Rémi Saurel

Modelling of French Emmental cheese water activity during salting and ripening periods

Abstract Water sorption isotherms of French Emmental cheese have been determined at various temperatures (between 4 and 28 °C) for core or rind samples taken at various ripening stages during a standard 74 days ripening period. The differences observed between all the experimental isotherms are probably due to the evolution of product composition during the cheese ripening. Experimental data concerning the water activity profiles for industrially processed Emmental cheese have been collected after different brining experiments carried out at usual temperature conditions (4–18 °C) and all along the ripening cycle. The local water activity profiles during the Emmental brining and ripening periods are strongly dependent on the local NaCl concentration distribution. All these profiles vary in relation to the process step and of the process time. For the brining step, a precise correlation was proposed to express the influence of the water content and of the NaCl concentration. For the ripening period, a more general correlation expressing the water activity as a function of the water, NaCl and free NH 2 group concentrations was fitted to our experimental data. This last correlation was able to interpret the activity profiles more precisely during an Emmental ripening cycle in industrial conditions than the physical models of sorption fitted to the water sorption curves.

Experimental study and modeling of effective NaCl diffusion coefficients values during Emmental cheese brining

Abstract Experimental data concerning NaCl concentration profiles for industrially processed Emmental cheese were determined with cylindrical samples ensuring semi-infinite unidirectional mass transfer with saturated brining aqueous solutions for different temperature conditions in the range between 4 and 18 °C. Considering a binary diffusion system constituted by the cheese water and the NaCl solute, the NaCl diffusion kinetics were analyzed at each investigated temperature by three different approaches: firstly, using the second Fick’s law with a constant NaCl diffusivity, which gave a poor interpretation of the experimental data; secondly, using the Boltzmann’s method with a concentration dependent NaCl diffusion coefficient and thirdly, by numerical identification from the diffusion equation with an empirical variation of the NaCl diffusivity with the salt concentration (2nd order polynomial) and a boundary condition expressing the mass flux continuity at the interface. As a whole, the derived diffusivity values were found to be in good agreement with the previous published data concerning different cheese types. These two last analyses clearly indicated a decrease of the NaCl effective diffusivity with the salt concentration and, secondly, an increase of this diffusivity with the temperature. Finally, the last modelling, based on a quadratic reduction in the salt diffusivity with the salt concentration and on a Dirichlet (type II) limit condition lead to the best fitting between the measured and the predicted NaCl concentration profiles.

Firming of fruit tissues by vacuum-infusion of pectin methylesterase: Visualisation of enzyme action.

Apple pieces were vacuum-impregnated with either a pectin methylesterase (PME) and calcium solution or with water prior to pasteurization. Pasteurized apple pieces impregnated with PME and calcium showed a significantly higher firmness. Moreover, solid state (13)C NMR spectroscopy of apple cell wall residues revealed an increase of their molecular rigidity. Exogenous PME addition involved a decrease from 82% to 45% of apple pectin degree of methyl-esterification. Microscopic observations of apple slices immunolabelled with antibodies specific for pectins showed that (i) demethyl-esterification was more intense in the cell wall region lining intercellular spaces (demonstrating a key role for these intercellular channels in the enzyme penetration in the tissue during vacuum-infusion) and that (ii) the number of calcium-dimerized deesterified homogalacturonan chains increased. The results corroborate the hypothesis that vacuum-impregnated PME action liberates free carboxyl groups along pectin chains that could interact with calcium, increasing the rigidity of pectins and finally the mechanical rigidity of apple tissue.

Modeling of dewatering and impregnation soaking process (osmotic dehydration)

Abstract Partial dewatering and simultaneous solute impregnation can be obtained by immersion of food pieces in concentrated solutions. Recently, further understanding of cross mass transfer was achieved by experimental study and modelling on agar model gel foods.

Thermal Denaturation of Pea Globulins (Pisum sativum L.)—Molecular Interactions Leading to Heat-Induced Protein Aggregation

The heat-induced denaturation and aggregation of mixed pea globulins (8%, w/w) were investigated using differential scanning calorimetry (DSC), SDS-PAGE, and size-exclusion chromatography (SEC-HPLC). DSC data showed that the pea proteins denaturation temperature (T(d)) was heating-rate dependent. The T(d) value decreased by about 4 °C by lowering the heating rate from 10 to 5 °C/min. The SDS-PAGE analysis revealed that protein denaturation upon heating at 90 °C was mainly governed by noncovalent interaction. The SEC-HPLC measurements indicated that low-denatured legumin (≈350-410 kDa) and vicilin/convicilin (≈170 kDa) globulins were heat-denatured and most of their subunits reassociated into high-molecular weight, soluble aggregates (>700 kDa). The addition of N-ethylmaleimide slightly modified the aggregation route of pea globulins. However, partially insoluble macroaggregates were produced in the presence of dithiothreitol, reflecting the stabilizing effect of disulfide bonds within legumin subunits.

Preferential localization of Lactococcus lactis cells entrapped in a caseinate/alginate phase separated system

This study aimed to entrap bioprotective lactic acid bacteria in a sodium caseinate/sodium alginate aqueous two-phase system. Phase diagram at pH=7 showed that sodium alginate and sodium caseinate were not miscible when their concentrations exceeded 1% (w/w) and 6% (w/w), respectively. The stability of the caseinate/alginate two-phase system was also checked at pH values of 6.0 and 5.5. Lactococcus lactis subsp. lactis LAB3 cells were added in a 4% (w/w) caseinate/1.5% (w/w) alginate two-phase system at pH=7. Fluorescence microscopy allowed to observe that the caseinate-rich phase formed droplets dispersed in a continuous alginate-rich phase. The distribution of bacteria in such a system was observed by epifluorescence microscopy: Lc. lactis LAB3 cells stained with Live/Dead(®) Baclight kit™ were located exclusively in the protein phase. Since zeta-potential measurements indicated that alginate, caseinate and bacterial cells all had an overall negative charge at pH 7, the preferential adhesion of LAB cells was assumed to be driven by hydrophobic effect or by depletion phenomena in such biopolymeric systems. Moreover, LAB cells viability was significantly higher in the ternary mixture obtained in the presence of both caseinate and alginate than in single alginate solution. Caseinate/alginate phase separated systems appeared thus well suited for Lc. lactis LAB3 cells entrapment.

Heat transfer study and modeling during Emmental ripening

Firstly, thermal properties were determined on various Emmental cheese samples in order to check the literature empirical relationships for dairy products. Thermal conductivity or diffusivity values were in good agreement with the predicted data considering the temperature effect and the cheese composition in terms of water, fat and non-fat concentrations. Furthermore, the external convective heat transfer coefficients values were experimentally determined by using the psychrometric method with different air flow patterns similar to industrial round cheese ripening conditions. These coefficients are globally higher than published data concerning heat exchange between air and cylindrical bodies. For a temperature change step from 5 to 16 °C applied in our ripening cellar, the experimental temperature profiles measured in the round cheese were in good agreement with the data predicted by numerical simulation by using a 2-D finite difference method. For the temperature step higher than 16 °C, the model could be improved by taking into account the endothermic source terms corresponding probably to melting enthalpies of cheese fat fractions.

Size measuring techniques as tool to monitor pea proteins intramolecular crosslinking by transglutaminase treatment

In this work, techniques for monitoring the intramolecular transglutaminase cross-links of pea proteins, based on protein size determination, were developed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles of transglutaminase-treated low concentration (0.01% w/w) pea albumin samples, compared to the untreated one (control), showed a higher electrophoretic migration of the major albumin fraction band (26 kDa), reflecting a decrease in protein size. This protein size decrease was confirmed, after DEAE column purification, by dynamic light scattering (DLS) where the hydrodynamic radius of treated samples appears to be reduced compared to the control one.

Partition of volatile compounds in pea globulin–maltodextrin aqueous two-phase system

This study is based on the assumption that the off-flavour of pea proteins might be decreased using the retention of volatile compounds by a mixture with another biopolymer. The partition of volatile compounds in an aqueous system containing pea protein and maltodextrins was followed under thermodynamic incompatibility conditions. Firstly, the phase diagram of the system was established. Then, the partition of aroma compounds between the phase rich in protein and the phase rich in maltodextrin was measured by SPME-GC-MS. There was a transfer of volatile compounds during phase separation. Variations of pH were also used to vary the retention of volatile compounds by proteins. The concentration of volatile compounds in protein solution at pH 2.4 was higher than at pH 7.2. It was possible to increase the transfer of volatile compounds from the phase rich in protein to the phase rich in maltodextrin using the effect of pH on protein denaturation.