Development of a structure-based model for the competitive growth of Listeria innocua in minced chicken breasts.

Most predictive models are based on planktonic microbial growth in broth and, therefore, do not deal with diffusional limitations generated by the solid-food structure. Recently, a few approaches have incorporated the diffusional component, but still on the basis of pure cultures growing in homogenous supports. In this work, a transport-phenomenon model that considers the complex and dynamic structure of real foods, as well as its background microflora, has been developed. This structure-based model was applied to the competitive growth of Listeria innocua in minced chicken breasts, considering oxygen as the limiting substrate. Physical structure and microbial growth in minced meat were compared to those reported for meat-based kappa-carrageenan gels by means of scanning electron microscopy (SEM) and viable counts. In both systems, microbial growth was affected by diffusional limitations, as compared with broth cultures. However, significant differences in their physical structure and a key effect of the indigenous microflora and the meat tissue itself, resulted in different extent and distribution of microbial growth. The approach here developed, useful for food safety purposes, has been shown to provide a reasonably good simulation of experimental data in minced breasts.

Decisive role of structure in food microbial colonization and implications for predictive microbiology.

Predictive models must consider the significant effect of the physical structure of the food on the magnitude and type of microbial growth. Before such models are developed, a thorough characterization of the food structure is mandatory because this information will determine the modeling approach. In this work, several physical structures common in poultry products were classified and described. Chicken breast skin and flesh and minced breasts were examined by scanning electron microscopy and compared with a meat-based model food. Such systems were surface or internally inoculated with Listeria innocua and incubated at 25 degrees C for 24 h. Different structures, including several substructures, found in the studied systems affected microbial distribution and growth. Based on these experimental findings, the most suitable type of model for each physical structure was determined. This information provides further clarification for predictive microbiology models.

Mixed cultures of Serratia marcescens and Kluyveromyces fragilis for simultaneous protease production and COD removal of whey.

AIMS: The aim of this study was to investigate the behaviour of a Serratia marcescens-Kluyveromyces fragilis mixed culture in whey, with the objective of proteases production and organic waste reduction. METHODS AND RESULTS: Discontinuous aerobic fermentations in whey were carried out using individual pure cultures and mixed cultures of S. marcescens and K. fragilis. Cell growth, protease production, lactose and proteins consumption and COD/TOC reduction were monitored. Lactose and protein content of the fermenting medium was almost depleted in the mixed cultures, achieving a reduction in the organic content much higher than in both pure cultures. Interestingly, proteolytic activity in the mixed cultures was similar to that obtained for S. marcescens in pure culture. In addition, protease stability was increased in the mixed cultures. Kinetic models were developed fitting well with the experimental results. CONCLUSIONS: Mixed cultures were found to maintain the achievements of each individual fermentation, yielding a high and stable production of proteases and a significant reduction of COD/TOC. SIGNIFICANCE AND IMPACT OF THE STUDY: Mixed cultures tested in this work have shown a synergistic effect with possible industrial applications. These results lead to a gain in the chain value for enzyme production with an environmentally friendly operation.

Controlled malolactic fermentation in cider using Oenococcus oeni immobilized in alginate beads and comparison with free cell fermentation.

Cells of Oenococcus oeni (formerly Leuconostoc oenos) immobilized in alginate beads were used as starter culture to conduct malolactic fermentation in cider production. Concentrations of major organic acids and volatile compounds were monitored during the process, and results were compared to those obtained when using free cells in the same conditions. The rates of malic acid consumption were similar but lower ethanoic acid content and higher concentration of alcohols were detected with immobilized cells. These features have beneficial effects on the organoleptic properties of cider. A comparison between the kinetic behavior in immobilized and free cells, based on the data obtained for the malic acid consumption, has been developed solving the homogeneous diffusion model when it is applied to the system with immobilized cells.

Analysis and description of the evolution of alginate immobilised cells systems.

Different immobilised cells models, including very simple ones, can be useful in the fitting of experimental results. However, goodness or the ability to extrapolate results needs to be in accordance with basic observations and these will also suggest models to be proposed. In this paper, observations of calcium alginate/bacteria systems are used to show the ability of basic models to fit classic observations, as well as how new observations, in this case from electronic microscopy, oblige us to think about more complex mechanisms and mathematical treatments. Nevertheless it is not only important to discuss the model type, but also the type of kinetics assumed in the interior of the beads, as well as the internal structure, the boundary conditions related to bead shredding and cell escape and finally, geometrical effects.