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Project
Integration of microscopic information in predictive models for food safety and spoilage.
Although the effect of food structure on microbial growth is generally accepted in predictive microbiology, the specific effect of the additionof biopolymer mixtures, making up a big share of our food products, is overlooked. This thesis investigates the effect of the addition of (mixtures of) food biopolymers on microbial growth.
A first study in this dissertation focuses on the growth of Escherichia</> coli </> and Salmonella </>Typhimurium in (mixtures of) biopolymers, i.e., gelatin, xanthan gum, gelatinxanthan gum and gelatincarrageenan, at salt concentrations between 0 and 5%. Biopolymers can interact with other components in food (model) systems, added to introduce stress and limit microbial outgrow. This is observed for xanthan gum andgelatin. Both biopolymers interact with the salt present in the medium,reducing the apparent salt concentration. For E. coli</> and S.</> Typhimurium this leads to a shorter lag phase than observed in liquid systems. Contrary to what is generally observed in literature,the addition of the used biopolymers resulting in immobilized cell growth, does not lead to a decrease in growth rate.
Next, a phase-separated biopolymer system, exhibiting a heterogeneous microstructure, is developed. The biopolymers gelatin and dextran are used for this purpose. Microscopic images demonstrate that both E. coli</> and Saccharomyces cerevisiae </>are located in the dextran phase of the heterogeneous microstructure, possibly because of the repulsion of the negatively-charged cells from the negatively-charged gelatin phase in combination with the favorability of the hydrophilic cells to the hydrophilic dextran phase.
For E. coli</> growth in the heterogeneous gelatindextran systems, exhibiting a microstructure of a dextran phase dispersed in a gelatin matrix, a decrease in maximum cell density is observed when increasing the microscopic observed percentage of dextran phase. However, when changing the type of microstructure, this relation no longer holds.
The addition of (a mixture of) biopolymers has a positive influence on the observed growth rate of E. coli</> and S. cerevisiae</>, especially under conditions of salt stress. For theyeast S. cerevisia</>e, an increase of maximum cell density is observed, going from experiments performed in liquid to those performed in singular and binary biopolymer systems.
Generally, it can be stated that for the conditions tested, the addition of biopolymers cannotbe neglected when predicting microbial behavior. Moreover, when determining microbial growth in heterogeneous biopolymer systems, especially inthe presence of a stress factor, i.e., salt, it is not sufficient to perform experiments in the preferential phase, neglecting the influence ofa heterogeneous microstructure. Care has to be taken when using currentpredictive models as they are mostly based on experiments in liquid, not taking into account the effect of a (heterogeneous) structure. These models can give faildangerous results, posing a risk on food safety and quality.
A first study in this dissertation focuses on the growth of Escherichia</> coli </> and Salmonella </>Typhimurium in (mixtures of) biopolymers, i.e., gelatin, xanthan gum, gelatinxanthan gum and gelatincarrageenan, at salt concentrations between 0 and 5%. Biopolymers can interact with other components in food (model) systems, added to introduce stress and limit microbial outgrow. This is observed for xanthan gum andgelatin. Both biopolymers interact with the salt present in the medium,reducing the apparent salt concentration. For E. coli</> and S.</> Typhimurium this leads to a shorter lag phase than observed in liquid systems. Contrary to what is generally observed in literature,the addition of the used biopolymers resulting in immobilized cell growth, does not lead to a decrease in growth rate.
Next, a phase-separated biopolymer system, exhibiting a heterogeneous microstructure, is developed. The biopolymers gelatin and dextran are used for this purpose. Microscopic images demonstrate that both E. coli</> and Saccharomyces cerevisiae </>are located in the dextran phase of the heterogeneous microstructure, possibly because of the repulsion of the negatively-charged cells from the negatively-charged gelatin phase in combination with the favorability of the hydrophilic cells to the hydrophilic dextran phase.
For E. coli</> growth in the heterogeneous gelatindextran systems, exhibiting a microstructure of a dextran phase dispersed in a gelatin matrix, a decrease in maximum cell density is observed when increasing the microscopic observed percentage of dextran phase. However, when changing the type of microstructure, this relation no longer holds.
The addition of (a mixture of) biopolymers has a positive influence on the observed growth rate of E. coli</> and S. cerevisiae</>, especially under conditions of salt stress. For theyeast S. cerevisia</>e, an increase of maximum cell density is observed, going from experiments performed in liquid to those performed in singular and binary biopolymer systems.
Generally, it can be stated that for the conditions tested, the addition of biopolymers cannotbe neglected when predicting microbial behavior. Moreover, when determining microbial growth in heterogeneous biopolymer systems, especially inthe presence of a stress factor, i.e., salt, it is not sufficient to perform experiments in the preferential phase, neglecting the influence ofa heterogeneous microstructure. Care has to be taken when using currentpredictive models as they are mostly based on experiments in liquid, not taking into account the effect of a (heterogeneous) structure. These models can give faildangerous results, posing a risk on food safety and quality.
Date:7 Sep 2010 → 31 Dec 2014
Keywords:Spoilage, Food safety
Disciplines:Catalysis and reacting systems engineering, Chemical product design and formulation, General chemical and biochemical engineering, Process engineering, Separation and membrane technologies, Transport phenomena, Other (bio)chemical engineering
Project type:PhD project