Using scanning fluid dynamic gauging to study the mechanisms and kinetics of enzyme-based cleaning

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1 Using scanning fluid dynamic gauging to study the mechanisms and kinetics of enzyme-based cleaning P.W. Gordon a, A.D.M. Brooker b, Y.M.J. Chew c, J.M. Peralta d, D.W. York b, D.I. Wilson a a Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3RA, UK (pwg23@cam.ac.uk) b Procter & Gamble Technical Centres Ltd., Newcastle-upon-Tyne, NE12 9TS, UK c Department of Chemical Engineering, University of Bath, Bath, BA2 7AY, UK d Instituto de Desarrollo Tecnológico para la Industria Química (INTEC), Güemes 345, Santa Fe, Argentina ABSTRACT This paper reports the development of a scanning fluid dynamic gauge (FDG), a non-contact measurement technique developed to track the thickness and deformation behaviour of fouling layers on solid surfaces in real time. The device is computer controlled and yields a measurement accuracy of ± 5 μm with a measurement interval of 1-15 s. The scanning facility allows multiple points on the surface to be monitored simultaneously, increasing the amount of data that can be obtained from a single test. Here, scanning FDG is used to study the swelling and subsequent cleaning of a proteinaceous food soil (egg yolk) in a simple enzyme-based cleaning formulation. Quantitative measures of extents of swelling and swelling rates, as well as removal rates, are presented, as are comparisons with simple gravimetric tests. Dynamic ph variation is used to demonstrate real-time monitoring of novel cleaning procedures using sfdg. Enzymes are shown to interrupt the swelling process so that the layer can be readily removed by fluid shear. This approach allows the cleaning formulation and protocol for a particular soft deposit to be optimised. Keywords: cleaning; enzyme; fluid dynamic gauge; protein; swelling INTRODUCTION The build-up of fouling layers on processing equipment surfaces is a common issue in the food industry (Fryer and Asteriadou, 29). The removal of soft-solid layers on process surfaces immersed in liquids is of interest to many industrial sectors, where the layer may be a polymer coating, a biofilm or product residue. The study of these materials can pose challenges as they tend to be heterogeneous, deform on contact and collapse when removed from their native liquid environment. Considerable time and resources are expended in the cleaning of these layers, yet relatively few techniques exist to study and optimise cleaning procedures. Existing techniques are often expensive, rely on specific material properties, or do not work well in aqueous systems. Tuladhar et al. (2) developed the technique of fluid dynamic gauging (FDG) as an alternative method suitable for tracking the thickness of such soft layers, in situ and in real time. FDG exploits liquid flow behaviour to locate the position of the layer surface. A small nozzle of throat diameter is positioned close to the layer. The surrounding liquid is drawn into the nozzle at a low flow rate, such that the flow is in the laminar regime: the flow rate,, is usefully sensitive to the clearance,, when <.25. The thickness of the deposit layer,, is obtained by difference ( ), where is the location of the nozzle relative to the substrate. If the deposit is sufficiently weak, the shear stress induced by the gauging flow may disturb the deposit. FDG can exploit this effect to probe the strength of the deposit. By using computational fluid dynamics (CFD) simulations to understand the shear stress imposed on the deposit, the cleaning rate can be measured under controlled cleaning conditions of temperature, ph, shear stress and enzyme concentration. A scanning version of FDG (sfdg) has recently been developed (Gordon et al., 21), which retains the benefits of conventional FDG, whilst adding additional capabilities and improving the quality and quantity of

2 data generated. The sfdg nozzle can be moved across the surface, taking multiple thickness/strength measurements. This allows, for example, the cleaning of different layers to be compared side-by-side under identical conditions. The device is computer-controlled and can be programmed to sample in a raster pattern. This paper reports recent developments of the FDG technique, and demonstrates its use to study the enzymebased cleaning of protein (egg yolk) deposits. Enzyme-based cleaning is widely used in cleaning domestic and industrial food deposits (Boyce et al., 21), but the precise mechanisms by which enzymes effect cleaning of complex soils is relatively poorly understood. By investigating the rate at which the enzymes promote deposit removal under certain conditions, new insights into cleaning processes can be gained. MATERIALS & METHODS SFDG APPARATUS Figure 1 shows a schematic and a photograph of the sfdg system. All parts of the gauge in contact with the cleaning solution are constructed from glass, perspex, or 316L stainless steel. The vertical (z) location of the nozzle is controlled by a stepper motor (M z, 5 µm increments) and measured by an inductive sensor (I SEN, ± 1 µm). The scanning action is provided by two horizontal linear drives (M x, M y, ± 1 µm) which move the sample in the plane underneath the gauging nozzle (N). (a) (b) S T F M z I SEN I W P J H E T S N D M y M x Figure 1. sfdg apparatus: (a) schematic, and (b) photograph. Labels: D = drain, E = telescopic extension, F = flowmeter, I = liquid inlet, I SEN = inductive sensor, J = holding tank, M = stepper motor, N = nozzle, P = Perspex test tank, S = sample, S T = siphon tube, T = thermocouple, W = weir; subscripts x, y, z denote the directions of travel [after Gordon et al. (211)]. The liquid level in the test tank (P) is maintained constant by a peristaltic pump charging the tank from a temperature-controlled holding tank (J) and an overflow weir (W). The difference in level ( ) between the weir and the end of the siphon tube (S T ) provides the hydrostatic pressure head that drives the gauging flow. The design has recently been modified to include a telescopic extension (E) to this siphon tube, which allows adjustment of during an experiment, via a motorised linear drive (± 1 µm). A thermal flowmeter (F), which offers a low pressure drop, measures the liquid flow rate as it passes from the nozzle to the holding tank. The temperature and ph of the 7 L of liquid can be adjusted via the temperature jacket around the

3 holding tank and by direct addition of solution to the test tank. A PC running LabView software performs control and data collection, including unattended operation. When taking deposit thickness measurements, the siphon head,, is set and the flow rate,, measured. The telescopic extension allows to be varied during an experiment, enabling the shear stress exerted by the fluid on the sample,, to be controlled. By combining this with the scanning ability of the device, it is possible to compare the cleaning behaviour of several points on the layer surface under different shear stresses, but otherwise identical conditions, in a single experiment. The system takes ~ 1 s to reach steady state after moving the nozzle, and has a combined vertical measurement accuracy of ± 5 µm (Gordon et al., 21). SAMPLE AND SOLUTION PREPARATION Egg yolk samples were obtained from the Center for Testmaterials (product DM-21, CFT B.V., Vlaardingen, NL). These were prepared by spray coating egg yolk suspension on to melamine tiles. The yellow films appeared uniform and were approximately 7 µm thick. Baked samples were prepared by heating in an oven in air at 15 ºC for 1 or 2 h, and allowed to cool before use. The swelling and removal of baked and unbaked samples was measured in the same experiment, demonstrating the capability of the sfdg system to study several (different) samples challenged with the same solvent/environment in one test, thereby reducing the number of experiments needed in comparative studies. Protease solutions were prepared by dissolving 8.4 g of sodium carbonate in 7 L RO (reverse osmosis) water to give a ph 1.8 buffer solution..448 g of commercial protease was dissolved in this solution in the sfdg tank 1 min before starting the experiment. EXPERIMENTAL PROCEDURES Gravimetric tests were conducted in triplicate in a water bath at 4 C. Samples were immersed in 25 ml sodium carbonate buffer solution (ph 1.8) and removed at regular intervals (~ 2 min), shaken 2 times to remove the majority of surface water, and weighed. The mass of the substrate alone was measured separately following similar treatment, and subtracted to yield the mass of swollen deposit. A feedback control loop on the sfdg was used to locate the nozzle at a clearance of 14 ± 2 µm, in order to maintain a uniform shear stress on the layer during measurement. The thickness of each film was measured at several locations spaced at least 5 mm apart in order to assess any intra-sample variation. One location was measured half as often as the others were in order to track any influence of the gauging flow on the sample behaviour. The position of the substrate was determined after each experiment by gauging on the clean surface. The shear stresses employed in these tests are comparable with the mean shear stress exerted on a surface during cleaning-in-place (CIP) operations (assuming 1-3 m s -1 circulation velocities; Gordon et al., 211). The shear stress imposed by the gauging flow,, was calculated using the analytical approximation reported by Chew et al. (25), based on flow between parallel plates (Middleman, 1998); where is the fluid viscosity, the fluid density, and the radial position being considered. The shear stress reported is defined as that underneath the inner nozzle rim, i.e., when =.5 mm. CALIBRATION AND VERIFICATION The sfdg device was calibrated before each set of experiments by moving the nozzle incrementally towards a clean stainless steel plate, and recording the mass flow rate through the nozzle. This procedure is reported in detail in Gordon et al. (21). It is possible to scan over the surface of a substrate, and use the sfdg to identify its topology. As such, the sfdg can operate in an analogous manner to an AFM, albeit at a length (1)

4 δ [µm] m [g] scale of tens of micrometers. Figure 2 shows an example of the use of sfdg for 2D imaging of a brass plate with letters fashioned from ~ 1 µm thick vinyl. Image processing has not been employed: this could potentially enhance the resolution of the device further. (a) (b) Figure 2. FDG topography scanning: (a) photograph of a brass plate with vinyl letters, and (b) 2-D image reconstructed from sfdg measurements. Contours in z are separated by 5 µm; Δx, y = 1 mm. RESULTS AND DISCUSSION COMPARISON OF SFDG AND GRAVIMETRIC RESULTS The swelling of protein layers can be monitored gravimetrically as they swell by recording their mass as the solution penetrates the layer. Figure 3 shows a gravimetric swelling profile for egg yolk layers in an alkaline buffer solution at 4 C. The gravimetric measurements are comparable to sfdg thickness measurements. It should be noted that sfdg gives a local thickness measurement, whereas the gravimetric data are lumped measurements for the entire sample Figure 3. Comparison of sfdg and gravimetric measurements for the swelling of egg yolk deposits in Na 2 CO 3 buffer solution (ph 1.8, 4 C). Crosses show the layer thickness measured using sfdg: circles (with error bars) indicate gravimetric swelling data obtained separately. The dashed horizontal line indicates the initial dry thickness of the layer, measured by micrometer. ACTION OF A COMMERCIAL PROTEASE Removal of proteinaceous food soils such as egg yolk from hard surfaces can be challenging, particularly if the deposit has been dried, aged or heated during processing. These deposits will swell when immersed in alkali, but may not be removed unless the interactions between protein chains and/or the substrate are

5 δ [µm] δ [µm] interrupted. The use of enzyme-based cleaning formulations can enhance disruption of the swollen gel at relatively moderate temperatures and phs. Figure 4(a) compares the swelling and removal behaviour of egg yolk layers immersed in buffer and protease solutions at 4 ºC and ph 1.8. In the buffer solution the layer swells steadily from an initial thickness of 7 µm to a final thickness of ~ 1 mm. When protease is present swelling is interrupted after ~ 4 min, and after ~ 6 min removal is observed. The measured thickness decreased rapidly while measurements were being made in the latter phase: when the gauge was elsewhere there was relatively little change in deposit thickness. The layer was only removed at the locations being gauged, confirming that some level of fluid shear is required for removal. The layer was completely removed at the gauged locations after 14 min. (a) (b) Buffer only h, 3 Pa h, 1 Pa 1 h, 3 Pa 1 h, 1 Pa 2 h, 3 Pa 2 h, 1 Pa 6 4 Buffer + protease Figure 4. Egg yolk removal: (a) Swelling behaviour in ph 1.8 buffer and buffered protease solution at 4 C. Horizontal dashed line denotes the initial dry thickness of the protein layer. (b) Influence of baking on the removal of egg yolk layers for different shear stresses imposed by the gauging flow. The most challenging food deposits to remove are often those that have been burnt-on or aged, owing to dehydration and chemical alteration. The effect of baking time on the cleaning characteristics of egg yolk layers was investigated by monitoring the thickness of both unbaked and baked samples in the same experiment, allowing direct comparison of their behaviour under identical conditions and in real time. Samples baked for 1 h and 2 h were studied alongside unbaked samples immersed in protease solution under shear stresses of 1 and 3 Pa. Figure 4(b) shows that baking reduced the initial rate of swelling, the maximum extent of swelling, and the subsequent rate of removal by the protease. Baking for 2 h had a similar effect, but with lower rates of removal. All of the deposit was removed when a gauging shear stress of ~ 3 Pa was used (albeit more slowly for the aged material). When the shear stress exerted on the layer was reduced to ~ 1 Pa, the unbaked deposit took significantly longer (~ 2 min) to clean, whereas that aged for 2 h was never completely removed: a layer 16 µm thick remained after 4 min. The upper layers of baked material could be removed, but 1 Pa of fluid shear was insufficient to remove the deposit closest to the substrate, suggesting that ageing gave rise to a stiff deposit with a yield stress > 1 Pa. These results and conclusions are discussed further in Gordon et al., (211). COMPARISON OF CLEANING PROTOCOLS In addition to studying cleaning behaviour under steady conditions, the speed of the sfdg measurement allows optimisation of cleaning protocols. Two protocols are compared in Figure 5. The initially dry deposit is exposed to a strong alkali (ph 11.9 NaOH), promoting rapid initial swelling. After 17 min, HCl was added to the overflow tank, gradually reducing the ph in the gauging tank to A commercial protease was then added to the overflow tank after 3 min, aiding removal of the highly swollen deposit. In this case the total

6 δ [µm] cleaning time was less than half that required to clean the deposit with enzyme at ph 1.8, demonstrating the potential of the sfdg to monitor cleaning under variable conditions A E 4 2 Constant ph Variable Conditions Figure 5. Comparison of the swelling and removal behaviour of egg yolk deposits at 4 C. Swelling under variable conditions employed an initially highly alkaline solution (ph 11.9) to promote rapid swelling, before dropping the ph to 11.2 after 17 min (marked A), then adding enzymes after 3 min (marked E) to effect deposit removal. CONCLUSIONS The scanning fluid dynamic gauge (sfdg) system was modified to allow the shear force exerted on the deposit to be altered during an experiment. The cleaning behaviour of egg yolk layers in simple protease/buffer solutions was studied: unbaked egg yolk layers swelled in alkali, but did not dissolve or erode when subject to flow stresses alone. In the presence of enzymes the swelling behaviour was interrupted and some fluid shear was required to remove the deposit. Baking modified the deposit, causing it to swell less quickly extending the cleaning time. Yield stress behaviour was evident after 2 h baking, where a shear stress of 1 Pa was not able to remove the remaining layer of deposit. The sfdg has been used to illustrate the potential benefits of more complex cleaning protocols, such as the variation of ph and addition of enzymes at specific times in the cleaning cycle. ACKNOWLEDGEMENTS The contributions of Andy Hubbard and Surinder Sall to the construction of the sfdg; an EPSRC CASE studentship with P&G Technical Centres Ltd. for PWG, and a Royal Academy of Engineering/EPSRC Research Fellowship for YMJC, are all gratefully acknowledged. REFERENCES Boyce, A., Piterina, A.V., Walsh, G., 21. Assessment of the potential suitability of selected commercially available enzymes for cleaning-in-place (CIP) in the dairy industry. Biofouling 26, Chew, J.Y.M., Höfling, V., Augustin, W., Paterson, W.R., Wilson, D.I., 25. A method for measuring the strength of scale deposits on heat transfer surfaces. Dev. Chem. Eng. Min. Proc. 13, Fryer, P., Asteriadou, K., 29. A prototype cleaning map: A classification of industrial cleaning processes. Trends Food Sci. Tech. 2, Gordon, P.W., Brooker, A.D.M., Chew, Y.M.J., Wilson, D.I., York, D.W., 21. A scanning fluid dynamic gauging technique for probing surface layers. Meas. Sci. Technol. 21, Gordon, P.W., Brooker, A.D., Chew, Y.M.J., York, D.W., Wilson, D.I., (211). Elucidating enzyme-based cleaning of protein soils using scanning FDG. Submitted to Chem. Eng. Res. Des. Middleman, S., An Introduction to Fluid Dynamics: Principles of Analysis and Design. Wiley, New York. Tuladhar, T.R., Paterson, W.R., Macleod, N., Wilson, D.I., 2. Development of a novel non-contact proximity gauge for thickness measurement of soft deposits and its application in fouling studies. Can. J. Chem. Eng. 78,