Methods of Detecting Fouling Caused by Heating of Milk

Fouling is the deposition of milk solids on heat transfer surfaces, particularly heat exchangers. It is a major industrial problem, which causes a decrease in heat transfer efficiency and shortens run times. The resultant effect is a decrease in process efficiency and economy. For studying and monitoring deposit formation, suitable fouling detectors or methods of measuring the deposit are required. This can be achieved through direct means, whereby the deposit is analyzed after a certain time, or indirectly through instrumentation for monitoring parameters such as temperature, pressure, flow rate, overall heat transfer coefficient, heat flux, and other physical properties. This article reviews the various reported fouling detection methods.     

Methods of Detecting Fouling Caused by Heating of Milk

Fouling is the deposition of milk solids on heat transfer surfaces, particularly heat exchangers. It is a major industrial problem, which causes a decrease in heat transfer efficiency and shortens run times. The resultant effect is a decrease in process efficiency and economy. For studying and monitoring deposit formation, suitable fouling detectors or methods of measuring the deposit are required. This can be achieved through direct means, whereby the deposit is analyzed after a certain time, or indirectly through instrumentation for monitoring parameters such as temperature, pressure, flow rate, overall heat transfer coefficient, heat flux, and other physical properties. This article reviews the various reported fouling detection methods.     

Investigating the effect of starting mode on food fat gel layer formation on cold surfaces

In a previous study, Huang et al. (2012) employed a spinning disc apparatus to study deposition of fats on cooled stainless steel surfaces using model solutions of tripalmitin in non-crystallising paraffin. The apparatus was modified to give more accurate estimation of the test surface temperature, allowing the surface temperature and shear stress to be manipulated independently. The effect of different starting modes, simulating various situations which arise in pipelines, was studied. When fouling is induced by a step change in the test surface temperature, the presence of a subcooled surface promotes the rapid formation, initially, of a gel layer, followed by a period of linear fouling, and eventually falling rate fouling behaviour. When fouling is driven by a change in concentration the initial gel formation step is absent. The linear fouling regime was relatively insensitive to temperature, shear stress and starting mode for the conditions studied here. In the falling rate regime, the rate was determined by the deposit/solution interface temperature, following normal growth kinetics. At low tripalmitin concentrations, of 2.5 wt.%, sigmoidal growth behaviour was observed, which is attributed to the wall shear stress being initially high enough to disrupt the gel. As time proceeded, the gel increased in strength and was able to grow as before.     

Fluid dynamic gauging of microfiltration membranes fouled with sugar beet molasses

Fluid dynamic gauging (FDG) was used to track the thickness of the cake layer formed during the microfiltration of a 45° Brix molasses solution using a 1.5 μm polysulphone membrane. A simultaneous measure of flux and deposit thickness throughout the full membrane operating cycle is reported. Asymptotic fouling thicknesses of ca. 100 μm are developed after 30 min of filtration. Accordingly, flux declines are severe at ca. 93%. Permeate line closed (PLC) operation leads to the complete removal of the deposit layer, and the recovery of 60% of the flux. However, permeate line open (PLO) operation leads to only a 50% flux recovery and an asymptotic deposit thickness of 10 μm. An initial increase and subsequent reduction in flux during chemical cleaning has also been recorded for both acid and alkali cleaning regimes. Effective cleaning regimes for membranes fouled by molasses require an alkali stage followed by an acid stage.     

Analysis by Raman spectroscopy of the conformational structure of whey proteins constituting fouling deposits during the processing in a heat exchanger

Whey protein fouling deposits generated on the hot wall downstream a plate heat exchanger were analyzed by micro Raman spectroscopy (MRS) carried out in the 800-1800 cm⁻¹ range. Deposits were formed using a model beta-lactoglobulin (BLG) fouling solution which was made using a whey protein isolate powder (89 wt.% in BLG) and a known amount of calcium. Thermal denaturation of the fouling solution was also analyzed by MRS as well as isolated BLG aggregates obtained by microfiltration of heated solutions. Specific Raman signatures of aggregates were identified, which were not detected in the Raman spectra of denatured (i.e. unfolded BLG molecule) solutions. MRS analyses at different depths of the deposit reveal a loss of α-helix structures, as observed in denatured BLG solutions, without the detection of aggregate signatures. For the range of calcium content investigated (from 97 to 160 mg l⁻¹), no effect of calcium ions on the molecular conformation of BLG within the deposit was shown. Of great significance, results suggest that, for our set of operating conditions used, the mass distribution of the fouling deposit in a plate heat exchanger is primarily controlled by the distribution of the unfolded protein generated by the denaturation process.     

COMPARISON of TWO OPERATING METHODS of A PLATE HEAT EXCHANGER UNDER CONSTANT HEAT FLUX CONDITION and THEIR EFFECT ON the TEMPERATURE PROFILE DURING MILK FOULING

Two methods of operating a plate heat exchanger under constant heat flux were studied: Method A) control by increasing the flow rate of the heating medium; and Method B) control by increasing the inlet temperature of the heating medium. the cross-sectional temperature variation in the plate used in this study was smaller than that of an intermating-type plate. A change in the axial temperature profile was discovered as a result of the uneven formation of milk deposit along the channel. the temperature changes of both the milk and the solid-milk interface (stainless steel-milk or deposit-milk) during fouling were larger when the effects of fouling were overcome by increased flow rate of heating medium than when the effects of fouling were overcome by increased temperature of the heating medium. A quadratic empirical model underpredicted the temperatures in the lightly fouled region and overpredicted the temperatures in the heavily fouled region.     

Effect of whey protein concentration on the fouling and cleaning of a heat transfer surface

In the studies of fouling and cleaning of heat exchange surfaces in dairy plants, whey protein deposits and heat induced whey protein gels (HIWPG) are considered as suitable model material to simulate the proteinaceous based type “A” milk fouling. Protein concentration of the fouling solution may significantly influence the formation of milk deposits on heat exchange surfaces, hence affecting the cleaning efficiency. In this study, a laboratory produced heat induced whey protein gels (HIWPG) and a pilot plant heat exchanger fouling/cleaning were used to investigate the effect of protein concentration on formation and cleaning of dairy fouling. Here, HIWPGs made from different protein concentrations were formed in capsules and then dissolved in aqueous sodium hydroxide (0.5 wt%). The dissolution rate calculation based on the UV spectrophotometer analysis. In the pilot-scale plant study, whey protein fouling deposits were formed by recirculating whey protein solutions with different concentrations through the heat exchange section in different runs, respectively. The deposit layers were then removed by recirculating aqueous sodium hydroxide (0.5 wt%) and the cleaning efficiency was monitored in the form of the recovery of heat transfer coefficient while both fluid electric conductivity and turbidity were monitored as indications of cleaning completion. It was found that increasing the protein concentration of the HIWPG significantly increased the gel hardness and the dissolution time. In addition, increasing the protein concentration significantly increased both, the amount of the fouling on the pilot-scale plant and the time required to clean the fouling deposit.     

Performance assessment of membrane distillation for skim milk and whey processing

Membrane distillation is an emerging membrane process based on evaporation of a volatile solvent. One of its often stated advantages is the low flux sensitivity toward concentration of the processed fluid, in contrast to reverse osmosis. In the present paper, we looked at 2 high-solids applications of the dairy industry: skim milk and whey. Performance was assessed under various hydrodynamic conditions to investigate the feasibility of fouling mitigation by changing the operating parameters and to compare performance to widespread membrane filtration processes. Whereas filtration processes are hydraulic pressure driven, membrane distillation uses vapor pressure from heat to drive separation and, therefore, operating parameters have a different bearing on the process. Experimental and calculated results identified factors influencing heat and mass transfer under various operating conditions using polytetrafluoroethylene flat-sheet membranes. Linear velocity was found to influence performance during skim milk processing but not during whey processing. Lower feed and higher permeate temperature was found to reduce fouling in the processing of both dairy solutions. Concentration of skim milk and whey by membrane distillation has potential, as it showed high rejection (>99%) of all dairy components and can operate using low electrical energy and pressures (<10 kPa). At higher cross-flow velocities (around 0.141 m/s), fluxes were comparable to those found with reverse osmosis, achieving a sustainable flux of approximately 12 kg/h·m² for skim milk of 20% dry matter concentration and approximately 20 kg/h·m² after 18 h of operation with whey at 20% dry matter concentration.     

Process conditions influence on characteristics of holding tube fouling due to cheese sauce

Cheese sauces, with formulations of 3 g/100 g or 4 g/100 g starch and 15 g/100 g or 35 g/100 g cheese, were processed through a scraped surface heat exchanger (SSHE) under two processing temperatures of 105°C and 121.1°C and mean velocities of 0.14, 0.24 and 0.34 m/s in 3, 5 and 7 h runs. Two test sections (3.548 and 3.569 cm in i.d., and 9.70 and 9.65 cm long, respectively) were installed, one in the holding section after the heating section of the SSHE and the other after cooling section of the SSHE. The fouling deposit was removed from the test sections after each run. Concentrations of protein, phosphorus, fat and mineral were determined. The amount of fat in the deposits was very low after 3 h of processing; it increased with time and almost doubled after 7 h of processing. Process conditions with high mean velocity, regardless of temperature, did not produce detectable fouling deposit. Maximum fouling was obtained in the holding section after the heating section of the SSHE when processing at 121.1°C and for 7 h. Formation of deposit was extremely sensitive to temperature. No fouling was found in the cooling sections.     

Milk fouling at direct ohmic heating

Direct ohmic heating (Joule’s heating) is a technology to warm up the food using an electric energy where electric current is passed through a material which gets heated by virtue of its electrical resistance. Advantages over conventional indirect heating methods are speed and uniformity of heating. On the other side, the direct ohmic heating of liquid foods has some problems, for example corrosion and deposits creation on electrodes (fouling). The present research on fouling concentrates upon the effects of material and surface properties of electrodes, and upon the effect of power frequency. This work evaluates influence of material (stainless steel, TiN and graphite electrodes), flow rate, electric current density (at constant frequency 50 Hz) and temperature (in a limited temperature range 65–75 °C), upon the fouling of skimmed milk. Results prove that the stainless steel electrodes are the worst while the graphite electrodes, where no fouling was observed, are the best, thus confirming the significant role of corrosion and electrochemical phenomena. The paper analyzes also another phenomenon typical for direct ohmic heating and it is an inner overheating of the formed fouling layer, representing a risk of overburning. Theoretical results show that the most intensive overheating occurs at relatively very thin deposit layers, depending upon the specific electrical conductivity of deposits.     

Experimental investigation and theoretical modelling of absolute convective drying of starch

Starch is a granular, hygroscopic, capillary-porous material with an intricate structure. It is known that drying starch till 0% moisture is a physical treatment that modifies starch properties. However, depending upon the heat and mass transfer mechanisms prevalent in the drying system, starch properties are modified in a unique manner. Therefore, the drying operation itself needs to be thoroughly understood. Potato starch was treated in an oven at three high-temperature levels (110, 130 and 140 °C) and for three sample thicknesses (2.8, 5.6 and 11.2 mm). It was observed that drying of starch with moisture content in the hygroscopic range occurred only in the 2nd falling rate period. Two existing theoretical, drying models, one based upon the Krischer Theory and the other based upon the Percolation Approach, were considered apt for modelling the oven drying. Both models describe the experimentally measured moisture profiles well. During drying, transition from one water sub-population to another was smooth. The effective diffusion coefficient decreased with moisture content and increased with temperature, but did not follow Arrhenius’ Law above 130 °C.     

Ultra-high-temperature processing of chocolate flavoured milk

Chocolate milk with different carrageenans (κappa and lambda) and sugar concentrations was heat treated indirectly at 145 °C for 6 s using a bench-top UHT plant. The temperature of the milk in the preheating and sterilizer sections, and the milk flow rate were determined to evaluate the overall heat transfer coefficient (OHTC) for monitoring fouling during UHT processing. Kappa-carrageenan was more effective than lambda-carrageenan in providing stability against fouling during UHT processing. By optimizing concentrations of κ-carrageenan and sugar, fouling could be minimized during UHT processing. The apparent viscosity and sedimentation of UHT-processed chocolate milk increased with increasing concentration of carrageenan and sugar.     

DMA thermal analysis of different parts of potato tuber

Dynamic mechanical analysis (DMA) was applied to three potato tissues (‘cortex’, ‘pith’, and ‘side’ surface) of two cultivars (more waxy ‘Nicola’ and more mealy ‘Saturna’) in temperature scans in the range 30–90 °C and constant air humidity of 90%. The obtained scans indicate peaks in both storage and loss module of elasticity (SM and LM, respectively) at temperatures higher than 70 °C – so called ‘starch’ peak (SP) – as was observed previously. The peak value increase with increasing potato dry matter (DM) content, below DM content approximately 14% no SP is observed (‘Nicola pith’). Slope analysis of the basic parameters: SM, LM, and loss tangent (LT) was performed and further characteristic points on the temperature plots were found: (i) in temperature range A (30–40 °C) maximum of SM and LM and minimum of LT, (ii) in temperature range B (40–50 °C) minimum of SM and LM slopes corresponding to point of inflection on SM-T and LM-T plots, (iii) at about 50 °C, big peak in LT in side tissue only, (iv) at about 70 °C just prior the ‘starch’ peak, big peak in LT that is more marked in ‘pith’; this peak denoted as ‘I’ influenced the ‘starch’ peak ‘II’. It was found that both cultivar and part of the tuber influences the DMA temperature plots.     

Heat treatment of whole milk by the direct joule effect--experimental and numerical approaches to fouling mechanisms

The development of alternative technologies such as the direct Joule effect to pasteurize and sterilize food products is of great scientific and industrial interest. Our objective was 1) to gain insight into the ability to ensure ultra-high-temperature treatment of milk and 2) to investigate the links among thermal, hydraulic, and electrical phenomena in relation to fouling in a direct Joule effect heater. The ohmic heater [OH; (where E is the electrical field and v is the velocity); P (power) = 15 kW] was composed of 5 flat rectangular cells [e (space between the plate and electrode) = 15 mm, w (wall) = 76 mm, and L (length of the plate in plate heat exchanger or electrode) = 246 mm]—3 active cells to ensure heating and 2 (at the extremities) for electrical insulation and the recovery of leakage currents. In the first step, the thermal performance of the OH was investigated vs. the flow regimen [50 < Re (Reynolds number) < 5,000], supplied power (0 < P < 15 kW), and electrical conductivity of fluids (0.1 < σ_20°C < 2 S/m) under clean conditions with model fluids. This protocol enabled a global thermal approach (thermal and electrical balance, modeling of the temperature profile of a fluid) and local analysis of the wall temperature of the electrode. An empirical correlation was established to estimate the temperature gradient, T_w − T_b (where T_w is the wall temperature and T_b is the product temperature) under clean conditions (without fouling) and was used to define operating conditions for pure-volume and direct-resistance heating. In the second step, the ability of OH to ensure the ultra-high-temperature treatment of whole milk was investigated and compared with a plate heat exchanger. Special care was taken to investigate the heat transfer phenomena occurring over a range of temperatures from 105 to 138°C. This temperature range corresponds to the part of the process made critical by protein and mineral fouling. The objectives were 1) to demonstrate the ability of an OH to ensure heat treatment of milk, 2) to study the thermal and hydraulic performance with an increasing power and temperature difference between the inlet and outlet of the OH, 3) to define and validate a criterion to follow heat dissipation efficiency, and 4) to compare the fouling propensity with the different configurations. A heat dissipation coefficient, Rh_CO, was defined and validated to monitor the fouling propensity through global electrical and thermal parameters. Finally, a numerical simulation was developed to analyze heat profiles (wall, deposit, bulk). Because of an increasing Joule effect in the static deposit, the simulation showed how wall overheating would definitively cause fouling to spiral out of control.     

Effect of membrane property and operating conditions on phytochemical properties and permeate flux during clarification of pineapple juice

The effects of membrane property on the permeate flux, membrane fouling and quality of clarified pineapple juice were studied. Both microfiltration (membrane pore size of 0.1 and 0.2 μm) and ultrafiltration (membrane molecular weight cut-off (MWCO) of 30 and 100 kDa) membranes were employed. Membrane filtration did not have significant effects on the pH, reducing sugar and acidity of clarified juice whereas the suspended solids and microorganism were completely removed. The 0.2 μm membrane gave the highest permeate flux, total vitamin C content, total phenolic content and antioxidant capacity as well as the highest value of irreversible fouling. Based on these results, the membrane with pore size of 0.2 μm was considered to be the most suitable membrane for the clarification of pineapple juice. The optimum operating conditions for the clarification pineapple juice by membrane filtration was a cross-flow velocity of 3.4 ms⁻¹ and transmembrane pressure (TMP) of 0.7 bar. An average flux of about 37 lm⁻² h⁻¹ was obtained during the microfiltration of pineapple juice under the optimum conditions using batch concentration mode.     

Thermal performance of a flat ohmic cell under non-fouling and whey protein fouling conditions

Temperature gradients, between electrode surfaces and bulk, in a continuous flat ohmic cell under whey protein fouling were studied. The temperature profiles in non-fouled cell were studied using two Newtonian fluids (water and an aqueous solution of sucrose at 55 g/100 g) and a pseudoplastic fluid (an aqueous solution of xanthan gum at 0.2 g/100 g). The temperature gradients were studied using two fouling fluids: an aqueous solution of β-lactoglobulin and an aqueous solution of β-lactoglobulin-xanthan gum mixture. Obtained result shows the existence of a temperature difference between electrode surfaces and the bulk when heating non-fouling fluids. The value and the shape of these gradients depend on the Reynolds number and the rheological behavior of the fluid. Under fouling conditions, the temperature gradient obtained at different Reynolds number exhibit a different trend. These differences could be explained by the effect of differential electrical conductivities between the bulk and the deposit, and the balance between heat generation by electrical power dissipation and thermal loss by convection (with the fluid) and conduction (with the electrode surfaces).

Significance for the science community and food industry

Food industry and in particularly the dairy industry, are faced with a severe problem due to equipment fouling during processing. Therefore, the development of alternative technologies for fouling limitation is of scientist and industrial relevance. Ohmic heating is one of these technologies, where the theoretical volume heating aspect should provide a considerable advantage to limit fouling phenomena. The present study evaluates the capability of a rectangular ohmic unit to provide a homogenous heat treatment of complexes dairy fluid (fluid rheology, flow rate and fouling presence).     

Are virgin olive oils obtained below 27 °C better than those produced at higher temperatures?

Within the European Union, indications of ‘first cold pressing’ and ‘cold extraction’ can only be used for virgin olive oil (VOO) obtained below 27 °C from mechanical processing. Three different malaxing temperatures (25, 35 and 45 °C) are here evaluated for the quality of the VOO obtained in a continuous industrial plant. The oils were stored at room temperature in the dark for 12 months. Initially, oil obtained from a blend of Frantoio/Leccino cultivars (F/L) had higher acidity and peroxide levels and lower phenolic content than a Coratina cultivar (Cor). The oxidative stability of the oils positively correlated with malaxation temperature (F/L, R² = 0.818; Cor, R² = 0.987) as the phenolic content was directly proportional to the temperature (F/L, R² = 0.887; Cor, R² = 0.992). Only oils obtained at 45 °C were rejected because of ‘heated or burnt’ off-flavour. Decarboxymethylation of secoiridoids and further hydrolysis of phenolic esters occurred during storage. The oxidation products of derivatives of tyrosol and hydroxytyrosol were detected after nine months in both the F/L and Cor samples. Thus, VOO obtained at a processing temperature lower than 27 °C does not show higher chemical and sensory qualities than VOO obtained at 35 °C.     

Analysis of the fouling mechanisms during cross-flow ultrafiltration of apple juice

The purpose of this work was to study the fouling mechanisms of a Carbosep^® M8 membrane during the cross-flow ultrafiltration of apple juice. A new fouling model has been developed that simultaneously considers membrane blocking within the pores, at the pore mouths and by cake formation at the membrane surface. Membrane fouling by apple juice was due to internal pore blocking as well as cake formation. When operating ultrafiltration at a transmembrane pressure of 150 kPa and a cross-flow velocity of 7 m/s, fouling was minimal with a gradual decrease of the relative contribution of cake formation; however, transmembrane pressure still exceeds critical pressure. The fouling model predicts no cake formation at a cross-flow velocity of 7.4 m/s and a transmembrane pressure of 150 kPa or at a cross-flow velocity of 7.0 m/s and a transmembrane pressure of 120 kPa. Under these conditions, internal membrane blocking would be the only mechanism responsible for the decrease of permeate flux.     

Role of β-lactoglobulin in the fouling of stainless steel surfaces by heated milk

Fouling occurred in sudden expansion (SE) test pieces in holding tubes downstream of a direct steam injection (DSI) heater. The fouling deposit contained high levels of protein, mainly β-lactoglobulin, and fat. Fouling during a run was monitored using heat flux sensors to determine the local fouling rate. The overall fouling rate was determined after a run from the weight of deposit and the overall time that fouling occurred during a run. Local and overall fouling rates were correlated. Fouling was high in SEs close to the DSI heater and diminished in SEs placed further downstream. The fouling rate was correlated with the level of native β-lactoglobulin remaining at the position of the SE. It is proposed that the fouling was due to a reactive intermediate of β-lactoglobulin that interacted with deposits on the SE surface. The considerable variation in fouling rates between runs suggests other environmental factors are also important.     

A physicochemical investigation of membrane fouling in cold microfiltration of skim milk

The main challenge in microfiltration (MF) is membrane fouling, which leads to a significant decline in permeate flux and a change in membrane selectivity over time. This work aims to elucidate the mechanisms of membrane fouling in cold MF of skim milk by identifying and quantifying the proteins and minerals involved in external and internal membrane fouling. Microfiltration was conducted using a 1.4-μm ceramic membrane, at a temperature of 6 ± 1°C, cross-flow velocity of 6 m/s, and transmembrane pressure of 159 kPa, for 90 min. Internal and external foulants were extracted from a ceramic membrane both after a brief contact between the membrane and skim milk, to evaluate instantaneous adsorption of foulants, and after MF. Four foulant streams were collected: weakly attached external foulants, weakly attached internal foulants, strongly attached external foulants, and strongly attached internal foulants. Liquid chromatography coupled with tandem mass spectrometry analysis showed that all major milk proteins were present in all foulant streams. Proteins did appear to be the major cause of membrane fouling. Proteomics analysis of the foulants indicated elevated levels of serum proteins as compared with milk in the foulant fractions collected from the adsorption study. Caseins were preferentially introduced into the fouling layer during MF, when transmembrane pressure was applied, as confirmed both by proteomics and mineral analyses. The knowledge generated in this study advances the understanding of fouling mechanisms in cold MF of skim milk and can be used to identify solutions for minimizing membrane fouling and increasing the efficiency of milk MF.