Deposit formation on heated surfaces: effect of interface energetics

A critical review on surface modifications mitigating dairy fouling

Thermal treatments performed in food processing industries generate fouling. This fouling deposit impairs heat transfer mechanism by creating a thermal resistance, thus leading to regular shutdown of the processes. Therefore, periodic and harsh cleaning-in-place (CIP) procedures are implemented. This CIP involves the use of chemicals and high amounts of water, thus increasing environmental burden. It has been estimated that 80% of production costs are owed to dairy fouling deposit. Since the 1970s, different types of surface modifications have been performed either to prevent fouling deposition (anti-fouling) or to facilitate removal (fouling-release). This review points out the impacts of surface modification on type A dairy fouling and on cleaning behaviors under batch and continuous flow conditions. Both types of anti-fouling and fouling-release coatings are reported as well as the different techniques used to modify stainless steel surface. Finally, methods for testing and characterising the effectiveness of coatings in mitigating dairy fouling are discussed.

A survey of state-of-the-art surface chemistries to minimize fouling from human and animal biofluids

Fouling of artificial surfaces by biofluids is a plague Biotechnology deeply suffers from. Herein, we inventory the state-of-the-art surface chemistries developed to minimize this effect from both human and animal biosamples.

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.

Antifouling Stainless Steel Surface: Competition between Roughness and Surface Energy

To increase the shelf-life qualities of dairy products, a heat treatment is usually done. However, heat treatments induce physico-chemical modifications of the products. Some of them lead to the expected product but an unwanted consequence of this process is the formation of a fouling deposit on the surfaces in contact with the processed fluid. To eliminate fouling, cleaning processes have to be done once a day. It increases the processing and maintenance costs. To control and to decrease the fouling are the main problems in food industries and an active research is carried out on efficient antifouling surface treatments. In the present study, a 316L 2B stainless steel was submitted to different surface treatments (Flame and plasma pre-treatments, Plasma Enhanced Chemical Vapour Deposition, hydrophobic coatings, mechanical polishing ...) to try to establish correlations between different surface parameters (roughness, hydrophobicity, nanostructuration, surface energy, ...) onto the fouling in heat exchangers. All the treated plates were then submitted to a fouling test using an aqueous solution of β-lactoglobulin at 1% (p/p) with a final calcium concentration of 910 mg/L and compared to a bare steel plate. The results obtained imply different influences of each parameter depending on the surface roughness: the effect of a non organized micrometric roughness is preponderant compared to the surface energy: the fouling comes from a mechanical effect mainly due to rubbing. However, when the surface is nanostructured, fouling decreases. When the roughness reaches the nanometer scale (between 100 and 400 nm), it is the surface energy and the polar/apolar components which become preponderant compared to the roughness. Fouling is this time mainly due to the hydrophilicity of the surface and to the adsorption of the β-lactoglobulin on acido-basic sites. Finally, when the roughness reaches less than 50 nm, polar/apolar components show no effect anymore, the preponderant parameter is the hydrophobicity of the surface.

A model heat exchange apparatus for the investigation of fouling of stainless steel surfaces by milk II. Deposition of fouling material at 140 °C, its adhesion and depth profiling

Formation and adhesion of fouling deposit during heating of milk at a stainless steel surface were studied separately using a model apparatus. The amount of deposit and proportion of minerals present increased with increase in surface temperature. At 140 °C, the amount of deposit formed increased linearly with heating time, becoming more uneven in appearance. The variations of deposit formation at 140 °C with pH of the milk, with fat removal and with preheating were similar to those observed in the final heating section of continuous UHT plants. Deposits containing a higher proportion of protein were easier to remove. The remaining inner layer had a smoother appearance and contained a higher proportion of minerals than the original deposit. Depth profiling, using secondary ion mass spectrometry, showed that protein was concentrated on the outside of deposits with calcium phosphate being concentrated closest to the steel surface.

A model heat-exchange apparatus for the investigation of fouling of stainless steel surfaces by milk I. Deposit formation at 100 °C

A model heat-exchange apparatus was used to investigate the factors affecting deposit formation from milk on a stainless steel surface at 100 °C. The structure and composition of the deposits were determined by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chemical analysis after solution in alkali. The effects of changing the pH, preheating and skimming of the milk were similar to those observed in a small-scale continuous ultra high temperature plant. The time course of deposit formation showed that a lag phase did not occur, but the deposit which formed after more than 45 min was more porous than that formed after shorter times. Most (50–90%) of the fresh deposit was readily removed by sonication, leaving a sublayer richer in minerais than the original. The results provide evidence for the two-layer model for deposit formation proposed by Tissier & Lalande (1986).

Preliminary stages of fouling from whey protein solutions

Fouling from milk fluids is a severe industrial problem which reduces the efficiency of process plant. The chemistry of fouling has been thoroughly investigated but the sequence of events that occur is not yet clear. Deposit contains both protein and minerals. Experiments have been carried out to determine the sequence of events in the fouling of stainless steel surfaces at 96 °C from turbulent flows of whey. Contact times between 4 and 210 s have been studied, and surface analysis techniques used to detect the distribution of elements. The first layer of deposit, formed after 4 s of contact between the fluid and the surface (fluid temperature 68 and 73 °C), consisted mainly of protein and was identified by X-ray photoelectron spectroscopy analysis. There was a lag phase of up to 150 s for a fluid temperature of 73 °C before deposit aggregates were observed to adsorb on to the surface. These aggregates were identified as protein and Ca by X-ray elemental mapping. No P was found in any experiments for this exposure. After 60 min contact time, however, both Ca and P were found at the interface between deposit and the stainless steel surface, irrespective of the Ca and P content of the test fluid.

Food materials adhesion: a review

This article reviews the various theories of adhesion mechanism and, more specifically, studies concerning foodstuffs adhesion to industrial equipment and packaging surfaces. Adhesion is governed by mechanical interlocking, wetting, electrostatic and chemical forces, and diffusion. Direct conclusions about the validity of one of these theories were seldom made in the empirical studies reviewed. The different food adhesion determination methods were detailed: direct observations, evaluations (weighting, UV absorbance, and adhesive loss), adhesion strength measurements, and indirect measurements via the wetting theory (tilted plane method, contact angle, and surface tension). The importance of proteins, product rheological properties, solid surface rugosity, and wetting phenomena in many adhesion cases is highlighted. Conclusions were made that fundamental mechanisms of food-contact surfaces interactions still need to be investigated to improve understanding in the science of food materials.