Molecular Understanding of Fouling Induction and Removal: Effect of the Interface Temperature on Milk Deposits

Abstract

Molecular details concerning the induction phase of milk fouling on stainless steel at an elevated temperature range were established to better understand the effect of temperature on surface fouling during pasteurization. The liquid–solid interface that replicates an industrial heat exchanger (≤75°C), including four stages (preheating, heating, holding, and cooling), was investigated using both a quartz crystal microbalance (QCM-D) and a customized flow cell. We found that the milk fouling induction process is rate-limited by the synergistic effects of bulk reactions, mass transfer, and surface reactions, all of which are controlled by both liquid and surface temperatures. Surface milk foulant becomes more rigid and compact as it builds up. The presence of protein aggregates in the bulk fluid leads to a fast formation of surface deposit with a reduced Young’s modulus. Foulant adhesion and cohesion strength was enhanced as both interfacial temperature and processing time increased, while removal force increased with an increasing deposit thickness. During cleaning, caustic swelling and removal showed semilinear correlations with surface temperature ( T S ), where higher T S reduced swelling and enhanced removal. Our findings evidence that adsorption kinetics, characteristics of the foulant, and the subsequent removal mechanism are greatly dependent on the temperature profile, of which the surface temperature is the most critical one.

Monitoring and Characterization of Milk Fouling on Stainless Steel Using a High-Pressure High-Temperature Quartz Crystal Microbalance with Dissipation

“…For thickness measurements, an AFM nanoindentation or scratching technique was employed . Using contact mode, a defined deflection force was applied to the NCHV-A cantilever resulting in a contact force of ∼62 μN for 20 s at 1 Hz with a 90 μm scan size.…”

“…For thickness measurements, an AFM nanoindentation or scratching technique was employed. 20 Using contact mode, a defined deflection force was applied to the NCHV-A cantilever resulting in a contact force of ∼62 μN for 20 s at 1 Hz with a 90 μm scan size. This resulted in the complete penetration of the foulant layer as indicated by the visibility of the SS surface after 20 s of contact with the surface.…”

“…With regard to milk fouling, QCM-D has been used at elevated temperatures (≤65 °C) to understand how the composition of a model milk-based fluid affects the rate of fouling. , In addition, QCM-D has been used to monitor fouling and cleaning during the stages of a simulated pasteurization process . The maximum temperature of fouling in these studies (65 °C) was constrained by instrumental limitations and did not exceed the denaturation temperature of whey proteins, such as β-lactoglobulin (≥70 °C, ∼pH 6.6), which has been closely linked to HTST fouling, − nor temperatures required to aggregate and coagulate casein micelles (>110 °C, ∼pH 6.5–6.7), which may have an effect on UHT fouling.…”

“…18,19 In addition, QCM-D has been used to monitor fouling and cleaning during the stages of a simulated pasteurization process. 20 The maximum temperature of fouling in these studies (65 °C) was constrained by instrumental limitations and did not exceed the denaturation temperature of whey proteins, such as β-lactoglobulin (≥70 °C, ∼pH 6.6), 21 which has been closely linked to HTST fouling, 22−24 nor temperatures required to aggregate and coagulate casein micelles (>110 °C, ∼pH 6.5−6.7), 25 which may have an effect on UHT fouling. To overcome this temperature limitation in this study, a high-pressure high-temperature (HPHT) attachment for QCM-D (QSense High Pressure, Biolin Scientific, Gothenburg, Sweden) with the capability of achieving temperatures and pressures up to 150 °C and 250 bar, respectively, was adapted to study biological samples at high temperatures (>100 °C) with advanced controls at low pressures (∼10−15 bar), which is critical for industrial applications such as UHT and HTST milk processing.…”

Assessing the Scalability of the High-Pressure High-Temperature Quartz Crystal Microbalance with Dissipation (HPHT QCM-D) for Milk Fouling Studies

“…For thickness measurements, contact mode AFM was used to nanomechanically remove the foulant using NSC15/Hard/Al BS tips (MikroMasch, Sofia, Bulgaria, k = 40 N/m, f 0 = 325 kHz). 29 Scan parameters during contact mode included a scan size of 90 μm, aspect ratio of 10, speed of 1 Hz, scan angle of 90°, and ∼70−120 s of contact time. Contact time was varied based on the time required to create a clean scratch.…”