graciela alvarez - Academia.edu (original) (raw)

Papers by graciela alvarez

Research paper thumbnail of Experimental laboratory-scale study of thermo-mechanical treatment of whey protein solution in industrial process-like conditions – Influence of shear

The aim of this study was to describe a thermal aggregation process at laboratory scale by using ... more The aim of this study was to describe a thermal aggregation process at laboratory scale by using an experimental process simulator. This device is able to impose fast variations of temperature and shear in the same range as in industrial equipments. Thermally-induced aggregation was studied on β- lactoglobulin (β-lg) solutions. We observed the influence of a moderate shear and of treatment duration, on the final product characteristics: aggregate size and concentration and residual native fraction. The first results provide information about the shear influence on β-lactoglobulin denaturation/aggregation during thermo-mechanical process: shear has no effect on the aggregated fraction but has a significant influence on the size distribution. The higher is the shear rate during heating, the higher is the concentration of large particles (20-1000 μm). These results are discussed in regards to the theoretical aspects of particles motion in suspension and the role of both perikinetic (Br...

Research paper thumbnail of Cryogenic heat exchangers for process cooling and renewable energy storage: A review

Applied Thermal Engineering, 2019

Pioneering synopsis of present cryogenic heat exchangers in energy storage systems. • First-of-it... more Pioneering synopsis of present cryogenic heat exchangers in energy storage systems. • First-of-its-kind review of trendy heat exchangers in a cryogenic technology context. • Spotlight on cryogenic energy storage as a novel technology to integrate renewables. • Deliberation upon the impact of heat exchangers' design on energy storage performance. • Outline of innovative modelling and design methods, alongside recent research trends.

Research paper thumbnail of Effect of dynamic storage temperatures on the microstructure of frozen carrot imaged using X-ray micro-CT

Journal of Food Engineering, 2018

Frozen vegetables are often exposed to dynamic temperature conditions during cold storage and dis... more Frozen vegetables are often exposed to dynamic temperature conditions during cold storage and distribution chain. The resulting ice recrystallization leads to microstructural changes, which is directly linked to the final vegetable quality. To this end, X-ray µCT was applied to visualize and quantify 3D ice crystal changes in carrot over a period of two months of frozen storage with dynamically changing temperature. The studied conditions revealed a significant increase in ice crystal size during the storage period. The equivalent diameter of the ice crystals increased from 246 ± 15.9 µm, to 342 ± 13.2 µm, 394 ± 18.5 µm, 525 ± 28.0 µm and 578 ± 27.6 µm at 0 d, 7 d, 14 d, 30 d and 60 d of storage, respectively, while the number of ice crystals decreased. The 3D data on the ice crystals and image analysis presented within this paper provide an insight making it possible to describe microstructure evolution, and for better control cold storage sector of frozen vegetables.

Research paper thumbnail of Life cycle assessment of salmon cold chains: comparison between chilling and superchilling technologies

Journal of Cleaner Production, 2016

The cold chain is defined as a set of refrigeration steps that maintain the quality and safety of... more The cold chain is defined as a set of refrigeration steps that maintain the quality and safety of food product. Refrigerant leakage and the use of fossil fuels to produce electrical power for refrigeration equipment contribute greatly to ozone depletion and global warming. Thus, new and emerging refrigeration technologies are developed to provide better energy efficient and environmentally friendly alternatives to current technologies. Superchilling is a concept where the temperature is reduced 1-2 °C below the initial freezing point of the product. The small amount of ice formed within the product (10-15%) serves as a heat sink, eliminating the need for ice during storage and transport. In this work, Life Cycle Assessment (LCA) is applied to the chilling and superchilling salmon cold chains. The superchilling cold chain presents an important improvement compared to the chilled one because of the augmentation of available volume for transportation. 1.

Research paper thumbnail of Thermal energy storage : a key technology for the food cold chain

Research paper thumbnail of Characterization of ice recrystallization in ice cream during storage using the focused beam reflectance measurement

Journal of Food Engineering, 2015

Ice recrystallization was investigated in two commercial and differently formulated ice creams us... more Ice recrystallization was investigated in two commercial and differently formulated ice creams using a rapid and real time granulometric method: the focused beam reflectance measurement. Ice cream A (with locust bean gum as primary stabilizer) and ice cream B (with carrageenan as primary stabilizer) were stored for 154 days at four different temperatures (-5,-8,-12 and-18°C) and three amplitudes of temperature fluctuations (± 0.1°C, ± 0.75°C, ± 2.5°C). Crystal size distribution was assessed at regular time intervals and recrystallization kinetic data was derived by fitting the experimental results to the asymptotic Ostwald ripening model. As expected, recrystallization rates increase with mean storage temperature and amplitude of temperature fluctuations. Carrageenan seems to be more effective than LBG in slowing down ice crystal growth during storage. Mean ice crystal size increased as a function of time 1/3 for both ice creams. The temperature dependence of recrystallization rate fitted Arrhenius well, with activation energies fairly similar for both ice creams.

Research paper thumbnail of Numerical study of energy performance of nanofluids used in secondary loops of refrigeration systems

International Journal of Refrigeration, 2015

A mathematical model was developed to predict the energy performances of refrigerating systems us... more A mathematical model was developed to predict the energy performances of refrigerating systems using nanofluids for application in refrigeration plants of cold chain. The model was based on a combination of the Effectiveness-Number of Transfer Units method and classical heat transfer and fluid hydrodynamic correlations. Simulations were done for a tubular heat exchanger in laminar and turbulent regimes, for various types of NPs and a wide range of volume fraction. It was shown that heat transfer coefficients significantly increased with the increase of NPs concentration whatever the flow regime, but the pressure drop also increased. Calculation of a Performance Evaluation Criterion has shown that the energy performance is strongly dependent on the type of NPs, their volume fraction and the flow regime. These criterion data also showed the existence of optimum parameters for which the use of nanofluids is energetically interesting. The model was validated using published data.

Research paper thumbnail of Influence of whey protein aggregation on the residence time distribution in a tubular heat exchanger and a helical holding tube during heat treatment process

Journal of Food Engineering, 2012

The residence time distribution (RTD) of a whey protein (WP) suspension flowing through a heat tr... more The residence time distribution (RTD) of a whey protein (WP) suspension flowing through a heat treatment system was studied. The experimental system is the tubular heat exchanger and the helical holding section of a laboratory scale thermal process. RTD was measured in isothermal conditions at 60°C, a temperature involving no WP aggregation, and at 87°C, temperature at which WP aggregates. Two flow rates (20 L/h and 49 L/h) were tested with two different lengths of holding tube in order to maintain the same order of magnitude of the holding time. Methylene blue was used as tracer and spectrophotometer analysis were performed to determine the outlet concentration. These investigations yielded to great differences between the RTD in aggregation and no aggregation conditions. The minimum residence time is shorter when WP aggregation occurs due to the modification of the velocity field inside the tubular devices with the local enhancement of the viscosity. Fitting the experimental results with a new compartment model based on the generalized convection model give very good agreement contrary to the more classical RTD models. This RTD model could be used in combination with a population balance model in order to predict the WP aggregates size dispersion.

Research paper thumbnail of In-line characterization of a whey protein aggregation process: Aggregates size and rheological measurements

Journal of Food Engineering, 2013

Research paper thumbnail of Online ice crystal size measurements during sorbet freezing by means of the focused beam reflectance measurement (FBRM) technology. Influence of operating conditions

Journal of Food Engineering, 2012

In ice cream and sorbet manufacturing small ice crystals are desired to deliver a product with a ... more In ice cream and sorbet manufacturing small ice crystals are desired to deliver a product with a smooth texture and good palatability. This research studied the influence of the operating conditions on the ice crystal size and the draw temperature of the sorbet during the freezing process. The evolution of ice crystal size was tracked with the focused beam reflectance measurement (FBRM) technique, which uses an in situ sensor that makes it possible to monitor online the chord length distribution (CLD) of ice crystals in sorbets containing up to 40% of ice. The refrigerant fluid temperature had the most significant influence on the mean ice crystal chord length, followed by the dasher speed, whereas the mix flow rate had no significant influence. A decrease in the refrigerant fluid temperature led to a reduction in ice crystal size, due to the growth of more small ice crystals left behind on the scraped wall from previous scrapings. Increasing the dasher speed slightly reduced the mean ice crystal chord length, due to the production of new small ice nuclei by secondary nucleation. For a given refrigerant fluid temperature and dasher speed, low mix flow rates resulted in lower draw temperatures, due to the fact that the product remains in contact with the freezer wall longer. High dasher speeds warmed the product slightly, due to the dissipation of frictional energy in the product, the effect of which was in part moderated by the improvement in the heat transfer coefficient between the product and the freezer wall.

Research paper thumbnail of Modeling flow and heat transfer in a scraped surface heat exchanger during the production of sorbet

Journal of Food Engineering, 2018

Research paper thumbnail of Effect of process parameters on ice crystals and air bubbles size distributions of sorbets in a scraped surface heat exchanger

International Journal of Refrigeration, 2018

Research paper thumbnail of Phase change material for the thermal protection of ice cream during storage and transportation

International Journal of Refrigeration, 2015

Ice cream is a very temperature sensitive product and temperature fluctuations during the storage... more Ice cream is a very temperature sensitive product and temperature fluctuations during the storage and distribution steps of the food cold chain may result in changes in ice crystal size due to recrystallization phenomenon. An excessive increase of the mean ice crystal size results in a reduction of ice cream quality and a shortening of its shelf life. It is possible to improve the ice cream storage and transportation conditions by using an additional packaging with a low thermal diffusivity. Insulation material is usually chosen to reduce temperature fluctuations, but this paper proposes to evaluate the thermal protection of a phase change material (PCM) packaging and to compare its performance to a polystyrene configuration. The impact on temperature fluctuations and ice crystal size was characterized experimentally for a 5 months storage period and 3 different packaging materials: cartonboard, polystyrene and PCM. Measurements during a temperature abuse for 40 minutes at 20°C were also performed to evaluate the thermal protection provided by the different types of packaging. The results demonstrate that the use of an additional PCM packaging can have a significant impact on the final quality of the product during long term storage or for a product exposed to heat shock.

Research paper thumbnail of Online ice crystal size measurements by the focused beam reflectance method (FBRM) during sorbet freezing

Procedia Food Science, 2011

The ice crystal size distribution determines in part the textural properties of sorbet and ice cr... more The ice crystal size distribution determines in part the textural properties of sorbet and ice cream. During sorbet and ice cream manufacturing, a narrow ice crystal size distribution with a small mean size is desired, in order to obtain a smooth texture in the final product. This research studied the influence of the mix flow rate, the evaporation temperature of the refrigerant fluid and the dasher speed on the ice crystal size and the draw temperature during sorbet freezing, so as to identify optimal operating conditions. The evolution of the ice crystal size was followed by the focused beam reflectance method (FBRM), which uses an in situ sensor that provides accurate and repeatable information about the chord length distribution (CLD) of ice crystals. Our results showed that the FBRM sensor is a promising tool which makes it possible to monitor online the development of the ice crystals in sorbets containing up to 40% of ice. Decreasing the refrigerant fluid temperature allows us to reduce the ice crystal size and to lower the product's temperature, due to the increase of the supercooling driving force. High dasher speeds slightly decrease the ice crystal chord length, due to the attrition of the bigger ice crystals, which produces new smaller ice nuclei by secondary nucleation. Also, an increase of the dasher speed slightly warms the product, due to the dissipation of frictional energy into the product. Low mix flow rates result in lower draw temperatures because the product remains longer in contact with the freezer wall extracting thus more heat from the product.

Research paper thumbnail of Refrigerated warehouses as intelligent hubs to integrate renewable energy in industrial food refrigeration and to enhance power grid sustainability

Trends in Food Science & Technology, 2016

Background Independence from fossil fuels, energy diversification, decarbonisation and energy eff... more Background Independence from fossil fuels, energy diversification, decarbonisation and energy efficiency are key prerequisites to make a national, regional or continental economy competitive in the global marketplace. As Europe is about to generate 20% of its energy demand from Renewable Energy Sources (RES) by 2020, adequate RES integration and renewable energy storage throughout the entire food cold chain must properly be addressed. Scope and approach Refrigerated warehouses for chilled and frozen foods are large energy consumers and account for a significant portion of the global energy demand. Nevertheless, the opportunity for RES integration in the energy supply of large food storage facilities is often neglected. In situ power generation using RES permits capture of a large portion of virtually free energy, thereby reducing dramatically the running costs and carbon footprint, while enhancing the economic competitiveness. In that context, there exist promising engineering solutions to exploit various renewables in the food preservation sector, in combination with the emerging sustainability-enhancing technology of Cryogenic Energy Storage (CES). Key Findings and Conclusions Substantial research endeavours are driven by the noble objective to turn the Europe's Energy Union into the world's number one in renewable energies. Integrating RES, in synchrony with CES development and proper control, is capable of both strengthening the food refrigeration sector and improving dramatically the power grid balance and energy system sustainability. Hence, this article ACCEPTED MANUSCRIPT aims to familiarise stakeholders of the European and global food preservation industry with state-ofthe-art knowledge, know-how, opportunities and professional achievements in the concerned field.

Research paper thumbnail of Control of a nonlinear ice cream crystallization process

In the ice cream industry, the type of final desired product (large cartons (sqrounds) or ice cre... more In the ice cream industry, the type of final desired product (large cartons (sqrounds) or ice creams on a stick) determine the viscosity at which the ice cream has to be produced. One of the objectives of the ice cream crystallization processes is therefore to produce an ice cream of specified viscosity. In this paper, a nonlinear control strategy is proposed for the control of the viscosity of the ice cream in a continuous crystallizer. It has been designed on the basis of a reduced order model obtained by application of the method of moments, on a population balance equation describing the evolution of the crystal size distribution. The control strategy is based on a linearizing control law coupled with a Smith predictor to account for the measurement delay. It has been validated on a pilot plant located at IRSTEA (Antony, France).

Research paper thumbnail of Steady-state and stability analysis of a population balance based nonlinear ice cream crystallization model

The process of crystallization can be modelled by a population balance equation coupled with an e... more The process of crystallization can be modelled by a population balance equation coupled with an energy balance equation. Such models are highly complex to study due to the infinite dimensional and nonlinear characteristics, especially when all the phenomena of nucleation, growth and breakage are considered. In the present paper, we have performed the stability analysis on a reduced order model obtained by the method of moments, which remains still highly complex. The considered model has been developed by the Cemagref and validated on experimental data. After computation, we get a scalar equation whose solutions correspond to the equilibrium points of the system. This equation is finally solved numerically for a concrete physical configuration of the crystallizer. We show that in most instances, there is only one steady state. The possibility of multiple steady-states is discussed.

Research paper thumbnail of Model identification and reduction for the control of an ice cream crystallization process

Chemical Engineering Science, 2014

Ice cream crystallization processes can be modeled by some population and energy balance equation... more Ice cream crystallization processes can be modeled by some population and energy balance equations. Due to the infinite dimensional and nonlinear characteristics, such models are highly complex, especially when all the phenomena of nucleation, growth and breakage are considered. Depending on the control problem under consideration, such a complexity can be useless and the control law can be designed on the basis of an input-output reduced order model of the process. In the present paper, we first consider a reduced order model of 6 ordinary differential equations obtained by the method of moments. By means of a sensitivity analysis and a parameter identification, it is shown that, to accurately describe the input-output behavior of the system whatever the conditions are, it is sufficient to change the values of only two parameters of this model, which is really interesting from a control point of view. However, when looking at the simulated data, the complexity of this moments model appears useless, from the input-output point of view. A second model reduction is therefore performed, based on physical assumptions. We finally get a new model with 3 ordinary differential equations, which is validated first on experimental data and then by comparison with the initial moments model.

Research paper thumbnail of Study of crystal size evolution by focused-beam reflectance measurement during the freezing of sucrose/water solutions in a scraped-surface heat exchanger

Process Biochemistry, 2010

This study examines the freezing step that occurs in a scraped-surface heat exchanger during the ... more This study examines the freezing step that occurs in a scraped-surface heat exchanger during the manufacturing of sorbet. During this step, the product enters the exchanger as a liquid, then it is cooled and partially crystallized before exiting the exchanger as a mixture of liquid and ice, also called sorbet. The freezing step governs the final quality of the product, particularly its texture. Most existing studies have focused on the product after freezing at the exchanger exit. The aim of this work was both to follow the evolution of the ice crystals' granulometry during the freezing of sorbet in the exchanger and to relate this evolution to process parameters such as refrigeration temperature, scraper speed and initial sucrose concentration. Few in situ sensors exist to follow granulometry, and this fact is especially true for rapid kinetics and concentrated suspensions. Focused-beam reflectance measurement (FBRM), an original tool, was used in this study. FBRM is currently used in the chemical and pharmaceutical industries to follow product granulometry, but it is not used in food-related applications. In our study, an experimental protocol was developed to assure identical initial thermal and crystallization conditions. First, the sensor sensitivity and the repeatability of the results were verified. The measurements performed with the FBRM probe showed that this technique can be used to follow crystal granulometry in a sorbet consisting of up to 30% of ice. The effect of process parameters was then analyzed. It appears that a decrease in refrigeration temperature accelerates ice crystallization and yields slightly smaller crystals. The same effect is observed with increasing scraper speed. Additionally, when the initial sucrose concentration in the solution is increased, the ice fraction increases more slowly but the mean chord length is smaller.

Research paper thumbnail of Analysis of heat transfer during mist chilling of a packed bed of spheres simulating foodstuffs

Journal of food engineering, 2001

Heterogeneity of heat transfer was studied in a packed bed of 8 mm spheres cooled by a two-phase ... more Heterogeneity of heat transfer was studied in a packed bed of 8 mm spheres cooled by a two-phase flow composed of air and fine (8 μm diameter) water droplets. Local heat fluxes in the packed bed were measured in both single-and two-phase flow and analyzed as a ...

Research paper thumbnail of Experimental laboratory-scale study of thermo-mechanical treatment of whey protein solution in industrial process-like conditions – Influence of shear

The aim of this study was to describe a thermal aggregation process at laboratory scale by using ... more The aim of this study was to describe a thermal aggregation process at laboratory scale by using an experimental process simulator. This device is able to impose fast variations of temperature and shear in the same range as in industrial equipments. Thermally-induced aggregation was studied on β- lactoglobulin (β-lg) solutions. We observed the influence of a moderate shear and of treatment duration, on the final product characteristics: aggregate size and concentration and residual native fraction. The first results provide information about the shear influence on β-lactoglobulin denaturation/aggregation during thermo-mechanical process: shear has no effect on the aggregated fraction but has a significant influence on the size distribution. The higher is the shear rate during heating, the higher is the concentration of large particles (20-1000 μm). These results are discussed in regards to the theoretical aspects of particles motion in suspension and the role of both perikinetic (Br...

Research paper thumbnail of Cryogenic heat exchangers for process cooling and renewable energy storage: A review

Applied Thermal Engineering, 2019

Pioneering synopsis of present cryogenic heat exchangers in energy storage systems. • First-of-it... more Pioneering synopsis of present cryogenic heat exchangers in energy storage systems. • First-of-its-kind review of trendy heat exchangers in a cryogenic technology context. • Spotlight on cryogenic energy storage as a novel technology to integrate renewables. • Deliberation upon the impact of heat exchangers' design on energy storage performance. • Outline of innovative modelling and design methods, alongside recent research trends.

Research paper thumbnail of Effect of dynamic storage temperatures on the microstructure of frozen carrot imaged using X-ray micro-CT

Journal of Food Engineering, 2018

Frozen vegetables are often exposed to dynamic temperature conditions during cold storage and dis... more Frozen vegetables are often exposed to dynamic temperature conditions during cold storage and distribution chain. The resulting ice recrystallization leads to microstructural changes, which is directly linked to the final vegetable quality. To this end, X-ray µCT was applied to visualize and quantify 3D ice crystal changes in carrot over a period of two months of frozen storage with dynamically changing temperature. The studied conditions revealed a significant increase in ice crystal size during the storage period. The equivalent diameter of the ice crystals increased from 246 ± 15.9 µm, to 342 ± 13.2 µm, 394 ± 18.5 µm, 525 ± 28.0 µm and 578 ± 27.6 µm at 0 d, 7 d, 14 d, 30 d and 60 d of storage, respectively, while the number of ice crystals decreased. The 3D data on the ice crystals and image analysis presented within this paper provide an insight making it possible to describe microstructure evolution, and for better control cold storage sector of frozen vegetables.

Research paper thumbnail of Life cycle assessment of salmon cold chains: comparison between chilling and superchilling technologies

Journal of Cleaner Production, 2016

The cold chain is defined as a set of refrigeration steps that maintain the quality and safety of... more The cold chain is defined as a set of refrigeration steps that maintain the quality and safety of food product. Refrigerant leakage and the use of fossil fuels to produce electrical power for refrigeration equipment contribute greatly to ozone depletion and global warming. Thus, new and emerging refrigeration technologies are developed to provide better energy efficient and environmentally friendly alternatives to current technologies. Superchilling is a concept where the temperature is reduced 1-2 °C below the initial freezing point of the product. The small amount of ice formed within the product (10-15%) serves as a heat sink, eliminating the need for ice during storage and transport. In this work, Life Cycle Assessment (LCA) is applied to the chilling and superchilling salmon cold chains. The superchilling cold chain presents an important improvement compared to the chilled one because of the augmentation of available volume for transportation. 1.

Research paper thumbnail of Thermal energy storage : a key technology for the food cold chain

Research paper thumbnail of Characterization of ice recrystallization in ice cream during storage using the focused beam reflectance measurement

Journal of Food Engineering, 2015

Ice recrystallization was investigated in two commercial and differently formulated ice creams us... more Ice recrystallization was investigated in two commercial and differently formulated ice creams using a rapid and real time granulometric method: the focused beam reflectance measurement. Ice cream A (with locust bean gum as primary stabilizer) and ice cream B (with carrageenan as primary stabilizer) were stored for 154 days at four different temperatures (-5,-8,-12 and-18°C) and three amplitudes of temperature fluctuations (± 0.1°C, ± 0.75°C, ± 2.5°C). Crystal size distribution was assessed at regular time intervals and recrystallization kinetic data was derived by fitting the experimental results to the asymptotic Ostwald ripening model. As expected, recrystallization rates increase with mean storage temperature and amplitude of temperature fluctuations. Carrageenan seems to be more effective than LBG in slowing down ice crystal growth during storage. Mean ice crystal size increased as a function of time 1/3 for both ice creams. The temperature dependence of recrystallization rate fitted Arrhenius well, with activation energies fairly similar for both ice creams.

Research paper thumbnail of Numerical study of energy performance of nanofluids used in secondary loops of refrigeration systems

International Journal of Refrigeration, 2015

A mathematical model was developed to predict the energy performances of refrigerating systems us... more A mathematical model was developed to predict the energy performances of refrigerating systems using nanofluids for application in refrigeration plants of cold chain. The model was based on a combination of the Effectiveness-Number of Transfer Units method and classical heat transfer and fluid hydrodynamic correlations. Simulations were done for a tubular heat exchanger in laminar and turbulent regimes, for various types of NPs and a wide range of volume fraction. It was shown that heat transfer coefficients significantly increased with the increase of NPs concentration whatever the flow regime, but the pressure drop also increased. Calculation of a Performance Evaluation Criterion has shown that the energy performance is strongly dependent on the type of NPs, their volume fraction and the flow regime. These criterion data also showed the existence of optimum parameters for which the use of nanofluids is energetically interesting. The model was validated using published data.

Research paper thumbnail of Influence of whey protein aggregation on the residence time distribution in a tubular heat exchanger and a helical holding tube during heat treatment process

Journal of Food Engineering, 2012

The residence time distribution (RTD) of a whey protein (WP) suspension flowing through a heat tr... more The residence time distribution (RTD) of a whey protein (WP) suspension flowing through a heat treatment system was studied. The experimental system is the tubular heat exchanger and the helical holding section of a laboratory scale thermal process. RTD was measured in isothermal conditions at 60°C, a temperature involving no WP aggregation, and at 87°C, temperature at which WP aggregates. Two flow rates (20 L/h and 49 L/h) were tested with two different lengths of holding tube in order to maintain the same order of magnitude of the holding time. Methylene blue was used as tracer and spectrophotometer analysis were performed to determine the outlet concentration. These investigations yielded to great differences between the RTD in aggregation and no aggregation conditions. The minimum residence time is shorter when WP aggregation occurs due to the modification of the velocity field inside the tubular devices with the local enhancement of the viscosity. Fitting the experimental results with a new compartment model based on the generalized convection model give very good agreement contrary to the more classical RTD models. This RTD model could be used in combination with a population balance model in order to predict the WP aggregates size dispersion.

Research paper thumbnail of In-line characterization of a whey protein aggregation process: Aggregates size and rheological measurements

Journal of Food Engineering, 2013

Research paper thumbnail of Online ice crystal size measurements during sorbet freezing by means of the focused beam reflectance measurement (FBRM) technology. Influence of operating conditions

Journal of Food Engineering, 2012

In ice cream and sorbet manufacturing small ice crystals are desired to deliver a product with a ... more In ice cream and sorbet manufacturing small ice crystals are desired to deliver a product with a smooth texture and good palatability. This research studied the influence of the operating conditions on the ice crystal size and the draw temperature of the sorbet during the freezing process. The evolution of ice crystal size was tracked with the focused beam reflectance measurement (FBRM) technique, which uses an in situ sensor that makes it possible to monitor online the chord length distribution (CLD) of ice crystals in sorbets containing up to 40% of ice. The refrigerant fluid temperature had the most significant influence on the mean ice crystal chord length, followed by the dasher speed, whereas the mix flow rate had no significant influence. A decrease in the refrigerant fluid temperature led to a reduction in ice crystal size, due to the growth of more small ice crystals left behind on the scraped wall from previous scrapings. Increasing the dasher speed slightly reduced the mean ice crystal chord length, due to the production of new small ice nuclei by secondary nucleation. For a given refrigerant fluid temperature and dasher speed, low mix flow rates resulted in lower draw temperatures, due to the fact that the product remains in contact with the freezer wall longer. High dasher speeds warmed the product slightly, due to the dissipation of frictional energy in the product, the effect of which was in part moderated by the improvement in the heat transfer coefficient between the product and the freezer wall.

Research paper thumbnail of Modeling flow and heat transfer in a scraped surface heat exchanger during the production of sorbet

Journal of Food Engineering, 2018

Research paper thumbnail of Effect of process parameters on ice crystals and air bubbles size distributions of sorbets in a scraped surface heat exchanger

International Journal of Refrigeration, 2018

Research paper thumbnail of Phase change material for the thermal protection of ice cream during storage and transportation

International Journal of Refrigeration, 2015

Ice cream is a very temperature sensitive product and temperature fluctuations during the storage... more Ice cream is a very temperature sensitive product and temperature fluctuations during the storage and distribution steps of the food cold chain may result in changes in ice crystal size due to recrystallization phenomenon. An excessive increase of the mean ice crystal size results in a reduction of ice cream quality and a shortening of its shelf life. It is possible to improve the ice cream storage and transportation conditions by using an additional packaging with a low thermal diffusivity. Insulation material is usually chosen to reduce temperature fluctuations, but this paper proposes to evaluate the thermal protection of a phase change material (PCM) packaging and to compare its performance to a polystyrene configuration. The impact on temperature fluctuations and ice crystal size was characterized experimentally for a 5 months storage period and 3 different packaging materials: cartonboard, polystyrene and PCM. Measurements during a temperature abuse for 40 minutes at 20°C were also performed to evaluate the thermal protection provided by the different types of packaging. The results demonstrate that the use of an additional PCM packaging can have a significant impact on the final quality of the product during long term storage or for a product exposed to heat shock.

Research paper thumbnail of Online ice crystal size measurements by the focused beam reflectance method (FBRM) during sorbet freezing

Procedia Food Science, 2011

The ice crystal size distribution determines in part the textural properties of sorbet and ice cr... more The ice crystal size distribution determines in part the textural properties of sorbet and ice cream. During sorbet and ice cream manufacturing, a narrow ice crystal size distribution with a small mean size is desired, in order to obtain a smooth texture in the final product. This research studied the influence of the mix flow rate, the evaporation temperature of the refrigerant fluid and the dasher speed on the ice crystal size and the draw temperature during sorbet freezing, so as to identify optimal operating conditions. The evolution of the ice crystal size was followed by the focused beam reflectance method (FBRM), which uses an in situ sensor that provides accurate and repeatable information about the chord length distribution (CLD) of ice crystals. Our results showed that the FBRM sensor is a promising tool which makes it possible to monitor online the development of the ice crystals in sorbets containing up to 40% of ice. Decreasing the refrigerant fluid temperature allows us to reduce the ice crystal size and to lower the product's temperature, due to the increase of the supercooling driving force. High dasher speeds slightly decrease the ice crystal chord length, due to the attrition of the bigger ice crystals, which produces new smaller ice nuclei by secondary nucleation. Also, an increase of the dasher speed slightly warms the product, due to the dissipation of frictional energy into the product. Low mix flow rates result in lower draw temperatures because the product remains longer in contact with the freezer wall extracting thus more heat from the product.

Research paper thumbnail of Refrigerated warehouses as intelligent hubs to integrate renewable energy in industrial food refrigeration and to enhance power grid sustainability

Trends in Food Science & Technology, 2016

Background Independence from fossil fuels, energy diversification, decarbonisation and energy eff... more Background Independence from fossil fuels, energy diversification, decarbonisation and energy efficiency are key prerequisites to make a national, regional or continental economy competitive in the global marketplace. As Europe is about to generate 20% of its energy demand from Renewable Energy Sources (RES) by 2020, adequate RES integration and renewable energy storage throughout the entire food cold chain must properly be addressed. Scope and approach Refrigerated warehouses for chilled and frozen foods are large energy consumers and account for a significant portion of the global energy demand. Nevertheless, the opportunity for RES integration in the energy supply of large food storage facilities is often neglected. In situ power generation using RES permits capture of a large portion of virtually free energy, thereby reducing dramatically the running costs and carbon footprint, while enhancing the economic competitiveness. In that context, there exist promising engineering solutions to exploit various renewables in the food preservation sector, in combination with the emerging sustainability-enhancing technology of Cryogenic Energy Storage (CES). Key Findings and Conclusions Substantial research endeavours are driven by the noble objective to turn the Europe's Energy Union into the world's number one in renewable energies. Integrating RES, in synchrony with CES development and proper control, is capable of both strengthening the food refrigeration sector and improving dramatically the power grid balance and energy system sustainability. Hence, this article ACCEPTED MANUSCRIPT aims to familiarise stakeholders of the European and global food preservation industry with state-ofthe-art knowledge, know-how, opportunities and professional achievements in the concerned field.

Research paper thumbnail of Control of a nonlinear ice cream crystallization process

In the ice cream industry, the type of final desired product (large cartons (sqrounds) or ice cre... more In the ice cream industry, the type of final desired product (large cartons (sqrounds) or ice creams on a stick) determine the viscosity at which the ice cream has to be produced. One of the objectives of the ice cream crystallization processes is therefore to produce an ice cream of specified viscosity. In this paper, a nonlinear control strategy is proposed for the control of the viscosity of the ice cream in a continuous crystallizer. It has been designed on the basis of a reduced order model obtained by application of the method of moments, on a population balance equation describing the evolution of the crystal size distribution. The control strategy is based on a linearizing control law coupled with a Smith predictor to account for the measurement delay. It has been validated on a pilot plant located at IRSTEA (Antony, France).

Research paper thumbnail of Steady-state and stability analysis of a population balance based nonlinear ice cream crystallization model

The process of crystallization can be modelled by a population balance equation coupled with an e... more The process of crystallization can be modelled by a population balance equation coupled with an energy balance equation. Such models are highly complex to study due to the infinite dimensional and nonlinear characteristics, especially when all the phenomena of nucleation, growth and breakage are considered. In the present paper, we have performed the stability analysis on a reduced order model obtained by the method of moments, which remains still highly complex. The considered model has been developed by the Cemagref and validated on experimental data. After computation, we get a scalar equation whose solutions correspond to the equilibrium points of the system. This equation is finally solved numerically for a concrete physical configuration of the crystallizer. We show that in most instances, there is only one steady state. The possibility of multiple steady-states is discussed.

Research paper thumbnail of Model identification and reduction for the control of an ice cream crystallization process

Chemical Engineering Science, 2014

Ice cream crystallization processes can be modeled by some population and energy balance equation... more Ice cream crystallization processes can be modeled by some population and energy balance equations. Due to the infinite dimensional and nonlinear characteristics, such models are highly complex, especially when all the phenomena of nucleation, growth and breakage are considered. Depending on the control problem under consideration, such a complexity can be useless and the control law can be designed on the basis of an input-output reduced order model of the process. In the present paper, we first consider a reduced order model of 6 ordinary differential equations obtained by the method of moments. By means of a sensitivity analysis and a parameter identification, it is shown that, to accurately describe the input-output behavior of the system whatever the conditions are, it is sufficient to change the values of only two parameters of this model, which is really interesting from a control point of view. However, when looking at the simulated data, the complexity of this moments model appears useless, from the input-output point of view. A second model reduction is therefore performed, based on physical assumptions. We finally get a new model with 3 ordinary differential equations, which is validated first on experimental data and then by comparison with the initial moments model.

Research paper thumbnail of Study of crystal size evolution by focused-beam reflectance measurement during the freezing of sucrose/water solutions in a scraped-surface heat exchanger

Process Biochemistry, 2010

This study examines the freezing step that occurs in a scraped-surface heat exchanger during the ... more This study examines the freezing step that occurs in a scraped-surface heat exchanger during the manufacturing of sorbet. During this step, the product enters the exchanger as a liquid, then it is cooled and partially crystallized before exiting the exchanger as a mixture of liquid and ice, also called sorbet. The freezing step governs the final quality of the product, particularly its texture. Most existing studies have focused on the product after freezing at the exchanger exit. The aim of this work was both to follow the evolution of the ice crystals' granulometry during the freezing of sorbet in the exchanger and to relate this evolution to process parameters such as refrigeration temperature, scraper speed and initial sucrose concentration. Few in situ sensors exist to follow granulometry, and this fact is especially true for rapid kinetics and concentrated suspensions. Focused-beam reflectance measurement (FBRM), an original tool, was used in this study. FBRM is currently used in the chemical and pharmaceutical industries to follow product granulometry, but it is not used in food-related applications. In our study, an experimental protocol was developed to assure identical initial thermal and crystallization conditions. First, the sensor sensitivity and the repeatability of the results were verified. The measurements performed with the FBRM probe showed that this technique can be used to follow crystal granulometry in a sorbet consisting of up to 30% of ice. The effect of process parameters was then analyzed. It appears that a decrease in refrigeration temperature accelerates ice crystallization and yields slightly smaller crystals. The same effect is observed with increasing scraper speed. Additionally, when the initial sucrose concentration in the solution is increased, the ice fraction increases more slowly but the mean chord length is smaller.

Research paper thumbnail of Analysis of heat transfer during mist chilling of a packed bed of spheres simulating foodstuffs

Journal of food engineering, 2001

Heterogeneity of heat transfer was studied in a packed bed of 8 mm spheres cooled by a two-phase ... more Heterogeneity of heat transfer was studied in a packed bed of 8 mm spheres cooled by a two-phase flow composed of air and fine (8 μm diameter) water droplets. Local heat fluxes in the packed bed were measured in both single-and two-phase flow and analyzed as a ...