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Theys T.E.,Chemical and Biochemical Process Technology | Geeraerd A.H.,Catholic University of Leuven | Devlieghere F.,Ghent University | Van Impe J.F.,Chemical and Biochemical Process Technology
Food Microbiology | Year: 2010

Several studies have shown that food structure causes slower growth rates and narrower growth boundaries of bacteria compared to laboratory media. In predictive microbiology, both aw or corresponding solute concentration (mainly NaCl) have been used as a growth influencing factor for kinetic models or growth/no growth interface models. The majority of these models have been based on data generated in liquid broth media with NaCl as the predominant aw influencing solute. However, in complex food systems, other aw influencing components might be present, next to NaCl. In this study, the growth rate of Salmonella typhimurium was studied in the growth region and the growth/no growth response was tested in Tryptic Soy Broth at 20 °C at varying gelatin concentration (0, 10, 50 g L-1 gelatin), pH (3.25-5.5) and water activity (aw) (0.929-0.996). From the viewpoint of water activity, the results suggest that NaCl is the main aw affecting compound. However, gelatin seemed to have an effect on medium aw too. Moreover, there is also an interaction effect between NaCl and gelatin. From the microbial viewpoint, the results confirmed that the aw decreasing effect of gelatin is less harmful to cells than the effect of Na+ ions. The unexpected shift of the growth/no growth interface to more severe conditions when going from a liquid medium to a medium with 10 g L-1 gelatin is more pronounced when formulating the models in terms of aw than in terms of NaCl concentrations. At 50 g L-1 gelatin, the model factored with NaCl concentration shifts to milder conditions (concordant to literature results) while the model with aw indicates a further shift to more severe conditions, which is due to the water activity lowering effect of gelatin and the interaction between gelatin and NaCl. The results suggest that solute concentration should be used instead of aw, both for kinetic models in the growth region and for growth/no growth interface models, if the transferability of models to solid foods is to be increased. © 2009.

Dang T.D.T.,Ghent University | Mertens L.,Chemical and Biochemical Process Technology | Vermeulen A.,Ghent University | Geeraerd A.H.,Catholic University of Leuven | And 3 more authors.
International Journal of Food Microbiology | Year: 2010

The aim of the study was to develop mathematical models describing growth/no growth (G/NG) boundaries of the highly resistant food spoilage yeast-Zygosaccharomyces bailii-in different environmental conditions, taking acidified sauces as the target product. By applying these models, the stability of products with characteristics within the investigated pH, a w and acetic acid ranges can be evaluated. Besides, the well-defined no growth regions can be used in the development of guidelines regarding formulation of new shelf-stable foods without using chemical preservatives, which would facilitate the innovation of additive-free products. Experiments were performed at different temperatures and periods (22 °C for 45 and 60 days, 30 °C for 45 days) in 150 modified Sabouraud media characterized by high amount of sugars (glucose and fructose, 15% (w/v)), acetic acid (0.0-2.5% (v/v), 6 levels), pH (3.0-5.0, 5 levels) and a w (0.93-0.97, 5 levels). These time and temperature combinations were chosen as they are commonly applied for shelf-stable foods. The media were inoculated with ca. 4.5 log CFU/ml and yeast growth was monitored daily using optical density measurements. Every condition was examined in 20 replicates in order to yield accurate growth probabilities. Three separate ordinary logistic regression models were developed for different tested temperatures and incubation time. The total acetic acid concentration was considered as variable for all models. In general, when one intrinsic inhibitory factor became more stringent, the G/NG boundary shifted to less stressful conditions of the other two factors, resulting in enlarged no growth zones. Abrupt changes of growth probability often occurred around the transition zones (between growth and no growth regions), which indicates that minor variations in environmental conditions near the G/NG boundaries can cause a significant impact on the growth probability. When comparing growth after 45 days between the two tested temperatures, an unexpected phenomenon was observed: the no growth region at 30 °C was larger than the one at 22 °C, though it is known that 30 °C is the optimal growth temperature for Z. bailii. These results show that lowering temperature does not always lead to a reduced growth of the yeast (i.e. more stable foods) and storing shelf-stable products at the higher temperature (30 °C) is not always the worst case. In addition, at 22 °C, there was no significant difference in no growth zones between the two incubation periods (45 and 60 days), implying that the no growth zones remain unchanged if the experimental time is sufficiently long. © 2009 Elsevier B.V. All rights reserved.

Appels L.,Chemical and Biochemical Process Technology | Appels L.,Catholic University of Leuven | Degreve J.,Chemical and Biochemical Process Technology | Van der Bruggen B.,Catholic University of Leuven | And 3 more authors.
Bioresource Technology | Year: 2010

In this work, the influence of a low temperature (70-90 °C) thermal treatment on anaerobic digestion is studied. Not only the increase in biogas production is investigated, but attention is also paid to the solubilisation of the main organic (proteins, carbohydrates and volatile fatty acids) and inorganic (heavy metals, S and P) sludge constituents during thermal treatment and the breakdown of the organic components during the subsequent anaerobic digestion. Taking into account the effects of the treatment on the sludge composition is of prime importance to evaluate its influence on the subsequent anaerobic digestion and biogas production using predictive models. It was seen that organic and inorganic compounds are efficiently solubilised during thermal treatment. In general, a higher temperature and a longer treatment time are beneficial for the release. The efficiency of the subsequent anaerobic digestion slightly decreased for sludge pre-treated at 70 °C. At higher pre-treatment temperatures, the biogas production increased significantly, up to a factor 11 for the 60 min treatment at 90 °C. © 2010 Elsevier Ltd. All rights reserved.

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