Monitoring cleaning cycles of fouled ducts using ultrasonic coda wave interferometry (CWI).

Fouling in heat exchangers is the buildup of deposits on the solid surfaces. These deposits reduce the eco-efficiency of the processing equipment and increase the risk of subsequent surface contamination with the formation of biofilms. In the agro-food and water supplier sectors, which are our main concern, fouling on the hot walls of processing heat exchangers is a common occurrence and requires frequent cleaning cycles to ensure hygiene requirements are met. This results in a considerable ecological footprint. Sensors and diagnostic tools for monitoring fouling are thus of utmost importance to ensure the rational validation of the cleaning end-point and to decrease the environmental impact of the cleaning cycles. In this paper, a non-destructive ultrasonic monitoring technique using coda waves and the associated signal processing was tested to monitor the evolution over time of a deposit layer on a solid wall during cleaning. To ascertain the feasibility of the method, a piece of wax of controlled thickness was deposited to simulate the initial fouling state and a cleaning cycle was launched. The decorrelation coefficient was used as an indicator to monitor fouling. This article presents the principle of this unprecedented technique for measuring the degree of fouling. The results of the experiments show that this non-destructive monitoring technology is sensitive to changes in fouling and that the decorrelation coefficient curves are in agreement with the cleaning kinetics captured using a video camera, thus ascertaining the pertinence of the diagnostic tool proposed.

Contribution of the shear wave ultrasonic reflectometry to the stickiness measurements.

Today, non-invasive quantification of the adhesion of a deposit to a surface is always a challenge and, unfortunately, few tools are available in this area. This is an obstacle, in several industrial processes, to the identification of conditions limiting the fouling and to the establishment of eco-efficient cleaning strategies. In this paper, a non-invasive ultrasonic technique was developed in the aim of characterizing the adhesion of viscoelastic fluids or solid deposited on a substrate. We adopted the idea that the more a deposit is difficult to clean the more adherent it is. From this point of view the value of the reflection coefficient of an ultrasonic shear wave informs us about the adhesion of the deposit on a surface. A large bibliography on the adhesion measurement is given. Then the principle of ultrasonic test is presented and cares required for the measurement of the reflection coefficient are widely discussed. The ultrasonic reflection coefficients obtained with different controlled samples covering a wide range of interfaces (liquid/substrate, solid/substrate) are presented and compared with other indicators of adhesion. All the data on various samples showed that the ultrasonic test is a tool to discriminate non-destructively a large range of interface quality, allowing ranking according to the adhesive strength.

Predicting the distribution of whey protein fouling in a plate heat exchanger using the kinetic parameters of the thermal denaturation reaction of ß-lactoglobulin and the bulk temperature profiles.

Fouling of plate heat exchangers (PHE) is a severe problem in the dairy industry, notably because the relationship between the build-up of protein fouling deposits and the chemical reactions taking place in the fouling solution has not yet been fully elucidated. Experiments were conducted at pilot scale in a corrugated PHE, and fouling deposits were generated using a model ß-lactoglobulin (ß-LG) fouling solution for which the ß-LG thermal denaturation reaction constants had been previously determined experimentally. Then 18 different bulk temperature profiles within the PHE were imposed. Analysis of the fouling runs shows that the dry deposit mass per channel versus the ratio R=kunf/kagg (with kunf and kagg representing, respectively, the unfolding and aggregation rate constants computed from both the identification of the ß-LG thermal denaturation process and knowledge of the imposed bulk temperature profile into the PHE channel) is able to gather reasonably well the experimental fouling mass data into a unique master curve. This type of representation of the results clearly shows that the heat-induced reactions (unfolding and aggregation) of the various ß-LG molecular species in the bulk fluid are essential to capture the trend of the fouling mass distribution inside a PHE. This investigation also illustrates unambiguously that the release of the unfolded ß-LG (also called ß-LG molten globule) within the bulk fluid (and the absence of its consumption in the form of aggregates) is a key phenomenon that controls the extent of protein fouling as well as its location inside the PHE.

Inline high frequency ultrasonic particle sizer.

This paper reports the development of a new method of particle sizing in a liquid. This method uses high frequency focused ultrasounds to detect particles crossing the focal zone of an ultrasonic sensor and to determine their size distribution by processing the reflected echoes. The major advantage of this technique compared to optical sizing methods is its ability to measure the size of particles suspended in an opaque liquid without any dedicated sample preparation. Validations of ultrasonic measurements were achieved on suspensions of polymethyl methacrylate beads in a size range extending from a few micrometer to several hundred micrometer with a temporal resolution of 1 s. The inline detection of aggregate formation was also demonstrated.

Toward the understanding of the interfacial dairy fouling deposition and growth mechanisms at a stainless steel surface: a multiscale approach.

The microstructures of two dairy fouling deposits obtained at a stainless steel surface after different processing times in a pilot plate heat exchanger were investigated at different scales. Electron-Probe Micro Analysis, Time-of-Flight Secondary Ion Mass Spectrometry, Atomic Force Microscopy, and X-Ray Photo-electron Spectroscopy techniques were used for this purpose. The two model fouling solutions were made by rehydrating whey protein in water containing calcium or not. Results on samples collected after 2h processing show that the microstructure of the fouling layers is completely different depending on calcium content: the layer is thin, smooth, and homogeneous in absence of calcium and on the contrary very thick and rough in presence of calcium. Analyses on substrates submitted to 1 min fouling reveal that fouling mechanisms are initiated by the deposit of unfolded proteins on the substrate and start immediately till the first seconds of exposure with no lag time. In presence of calcium, amorphous calcium carbonate nuclei are detected in addition to unfolded proteins at the interface, and it is shown that the protein precedes the deposit of calcium on the substrate. Moreover, it is evidenced that amorphous calcium carbonate particles are stabilized by the unfolded protein. They are thus more easily trapped in the steel roughnesses and contribute to accelerate the deposit buildup, offering due to their larger characteristic dimension more roughness and favorable conditions for the subsequent unfolded protein to depose.

Advances in food powder agglomeration engineering.

Food powders are used in everyday life in many ways and offer technological solutions to the problem of food production. The natural origin of food powders, diversity in their chemical composition, variability of the raw materials, heterogeneity of the native structures, and physicochemical reactivity under hydrothermal stresses contribute to the complexity in their behavior. Food powder agglomeration has recently been considered according to a multiscale approach, which is followed in the chapter layout: (i) at the particle scale, by a presentation of particle properties and surface reactivity in connection with the agglomeration mechanisms, (ii) at the mechanisms scale, by describing the structuration dynamics of agglomerates, (iii) at the process scale, by a presentation of agglomeration technologies and sensors and by studying the stress transmission mode in the powder bed, and finally (iv) by an integration of the acquired knowledge, thanks to a dimensional analysis carried out at each scale.

Granulomorphometry: a suitable tool for identifying hydrophobic and disulfide bonds in ß-lactoglobulin aggregates. Application to the study of ß-lactoglobulin aggregation mechanism between 70 and 95°C.

This work deals with the investigation of ß-lactoglobulin (ß-LG) aggregation by granulomorphometry. In the first part of this study, we showed that the binding interactions involved in aggregate structure could be identified by their appearance in granulomorphometric pictures. The reliability of this analytical approach was demonstrated by comparing the appearance of ß-LG aggregates in the presence and absence of a thiol-blocking agent (N-ethylmaleimide). The translucency of the aggregates was associated with hydrophobic interactions and their opacity was associated with disulfide bonds. We state, based on the morphology of the aggregates, along with the color of protein aggregates and insoluble materials, that hydrophobic interactions had a better water-holding capacity than disulfide bonds. Additionally, our results suggest that disulfide and hydrophobic bonds compete for ß-LG aggregate shaping. In the second part of this work, interesting features of granulomorphometry useful for identifying aggregate binding interactions were highlighted to clarify the effect of temperature on the aggregation mechanisms occurring in a ß-LG concentrate with a moderate calcium content (6.6mmol·L(-1)). Heat treatment experiments were performed between 70 and 95°C, and granulomorphometric measurements (aggregate size, aggregate number, and gray level of the picture) were conducted at different sampling times up to 4h. Results, which were interpreted in light of calculated ß-LG denaturation levels, revealed that the aggregation mechanism could be split into 2 steps. Initially, ß-LG denatured quickly, leading to fast ß-LG aggregation by disulfide bonds. The denaturation rate then declined, which drastically slowed the disulfide aggregation mechanism. From that point on, a second aggregation path became preponderant. It consisted of the agglomeration of small aggregates by hydrophobic interactions and resulted in the formation of large aggregates containing both interaction types. This second aggregation mechanism was clearly favored at high temperatures because it was not detected in our experiments at temperatures below 85°C.

Influence of calcium on ß-lactoglobulin denaturation kinetics: Implications in unfolding and aggregation mechanisms.

Much research dealing with the processing of milk by-products in heat exchangers has noted the key role of calcium in ß-lactoglobulin (ß-LG) fouling behavior. Nevertheless, the manner by which Ca affects ß-LG denaturation has rarely been quantified using reliable kinetic and thermodynamic data. To this end, the influence of Ca on ß-LG denaturation mechanisms in simulated lactoserum concentrates was studied on the laboratory-scale under 100°C by HPLC analysis. The heat-treated solutions were composed of 53.3g/L ß-LG and were enriched in Ca at various concentrations (0, 66, 132, and 264 mg/kg). The kinetic parameters (reaction order, activation energy, and frequency factor) associated with ß-LG denaturation, along with the unfolding and aggregation thermodynamic parameters were deduced from these experiments and discussed with respect to Ca content. We found that the multistage process characterizing ß-LG thermal denaturation is not greatly affected by Ca addition. In fact, the general model subdividing ß-LG denaturation mechanisms in 2 steps, namely, unfolding and aggregation, remained valid for all tested Ca concentrations. The change in the predominant mechanism from unfolding to aggregation was observed at 80°C across the entire Ca concentration range. Moreover, the classical 1.5 reaction order value was unaffected by the presence of Ca. Interpretation of the acquired kinetic data showed that Ca addition led to a significant increase in kinetic rate, and more so in the aggregation temperature range. This indicates that Ca principally catalyzes ß-LG aggregation, by lowering the Coulombian repulsion between the negatively charged ß-LG reactive species, bridging ß-LG proteins, or via an ion-specific conformational change. To a lesser extent, Ca favors ß-LG unfolding, probably by disturbing the noncovalent binding network of native ß-LG. Simultaneously, Ca has a slight protective role on the native and unfolded ß-LG species, as shown by the increase in activation energy with Ca concentration. The calculation of thermodynamic parameters related to ß-LG denaturation confirmed this observation. A threshold effect in Ca influence was noted in this study: no further significant kinetic rate change was observed above 132 mg/kg of Ca; at this concentration, the studied solution was an almost equimolar mixture of ß-LG and Ca. Finally, we simulated the temporal evolution of ß-LG species concentrations at diverse Ca contents at 3 holding temperatures. The simulations were based on the acquired kinetic parameters. This permitted us to highlight the greater effect of Ca on ß-LG denaturation at high Ca content or for short-time heat treatments at temperatures near 100°C, as in heat exchangers.