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 (TS), where higher TS 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.
Highlights
Deposition of proteinaceous compounds onto solid substrates is a serious concern for food processing as well other sectors such as biomedical devices and marine industry, whereby surfaceanchored proteins could build-up to form thick foulant and promote biofilm growth
We suggest that the differences between foulants could be attributed to the molecular packing during the build-up, which is controlled by the temperature at the interface: (a) When the liquid (TL) is below the denaturation point of proteins, surface fouling involves milk components adsorbing and saturating the stainless steel surface and rearrangement in their interfacial configuration (Figure 1), which is significantly controlled by surface temperature, TS. (b) Once TL is increased to 75°C, the diffusion coefficient of the protein molecules in the bulk solution increases ca. 10% according to the Stokes−Einstein equation (D = kBT/ 6πηr) favoring surface adsorption and reducing the time required to reach surface saturation, where an increased TS favors the chemisorption of milk compounds
This work presented a molecular understanding of milk fouling process under various temperature profiles, which underpins different stages of a pasteurization process
Summary
Deposition of proteinaceous compounds onto solid substrates is a serious concern for food processing as well other sectors such as biomedical devices and marine industry, whereby surfaceanchored proteins could build-up to form thick foulant and promote biofilm growth. Pasteurization of raw milk (e.g., 71.7°C for at least 15 s) is essential to the dairy industry as it deactivates pathogens and microorganisms to ensure food safety and extend shelf life for dairy products. Such mild heat treatment favors fouling on food-contact surfaces (e.g., stainless steel), which is a significant challenge for the food industry. At a mildly elevated temperature (60−70°C), there is an alteration to the tertiary structure of β-Lg by breaking the non-covalent bonds, exposing the hidden S−S bond, which favors interactions between −SH groups and solid surfaces,[5−7] leading to a notable protein aggregation and surface deposition. The characteristics of the solid substrate play a critical role in this process, the effect of substrate temperature on the fouling process is unclear, with little knowledge of the underpinning kinetics
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