Pierre-Sylvain Mirade

Three-dimensional CFD calculations for designing large food chillers

In industry solid foods such as vegetables and meat are mostly air-chilled and stored in refrigerated rooms. Plant performance depends on heat and water exchanges between the air and the products and thus on the spatial distribution of the air parameters: velocity, turbulence intensity, temperature and relative humidity. In theory the transient behaviour of chillers could be calculated using computational fluid dynamics (CFD) codes, but current computer capacity and the inaccuracy of models for evaluating heat and mass transfer coefficients make them inapplicable to very large rooms containing hundreds of individual products. To overcome these limits a four-step calculation is proposed to assess how changes in chiller design and operating conditions affect chilling kinetics: (1) 3D calculation of the air velocity field with a CFD code, (2) deduction of an average air velocity map, (3) determination of heat and mass transfer coefficients in relation to the air velocity map using independent measurements, and (4) calculation of the product chilling kinetics using specific software. This procedure was applied to a pork chiller containing 290 carcasses. Two design cases differing in inlet air direction and flow rate, and two functioning modes, batch and continuous, were analysed. A fairly good agreement was observed between the calculated and measured air velocities. It was shown from temperature and weight loss kinetics that with this type of design, batch chilling can only be achieved overnight at the cost of overcooling for low weight carcasses, and increased weight losses.

Prediction of the air velocity field in modern meat dryers using unsteady computational fluid dynamics (CFD) models

To even out drying conditions and thereby improve meat product processing, modern dryers run with a ventilation cycle, during which, while the total airflow rate remains steady, the airflow rate ranges reciprocally from a low to a high level in each of two blower ducts located on either side of the plant. In this work, using a two-dimensional computational fluid dynamics model with time-dependent boundary conditions (i.e. an unsteady model), the homogeneity of the distribution of the air velocity in an industrial meat dryer is discussed for several low and high levels of the ventilation cycle, and two forms of this cycle (linear and sinusoidal). The changes in the airflow structure due to dissymmetry occurring in the ventilation cycle are also evaluated. All airflow simulations are consistent with the heterogeneity of drying usually observed in practice. This work can provide professionals with a rational approach to help them in the operation and design of modern dryers.

Quantitative study of the relationships among proteolysis, lipid oxidation, structure and texture throughout the dry-cured ham process.

Temperature, salt and water contents are key processing factors in dry-cured ham production. They affect how proteolysis, lipid oxidation, structure and texture evolve, and thus determine the sensory properties and final quality of dry-cured ham. The aim of this study was to quantify the interrelationships and the time course of (i) proteolysis, (ii) lipid oxidation, (iii) five textural parameters: hardness, fragility, cohesiveness, springiness and adhesiveness and (iv) four structural parameters: fibre numbers, extracellular spaces, cross section area, and connective tissue area, during the dry-cured ham process. Applying multiple polynomial regression enabled us to build phenomenological models relating proteolysis, salt and water contents to certain textural and structural parameters investigated. A linear relationship between lipid oxidation and proteolysis was also established. All of these models and relationships, once combined with salt penetration, water migration and heat transfer models, can be used to dynamically simulate all of these phenomena throughout dry-cured ham manufacturing.

A new experimental method for measuring and visualising air flow in large food plants

Abstract A new experimental method was developed to visualise air flow patterns and spatial distribution of the mean air velocity in large food plants. The continuous motion of an anemometer measuring air velocity at regular intervals made it possible to rapidly obtain measurements at several thousands of points. The processing of these measurements by Fouriers series and a low-pass filter eliminated the time-fluctuations due to air flow unsteadiness and only the spatial variations of the mean air velocity were kept afterwards. This paper shows how this method was applied in the food industry to a typical meat chiller. The graphical representation of the mean air velocity distribution brought to light dysfunction inside the chiller. The experimental tool presented here can help specialists to carry out diagnoses of the air flow in food plants.

Development of a rapid, specific and efficient procedure for the determination of proteolytic activity in dry-cured ham: Definition of a new proteolysis index

A method was adapted to determine proteolytic activity in dry-cured ham using fluorescamine-specific labelling of N-terminal α-amino groups of peptides and amino acids. Fluorescence of the complex was measured using a microplate procedure and optimum excitation and emission wavelengths of 375nm and 475nm, respectively. A new proteolysis index (PI) was defined as the percentage ratio of the N-terminal α-amino group content to the total protein content of the ham extract. The robustness of the method was evaluated by measuring PI in pork meat samples subjected to standardized processing conditions and in samples extracted from industrial hams taken at different stages of processing. For the industrial samples, a comparison with the classic nitrogen procedure of PI determination was performed and a formula relating the two PIs was established. The rapidity, sensitivity and specificity of the procedure make it a good candidate for a screening test to evaluate ham quality in industry.

Effect of Combined Salt and Animal Fat Reductions on Physicochemical and Biochemical Changes During the Manufacture of Dry-Fermented Sausages

Reducing salt and fat contents in dry-fermented sausages could benefit consumer health. This study aimed to quantify, from an experimental design, the effects of salt and fat contents and combined salt and fat reductions on the time-course of several physicochemical (product weight loss, mean water activity and pH values) and biochemical (proteolysis, lipolysis and lipid and protein oxidations) parameters. Statistical analyses found that time, salt and fat contents had a very significant impact on weight loss and aw and that time and salt content (not fat content) had a significant impact on pH. Biochemical results indicated that proteolysis was salt-content-dependent and amplified by combined salt and fat reductions. Intensity of lipolysis was mainly dependent on fat content. Lipid and protein oxidations were more intense in higher-fat formulations. Combined salt and fat reductions in dry-fermented sausages increased acidification, weight losses and aw, leading to more proteolysis, less lipolysis and less oxidation. Sensory studies are now required to investigate consumer acceptability of these healthier sausages. However, the present results constitute a valuable set of data for helping professionals wishing to reduce salt and fat contents in dry-fermented sausages.

Building phenomenological models that relate proteolysis in pork muscles to temperature, water and salt content

Throughout dry-cured ham production, salt and water content, pH and temperature are key factors affecting proteolysis, one of the main biochemical processes influencing sensory properties and final quality of the product. The aim of this study was to quantify the effect of these variables (except pH) on the time course of proteolysis in laboratory-prepared pork meat samples. Based on a Doehlert design, samples of five different types of pork muscle were prepared, salted, dried and placed at different temperatures, and sampled at different times for quantification of proteolysis. Statistical analysis of the experimental results showed that the proteolysis index (PI) was correlated positively with temperature and water content, but negatively with salt content. Applying response surface methodology and multiple linear regressions enabled us to build phenomenological models relating PI to water and salt content, and to temperature. These models could then be integrated into a 3D numerical ham model, coupling salt and water transfers to proteolysis.

Toward Quantitative CFD Prediction of Contaminant Particle Deposition against Surfaces in Large Forced-Ventilation Food Plants

This article discusses the application of a computational fluid dynamics (CFD) approach for constructing a numerical methodology based on Reynolds-Averaged Navier-Stokes (RANS) equations in order to accurately determine particle deposition rates against surfaces subjected to a low-velocity turbulent airflow. We began by studying a horizontal duct flow configuration and validating the numerical output for airflow pattern and particle deposition rate predictions against published numerical data. The main outcome of this first part of the study was that when starting from a one-way coupling Lagrangian formulation, accurate prediction of particle deposition rates strictly requires the construction of a sufficiently fine mesh (characterized by a dimensionless “distance to wall” criterion y+ lower than 4) and the use of a turbulence model accounting for turbulence anisotropy in combination with a near-wall “two-layer zonal” boundary condition. The CFD methodology set up was then used to predict airflow patterns together with airborne deposition of spherical 1–50 μm particles against all the surfaces in an 87 m3 ventilated cold room filled with 9 big rectangular parallelepipeds. The results demonstrated close agreement in air velocity predictions and showed particle deposition rates varying with both room wall surface type and orientation and with particle diameter due to the different physical phenomena involved. This paper is the first attempt to predict particle deposition in such a large volume, corresponding to a food-processing environment.

SENSITIVITY ANALYSIS OF A SIMPLIFIED CHEESE RIPENING MASS LOSS MODEL

Abstract This paper is focused on the mass loss rate during the ripening of surface-mould cheese. From mechanistic laws, the idea is to carry out simplifications without loss of accuracy and to perform sensitivity analysis for model inputs. The high significance of accurate relative humidity and gas composition measurements and the low influence of the atmospheric temperature are pointed out. The model reliability according to three key parameters (emissivity, surface water activity and average heat transfer coefficient) is also described.