Current knowledge in hygienic design: can we minimize fouling and speed cleaning?

Procedia Food Science 1 (2011) 1753 – 1760

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2211–601X © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of 11th International Congress on Engineering and Food (ICEF 11) Executive Committee.doi:10.1016/j.profoo.2011.09.258

Available online at www.sciencedirect.com

Procedia – Food Science 00 (2011) 000–000

ProcediaFood Science

www.elsevier.com/locate/procedia

11th International Congress on Engineering and Food (ICEF11)

Current knowledge in hygienic design: can we minimize fouling and speed cleaning?

Peter J. Fryer*a, Phillip T. Robbins, Konstantina Asteriadou

School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK

Abstract

Fouling of process plant from food fluids is a major practical problem that lowers plant efficiency and endangers product safety. As a result, frequent process plant cleaning is needed, and cleaning-in-place (CIP) protocols are well-developed. It is less clear whether they are optimal, however. Recent progress in fouling and cleaning research is reviewed. Advances in computational modelling and nanotechnology may enable developments in modelling cleanability at the design stage and in developing surfaces that resist fouling and speed cleaning.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ICEF11 Executive Committee Members

Keywords: Fouling, cleaning, hygienic design, nanocoatings

1. Introduction

Fouling is a significant problem in the process industries in all areas of heat transfer. Deposits form due to adhesion of species to the surface followed by cohesion between elements of the material that resists removal. Fouling research has formed an important part of heat transfer research for many years [1] and after it was described as ‘the last unresolved problem in heat transfer’ [2] many papers and conference proceedings have discussed advances in the area (for recent conference proceedings see [3]). Many of the components of foods are thermally unstable. As a result, during thermal processing, the formation of fouling deposits from is a severe problem. Hygienic design of processes is a complex subject, and a number of guidelines have been produced by organisations such as EHEDG (www.ehedg.org).

Cleaning processes in the food industry are commonplace but rarely optimised, because of the difficulty of validating cleaning end-point. This paper reviews recent work in the classification and study of cleaning.

* Corresponding author. Tel.: +44-121-414-5451; fax: +44-121-414-5377. E-mail address:[email protected].

© 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of 11th International Congress on Engineering and Food (ICEF 11) Executive Committee.

1754 Peter J. Fryer et al. / Procedia Food Science 1 (2011) 1753 – 17602 Author name / Procedia – Food Science 00 (2011) 000–000

2. Fouling and cleaning

A range of organic and mineral materials can be deposited on process surfaces. Epstein [4] developed a general classification of fouling mechanisms, and many of these can be identified in foods: including (i) reaction fouling: where fouling results from reaction such as protein denaturation [5]; for example, the whey protein -lactoglobulin is deposited preferentially forms a significant fraction of milk pasteuriser deposits, (ii) Biological fouling: from adhesion of single organisms to the growth of thick biofilms [6] and (iii) Crystallisation or precipitation fouling: here a dissolved component of the fluid is deposited such as calcium carbonate from boiling water or calcium phosphate from milk at UHT temperatures. A subset is Solidification fouling: solidification of the fluid or components of it [7] such as ice from water or starch from a food fluid;

Fouling from foods may result from more than one mechanism – for example proteins and calcium phosphate interact to create milk fouling [8]. Fouling profiles can also be significantly affected by small changes in fluid composition: for example, adding calcium phosphate to whey protein concentrate changes the rate and extent of fouling and the deposit composition [9]. Several authors [10-14] have proposed methods for predicting fouling and reducing the severity of the problem, but integration of these with cleaning has not been reported.

It is usually not possible to run food lines for longer than 24 hours before cleaning. Cleaning is needed because (i) of degradation of performance caused by fouling; (ii) of possible damage to product caused by microbial growth in the plant, and (iii) of the need to changeover between different products, to avoid cross-contamination when many products are made on the same line. Cleaning takes several hours, and thus forms a large fraction of total production time – the greatest cost may be loss of product that cannot be made as the plant is being cleaned. Minimising cleaning costs and environmental effects is industrially critical.

The focus of the food industry is on cleaning efficiency, as cleaning can take up to 25% of operational time. Cleaning processes are often highly automated, with cleaning-in-place (CIP) technologies commonplace [15]. Designing a protocol for a given situation is still however semi-empirical. Practical cleaning protocols are commonly overdesigned – but there is little incentive to experiment with safety margins, which might endanger safe production. However, as the costs of utilities and of disposal of waste increase, minimising the environmental impact of processes becomes more important. Cleaning can generate large volumes of dirty water of very high or low pH, forming a significant part of the effluent load of a process. To make the process more environmentally sustainable, better understanding of cleaning is needed that can be used both to design better cleaning processes and to minimise the cleaning and effluent costs. Determination of the endpoint of cleaning is critical – whilst sensors for fouling are relatively common (such as [16]) measuring the endpoint of cleaning is very difficult.

3. A prototype cleaning map

Significant problems in cleaning practice (and research) are: the difficulty of comparing cleaning of different fouling deposits, for example there is currently no

way to allow the cleaning time of one deposit to be reliably predicted from that of another; the lack of effective online measurement methods to identify the cleaning endpoint; scale up – if the mechanism of cleaning is not understood then scaling up lab or pilot plant data to

production is basically empirical. Model deposits can give different results to real deposits, for example using whey proteins to simulate pasteuriser behaviour [17];

The practical consequences of this are that (i) the best way of cleaning each type of soil has been developed largely independently, (ii) CIP protocols are set and operated without effective measurement of the endpoint. The long-term aim of cleaning research is to find ways to predict cleaning behaviour that

1755Peter J. Fryer et al. / Procedia Food Science 1 (2011) 1753 – 1760 Author name / Procedia – Food Science 00 (2011) 000–000 3

can be used to design equipment [18,19]. To do this needs better understanding of cleaning processes [20].

Fig. 1. A prototype cleaning map: from the review of Fryer and Asteriadou [21]

We have recently [21] tried to classify the cleaning problems of the food and personal care industries by the severity of the problem and the mechanisms of removal. Figure 1 shows cleaning problems classified by: (i) fouling deposit, and (ii) the cleaning agent needed. The chemical and mechanical properties of the deposit control the type of chemical needed. The fouling deposits most difficult to clean are:

Type 1: cleaning of highly viscous or viscoelastic or viscoplastic fluids by water. In some cases fouling forms basically by solidification, such as for viscous foods, starch-based sauces and confectionery fluids such as chocolates or crème fillings. These materials can however be removed by the action of water alone – the need is to optimise water usage and product recovery. Type 2: cleaning and killing of biofilms. Although it is possible to remove some biofilms with water alone [22], normal practice is to use biocide, which changes the removal characteristics of the deposit [23]. Type 3: cleaning of solid deposits by chemical action. Here solids are formed by deposition of components of the process fluid. The nature of the deposit will determine the type of cleaning chemical used; such as sodium hydroxide to remove organic films and acids to remove mineral scales.

Cleaning-in-place involves fluid flow across the deposit and thus both fluid mechanical and chemical forces act on the deposit. Cleaning must overcome both the (i) cohesive forces that bind elements of deposit together, as well as (ii) the adhesion forces between deposit and surface. This may be done by fluid action alone, or by the combined effects of fluid and chemicals.


3.1 Fluid mechanical removal.

Type 1 deposits can be removed by fluid alone. The rheology of the deposit and the rheological and flow properties (such as the shear force applied to the deposit) of the cleaning fluid will thus be the factors that removal. It should be possible to predict the cleaning time from the fluid dynamics of the flows [24]. However, only limited data on the displacement of one fluid by another with significantly different viscosity is available [25]. Henningsson et al [26] describe rinsing yoghurt out of a tube using water, and used a VOF (volume-of fluid) CFD model to simulate rinsing; as expected, the final stages of cleaning, with a thin layer on the wall, were most difficult to measure and to model.

Fig. 2. Weight of toothpaste remaning in a 1m pipe undergoing cleaning by water, showing four repeats for the same conditions; there are three stages of the process, (i) water breakthrough, too short to see, (ii) an exponential stage of removal and (iii) a linear rate of cleaning

Cole et al [27] show for the removal of toothpaste from pipework (a similar problem to the removal of viscous foods) that cleaning time is inversely proportional to shear stress, and is not a function of tube length. Figure 2 shows the weight of toothpaste remaining in a pipe after water has been flowing through it for different lengths of time. There are three stages (i) a short bulk cleaning stage (on the order of the length of the fluid residence time in the pipe, here < 10s and thus not seen in the Figure) before water breaks through, (ii) a stage of loss of material which follows the form of an exponential decay process,(iii) a final film removal stage controlled by slow removal of a very thin film from the wall. The need is to maximise the product recovery that may be possible in the first two stages, and then minimise the cleaning time to remove the final film. The process is complex – it is clear, for example, why the final cleaning time does not depend on the pipe length, as the final stage correspond to the removal of this layers or islands of soil, which takes place over a much loner period than the bulk removal of soil.


3.2 Diffusion-reaction removal.

For both types 2 and 3 deposits, chemicals are needed for removal. The cleaning process will thus involve diffusion of the chemical to and/or into the deposit, and some reaction that transforms it into a removable form. Three different mechanisms are: Dissolution– for example the dissolving of mineral salts in acid, proteins in alkali or sugars in water. Bulk cohesive failure: removal through breakdown of cohesive forces between parts of the deposit; Surface adhesive failure: removal by breakdown of adhesive forces that bind deposit to the surface.

In some cases one mechanism will predominate; for example, some starch (baked tomato paste) films exhibit removal by adhesive failure between the surface and the deposit. In practice, different mechanisms may control under different conditions. As well as fluid shear, cleaning will depend on Mass transfer of chemical to the deposit-fluid interface or of removed material; Diffusion of active species through fouling deposit; [28-29] give estimates for whey protein gels; The rate of reaction; little information is commonly available although [30] give qualitative data on

the effect of alkali on whey protein gels and show the effects of temperature predominate; The effect of the reaction on the deposits and surface. In protein cleaning reaction with alkali can

produces an expanded deposit that is easier to remove, but some deposits (such as [31]) excessive concentration of alkali gives a shrunken and gelled cohesive deposit that is difficult to clean.

Most mechanistic studies on diffusion-reaction films have used milk or milk simulants because of the commercial importance of milk processing. Milk protein swells on contact with alkali (save at very high pH) into a form that that can be removed. Gillham [32,33] studied alkali based cleaning of whey protein deposits on stainless steel, and found different mechanisms controlled cleaning at different stages removal of the last layers was largely controlled by shear, with flow reversal significantly increasing removal. Bulk protein behaviour was studied by Mercade-Prieto [34] who investigated whey protein and b-lactoglobulin gels and solutions at high pH and relate a number of effects seen in cleaning to protein breakdown. They also show that dissolution depends on the aggregation structure, i.e. on the fouling conditions. Full kinetic models for cleaning of these systems are not however yet available.

4. Modification of surfaces

The balance between adhesive and cohesive effects can be changed by changing the nature of the food contact surface. It is well known that bacterial adhesion can be minimised by changing surface energy [35] and it is possible to control marine biofouling by surface modification [36] Changing surface energy can also reduce mineral fouling [37].

Amongst many other studies, a large EU project [38] has studied the effects of different surfaces on milk deposit adhesion, and found that it was possible to change the rate and extent of fouling and cleaning. The forces involved in removal from different surfaces have been studied using dynamic gauging and micro-manipulation probes [39]. The authors found that adhesive failure occurred preferentially over a range of surface energies (22-28 mN/m) where surface binding was least strong. Liu et al [40-43] used micro-manipulation probes on a range of deposits, including milk and starch, and identified cohesive and adhesive failure modes, as a function of variables such as deposit thickness and interfacial energy. Cohesive failure occurs preferentially at high temperature and chemical concentration, i.e. where reaction rates are high.

It is thus clear that different modes of failure exist which result in cleaning; the kinetics of the processes that take place are needed, particularly under circumstances where both cohesion and adhesive failure can occur. Nanotechnology is increasingly making it possible to develop a wide range of different surface films; it is likely that some of these will have application in food processing by making it possible


either to resist fouling or minimise cleaning time. For example, Bani Kanaheh [44] show reductions in cleaning time for dairy deposits by 80% for electropolished surfaces and surfaces coated in polyurethane. The surface must however be cheap enough to make and stable enough to survive industrial use – but once such surfaces are developed the fouling and cleaning problem will diminish significantly. Innovation in this area is rapid because of the need to make surfaces for medical and pharmaceutical applications.

5. Conclusion: future work needed

The problems of fouling and cleaning have briefly been reviewed. As the importance of making food processes sustainable increases, losses due to cleaning will become more significant, and the need to optimise cleaning will become more important. Fouling is well understood in comparison to cleaning, especially in the dairy industry. Cleaning processes have been classified by deposit severity and the corresponding cleaning fluid needed. This may allow development of models for the different processes that occur. Future research needs include:

Understanding fluid mechanical removal, to maximise product recovery and minimise the cleaning time;

Developing efficient models for diffusion-reaction removal which can predict cleaning times for different materials;

Incorporating both kinds of model into design codes for items of equipment and for process plant, and thus to enable inherently cleanable equipment to be developed at the design stage;

Building this understanding into codes developed by groups such as the EHETG

Acknowledgements

This paper reports results from the ZEAL project TP//ZEE/6/1/21191, which involves; Alfa Laval, Cadbury Ltd., Ecolab Ltd., Newcastle University, Scottish & Newcastle Ltd., GEA Process Engineering Ltd., Unilever UK Central Resources Ltd., Imperial College of Science Technology and Medicine, GlaxoSmithKline, Bruker Optics Ltd. and the University of Birmingham. The project is co-funded by the Technology Strategy Board’s Collaborative Research and Development programme, following open competition. For more information visit http://www.innovateuk.org

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Presented at ICEF11 (May 22-26, 2011- Athens, Greece) as paper HDO488.

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Current knowledge in hygienic design: can we minimize fouling and speed cleaning?