Few investigations have attempted to connect the mechanism of dairy fouling to the chemical reaction of denaturation (unfolding and aggregation) occurring in the bulk. The objective of this study is to contribute to this aspect in order to propose innovative controls to limit fouling deposit formation. Experimental investigations have been carried out to observe the relationship between the deposit mass distribution generated in plate heat exchangers (PHE) by a whey protein isolate (WPI) mainly composed of
In the dairy industry, heat treatments are carried out in order to ensure food security and to impart several functionalities to milk and its derivatives, like thermal stability, viscosity, or gelation [
Fouling deposit formation on heat exchanger surfaces is a major industrial problem of milk processing plants, which involves frequent cleaning of the installations, thereby resulting in excessive rinsing water and harsh chemicals use. A number of studies have reported the drastic economic costs of fouling. Fouling and the resulting cleaning of the process equipment account for about 80% of the total production costs [
Milk fouling deposit is complex in nature. Deposit is formed by a mixture of inorganic salts (mainly calcium) and proteins (largely whey proteins). The key role played by
The fouling mechanisms are complicated and involve chemical reactions and heat and mass transfer processes [ unfolding and aggregation of proteins in the bulk; transport of the unfolded and aggregated proteins to the surface; surface reactions resulting in incorporation of protein into the deposit layer; possible reentrainment or removal of deposit toward the bulk.
At the state of the art, the possible limiting processes controlling fouling phenomena (bulk reaction regarding the temperature profiles, surface reaction concerning the flow conditions, and mass transfer of the different protein species occurring in the bulk) are not clearly elucidated and ambiguity on foulant precursor (unfolded and/or aggregated species) also exists.
Belmar-Beiny et al. [
So, there is still a lack of knowledge between the chemical reactions occurring in the bulk (unfolding and aggregation of
In this study, we propose to partially fill this gap by investigating the chemical reactions of
The main objective of this work is to investigate whether a relationship can be established between the distribution of the dry fouling deposit mass in each PHE channel and the
The model fluids used in this study were reconstituted from WPI Promilk 852FB1 supplied by Ingredia (France). The composition of the powder is shown in Table
Composition of WPI powder.
Component | Promilk 852FB1 |
---|---|
Total proteins | 80.1 |
66.0 | |
13.3 | |
Minerals | 2.9 |
In each experiment, 1% (w/w)
Only a small range of calcium content was studied because it is admitted that a very slight chemical variation results in a large variation in the fouling formation [
All thermal denaturation experiments were conducted on twelve samples of 2 mL that were put in stainless steel tubes (350 mm length, 10 mm core diameter, 1 mm wall thickness, and 0.3
Before submitting samples to the desired holding temperature, the samples were preheated at 60°C for the range of desired temperatures below 80°C and 65°C for the range of desired temperatures over 80°C in a first water bath. The choice of this water bath temperature is not trivial. The
The temperature increase from the preheating temperature to the desired holding temperature was performed by placing the samples in a second water bath whose temperature was maintained until 20°C higher than the holding temperature. The second water bath was used in order to reduce the heat increase time and the denaturation level before sampling. The first sample, corresponding to time zero, was taken when the sample temperature was equal to the desired holding value.
The eleven other samples were maintained during a time sufficient in a third water bath, taken off at different times, and cooled down immediately in a beaker with melting ice to stop further
The temperature profile in samples placed in the three water baths was determined using a sensor connected to a temperature measurement acquisition system, placed in a stainless steel tube filled with water (Figure
Imposed temperature profiles (in the three water baths) to carry out thermal denaturation at a constant holding temperature.
The soluble (native and unfolded)
The mobile phases used in HPLC were 0.1% (v/v) trifluoroacetic acid (99%, Acros Organics, Thermo Fisher Scientific, Waltham, MA, USA) in Milli-Q water and 0.1% trifluoroacetic acid in a mixture of 80% acetonitrile (HPLC grade, Thermo Fisher Scientific, Waltham, MA, USA) and 20% Milli-Q water.
The HPLC analyses were carried out at the following conditions: flow rate 1 mL·min−1, injection volume 20
The reaction model used in this study is derived from the work of Tolkach and Kulozik [
For each temperature, the corresponding denaturation rate constant was determined from the Arrhenius plot. The relation between the denaturation kinetic rate and the heat treatment temperature is given by
The value of
The slope of this linear representation is equal to
The value of
Fouling experiments were carried out in a pilot plant (Figure
Schematic diagram of the experimental setup carried out for fouling runs with the 1% (w/w) WPI solutions.
The PHE setup consisted of 10 plates; that is, 5 passes (one channel per pass) of about 0.074 m2 projected heat transfer area per plate (0.495 m length and 0.15 m width) were installed in a countercurrent configuration to optimize the heat transfer, as represented in Figure
Plate heat exchanger flow arrangement.
The temperature profile inside the heat exchanger was simulated with Sphere software (previously developed at our laboratory): temperatures in all passes of hot and cold fluids were calculated from the knowledge of fluids’ inlet temperature and flow rate, plate properties, and heat exchanger design. The temperature profile is controlled by the heat exchanger inlet parameters: product and hot fluid inlet temperatures (
Operating conditions investigated with the pilot scale experimental set: mean inlet and outlet temperatures and flow rates of
Thermal profile number | Total calcium content (ppm) |
|
|
|
|
---|---|---|---|---|---|
#1 | 100 | 65 | 85 | 300 | 300 |
#2 | 100 | 65 | 85 | 300 | 900 |
#3 | 100 | 65 | 85 | 300 | 150 |
#4 | 100 | 60 | 75 | 300 | 300 |
#1 | 120 | 65 | 85 | 300 | 300 |
#2 | 120 | 65 | 85 | 300 | 900 |
#3 | 120 | 65 | 85 | 300 | 150 |
#4 | 120 | 60 | 75 | 300 | 300 |
The temperature profiles displayed in Figure
The imposed thermal profiles in the PHE (Sphere simulations).
Heat exchanger plates were weighted before each heat treatment experiment. After being dried in an air oven at 50°C, fouled plates were weighted at ambient temperature and the dry deposit mass on each plate was deduced by subtraction.
The amount of fouling was also monitored by calculating the fouling resistance. A linear relationship was visible between the average fouling resistance
A logarithmic mean temperature difference (LMTD) method was used to relate the heat transfer rate to the overall heat transfer coefficient. In the case of a no pure cross countercurrent flow inside the plate heat exchanger, the correction factor
Leuliet et al. [
The Arrhenius plots for the denaturation reaction of the WPI model fouling solutions at two calcium concentrations (100 and 120 ppm) were presented in Figure
Arrhenius plot for the
Two mechanisms appear in Figure
These results are in agreement with Petit et al. [
Figure
The frequency factor logarithms (
Denaturation parameters at the two calcium concentrations.
Denaturation parameter | 100 ppm total calcium | 120 ppm total calcium |
---|---|---|
|
||
|
124.8 | 117.2 |
|
384.5 | 271.2 |
|
||
|
86.3 | 83.1 |
|
360.7 | 260.4 |
Figure
Pictures of the deposit collected on heat exchanger surface in the second and last channels of the PHE.
Figure
Fouling mass distribution, related to the projected heat transfer area per plaque, in each channel of the PHE for the two fouling solutions. 1% (w/w) WPI solution containing (a) 100 ppm calcium and (b) 120 ppm calcium (
The deposit mass is negligible, for the thermal profile #4 at 100 and 120 ppm calcium, owing to the lack of denatured
Figure
Total amount of the deposit mass in the PHE per unit area for 1% (w/w) WPI fouling solutions at various calcium concentrations (100 and 120 ppm) and for different temperature profiles (
This difference of fouling distributions obtained at various calcium concentrations demonstrates the major role of the temperature profile on
Figure
Fouling resistance evolution with time measured along the PHE. Various symbols refer to various fouling runs with varying thermal profiles and various calcium content for the 1% w/w WPI solutions (closed symbols: 100 ppm calcium; open symbols: 120 ppm calcium) (
The results also show that the fouling potential of WPI in the PHE increases with the increasing temperature. Indeed, for temperature range of 65–85°C, fouling rate is altered and favoured by higher amount of calcium in the model fluid. It can be noted that, for the thermal profile #4 (60–75°C), fouling resistance curves for calcium content of 100 and 120 ppm calcium were superposed. These results are not contradictory with the assumption of Daufin et al. [
To study the relationship between the chemical reaction of the
Plots of the dry deposit masses per unit area versus the ratio of unfolding and aggregation rate constants for the 1% w/w WPI solutions (closed symbols: 100 ppm calcium; open symbols: 120 ppm calcium) (
For each fouling solution, it could be observed that an
The two master curves, representing the deposit mass per channel versus the ratio an unfolding limited zone with a sharp increase of the deposit mass per channel with an aggregation limited zone where the deposit mass per channel seems to reach a limiting value.
Indeed, the curve of the dry deposit mass at 100 ppm calcium showed a sharp increase at values of
In summary, for the two fouling model solutions and the operating conditions investigated, it can be observed that a sharp increase of deposit mass occurs, when
Unfortunately, it was not possible to perform pilot scale experiments at higher values of
To ascertain validity of the master curve independently of the PHE configuration, further experiments were conducted at PHE consisting of 10 passes (one channel per pass) for a 1% (w/w) WPI model solution at 100 ppm calcium. One thermal profile (#1), out of the four tested for the five channels of the PHE, was imposed. Figure
Variation of the dry deposit mass per unit area in the various channels with the ratio
Also in this case, the fouling mass is reasonably well correlated to the ratio the importance of determining this indicator for predicting fouling mass distribution; the robustness of the approach.
Fouling experiments were performed with 1% (w/w) WPI solutions, at two different calcium concentrations, in order to investigate the effect of the operating conditions associated with the chemical denaturation reactivity of heat treatment in a PHE on the deposit formation. The extent of fouling deposit was monitored by weighing the mass of the dry fouling deposit on the plates.
It was shown that an increase of the calcium content in the fouling solution induced a strong increase in the the fouling mass distribution also depends on the thermal profiles imposed on the fouling solutions; the dry deposit mass on each pass of the PHE seems to be correlated with
This work clearly shows that
Adjusted parameter in the nonlinear regression using (
Adjusted parameter in the nonlinear regression using (
Adjusted parameter in the nonlinear regression using (
Calcium concentration, ppm
Concentration of the total soluble
Specific heat for the product, J kg−1 K−1
Specific heat for the hot water, J kg−1 K−1
Adjusted parameter in the nonlinear regression using (
Activation energy, J mol−1
Logarithmic mean temperature difference, -
Denaturation rate constant, g1−
Denaturation frequency factor, g1−
Unfolding rate constant, g1−
Aggregation rate constant, g1−
Measured dry fouling deposit mass in a channel along the PHE, g
Mass flow rates for the product, kg s−1
Mass flow rates for the hot water, kg s−1
Heat-induced denaturation reaction order
WPI fouling solution flow rate, L h−1
Hot water flow rate, L h−1
Ratio between the unfolding and aggregation rate constants, -
The universal gas constant equal to 8.314, J mol−1 K−1
Fouling resistance, m2
Heat transfer area, m2
Hot water temperature at the PHE inlet, K
WPI fouling solution temperature at the PHE inlet, K
Hot water temperature at the PHE outlet, K
Hot water temperature at the PHE outlet, K
Overall heat transfer coefficient, W m−2 K−1
Logarithmic mean temperature difference, K.
Aggregation
Channel number
Hot water
Product
Plate number
Plate heat exchanger
Unfolding
Whey protein isolate
Thermal profile number.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors are indebted to the Scientific Committee of Agrocampus Ouest Rennes and HEI (Hautes Etudes d’Ingénieur, Lille) for stimulating discussions and financial support for the Ph.D. Thesis of the first author.