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A critical review of milk fouling in heat exchangers

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  • 1. A Critical Review of Milk Fouling in Heat Exchangers Bipan Bansal and Xiao Dong Chen ABSTRACT: Fouling of heat exchangers is a problem in the dairy industry and costs billions of dollars ever y year. It has ABSTRACT CT: exchangers problem dairy industry ever year It ery ear. been studied extensively by researchers around the world, and a large number of studies are reported in the literature. This review focuses on the mechanisms of milk fouling, investigating the role of protein denaturation and aggregation well transfer ansfer. endeavor review have as well as mass transfer. We also endeavor to review the effect of a number of factors which have been classified into 5 categories: quality, operating conditions, character acteristics exchangers changers, presence categories: (1) milk quality, (2) operating conditions, (3) type and characteristics of heat exchangers, (4) presence of microorganisms, and (5) transfer of location where fouling takes place. Different aspects have been discussed with the view of possible industrial applications and future direction for research. It may not be possible to alter the properties of milk since they are dependent on the source, collection schedule, season, and many other factors. Lowering the 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 newer technologies like microwave heating and ohmic heating is gaining momentum because these result in lower fouling; however further resear er, esearch requir equired realiz ealize presence microor oorganisms creates problem. however, further research is required to realize their full potential. The presence of microorganisms creates problem. The situation gets worse when the microorganisms get released into the process stream. The location where fouling takes place is of paramount importance because controlling fouling within the heat exchanger may yield little benefit in case fouling starts taking place elsewhere in the plant.Introduction processing plants such as petroleum, petrochemical, and so forth Thermal processing is an energy-intensive process in the dairy need to be cleaned only once or twice a year. According to Geor-industry because every product is heated at least once (de Jong giadis and others (1998), in the dairy industry the cost due to the1997). Processing of billions of liters of milk every year in coun- interruption in production can be dominant compared with thetries such as India, the United States, and New Zealand means the cost due to reduction in performance efficiency. Along with theefficiency of the heating process is of paramount importance (FCG cost, quality issues are equally important, and in fact many times a2004). Fouling of heat exchangers is an issue because it reduces shutdown is required due to concerns of product quality/contam-heat transfer efficiency and increases pressure drop and hence af- ination instead of the performance of a heat exchanger. Accordingfects the economy of a processing plant (Toyoda and others to van Asselt and others (2005), about 80% of the total produc-1994; Müller-Steinhagen 1993). As a result of fouling, there is a tion costs in the dairy industry can be attributed to fouling andpossibility of deterioration in product quality because the process cleaning of the process equipment.fluid cannot be heated up to the required temperature (for pas- In this study, we endeavored to review a wide range of articlesteurization or sterilization). Also the deposits dislodged by the reported in literature and interpret the given information on foul-flowing fluid can cause contamination. ing in heat exchangers. The aim was to generate some new inter- Fouling-related costs are additional energy, lost productivity, ad- est in this field and to elaborate on some possible new directionditional equipment, manpower, chemicals, and environmental for research. It is not intended at all to suggest that this article pro-impact (Gillham and others 2000). Generally, milk fouling is so poses the only way to understand the problem.rapid 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 Mechanisms of Milk Fouling Milk is a complicated biological fluid and contains a number ofMS 20050437 Submitted 7/20/05, Revised 9/15/05, Accepted 1/3/06. The species. Its average composition is given in Table 1. Thermal re-authors are with Dept. of Chemical and Materials Engineering, Univ. of sponses of the constituents generally differ from each other. MilkAuckland, Auckland, New Zealand. Fonterra Cooperative Group Limited,Palmerston North, New Zealand. Direct inquiries to author Bansal (E-mail: fouling can be classified into 2 categories known as type A andb.bansal@fonterra.com) or Chen (E-mail: d.chen@auckland.ac.nz) type B (Burton 1968; Lund and Bixby 1975; Changani and others 1997; Visser and Jeurnink 1997). Type A (protein) fouling takes© 2006 Institute of Food Technologists Vol. 5, 2006—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 27
  • 2. CRFSFS: Comprehensive Reviews in Food Science and Food SafetyTable 1—Average composition of milk (Bylund 1995) riod varies between 1 and 60 min for tubular heat exchangers (deConstituents Average concentration (%) Jong 1997) but is much shorter or even instantaneous in plate heat exchangers where intense mixing of fluid takes place due toWater 87.5 higher turbulence (Belmar-Beiny and others 1993). Activation en-Total solids 13 ergies of deposition reactions for both types of heat exchangers Proteins 3.4 Lactose 4.8 are reported to be similar, which suggests that the underlying pro- Minerals 0.8 cesses are same in both cases (Fryer and Belmar-Beiny 1991). Fat 3.9 The native proteins may attach on the heat transfer surface at lowProteins 3.4 temperatures (even at room temperature) with coverage of about 2 Casein 2.6 mg/m2 but this does not result in any further deposition (Arnebrant ␤-Lactoglobulin (␤-Lg) 0.32 and others 1987; Wahlgren and Arnebrant 1990, 1991; Jeurnink ␣-Lactalbumin (␣-La) 0.12 and others 1996b). Denaturation of native proteins in heat ex- changers starts only at temperatures above 70 °C to 74 °C (Fryer and Belmar-Beiny 1991). According to Delsing and Hiddink (1983) and Visser and Jeurnink (1997), mainly proteins form the 1st de-place at temperatures between 75 °C and 110 °C. The deposits are posit layer. Belmar-Beiny and Fryer (1993) analyzed the depositswhite, soft, and spongy (milk film), and their composition is 50% to with contact heating times down to 4 s and observed that the 1st70% proteins, 30% to 40% minerals, and 4% to 8% fat. Type B layer was made of proteinaceous material. Analysis of deposits after(mineral) fouling takes place at temperatures above 110 °C. The de- fouling for an extended period usually shows that the deposits nearposits are hard, compact, granular in structure, and gray in color the surface contain a higher proportion of minerals. This is caused(milk stone), and their composition is 70% to 80% minerals (mainly by the diffusion of minerals through the deposits to the surface rath-calcium phosphate), 15% to 20% proteins, and 4% to 8% fat. er than minerals forming 1st on the surface (Belmar-Beiny and oth- Whey proteins constitute only about 5% of the milk solids, but ers 1993). In contrast, according to Tissier and Lalande (1986), Fos-they account for more than 50% of the fouling deposits in type A ter and others (1989), and Fryer and Belmar-Beiny (1991), a dense,fouling. ␤-Lactoglobulin (␤-Lg) and ␣-lactalbumin (␣-La) are the 2 high-mineral-containing sublayer forms 1st and is followed by amajor whey proteins, but the dominant protein in heat-induced more spongy, proteinaceous layer.fouling is only ␤-Lg. It has high heat sensitivity and hence figures Fouling in a heat exchanger depends on bulk and surface process-prominently in the fouling process (Lyster 1970; Lalande and others es. The deposition is a result of a number of stages (Belmar-Beiny and1985; Gotham and others 1992; Delplace and others 1994; By- Fryer 1993). The 1st stage involves denaturation and aggregation oflund 1995). Caseins are resistant to thermal processing but do pre- proteins in the bulk followed by the transport of the aggregated pro-cipitate upon acidification (Fox 1989; Visser and Jeurnink 1997). teins to the heat-transfer surface. Then surface reactions take place, Although the exact mechanisms and reactions between differ- resulting in incorporation of the proteins into the deposit layer. Theent milk components are not yet fully understood, a relationship deposit layer is subjected to fluid hydrodynamic forces and as a re-between the denaturation of native ␤-Lg and fouling of heat ex- sult there is possible re-entrainment or removal of the deposits.changers has been established (Dalgleish 1990). Upon heating of The step controlling the overall fouling may either be related tomilk, the native proteins (␤-Lg) 1st denature (unfold) and expose physical/chemical changes in the proteins or the mass transfer of thethe core containing reactive sulphydryl groups. The denatured or proteins between the fluid and the heat-transfer surface. In some cas-unfolded protein molecules then react with the similar or other es, it may be a combination of both. Belmar-Beiny and others (1993)types of protein molecules such as casein and ␣-La and form ag- and Schreier and Fryer (1995) proposed that fouling was dependentgregates (Jeurnink and de Kruif 1993). The rate of fouling may be on the bulk and surface reactions and not on the mass transfer. It wasdifferent for the denatured and aggregated proteins. Also, being also proposed that the fouling rate was independent of the concen-larger in size, the transport of the aggregated proteins from the tration of foulant in the liquid (Schreier and Fryer 1995). de Jong andbulk to the heat transfer surfaces may be more difficult compared van der Linden (1992), de Jong and others (1992), and Grijspeerdtwith the denatured proteins (Treybal 1981; Chen 2000). Delplace and others (2004) observed that the fouling process was reaction-and others (1994) experimentally observed that only 3.6% of the controlled and was not limited by mass transfer. Sahoo and othersdenatured ␤-Lg was involved in deposit formation. Lalande and (2005) and Nema and Datta (2005) used a similar concept in theirothers (1985) found this figure to be about 5%. However, it is not fouling model. Toyoda and others (1994), Georgiadis and othersclear whether fouling is primarily caused by the aggregated pro- (1998), Georgiadis and Macchietto (2000), Chen and others (1998a,teins or the denatured proteins deposit 1st on the heat-transfer 2000, 2001), Chen (2000), Bansal and Chen (2005), and Bansal andsurfaces and the aggregation takes place subsequently. According others (2005) considered that fouling is dependent on mass transferto Changani and others (1997), fouling occurs when the aggrega- as well as bulk and surface reactions.tion takes place next to the heated surfaces. Toyoda and others According to Lalande and others (1985), Hege and Kessler(1994) modeled the milk fouling process based on the assump- (1986), Arnebrant and others (1987), and Kessler and Beyertion that only aggregated proteins resulted in fouling. According (1991), protein denaturation is the governing reaction. On theto Delplace and others (1997), fouling is controlled by the aggre- other hand, Lalande and René (1988) and Gotham and othersgation reaction of proteins. de Jong and others (1992) found that (1992) observed that protein aggregation is the governing reac-the formation of protein aggregates reduces fouling. van Asselt tion. de Jong and others (1992) found that the deposition of milkand others (2005) believe that ␤-Lg aggregates are not involved in constituents in heat exchangers is reaction-controlled adsorptionfouling reactions. Chen and others (1998a, 2000, 2001), Bansal of denatured proteins. According to Toyoda and others (1994),and Chen (2005), and Bansal and others (2005) in their mathe- only aggregated proteins present in thermal boundary layer arematical modeling considered that along with aggregated proteins, able to cause deposition. Delplace and others (1997) believeddenatured proteins also took part in deposit formation. that the formation of aggregates reduces fouling and that mixing Usually an induction period is required for the formation of the can be used to promote aggregation and hence control fouling.protein aggregates or insoluble mineral complexes before notice- de Wit and Swinkles (1980), Anema and McKenna (1996), Chan-able amount of deposits are formed (Elofsson and others 1996; gani and others (1997), and Chen and others (1998a) suggestedVisser and Jeurnink 1997; de Jong and others 1998). This time pe- that the protein unfolding or denaturation step is reversible28 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 5, 2006
  • 3. Milk fouling in heat exchangers . . .Table 2—Important aspects of fouling mechanismsAspects ReferencesProtein 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 foulingOnly 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)whereas Ruegg and others (1977), Lalande and others (1985), heat stability of milk proteins decreases with a reduction in pH (FosterArnebrant and others (1987), Gotham and others (1992), and and others 1989; Xiong 1992; Corredig and Dalgleish 1996; de JongRoefs and de Kruif (1994), and Karlsson and others (1996) have and others 1998). A decrease in pH will also result in an increase infound evidence that the denaturation step is irreversible. In com- concentration of ionic calcium, possibly due to the dissolution ofparison, the protein aggregation step has been reported to be al- calcium phosphate from casein micelle and its increased solubilityways irreversible (Mulvihill and Donovan 1987, Anema and McK- (Lewis and Heppell 2000). A slight increase in pH has been observedenna 1996, Changani and others 1997, and Chen and others to increase processing time (Skudder and others 1986).1998a). Chen (2000), Chen and others (2000, 2001), Bansal and The calcium ions present in milk influence the denaturation tem-Chen (2005), and Bansal and others (2005) suggested that fouling perature of ␤-Lg, promote aggregation by attaching to ␤-Lg, and en-is caused by both denatured and aggregated proteins and per- hance the deposition by forming bridges between the proteins ad-haps primarily influenced by the presence of the denatured pro- sorbed on the heat transfer surface and aggregates formed in theteins in the bulk. The simulated results of Chen (2000) and Chen bulk (Xiong 1992; Changani and others 1997; Christian and othersand others (2000, 2001) show that for hot surface–cold fluid sce- 2002). In addition, the solubility of calcium phosphate decreasesnario, different combinations of unfolded and aggregated proteins with heating. The addition of calcium ions enhances depositionlead to similar accuracy of the fouling predictions. However, for and there is a greater amount of caseins present in the deposits,cold surface–hot fluid scenario, different mechanisms lead to dif- suggesting an increased instability of casein micelles (Delsing andferent predictions. Hence, there is a need for further study of the Hiddink 1983; Daufin and others 1987; Grandison 1988; de Jongcold surface effect. In general, a cold surface is not expected to 1997; de Jong and others 1998). The enrichment of milk with calci-promote aggregation to the extent that a hot surface would do. um salts is gaining interest to achieve higher calcium intake perAlso, aggregated proteins would have a lesser tendency to deposit serving; however, its impact on the heat stability of milk depends onon a surface compared with denatured proteins due to their rela- the source and level of calcium fortification (Vyas and Tong 2004).tively compact structure and perhaps a cold surface would not According to Jeurnink and de Kruif (1995), both increasing as wellprovide any further assistance. Table 2 summarizes important as- as decreasing the calcium content of milk compared with normalpects of the fouling mechanisms mentioned above. milk 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 pHFactors Affecting Milk Fouling is found to increase the amount of fat within the deposits (Lewis Fouling depends on various parameters such as heat transfer and Heppell 2000). Lactose is not involved in the fouling processmethod, hydraulic and thermal conditions, heat transfer surface as such until it is involved in the Maillard reaction at a high tem-characteristics, and type and quality of milk along with its pro- perature (Visser and others 1997). Additives may reduce foulingcessing history. These factors can be broadly classified into 5 ma- by enhancing the heat stability of milk but may not be permittedjor categories: milk composition, operating conditions in heat ex- in many countries (Lyster 1970; Skudder and others 1981; Chan-changers, type and characteristics of heat exchangers, presence of gani and others 1997).microorganisms, and location of fouling. Holding milk for up to 24 h at 4 °C before processing results in less fouling, although further aging increases fouling (BurtonMilk composition 1968; Changani and others 1997; Lewis and Heppell 2000). Pro- The composition of milk depends on its source and hence may longed storage of milk for a few days may enhance fouling due tonot be possible to change. A seasonal variation in milk fouling is the action of proteolytic enzymes (Burton 1968; de Jong 1997).attributed to differences in its composition (Burton 1967; Belmar- However, storage at a lower temperature of 2 °C for more than 14Beiny and others 1993; de Jong 1997). Increasing the protein d has been found to have no significant impact on fouling (Lewisconcentration results in higher fouling (Toyoda and others 1994; and Heppell 2000).Changani and others 1997; Kessler 2002). Reconstituted milk gives much less fouling because about 25% The effect of pH on fouling is not straightforward. In general, the of ␤-Lg is denatured during the production of milk powders Vol. 5, 2006—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 29
  • 4. CRFSFS: Comprehensive Reviews in Food Science and Food Safety(Changani and others 1997; Visser and Jeurnink 1997). The con- perature was the most important factor in initiating fouling. Whencentration of calcium is reported to be 9% less in the reconstitut- the surface temperature was less than 68 °C, no fouling was ob-ed milk, which would result in less fouling (Changani and others served, even though the bulk temperature was up to 84 °C.1997). In contrast, Newstead and others (1998) found that Ultra Preheating of milk (often termed forewarming) causes denatur-High Temperature (UHT) fouling rates of the recombined milk in- ation and aggregation of proteins before the heating section, whichcreased with increasing preheat treatment (preheating temperature then leads to lower fouling in heat exchangers (Bell and Sanders× preheating time). The fouling deposits also had high levels of fat 1944; Burton 1968; Mottar and Moermans 1988; Foster and others(up to 60% or more) compared with the deposits formed during 1989). The main effect of forewarming is the denaturation of ␤-Lgfresh milk processing (10% or less). The difference is attributed to and its association with casein micelle and hence a reduction in thechanges in fat globule membranes. Fung and others (1998) stud- amount of type A deposits. Also, there is a reduction in the avail-ied the effect of the damage to milk fat globule membrane by a ability of ionic calcium with preheating as calcium phosphate getscavitating pump on fouling of whole milk. The fouling rate was en- attached to casein micelle (Lewis and Heppell 2000).hanced and the argument was that the damage to the membranesresults in the fat globules to coalesce, which then tend to migrate Type and characteristics of heat exchangersfaster toward the heated wall. Plate heat exchangers are used commonly in the dairy industry because they offer advantages of superior heat-transfer performance,Operating conditions in heat exchangers lower temperature gradient, higher turbulence, ease of maintenance, Important operating parameters that can be varied in a heat ex- and compactness over tubular heat exchangers. However, plate heatchanger are air content, velocity/turbulence, and temperature. exchangers are prone to fouling because of their narrow flow chan- The presence of air in milk enhances fouling (Burton 1968; de nels (Delplace and others 1994) and contact points between adja-Jong 1997; de Jong and others 1998). However, fouling is en- cent plates (Belmar-Beiny and others 1993). Also, milk fouling in ahanced only when the air bubbles are formed on the heat-transfer heat exchanger is difficult to completely eliminate, simply due to thesurface, which then act as nuclei for deposit formation (Burton fact that the temperature of the heat-transfer surface needs to be con-1968; de Jong 1997). The solubility of air in milk decreases with siderably higher than the bulk temperature to have efficient heatheating as well as a reduction in the pressure (de Jong 1997; de transfer. Complex hydraulic and thermal characteristics in plate heatJong and others 1998). Also, the formation of air bubbles is en- exchangers make it very difficult to analyze milk fouling. The use ofhanced by mechanical forces induced by valves, expansion ves- co-current and counter-current flow passages within the same heatsels, and free-falling streams (de Jong 1997). Although it is usually exchanger further complicates the problem.reported that the presence of a deaerator will reduce fouling, there The heat-transfer surface to which the deposits stick affects foul-is no reported evidence (Lewis and Heppell 2000). ing (Wahlgren and Arnebrant 1990, 1991). It influences the adhe- Fouling decreases with increasing turbulence (Belmar-Beiny and sion of microorganisms as well (Flint and others 2000). The surfaceothers 1993; Santos and others 2003). According to Paterson and characteristics are generally important only until the surface getsFryer (1988) and Changani and others (1997), the thickness and covered with the deposits. The surface treatment can be of greatsubsequently the volume of laminar sublayer decrease with in- benefit in case fouling occurs after a time delay and the strength ofcreasing velocity and as a result, the amount of foulant depositing the adhesion of the deposits onto the metal surfaces is weaker, giv-on the heat-transfer surface decreases. Delplace and others (1997) ing way to an easier cleaning process. Stainless steel is the standardobserved that significant variations in Reynolds number and aver- material used for surfaces that are in contact with milk. Factors thatage boundary layer thickness had no effect on the fouling rate. may affect fouling of a stainless-steel surface are presence of a chro-Higher flow velocities also promote deposit re-entrainment through mium oxide or passive layer, surface charge, surface energy, surfaceincreased fluid shear stresses (Rakes and others 1986). Higher tur- microstructure (roughness and other irregularities), presence of ac-bulence and different flow characteristics are in fact found to result tive sites, residual materials from previous processing conditions,in a smaller induction period in plate heat exchangers compared and type of stainless steel used (Jeurnink and others 1996a; Visserwith tubular heat exchangers (Belmar-Beiny and others 1993). The and Jeurnink 1997). Modifications of the heat-transfer surface char-reason for this may be the presence of low-velocity zones near the acteristics through electro-polishing and surface coatings can re-contact points between the adjacent plates. The use of pulsatile flow duce fouling by altering the surface roughness, charge, and wetta-was found to mitigate fouling when only the wall region near the bility (Yoon and Lund 1994; Pie␤linger-Schweiger 2001; Santosheat-transfer surface was hot enough to cause the protein denatur- and others 2001, 2004; Beuf and others 2003; Rosmaninho andation and aggregation reactions (Bradley and Fryer 1992). The rea- others 2003, 2005; Ramachandra and others 2005, Rosmaninhoson was that the fluid spent less time near the wall due to higher and Melo 2006). It is generally reported that hydrophobic surfacesmixing. The pulsations, however, enhanced fouling when the bulk adsorb more protein than hydrophilic surfaces (Wahlgren andfluid was also hot enough for the protein reactions to take place. Arnebrant 1991). Increasing the surface roughness provides a larg-Chen and others (2001) predicted that mixing caused by in-line er effective surface area and results in a higher effective surface en-mixers can reduce fouling substantially. ergy than a smooth surface (Yoon and Lund 1994). As a result, the Temperature of milk in a heat exchanger is probably the single adhesion of deposits with a rough surface would be comparativelymost important factor controlling fouling (Burton 1968; Kessler stronger. The effect of different surface coatings tends to be less onand Beyer 1991; Belmar-Beiny and others 1993; Toyoda and oth- the deposit formation but more on their adhesion strength (Britteners 1994; Corredig and Dalgleish 1996; Elofsson and others and others 1988). Magnetic field treatment has been observed to1996; Jeurnink and others 1996b; Santos and others 2003). In- have no effect on the milk fouling rate (Yoon and Lund 1994).creasing the temperature results in higher fouling. Beyond 110 °C, There has been an increasing use of heat exchangers that foulthe nature of fouling changes from type A to type B (Burton 1968). comparatively less, for example, fluidized bed heat exchangersIt is worth mentioning that both the absolute temperature and (Klaren 2003), Helixchanger heat exchangers (Master and otherstemperature difference are important for fouling. This means that it 2003), and heat exchangers equipped with turbulence promotersis feasible to have fouling in coolers where the wall temperature is (Gough and Rogers 1987). However, the information availablelower than the bulk temperature. Chen and Bala (1998) investigat- about their use in thermal processing of dairy fluids is limited. Theed the effect of surface and bulk temperatures on fouling of whole use of pulsatile flow exchanger results in higher mass transfer thatmilk, skim milk, and whey protein and found that the surface tem- may enhance fouling in case the deposition process is mass trans-30 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY—Vol. 5, 2006
  • 5. Milk fouling in heat exchangers . . .fer controlled (Bradley and Fryer 1992). The use of a fluid bed overcome the wall effect and result in uniform heating, but it mayheat exchanger has been found to reduce the amount of fouling also promote fouling as the foulant is transferred easily from theand enhance the rate of heat transfer (Bradley and Fryer 1992). bulk to the surface. Cooling may be used to reduce the tempera- Direct heating methods such as steam injection and steam infu- tures of electrode surfaces that would help control fouling.sion allow an optimal selectivity between desired (nutritional val-ue) and undesired (surviving microorganisms) product transfor- Presence of microorganismsmations (de Jong and others 1998). These methods result in a low The formation of deposits promotes the adhesion of microor-fouling rate because the desired temperatures are achieved within ganisms to heat-transfer surface, resulting in bio-fouling. Further-a very short time due to high heating rates (de Jong 1997). Also, more, the deposits provide nutrients to microorganisms, ensuringhighly viscous fluids can be handled more easily (de Jong and their growth. It is worth mentioning here that a lot of processes inothers 1998). The absence of heat-transfer surface in such cases is the dairy industry are carried out at temperatures below 100 °C.also an advantage. The resulting dilution, however, may not be For example, pasteurization is generally achieved by heating milkdesirable. The direct injection of hot air/nitrogen has been found at 72 °C for 15 s in a continuous flow system. At this temperature,to give satisfactory performance in concentrating milk through only the pathogenic bacteria along with some vegetative cells areevaporation of water (Zaida and others 1987). killed. A higher temperature of 85 °C is required to kill the re- Microwave heating has been used in numerous industrial applica- maining vegetative cells. Spores are much more heat-resistant andtions for several years due to its advantages such as faster through- remain active well beyond this temperature. Their inactivation isput, better quality, energy saving, and less space requirement over important for the products with longer shelf life.conventional heating methods (Metaxas and Meredith 1988). How- Bio-fouling, either microorganism deposition or biofilm forma-ever, the limited lifespan of a microwave system can raise doubts tion, in a heat exchanger raises serious quality concerns. Flint andover its economic viability. A number of studies have been reported coworkers have investigated the effect of bio-fouling in dairy man-on heat treatment of milk using microwaves, but these are based on ufacturing plants (Flint and Hartley 1996; Flint and others 1997,general quality issues such as nutrients and microorganisms instead 1999, 2000). According to Bott (1993), bio-fouling takes placeof fouling (Thompson and Thompson 1990; Kindle and others 1996; through 2 different mechanisms: deposition of microorganismsSieber and others 1996; Villamiel and others 1996). directly on the heat-transfer surfaces of the heat exchanger, and In induction heating, heat is generated by placing the food ma- deposition/attachment/entrapment of microorganisms on/in theterial inside an electric coil. A high-frequency alternating current deposit layer forming on the heat-transfer surfaces. With the sup-is passed through the coil, which creates an electromagnetic field. ply of nutrients by the deposits, microorganisms multiply.This induces a current in the food material and heats it up. Induc- The presence of microorganisms in the process stream and/ortion heating produces high local temperatures very quickly, but its deposit layer not only affects the product quality, it influences theuse has been limited to the materials industry only. fouling process as well (Flint and others 1997, 1999; Yoo and oth- Ohmic heating or direct resistance heating is a heat-treatment ers 2005). When microorganisms get released into the processprocess in which an electrical current is passed through milk, and fluid due to hydrodynamic forces, they contaminate downstreamheat is generated within milk to achieve pasteurization/sterilization sections. This may also result in microbial growth in areas that(Quarini 1995). This technique offers the potential of thermal pro- otherwise are not conducive to bio-fouling. The release pattern ofcessing of materials without relying on an inefficient mechanism thermophilic bacteria Bacillus stearothermophilus into the pro-such as conduction of heat from a surface into the fluid. It also has cess stream has been studied in detail by Chen and othersan advantage over microwave processing where processing can be (1998b) and Yoo and Chen (2002).limited by the depth to which energy can penetrate the food materi-al (Fryer and others 1993). The resistance heating technique was Location of foulingused for milk pasteurization in the early 20th century (de Alwis and Protein denaturation and aggregation reactions take place asFryer 1990). In recent years, this technology has been in use again soon as milk is heated. The relative amounts of denatured and ag-after being abandoned for a major part of the 20th century. APV gregated proteins depend on a number of factors such as operat-Intl. Ltd. (England) developed commercial ohmic heating units for ing conditions, type and design of heat exchanger, and propertiescontinuous sterilization of food products (Skudder and Biss 1997). of heat transfer surface. The use of an efficient technology mayAyadi and others (2003, 2004a, 2004b, 2005a, 2005b) have in- help to mitigate fouling within a heat exchanger; however, thevestigated the performance of a plate-type ohmic heater for thermal processed milk at the exit of the heat exchanger would still have atreatment of dairy products. Bansal and others (2005) and Bansal lot of denatured and aggregated proteins. This would result in se-and Chen (2005) studied skim milk fouling in a concentric cylinder vere fouling at various locations further downstream. Hence, con-ohmic heater and also developed a mathematical model to simu- trolling fouling only within the heat exchanger may not yield ef-late the fouling process. fective results and an overall strategy is required to mitigate foul- Surface temperatures are lower in ohmic heating because heat is ing over the entire setup (Petermeier and others 2002; Grijspeerdtgenerated in the bulk fluid. Hence, less fouling should take place. and others 2004). An example of such a strategy is the preheatingHowever, when the deposits start attaching to the electrode surfac- of milk before the heating section as mentioned previously (Belles, the temperature profile changes dramatically (Ayadi and others and Sanders 1944; Burton 1968; Mottar and Moermans 1988;2004b; Bansal and Chen 2005; Bansal and others 2005). In con- Foster and others 1989). The installation of an additional sectionventional indirect heating methods, such as shell and tube or plate at a constant temperature (holding section) within the cascade ofheat exchangers, the deposit formation lowers the deposit/fluid in- heat exchangers is also known to reduce fouling (de Jong andterface temperature. In contrast, the deposit/fluid interface tempera- others 1992, 1994; de Jong and van der Linden 1992; de Jongture increases with deposit formation, which further promotes foul- 1997). This outcome is attributed to the fact that denatured ␤-Lg ising (Bansal and others 2005). The reason for this is that apart from transformed into aggregated ␤-Lg in the holding section. This ag-the bulk fluid, some heat is generated in the deposit layer as well, gregated form is inactive and is unable to form aggregates withdue to its own electrical resistance. Furthermore, this layer also re- other components of milk and hence does not play an active rolestricts the outward flow of heat from the bulk fluid. in the fouling process in the downstream sections (de Jong and There are 2 other issues that may be important in an ohmic van der Linden 1992). Hence, successful mitigation of fouling de-heating process. Better mixing of the fluid may be required to pends on controlling local thermal and hydraulic conditions as Vol. 5, 2006—COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 31
  • 6. CRFSFS: Comprehensive Reviews in Food Science and Food Safetywell as surface properties throughout the plant rather than just fect of Reynolds number and fluid temperature in whey protein fouling. J Food Eng 19:119–39.within the heat exchangers. Beuf M, Rizzo G, Leuliet JC, Müller-Steinhagen H, Yiantsios S, Karabelas A, Ben- ezech T. 2003. Potency of stainless steel modifications in reducing fouling and in improving cleaning of plate heat exchangers processing dairy products. In: Proceedings of heat exchanger fouling and cleaning—fundamentals and applica-Conclusions tions. Santa Fe, N.M, U.S.A. May 18-22: Engineering Conferences International, Fouling of heat exchangers in the dairy industry is a complex New York, U.S.A. Bott TR. 1993. Aspects of biofilm formation and destruction. Corrosion Rev 11(1/phenomenon and the mechanisms are not completely under- 2):1–24.stood. Although there is an established link between protein de- Bradley SE, Fryer PJ. 1992. 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