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Despite CO2 being one of the most important gases affecting the quality of most semi-hard cheeses, the thermodynamic properties of this molecule in relation to cheese ripening have rarely been investigated. In this study the CO2 solubility coefficient was experimentally assessed in semi-hard cheese as a function of the most relevant compositional and ripening variables. As expected, CO2 solubility was found to linearly decrease with temperature in the range 2-25 °C. Unexpectedly, solubility was not significantly different at 39% and 48% moisture, while it was found lower at 42%. Unavoidable changes in protein content of the three cheese variants is suspected to produce an interaction with water content, leading to complex interpretation of the results. Increasing salt content in cheese from 0 to 2.7%w/w significantly decreased CO2 solubility by about 25%, probably due to the increased bonded water molecules in the cheese water phase., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

A Doehlert design was performed to study the effect of calcium and phosphate supplementation at 0 to 25 mmol/kg and 0 to 16 mmol/kg, respectively, on the rennet gelation of reconstituted skim milk subjected to pH-reversible CO(2) acidification. Supplemented reconstituted skim milk samples were acidified to pH 5.80 by the addition of CO(2) under pressure and depressurized under vacuum to restore the initial pH value. The second-order polynomial models satisfactorily predicted the effect of salt addition on the micellar molar Ca:P ratio and the average diameter of the casein micelles, whereas only trends were used in the analysis of the rennet-clotting behavior of salt-supplemented, CO(2)-treated milk. Whether added Ca was the most determinant factor on the micellar molar Ca:P ratio, added Pi (a mixture of Na(2)HPO(4) and NaH(2)PO(4)) was the most determinant factor on the other responses studied, and its effect was most pronounced when Ca was simultaneously added. By comparison with control samples, changes observed in this study were essentially due to salt supplementation and not to the CO(2) treatment. Therefore, this CO(2) treatment could be considered as an entirely reversible treatment rather than only pH-reversible, and predictions might be applied to untreated milk. In the case of Ca-supplemented milk, the micellar molar Ca:P ratio increased, the average micellar diameter decreased, and the rennet-clotting properties were improved, whereas opposite effects were observed upon Pi supplementation. Since modification of the micellar molar ratio is the result of change in the chemical composition of micellar calcium phosphate, the effect of calcium and phosphate supplementation on the rennet clotting of milk was found to be also dependent on the nature of the interaction between caseins and colloidal calcium phosphate.

Response surface methodology was used to investigate the effect of salt supplementation on the micellar composition of reconstituted skim milk subjected to acidification by CO2 pressure to pH 5.8, followed by depressurization under vacuum. Using a Doehlert design, calcium and phosphate were added to skim milk in the range of 0 to 25 mmol/kg and 0 to 16 mmol/kg of milk, respectively, and the pH was adjusted to 6.65 +/- 0.02. After carbonation, the milk sample was depressurized, and the pH returned to its initial value without modification of the ionic strength. Micellar composition was assessed by the concentration of micellar Ca, P, Mg, and protein, and the buffering properties of milk. The second order polynomial models satisfactorily predicted the effect of salt supplementation on the micellar composition (R2adj > 0.75). Added calcium was the most determinant factor, and favored the removal of Ca, P, Mg, and proteins from the soluble phase to the micellar phase when this addition was less than 17.5 mmol/kg of milk. Above this concentration, only the concentration of micellar Ca increased. The buffering response surface showed that the amount of micellar calcium phosphate increased to a maximum upon addition of 17.5 mmol of Ca/kg. By comparison with a control sample (supplemented but untreated skim milk), changes were essentially due to salt supplementation and not to the CO2 treatment. We suggest that Ca formed micellar calcium phosphate when added at a concentration less than 17.5 mmol/kg; whereas above this concentration, Ca bound directly to micellar proteins.

We report here measured adsorption capacities for a natural chabazite zeolite at pressures ranging from (5 to 3000) kPa, at temperatures of (244 and 305) K, for pure N2, CH4, and CO2, and for gas mixtures of CH4 CO2. The pure gas data sets from this work and from the literature were in good agreement (10 %) and were regressed to Toth models over a wide range of pressure and temperature. We show that extrapolation of models that were fit only to low pressure data (below 120 kPa) can lead to a 30 % deviation in adsorption capacities predicted at high pressures. Similarly, models fit only to high pressure pure fluid data resulted in unreliable predictions for mixture adsorption capacities particularly when the component's partial pressure was low. The experimental results indicate that, while the chabazite is unlikely to be useful for N2/CH4separation, it may have potential for removing bulk CO2from natural gas, particularly at low temperatures. A feed gas mixture of 0.95CH4 0.05CO2placed in contact with the chabazite resulted in equilibrium vapor phases with CO2mole fractions of about (0.0013 and 0.0002) at (305 and 244) K, respectively. The ideal adsorbed solution theory was used to successfully describe the observed mixture behavior, although it was found to be sensitive to the data range over which the pure fluid models were regressed. [ABSTRACT FROM AUTHOR]

A Doehlert design was performed to study the effect of calcium and phosphate supplementation at 0 to 25 mmol/kg and 0 to 16 mmol/kg, respectively, on the rennet gelation of reconstituted skim milk subjected to pH-reversible CO2 acidification. Supplemented reconstituted skim milk samples were acidified to pH 5.80 by the addition of CO2 under pressure and depressurized under vacuum to restore the initial pH value. The second-order polynomial models satisfactorily predicted the effect of salt addition on the micellar molar Ca:P ratio and the average diameter of the casein micelles, whereas only trends were used in the analysis of the rennet-clotting behavior of salt-supplemented, CO2-treated milk. Whether added Ca was the most determinant factor on the micellar molar Ca:P ratio, added Pi (a mixture of Na2HPO4 and NaH2PO4) was the most determinant factor on the other responses studied, and its effect was most pronounced when Ca was simultaneously added. By comparison with control samples, changes observed in this study were essentially due to salt supplementation and not to the CO2 treatment. Therefore, this CO2 treatment could be considered as an entirely reversible treatment rather than only pH-reversible, and predictions might be applied to untreated milk. In the case of Ca-supplemented milk, the micellar molar Ca:P ratio increased, the average micellar diameter decreased, and the rennet-clotting properties were improved, whereas opposite effects were observed upon Pi supplementation. Since modification of the micellar molar ratio is the result of change in the chemical composition of micellar calcium phosphate, the effect of calcium and phosphate supplementation on the rennet clotting of milk was found to be also dependent on the nature of the interaction between caseins and colloidal calcium phosphate. [ABSTRACT FROM AUTHOR]

This study focuses on CO 2 diffusivity, which is one of the most important factors impacting eye growth in semi-hard cheese, but yet has scarcely been investigated in literature. The effect of compositional and ripening parameters on experimental CO 2 diffusivity in semi-hard cheese was studied. CO 2 diffusivity in semi-hard cheese showed a complex relationship with temperature in the range 4–25 °C, represented by a quadratic curve with the highest diffusivity at 13 °C. Salt was found to impact CO 2 diffusivity, with a decrease of about 60% for 0–2.7% w / w salt addition, which may be ascribed to viscosity increase of the cheese matrix and “salting in” effect of protein. CO 2 diffusivity was found to increase with moisture content from 39% to 42% w / w , then it decreased from 42% to 48% w / w . Inevitable changes in protein content of the three cheese variants may be responsible for the complex behaviour observed. [ABSTRACT FROM AUTHOR]

Despite CO 2 production and diffusion during ripening of semi-hard Swiss-type cheese are considered as important quality parameters, the research concerning key gas production and transfer in cheese remains widely overlooked. In this study, experimentally assessed CO 2 production was coupled with transfer coefficients in a mathematical model in order to predict CO 2 gradients formed inside the cheese during ripening. The permeability coefficient of CO 2 through the multilayer barrier packaging which wraps the cheese during ripening was also included in the model. The presented model was validated by assessing the CO 2 concentration in the cheese and its partial pressure in the packaging headspace. CO 2 production rate was found to be the most important input parameter affecting CO 2 gradients formed in cheese during ripening whereas the other input parameters (solubility, diffusivity, permeability) had little effect on the total CO 2 gradient. [ABSTRACT FROM AUTHOR]

Despite the fact that CO 2 production by Propionic Acid Bacteria (PAB) fermentation is the driving force for eye growth in most hard and semi-hard cheese types, few coefficients of CO 2 production have been assessed in literature. In this study, the diffusion free CO 2 production rate coefficient was experimentally assessed in semi-hard cheese as a function of salt content (0–1.3% w/w), ripening temperature (13–25 °C) and ripening time (2 weeks). The CO 2 production rate decreased linearly with increasing salt content due to the inhibition effects of salt on the PAB, while it increased with temperature, due to the vicinity with the PAB optimum growth temperature (25–30 °C). A lower rate of production was observed in the first days of ripening, followed by an increase in CO 2 production rate and consequent decrease due to the lower availability of the main substrate (lactate). We proposed a polynomial equation for predicting changes in the CO 2 production rate during ripening. This equation may be used to better describe the phenomenon of eye growth occurring in cheese with propionic acid fermentation. [ABSTRACT FROM AUTHOR]

CO 2 is produced by many microorganisms present in cheese and it can affect cheese quality both during processing and storage. The knowledge of the extent of CO 2 production by cheese microorganism (CO 2 production rate coefficients) may be used to predict gas exchange in cheese/packaging systems, helping dairy industries in the right choice of the packaging (higher/lower gas permeability) and mastering of cheese ripening. However very few coefficients for CO 2 production rate have been published and the ones assessed in vitro (not inside real food) may not well describe the activity of the microorganisms in situ. We have therefore developed a methodology for the in situ assessment of CO 2 production rate and applied it to cheese with propionic acid fermentation. The proposed methodology is based on infra-red measurement of CO 2 and it allows measuring its accumulation up to 1% with 0.001% resolution, while monitoring the level of oxygen. The method showed a good repeatability, with a low coefficient of variation within samples (6.6%) and between samples (8.4%) compared to 10–30% between samples found in literature. The method was compared with a gas chromatography based method, which is also described. [ABSTRACT FROM AUTHOR]