Vol. 5, 2006—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 27© 2006 Institute of Food Technologists

A CriticalReview of MilkFouling in Heat

ExchangersBipan Bansal and Xiao Dong Chen

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Different aspects have been discussed with themicroorganisms, and (5) transfer of location where fouling takes place. Different aspects have been discussed with themicroorganisms, and (5) transfer of location where fouling takes place. Different aspects have been discussed with themicroorganisms, and (5) transfer of location where fouling takes place. Different aspects have been discussed with themicroorganisms, and (5) transfer of location where fouling takes place. Different aspects have been discussed with theview of possible industrial applications and future direction for research. It may not be possible to alter the propertiesview of possible industrial applications and future direction for research. It may not be possible to alter the propertiesview of possible industrial applications and future direction for research. It may not be possible to alter the propertiesview of possible industrial applications and future direction for research. It may not be possible to alter the propertiesview of possible industrial applications and future direction for research. It may not be possible to alter the propertiesof milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering theof milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering theof milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering theof milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering theof milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering thesurface temperature and increasing the flow velocity tend to reduce fouling. Reducing the heat transfer surface rough-surface temperature and increasing the flow velocity tend to reduce fouling. Reducing the heat transfer surface rough-surface temperature and increasing the flow velocity tend to reduce fouling. Reducing the heat transfer surface rough-surface temperature and increasing the flow velocity tend to reduce fouling. Reducing the heat transfer surface rough-surface temperature and increasing the flow velocity tend to reduce fouling. Reducing the heat transfer surface rough-ness and wettability is likely to lower the tendency of the proteins to adsorb onto the surface. The use of newerness and wettability is likely to lower the tendency of the proteins to adsorb onto the surface. The use of newerness and wettability is likely to lower the tendency of the proteins to adsorb onto the surface. The use of newerness and wettability is likely to lower the tendency of the proteins to adsorb onto the surface. The use of newerness and wettability is likely to lower the tendency of the proteins to adsorb onto the surface. The use of newertechnologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling;technologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling;technologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling;technologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling;technologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling;hohohohohowwwwwevevevevevererererer, fur, fur, fur, fur, further rther rther rther rther researesearesearesearesearch is rch is rch is rch is rch is requirequirequirequirequired to red to red to red to red to realizealizealizealizealize their full potential. e their full potential. e their full potential. e their full potential. e their full potential. The prThe prThe prThe prThe presence of micresence of micresence of micresence of micresence of microorooroorooroorganisms crganisms crganisms crganisms crganisms creates preates preates preates preates problem.oblem.oblem.oblem.oblem.The situation gets worse when the microorganisms get released into the process stream. The location where foulingThe situation gets worse when the microorganisms get released into the process stream. The location where foulingThe situation gets worse when the microorganisms get released into the process stream. The location where foulingThe situation gets worse when the microorganisms get released into the process stream. The location where foulingThe situation gets worse when the microorganisms get released into the process stream. The location where foulingtakes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefittakes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefittakes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefittakes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefittakes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefitin case fouling starts taking place elsewhere in the plant.in case fouling starts taking place elsewhere in the plant.in case fouling starts taking place elsewhere in the plant.in case fouling starts taking place elsewhere in the plant.in case fouling starts taking place elsewhere in the plant.

IntroductionThermal processing is an energy-intensive process in the dairy

industry because every product is heated at least once (de Jong1997). Processing of billions of liters of milk every year in coun-tries such as India, the United States, and New Zealand means theefficiency of the heating process is of paramount importance (FCG2004). Fouling of heat exchangers is an issue because it reducesheat transfer efficiency and increases pressure drop and hence af-fects the economy of a processing plant (Toyoda and others1994; Müller-Steinhagen 1993). As a result of fouling, there is apossibility of deterioration in product quality because the processfluid cannot be heated up to the required temperature (for pas-teurization or sterilization). Also the deposits dislodged by theflowing fluid can cause contamination.

Fouling-related costs are additional energy, lost productivity, ad-ditional equipment, manpower, chemicals, and environmentalimpact (Gillham and others 2000). Generally, milk fouling is sorapid that heat exchangers need to be cleaned every day to main-tain production capability and efficiency and meet strict hygienestandards. In comparison, the heat exchangers in other major

processing plants such as petroleum, petrochemical, and so forthneed to be cleaned only once or twice a year. According to Geor-giadis and others (1998), in the dairy industry the cost due to theinterruption in production can be dominant compared with thecost due to reduction in performance efficiency. Along with thecost, quality issues are equally important, and in fact many times ashutdown is required due to concerns of product quality/contam-ination instead of the performance of a heat exchanger. Accordingto van Asselt and others (2005), about 80% of the total produc-tion costs in the dairy industry can be attributed to fouling andcleaning of the process equipment.

In this study, we endeavored to review a wide range of articlesreported in literature and interpret the given information on foul-ing in heat exchangers. The aim was to generate some new inter-est in this field and to elaborate on some possible new directionfor research. It is not intended at all to suggest that this article pro-poses the only way to understand the problem.

Mechanisms of Milk FoulingMilk is a complicated biological fluid and contains a number of

species. Its average composition is given in Table 1. Thermal re-sponses of the constituents generally differ from each other. Milkfouling can be classified into 2 categories known as type A andtype B (Burton 1968; Lund and Bixby 1975; Changani and others1997; Visser and Jeurnink 1997). Type A (protein) fouling takes

MS 20050437 Submitted 7/20/05, Revised 9/15/05, Accepted 1/3/06. Theauthors are with Dept. of Chemical and Materials Engineering, Univ. ofAuckland, Auckland, New Zealand. Fonterra Cooperative Group Limited,Palmerston North, New Zealand. Direct inquiries to author Bansal (E-mail:b.bansal@fonterra.com) or Chen (E-mail: d.chen@auckland.ac.nz)

28 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 5, 2006

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place at temperatures between 75 °C and 110 °C. The deposits arewhite, soft, and spongy (milk film), and their composition is 50% to70% proteins, 30% to 40% minerals, and 4% to 8% fat. Type B(mineral) fouling takes place at temperatures above 110 °C. The de-posits are hard, compact, granular in structure, and gray in color(milk stone), and their composition is 70% to 80% minerals (mainlycalcium phosphate), 15% to 20% proteins, and 4% to 8% fat.

Whey proteins constitute only about 5% of the milk solids, butthey account for more than 50% of the fouling deposits in type Afouling. �-Lactoglobulin (�-Lg) and �-lactalbumin (�-La) are the 2major whey proteins, but the dominant protein in heat-inducedfouling is only �-Lg. It has high heat sensitivity and hence figuresprominently in the fouling process (Lyster 1970; Lalande and others1985; Gotham and others 1992; Delplace and others 1994; By-lund 1995). Caseins are resistant to thermal processing but do pre-cipitate upon acidification (Fox 1989; Visser and Jeurnink 1997).

Although the exact mechanisms and reactions between differ-ent milk components are not yet fully understood, a relationshipbetween the denaturation of native �-Lg and fouling of heat ex-changers has been established (Dalgleish 1990). Upon heating ofmilk, the native proteins (�-Lg) 1st denature (unfold) and exposethe core containing reactive sulphydryl groups. The denatured orunfolded protein molecules then react with the similar or othertypes of protein molecules such as casein and �-La and form ag-gregates (Jeurnink and de Kruif 1993). The rate of fouling may bedifferent for the denatured and aggregated proteins. Also, beinglarger in size, the transport of the aggregated proteins from thebulk to the heat transfer surfaces may be more difficult comparedwith the denatured proteins (Treybal 1981; Chen 2000). Delplaceand others (1994) experimentally observed that only 3.6% of thedenatured �-Lg was involved in deposit formation. Lalande andothers (1985) found this figure to be about 5%. However, it is notclear whether fouling is primarily caused by the aggregated pro-teins or the denatured proteins deposit 1st on the heat-transfersurfaces and the aggregation takes place subsequently. Accordingto Changani and others (1997), fouling occurs when the aggrega-tion takes place next to the heated surfaces. Toyoda and others(1994) modeled the milk fouling process based on the assump-tion that only aggregated proteins resulted in fouling. Accordingto Delplace and others (1997), fouling is controlled by the aggre-gation reaction of proteins. de Jong and others (1992) found thatthe formation of protein aggregates reduces fouling. van Asseltand others (2005) believe that �-Lg aggregates are not involved infouling reactions. Chen and others (1998a, 2000, 2001), Bansaland Chen (2005), and Bansal and others (2005) in their mathe-matical modeling considered that along with aggregated proteins,denatured proteins also took part in deposit formation.

Usually an induction period is required for the formation of theprotein aggregates or insoluble mineral complexes before notice-able amount of deposits are formed (Elofsson and others 1996;Visser and Jeurnink 1997; de Jong and others 1998). This time pe-

riod varies between 1 and 60 min for tubular heat exchangers (deJong 1997) but is much shorter or even instantaneous in plateheat exchangers where intense mixing of fluid takes place due tohigher turbulence (Belmar-Beiny and others 1993). Activation en-ergies of deposition reactions for both types of heat exchangersare reported to be similar, which suggests that the underlying pro-cesses are same in both cases (Fryer and Belmar-Beiny 1991).

The native proteins may attach on the heat transfer surface at lowtemperatures (even at room temperature) with coverage of about 2mg/m2 but this does not result in any further deposition (Arnebrantand others 1987; Wahlgren and Arnebrant 1990, 1991; Jeurninkand others 1996b). Denaturation of native proteins in heat ex-changers starts only at temperatures above 70 °C to 74 °C (Fryerand Belmar-Beiny 1991). According to Delsing and Hiddink (1983)and Visser and Jeurnink (1997), mainly proteins form the 1st de-posit layer. Belmar-Beiny and Fryer (1993) analyzed the depositswith contact heating times down to 4 s and observed that the 1stlayer was made of proteinaceous material. Analysis of deposits afterfouling for an extended period usually shows that the deposits nearthe surface contain a higher proportion of minerals. This is causedby the diffusion of minerals through the deposits to the surface rath-er than minerals forming 1st on the surface (Belmar-Beiny and oth-ers 1993). In contrast, according to Tissier and Lalande (1986), Fos-ter and others (1989), and Fryer and Belmar-Beiny (1991), a dense,high-mineral-containing sublayer forms 1st and is followed by amore spongy, proteinaceous layer.

Fouling in a heat exchanger depends on bulk and surface process-es. The deposition is a result of a number of stages (Belmar-Beiny andFryer 1993). The 1st stage involves denaturation and aggregation ofproteins in the bulk followed by the transport of the aggregated pro-teins to the heat-transfer surface. Then surface reactions take place,resulting in incorporation of the proteins into the deposit layer. Thedeposit layer is subjected to fluid hydrodynamic forces and as a re-sult there is possible re-entrainment or removal of the deposits.

The step controlling the overall fouling may either be related tophysical/chemical changes in the proteins or the mass transfer of theproteins between the fluid and the heat-transfer surface. In some cas-es, it may be a combination of both. Belmar-Beiny and others (1993)and Schreier and Fryer (1995) proposed that fouling was dependenton the bulk and surface reactions and not on the mass transfer. It wasalso proposed that the fouling rate was independent of the concen-tration of foulant in the liquid (Schreier and Fryer 1995). de Jong andvan der Linden (1992), de Jong and others (1992), and Grijspeerdtand others (2004) observed that the fouling process was reaction-controlled and was not limited by mass transfer. Sahoo and others(2005) and Nema and Datta (2005) used a similar concept in theirfouling model. Toyoda and others (1994), Georgiadis and others(1998), Georgiadis and Macchietto (2000), Chen and others (1998a,2000, 2001), Chen (2000), Bansal and Chen (2005), and Bansal andothers (2005) considered that fouling is dependent on mass transferas well as bulk and surface reactions.

According to Lalande and others (1985), Hege and Kessler(1986), Arnebrant and others (1987), and Kessler and Beyer(1991), protein denaturation is the governing reaction. On theother hand, Lalande and René (1988) and Gotham and others(1992) observed that protein aggregation is the governing reac-tion. de Jong and others (1992) found that the deposition of milkconstituents in heat exchangers is reaction-controlled adsorptionof denatured proteins. According to Toyoda and others (1994),only aggregated proteins present in thermal boundary layer areable to cause deposition. Delplace and others (1997) believedthat the formation of aggregates reduces fouling and that mixingcan be used to promote aggregation and hence control fouling.de Wit and Swinkles (1980), Anema and McKenna (1996), Chan-gani and others (1997), and Chen and others (1998a) suggestedthat the protein unfolding or denaturation step is reversible

Table 1—Average composition of milk (Bylund 1995)

Constituents Average concentration (%)

Water 87.5

Total solids 13 Proteins 3.4 Lactose 4.8 Minerals 0.8 Fat 3.9

Proteins 3.4 Casein 2.6 �-Lactoglobulin (�-Lg) 0.32 �-Lactalbumin (�-La) 0.12

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Milk fouling in heat exchangers . . .

whereas Ruegg and others (1977), Lalande and others (1985),Arnebrant and others (1987), Gotham and others (1992), andRoefs and de Kruif (1994), and Karlsson and others (1996) havefound evidence that the denaturation step is irreversible. In com-parison, the protein aggregation step has been reported to be al-ways irreversible (Mulvihill and Donovan 1987, Anema and McK-enna 1996, Changani and others 1997, and Chen and others1998a). Chen (2000), Chen and others (2000, 2001), Bansal andChen (2005), and Bansal and others (2005) suggested that foulingis caused by both denatured and aggregated proteins and per-haps primarily influenced by the presence of the denatured pro-teins in the bulk. The simulated results of Chen (2000) and Chenand others (2000, 2001) show that for hot surface–cold fluid sce-nario, different combinations of unfolded and aggregated proteinslead to similar accuracy of the fouling predictions. However, forcold surface–hot fluid scenario, different mechanisms lead to dif-ferent predictions. Hence, there is a need for further study of thecold surface effect. In general, a cold surface is not expected topromote aggregation to the extent that a hot surface would do.Also, aggregated proteins would have a lesser tendency to depositon a surface compared with denatured proteins due to their rela-tively compact structure and perhaps a cold surface would notprovide any further assistance. Table 2 summarizes important as-pects of the fouling mechanisms mentioned above.

Factors Affecting Milk FoulingFouling depends on various parameters such as heat transfer

method, hydraulic and thermal conditions, heat transfer surfacecharacteristics, and type and quality of milk along with its pro-cessing history. These factors can be broadly classified into 5 ma-jor categories: milk composition, operating conditions in heat ex-changers, type and characteristics of heat exchangers, presence ofmicroorganisms, and location of fouling.

Milk compositionThe composition of milk depends on its source and hence may

not be possible to change. A seasonal variation in milk fouling isattributed to differences in its composition (Burton 1967; Belmar-Beiny and others 1993; de Jong 1997). Increasing the proteinconcentration results in higher fouling (Toyoda and others 1994;Changani and others 1997; Kessler 2002).

The effect of pH on fouling is not straightforward. In general, the

heat stability of milk proteins decreases with a reduction in pH (Fosterand others 1989; Xiong 1992; Corredig and Dalgleish 1996; de Jongand others 1998). A decrease in pH will also result in an increase inconcentration of ionic calcium, possibly due to the dissolution ofcalcium phosphate from casein micelle and its increased solubility(Lewis and Heppell 2000). A slight increase in pH has been observedto increase processing time (Skudder and others 1986).

The calcium ions present in milk influence the denaturation tem-perature of �-Lg, promote aggregation by attaching to �-Lg, and en-hance the deposition by forming bridges between the proteins ad-sorbed on the heat transfer surface and aggregates formed in thebulk (Xiong 1992; Changani and others 1997; Christian and others2002). In addition, the solubility of calcium phosphate decreaseswith heating. The addition of calcium ions enhances depositionand there is a greater amount of caseins present in the deposits,suggesting an increased instability of casein micelles (Delsing andHiddink 1983; Daufin and others 1987; Grandison 1988; de Jong1997; de Jong and others 1998). The enrichment of milk with calci-um salts is gaining interest to achieve higher calcium intake perserving; however, its impact on the heat stability of milk depends onthe source and level of calcium fortification (Vyas and Tong 2004).According to Jeurnink and de Kruif (1995), both increasing as wellas decreasing the calcium content of milk compared with normalmilk results in lower heat stability and hence more fouling.

The fat present in milk has little effect on fouling (Foster and oth-ers 1989; Visser and Jeurnink 1997). However, decreasing the pHis found to increase the amount of fat within the deposits (Lewisand Heppell 2000). Lactose is not involved in the fouling processas such until it is involved in the Maillard reaction at a high tem-perature (Visser and others 1997). Additives may reduce foulingby enhancing the heat stability of milk but may not be permittedin many countries (Lyster 1970; Skudder and others 1981; Chan-gani and others 1997).

Holding milk for up to 24 h at 4 °C before processing results inless fouling, although further aging increases fouling (Burton1968; Changani and others 1997; Lewis and Heppell 2000). Pro-longed storage of milk for a few days may enhance fouling due tothe action of proteolytic enzymes (Burton 1968; de Jong 1997).However, storage at a lower temperature of 2 °C for more than 14d has been found to have no significant impact on fouling (Lewisand Heppell 2000).

Reconstituted milk gives much less fouling because about 25%of �-Lg is denatured during the production of milk powders

Table 2—Important aspects of fouling mechanisms

Aspects References

Protein denaturation is reversible de Wit and Swinkles (1980), Anema and McKenna (1996), Changani and others (1997), Chen and others (1998a)

Protein denaturation is irreversible Ruegg and others (1977), Lalande and others (1985), Arnebrat and others (1987), Gotham and others (1992), Roef and de Kruif (1994), Karlsson and others (1996)

Protein aggregation is irreversible Mulvihill and Donovan (1987), Anema and McKenna (1996), Changani and others (1997), Chen and others (1998a)

Protein denaturation is the governing reaction Lalande and others (1985), Hege and Kessler (1986), Arnebrant and others (1987), Kessler and Beyer (1991), de Jong and others (1992)

Protein aggregation is the governing reaction Lalande and René (1988), Gotham and others (1992), Delplace and others (1997)

Formation of protein aggregates enable to de Jong and others (1992), Delplace and others (1997), van Asselt and others (2005) reduce fouling

Only protein aggregates cause fouling Toyoda and others (1994)

Fouling is considered to depend on protein de Jong and van del Linden (1992), de Jong and others (1992), Belmar-Beiny and others (1993), reactions only Delplace and others (1994, 1997), Schreier and Fryer (1995), Grijspeerdt and others (2004),

Sahoo and others (2005), Nema and Datta (2005)

Fouling is considered to depend on protein Toyoda and others (1994), Georgiadis and others (1998), Georgiadis and Macchietto (2000), reactions as well as mass transfer Chen and others (1998a, 2000, 2001), Bansal and Chen (2005), Bansal and others (2005)

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(Changani and others 1997; Visser and Jeurnink 1997). The con-centration of calcium is reported to be 9% less in the reconstitut-ed milk, which would result in less fouling (Changani and others1997). In contrast, Newstead and others (1998) found that UltraHigh Temperature (UHT) fouling rates of the recombined milk in-creased with increasing preheat treatment (preheating temperature× preheating time). The fouling deposits also had high levels of fat(up to 60% or more) compared with the deposits formed duringfresh milk processing (10% or less). The difference is attributed tochanges in fat globule membranes. Fung and others (1998) stud-ied the effect of the damage to milk fat globule membrane by acavitating pump on fouling of whole milk. The fouling rate was en-hanced and the argument was that the damage to the membranesresults in the fat globules to coalesce, which then tend to migratefaster toward the heated wall.

Operating conditions in heat exchangersImportant operating parameters that can be varied in a heat ex-

changer are air content, velocity/turbulence, and temperature.The presence of air in milk enhances fouling (Burton 1968; de

Jong 1997; de Jong and others 1998). However, fouling is en-hanced only when the air bubbles are formed on the heat-transfersurface, which then act as nuclei for deposit formation (Burton1968; de Jong 1997). The solubility of air in milk decreases withheating as well as a reduction in the pressure (de Jong 1997; deJong and others 1998). Also, the formation of air bubbles is en-hanced by mechanical forces induced by valves, expansion ves-sels, and free-falling streams (de Jong 1997). Although it is usuallyreported that the presence of a deaerator will reduce fouling, thereis no reported evidence (Lewis and Heppell 2000).

Fouling decreases with increasing turbulence (Belmar-Beiny andothers 1993; Santos and others 2003). According to Paterson andFryer (1988) and Changani and others (1997), the thickness andsubsequently the volume of laminar sublayer decrease with in-creasing velocity and as a result, the amount of foulant depositingon the heat-transfer surface decreases. Delplace and others (1997)observed that significant variations in Reynolds number and aver-age boundary layer thickness had no effect on the fouling rate.Higher flow velocities also promote deposit re-entrainment throughincreased fluid shear stresses (Rakes and others 1986). Higher tur-bulence and different flow characteristics are in fact found to resultin a smaller induction period in plate heat exchangers comparedwith tubular heat exchangers (Belmar-Beiny and others 1993). Thereason for this may be the presence of low-velocity zones near thecontact points between the adjacent plates. The use of pulsatile flowwas found to mitigate fouling when only the wall region near theheat-transfer surface was hot enough to cause the protein denatur-ation and aggregation reactions (Bradley and Fryer 1992). The rea-son was that the fluid spent less time near the wall due to highermixing. The pulsations, however, enhanced fouling when the bulkfluid was also hot enough for the protein reactions to take place.Chen and others (2001) predicted that mixing caused by in-linemixers can reduce fouling substantially.

Temperature of milk in a heat exchanger is probably the singlemost important factor controlling fouling (Burton 1968; Kesslerand Beyer 1991; Belmar-Beiny and others 1993; Toyoda and oth-ers 1994; Corredig and Dalgleish 1996; Elofsson and others1996; Jeurnink and others 1996b; Santos and others 2003). In-creasing the temperature results in higher fouling. Beyond 110 °C,the nature of fouling changes from type A to type B (Burton 1968).It is worth mentioning that both the absolute temperature andtemperature difference are important for fouling. This means that itis feasible to have fouling in coolers where the wall temperature islower than the bulk temperature. Chen and Bala (1998) investigat-ed the effect of surface and bulk temperatures on fouling of wholemilk, skim milk, and whey protein and found that the surface tem-

perature was the most important factor in initiating fouling. Whenthe surface temperature was less than 68 °C, no fouling was ob-served, even though the bulk temperature was up to 84 °C.

Preheating of milk (often termed forewarming) causes denatur-ation and aggregation of proteins before the heating section, whichthen leads to lower fouling in heat exchangers (Bell and Sanders1944; Burton 1968; Mottar and Moermans 1988; Foster and others1989). The main effect of forewarming is the denaturation of �-Lgand its association with casein micelle and hence a reduction in theamount of type A deposits. Also, there is a reduction in the avail-ability of ionic calcium with preheating as calcium phosphate getsattached to casein micelle (Lewis and Heppell 2000).

Type and characteristics of heat exchangersPlate heat exchangers are used commonly in the dairy industry

because they offer advantages of superior heat-transfer performance,lower temperature gradient, higher turbulence, ease of maintenance,and compactness over tubular heat exchangers. However, plate heatexchangers are prone to fouling because of their narrow flow chan-nels (Delplace and others 1994) and contact points between adja-cent plates (Belmar-Beiny and others 1993). Also, milk fouling in aheat exchanger is difficult to completely eliminate, simply due to thefact that the temperature of the heat-transfer surface needs to be con-siderably higher than the bulk temperature to have efficient heattransfer. Complex hydraulic and thermal characteristics in plate heatexchangers make it very difficult to analyze milk fouling. The use ofco-current and counter-current flow passages within the same heatexchanger further complicates the problem.

The heat-transfer surface to which the deposits stick affects foul-ing (Wahlgren and Arnebrant 1990, 1991). It influences the adhe-sion of microorganisms as well (Flint and others 2000). The surfacecharacteristics are generally important only until the surface getscovered with the deposits. The surface treatment can be of greatbenefit in case fouling occurs after a time delay and the strength ofthe adhesion of the deposits onto the metal surfaces is weaker, giv-ing way to an easier cleaning process. Stainless steel is the standardmaterial used for surfaces that are in contact with milk. Factors thatmay affect fouling of a stainless-steel surface are presence of a chro-mium oxide or passive layer, surface charge, surface energy, surfacemicrostructure (roughness and other irregularities), presence of ac-tive sites, residual materials from previous processing conditions,and type of stainless steel used (Jeurnink and others 1996a; Visserand Jeurnink 1997). Modifications of the heat-transfer surface char-acteristics through electro-polishing and surface coatings can re-duce fouling by altering the surface roughness, charge, and wetta-bility (Yoon and Lund 1994; Pie�linger-Schweiger 2001; Santosand others 2001, 2004; Beuf and others 2003; Rosmaninho andothers 2003, 2005; Ramachandra and others 2005, Rosmaninhoand Melo 2006). It is generally reported that hydrophobic surfacesadsorb more protein than hydrophilic surfaces (Wahlgren andArnebrant 1991). Increasing the surface roughness provides a larg-er effective surface area and results in a higher effective surface en-ergy than a smooth surface (Yoon and Lund 1994). As a result, theadhesion of deposits with a rough surface would be comparativelystronger. The effect of different surface coatings tends to be less onthe deposit formation but more on their adhesion strength (Brittenand others 1988). Magnetic field treatment has been observed tohave no effect on the milk fouling rate (Yoon and Lund 1994).

There has been an increasing use of heat exchangers that foulcomparatively less, for example, fluidized bed heat exchangers(Klaren 2003), Helixchanger heat exchangers (Master and others2003), and heat exchangers equipped with turbulence promoters(Gough and Rogers 1987). However, the information availableabout their use in thermal processing of dairy fluids is limited. Theuse of pulsatile flow exchanger results in higher mass transfer thatmay enhance fouling in case the deposition process is mass trans-

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Milk fouling in heat exchangers . . .

fer controlled (Bradley and Fryer 1992). The use of a fluid bedheat exchanger has been found to reduce the amount of foulingand enhance the rate of heat transfer (Bradley and Fryer 1992).

Direct heating methods such as steam injection and steam infu-sion allow an optimal selectivity between desired (nutritional val-ue) and undesired (surviving microorganisms) product transfor-mations (de Jong and others 1998). These methods result in a lowfouling rate because the desired temperatures are achieved withina very short time due to high heating rates (de Jong 1997). Also,highly viscous fluids can be handled more easily (de Jong andothers 1998). The absence of heat-transfer surface in such cases isalso an advantage. The resulting dilution, however, may not bedesirable. The direct injection of hot air/nitrogen has been foundto give satisfactory performance in concentrating milk throughevaporation of water (Zaida and others 1987).

Microwave heating has been used in numerous industrial applica-tions for several years due to its advantages such as faster through-put, better quality, energy saving, and less space requirement overconventional heating methods (Metaxas and Meredith 1988). How-ever, the limited lifespan of a microwave system can raise doubtsover its economic viability. A number of studies have been reportedon heat treatment of milk using microwaves, but these are based ongeneral quality issues such as nutrients and microorganisms insteadof fouling (Thompson and Thompson 1990; Kindle and others 1996;Sieber and others 1996; Villamiel and others 1996).

In induction heating, heat is generated by placing the food ma-terial inside an electric coil. A high-frequency alternating currentis passed through the coil, which creates an electromagnetic field.This induces a current in the food material and heats it up. Induc-tion heating produces high local temperatures very quickly, but itsuse has been limited to the materials industry only.

Ohmic heating or direct resistance heating is a heat-treatmentprocess in which an electrical current is passed through milk, andheat is generated within milk to achieve pasteurization/sterilization(Quarini 1995). This technique offers the potential of thermal pro-cessing of materials without relying on an inefficient mechanismsuch as conduction of heat from a surface into the fluid. It also hasan advantage over microwave processing where processing can belimited by the depth to which energy can penetrate the food materi-al (Fryer and others 1993). The resistance heating technique wasused for milk pasteurization in the early 20th century (de Alwis andFryer 1990). In recent years, this technology has been in use againafter being abandoned for a major part of the 20th century. APVIntl. Ltd. (England) developed commercial ohmic heating units forcontinuous sterilization of food products (Skudder and Biss 1997).Ayadi and others (2003, 2004a, 2004b, 2005a, 2005b) have in-vestigated the performance of a plate-type ohmic heater for thermaltreatment of dairy products. Bansal and others (2005) and Bansaland Chen (2005) studied skim milk fouling in a concentric cylinderohmic heater and also developed a mathematical model to simu-late the fouling process.

Surface temperatures are lower in ohmic heating because heat isgenerated in the bulk fluid. Hence, less fouling should take place.However, when the deposits start attaching to the electrode surfac-es, the temperature profile changes dramatically (Ayadi and others2004b; Bansal and Chen 2005; Bansal and others 2005). In con-ventional indirect heating methods, such as shell and tube or plateheat exchangers, the deposit formation lowers the deposit/fluid in-terface temperature. In contrast, the deposit/fluid interface tempera-ture increases with deposit formation, which further promotes foul-ing (Bansal and others 2005). The reason for this is that apart fromthe bulk fluid, some heat is generated in the deposit layer as well,due to its own electrical resistance. Furthermore, this layer also re-stricts the outward flow of heat from the bulk fluid.

There are 2 other issues that may be important in an ohmicheating process. Better mixing of the fluid may be required to

overcome the wall effect and result in uniform heating, but it mayalso promote fouling as the foulant is transferred easily from thebulk to the surface. Cooling may be used to reduce the tempera-tures of electrode surfaces that would help control fouling.

Presence of microorganismsThe formation of deposits promotes the adhesion of microor-

ganisms to heat-transfer surface, resulting in bio-fouling. Further-more, the deposits provide nutrients to microorganisms, ensuringtheir growth. It is worth mentioning here that a lot of processes inthe dairy industry are carried out at temperatures below 100 °C.For example, pasteurization is generally achieved by heating milkat 72 °C for 15 s in a continuous flow system. At this temperature,only the pathogenic bacteria along with some vegetative cells arekilled. A higher temperature of 85 °C is required to kill the re-maining vegetative cells. Spores are much more heat-resistant andremain active well beyond this temperature. Their inactivation isimportant for the products with longer shelf life.

Bio-fouling, either microorganism deposition or biofilm forma-tion, in a heat exchanger raises serious quality concerns. Flint andcoworkers have investigated the effect of bio-fouling in dairy man-ufacturing plants (Flint and Hartley 1996; Flint and others 1997,1999, 2000). According to Bott (1993), bio-fouling takes placethrough 2 different mechanisms: deposition of microorganismsdirectly on the heat-transfer surfaces of the heat exchanger, anddeposition/attachment/entrapment of microorganisms on/in thedeposit layer forming on the heat-transfer surfaces. With the sup-ply of nutrients by the deposits, microorganisms multiply.

The presence of microorganisms in the process stream and/ordeposit layer not only affects the product quality, it influences thefouling process as well (Flint and others 1997, 1999; Yoo and oth-ers 2005). When microorganisms get released into the processfluid due to hydrodynamic forces, they contaminate downstreamsections. This may also result in microbial growth in areas thatotherwise are not conducive to bio-fouling. The release pattern ofthermophilic bacteria Bacillus stearothermophilus into the pro-cess stream has been studied in detail by Chen and others(1998b) and Yoo and Chen (2002).

Location of foulingProtein denaturation and aggregation reactions take place as

soon as milk is heated. The relative amounts of denatured and ag-gregated proteins depend on a number of factors such as operat-ing conditions, type and design of heat exchanger, and propertiesof heat transfer surface. The use of an efficient technology mayhelp to mitigate fouling within a heat exchanger; however, theprocessed milk at the exit of the heat exchanger would still have alot of denatured and aggregated proteins. This would result in se-vere fouling at various locations further downstream. Hence, con-trolling fouling only within the heat exchanger may not yield ef-fective results and an overall strategy is required to mitigate foul-ing over the entire setup (Petermeier and others 2002; Grijspeerdtand others 2004). An example of such a strategy is the preheatingof milk before the heating section as mentioned previously (Belland Sanders 1944; Burton 1968; Mottar and Moermans 1988;Foster and others 1989). The installation of an additional sectionat a constant temperature (holding section) within the cascade ofheat exchangers is also known to reduce fouling (de Jong andothers 1992, 1994; de Jong and van der Linden 1992; de Jong1997). This outcome is attributed to the fact that denatured �-Lg istransformed into aggregated �-Lg in the holding section. This ag-gregated form is inactive and is unable to form aggregates withother components of milk and hence does not play an active rolein the fouling process in the downstream sections (de Jong andvan der Linden 1992). Hence, successful mitigation of fouling de-pends on controlling local thermal and hydraulic conditions as

32 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 5, 2006

CRFSFS: Comprehensive Reviews in Food Science and Food Safety

well as surface properties throughout the plant rather than justwithin the heat exchangers.

ConclusionsFouling of heat exchangers in the dairy industry is a complex

phenomenon and the mechanisms are not completely under-stood. Although there is an established link between protein de-naturation and fouling, the relative impact of the denatured andaggregated proteins on the deposit formation is not clear. In gener-al, it is believed that fouling is controlled by the aggregation reac-tion of proteins and the formation of protein aggregates reducesfouling. The mass transfer of proteins between the fluid and heattransfer surface also plays an important role.

It may not be possible to completely eliminate fouling in heat ex-changers simply due to the fact that denaturation and aggregationreactions initiate as soon as milk is subjected to heating. Fouling,however, can be controlled and mitigated by selecting appropriatethermal and hydraulic conditions. Both increasing the flow rate anddecreasing the temperature reduce fouling. Plate heat exchangerstend to have lower fouling compared with tubular heat exchangersbecause they have higher turbulence and the surface temperatureis also comparatively lower. Microwave heating and ohmic heatingalso result in less fouling; however, the information available aboutthese technologies is limited. Bio-fouling aggravates the problemand raises concerns about the product quality as well. The releaseof microorganisms from the fouling deposits into the processstream can contaminate the plant well beyond the heat exchangers.

Finally, it is important that a holistic approach is taken to miti-gate fouling because controlling fouling within the heat exchang-ers may be of little use in case it shifts to other parts of the plant.The impact of fouling needs to be assessed for the entire plant be-fore a mitigation strategy is devised. To date, a significant amountof research has been done, but still there is a lack of generalizedmethodologies and techniques to control fouling. Further con-centrated and joint efforts among industry, research institutes, andacademia are required to combat this serious problem.

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