmax values for shaken cultures in other studies appearing in literature. This indicates that under static incubation, the effect of the microstructure is often negligible as compared to the oxygen availability. However, for the most stressing experimental conditions, the combination of high salt concentrations and a solid(like) growth environment inhibits growth, and the order μmax, planktonic cells≥. μmax, surface colonies is respected. This is for instance illustrated in the case of S. Typhimurium at pH. 5.5 and 6% (w/v) NaCl: μmax, planktonic cells reaches a value of 0.183 1/h while μmax, surface colonies is only 0.142 1/h. Also for L. monocytogenes at pH 6.0 and 8% (w/v) NaCl, μmax, planktonic cells (0.145 1/h) is higher than μmax, surface colonies (0.113 1/h). This study indicates the relevance of intrinsic factors on the growth dynamics and addresses the importance to include growth morphology, determined by the intrinsic food structure, in predictive models. © 2015 Elsevier Ltd. Source


Skara T.,NOFIMA | Cappuyns A.M.,BioTeC Chemical and Biochemical Process Technology and Control | Van Derlinden E.,BioTeC Chemical and Biochemical Process Technology and Control | Rosnes J.T.,NOFIMA | And 2 more authors.
Journal of Food Protection | Year: 2012

The growth dynamics of Listeria monocytogenes strains isolated from salmon or a salmon processing environment and two reference Listeria innocua strains were investigated at refrigerated and close-to-optimal growth temperatures. Estimates for the growth rates and the lag-phase duration at 4, 8, 12, and 30°C were obtained for optical density measurements by using different growth parameter estimation methods, i.e., the serial dilution (SD) method and the relative rate to detection (RRD) method. Both single L. innocua and L. monocytogenes strains and mixtures of L. monocytogenes strains (cocktails) were studied. Both methods show an increase in maximum growth rate (mmax) of Listeria with increasing temperatures. Generally, single-strain growth rate estimates were quite similar for both species, although L. monocytogenes showed slightly higher mmax estimates at 4°C. The SD method gave the highest estimates for the growth rate, i.e., the estimates from the RRD method were 10 to 20% lower. This should lead to caution when using the latter method for Listeria, particularly at lower temperatures. Overall, the SD method is preferred as this method yields mmax estimates close to the biological value and provides estimates for the duration of lag time (l). For discrimination between different strains, l appeared to be a more suitable parameter than mmax. This effect was most prominent for L. innocua. Significant differences were observed between mmax and/or l of L. monocytogenes cocktails and single strains at all temperatures investigated. At 4°C, the average growth rate of cocktails was higher than that of single strains. At 8 and 30°C, this trend was reversed. The average l of single strains were more than twice as long as those of cocktails at 4°C. At 8 and 30°C, the l of cocktails were significantly slower than those of single strains, but the variation was considerably less and the differences were less pronounced. Copyright © International Association for Food Protection. Source

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