EFFECT OF SORBET FREEZING PROCESS ON DRAW TEMPERATURE AND ICE CRYSTAL SIZE USING FOCUSED BEAM REFLECTANCE METHOD (FBRM) ONLINE MEASUREMENTS

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1 ICR 2011, August Prague, Czech Republc ID: 949 EFFECT OF SORBET FREEZING PROCESS ON DRAW TEMPERATURE AND ICE CRYSTAL SIZE USING FOCUSED BEAM REFLECTANCE METHOD (FBRM) ONLINE MEASUREMENTS M. ARELLANO A,B, J. E. GONZALEZ A,B, D. LEDUCQ A, H. BENKHELIFA B, D. FLICK B, G. ALVAREZ A a Cemagref. UR Géne des Procédés Frgorfques. Cemagref, 1 rue Perre-Glles de Gennes, Antony, 92160, France Antony, France. gracela.alvarez@cemagref.fr b AgroParsTech.UMR n 1145 Ingénere Procédés Alments. Pars, France. benkhel@agroparstech.fr ABSTRACT Durng sorbet and ce cream manufacturng s desred to obtan, a narrow ce crystal sze dstrbuton wth a small mean sze, n order to get a smooth texture n the fnal product. We studed the nfluence of the mx flow rate, the evaporaton temperature of the refrgerant flud and the dasher speed the draw temperature and on the ce crystal sze durng sorbet freezng n order to dentfy optmal operatng condtons. We used the focused beam reflectance method (FBRM) to follow the evoluton of the ce crystal sze. The FBRM probe, provdes accurate and repeatable nformaton about the chord length dstrbuton (CLD) of ce crystals. Wth ths probe t s possble to follow onlne the development of the ce crystals n sorbets or slurres contanng up to 40% of ce. The effect of refrgerant flud temperature was very mportant; by decreasng refrgerant temperature we reduce sgnfcantly the ce crystal sze and we obtan as expected lower draw temperature. Hgh dasher speeds slghtly decrease the ce crystal chord length, due to the attrton of the bgger ce crystals, whch produces new smaller ce nucle by secondary nucleaton. Also, an ncrease of the dasher speed slghtly warms the product, due to the dsspaton of frctonal energy nto the product. As expected low mx flow rates result n lower draw temperatures because the product remans longer n contact wth the freezer wall, extractng thus more heat from the product. Nevertheless, the mx flow rate dd not show a sgnfcant effect on the ce crystal sze. Keywords: Ice crystal sze; sorbet; ce cream; Focused beam reflectance method; Freezng; Scraped surface heat exchanger. 1. INTRODUCTION Durng the freezng process of sorbet and ce cream, t s necessary to obtan a narrow ce crystal sze dstrbuton (CSD) and small ce crystals (<50 µm). The qualty of the fnal product s related to the ce crystal sze because t confers a smooth texture and good palatablty to the product. Therefore, n order to mprove the qualty of the fnal product, we need to assess the process condtons that affect the most drectly the ce crystal sze. The process condtons that nfluence the mechansm of ce crystallzaton wthn a freezer are the evaporaton temperature of the refrgerant flud, the dasher rotatonal speed and the mx flow rate. The evaporaton temperature of the refrgerant flud provdes the degree of supercoolng enough to trgger ce nucleaton and t also determnes the heat removal rate of the system. The resdence tme of the product wthn the freezer s determned by the mx flow rate, whch would also affect the heat removal rate, and consequently the nucleaton and growth mechansms of the ce crystals. The scrapng acton of the blades orgnates frctonal heat whch may be dsspated nto the product and lead to the ncrease of ts draw temperature (Russell et al., 1999). However, the rotaton of the dasher helps also to mprove the heat transfer coeffcent between the freezer wall and the product, leadng to the reducton of the product s ext temperature (Ben Lakhdar et al., 2005).

2 ICR 2011, August Prague, Czech Republc Several methods have been used n lterature, to characterze the ce CSD. Nevertheless, some of these methods partally destroy the ce crystal structure durng sample preparaton, and none of these methods has been able to measure drectly onlne the ce crystal sze durng the freezng process. Recently, Haddad A. et al. (2010) have used the onlne focused beam reflectance method (FBRM), to successfully follow the evoluton of the ce CSD durng the batch freezng of sucrose/water solutons. The FBRM probe s a laser reflecton technque, whch provdes real tme nformaton about the chord length dstrbuton (CLD) of the ce crystals. One of the man advantages of ths technque s ts sutablty for n stu measurements of partculates at hgh sold concentratons. Despte the fact that the FBRM technque gves no nformaton about the partcle s morphology and t measures a CLD rather than a CSD, ths measure can be useful to follow the evoluton of crystals' sze durng the freezng process. The am of ths research s to use the FBRM technque to study onlne the nfluence of the process condtons on the ce crystal chord length as well as on the draw temperature durng the freezng of lemon sorbet. 2. MATERIALS & METHODS 2.1 Expermental setup - Sorbet freezng Lemon sorbet (25.7 % w/w sweeteners solds, 0.5 % w/w locust bean gum/guar gum/ hypromellose stablser blend) was produced n a contnuous plot freezer at a laboratory scale (WCB MF 50) wth a maxmum capacty of kg s -1. The dasher speed was studed from 57 to 105 rad.s -1 and the evaporaton temperature of the refrgerant flud R22 (Chlorodfluoromethane) was changed between -10 to -20 C accordng to the expermental condtons. 2.2 Expermental desgn and data treatment An expermental desgn of type central composte was used to assess the nfluence of the process condtons, such as the mx flow rate (MFR), the dasher rotatonal speed (DRS) and the evaporaton temperature of R22 (TR22), on the response varables of mean ce crystal chord length (MCL) and draw (ext) temperature (DT) of sorbet. The central composte expermental desgn was composed of a 2 3 factoral desgn wth expermental ponts at ±1, a 'composte' desgn wth ponts at the extremes of the expermental regon (±α, wth α = 1.68) and a common central pont of the two desgns at zero (Sablan et al., 2007). The values of the expermental desgn and the real freezng condtons are shown n Table 1. Table 1. Operatng condtons for the central composte expermental desgn a. Process Condtons Coded Values Factors Coded varables - α α MFR (kg.s -1 ) X TR22 ( C) X DRS (rad.s -1 ) X a MFR = mx flow rate; TR22 = evaporaton temperature of R22; DRS = dasher rotatonal speed. Analyss of varance (ANOVA) at a 95% confdence nterval was performed to fnd relatonshps between the process condtons and the responses of MCL and DT durng sorbet freezng. The second-order polynomal used to predct the expermental behavour of the responses was gven by equaton 1: y X 3 1 X 2 3 j1 X j X j (1) where y s the response, β 0, β, β and β j are regresson coeffcents for ntercepton, lnear, quadratc and nteracton effects, respectvely, and X X j are coded levels of the expermental factors.

ICR 2011, August 21-26 - Prague, Czech Republc 2.3. Ice crystal sze measurements A Mettler-Toledo Lasentec FBRM probe (Model S400A-8) was used to measure ce crystal chord length onlne.

When a partcle s ntersected by the laser beam, t reflects the laser lght durng the tme t s been scanned (cf. fgure 1a).

3 ICR 2011, August Prague, Czech Republc 2.3. Ice crystal sze measurements A Mettler-Toledo Lasentec FBRM probe (Model S400A-8) was used to measure ce crystal chord length onlne. Ths devce generates a focused laser beam (780 nm) whch scans a crcular path at the nterface between the probe's wndow and the partcles n suspenson. When a partcle s ntersected by the laser beam, t reflects the laser lght durng the tme t s been scanned (cf. fgure 1a). Smultaneously, the tme perod of reflecton s detected by the FBRM probe and then multpled by the tangental speed of the laser beam, yeldng thus a dstance across the partcle, whch s a chord length. The FBRM probe measures thousands of chords per second provdng a CLD (number of counts per second sorted by chord length nto 100 logarthmc sze classes). Departng from ths nformaton, the mean chord length of the ce crystals was obtaned by the equaton 2: c n c / n, (2) mean where n s the number of partcles for each of the sze classes of chord length c. 1 The FBRM probe was nserted nto the outlet ppe of the freezer wth a 45 angle relatve to the flow (cf. fgure 1b), so as to renew contnually the sorbet flow that was beng measured. In order to avod condensaton at the nsde surface of the FBRM probe wndow, a purge was carred out wth ntrogen at 1 bar, wth a flow rate of 5 l/mn. Once the steady state of the freezer was establshed, the chord length acquston data was synchronzed wth draw temperature and recorded every 5 seconds for a perod of 10 mnutes. Fgure 1a. Measurement prncple of a partcle's chord length by the FBRM probe (Mettler-Toledo). Fgure 1b. FBRM probe nserted at the outlet ppe of the freezer. 2.4 Draw temperature measurements We measured the product's draw (ext) temperature onlne by usng a calbrated Pt100 probe (Baumer, accuracy of 0.1 C). The Pt100 probe was nserted nto the outlet ppe of the freezer before the product's ext (cf. fgure 1b). The draw temperature determnes the ce mass fracton wthn sorbet. A decrease of the draw temperature wll be accompaned by an ncrease of the ce mass fracton. The relatonshp between the draw temperature of sorbet and ts ce mass fracton s establshed by the sorbet's equlbrum freezng pont curve, or "lqudus curve" of the mx, whch was determned prevously n our laboratory, based on dfferental scannng calormetrc (DSC) measurements. 3. RESULTS & DISCUSSION

4 ICR 2011, August Prague, Czech Republc Table 2 shows the real process condtons under whch the onlne ce crystal CLD and draw temperature measurements were taken, as well as the mean values of the obtaned responses. Each condton was performed twce and 5 tmes for run 10. It can be seen from table 2, that the use of the FBRM sensor enables us to follow onlne the development of the ce crystals n sorbets contanng up to 40% of ce, whch was one of the objectves of ths research. Table 2. Real freezng condtons durng measurements and obtaned responses a. Coded values Factors Responses Run MFR TR22 DRS MCL DT IMF MFR TR22 DRS kg.s -1 C rad.s -1 µm ºC % α α α α α α a MFR = Mx flow rate; TR22 = Evaporaton temperature of R22; DRS = Dasher rotatonal speed; MCL = Mean chord length; DT = Draw temperature; IMF = Ice mass fracton. Accordng to the global ANOVA analyss shown n table 3, the mean chord length response showed a sgnfcant model regresson (P < ) wth a value of R 2 = 0.94, a varaton coeffcent CV = 2.36% and dd not show a sgnfcant lack of ft (P = 0.62). In the same manner, the draw temperature of sorbet had a sgnfcant regresson model (R 2 = 0.99, CV = 2.24%, P < ) and dd not show a sgnfcant lack-of-ft (P = 0.83). Thus, these models can be used to predct the expermental behavour of mean chord length and draw temperature responses, respectvely. Table 3. Global Analyss of Varance for Responses of MCL and DT a Response R 2 adjusted CV (%) F Value P-value (model) Lack-of-Ft MCL < * 0.62 DT < * 0.83 a MCL = Mean Chord Length; DT = Draw temperature; CV = Coeffcent of varaton; * = sgnfcant nfluence at 95% confdence nterval. The ANOVA analyss of the draw temperature response shown n table 4, ndcates that t was sgnfcantly nfluenced at a 95% confdence nterval by the mx flow rate (β 1 )and the evaporaton temperature of the refrgerant flud (β 2 ), for both ther lnear and quadratc terms (P < for β 1, P < for β 2, P = for β 11 and P = for β 22 ), as well as ther nteracton (P < for β 12 ). In the same manner, the dasher speed had a sgnfcant effect on the draw temperature for ts lnear term (P = for β 3 ) and for ts nteracton wth the mx flow rate (P = for β 13 ). Regardng the mean chord length, we can observe from the ANOVA analyss shown n table 4, that the evaporaton temperature sgnfcantly affected MCL at a 95% confdence nterval n ts lnear and quadratc terms (P < for β 2 and P < for β 22 ). The dasher speed also sgnfcantly affected the MCL response n ts lnear term (P = for β 3 ). Whlst the mx flow rate dd not show a sgnfcant effect. Table 4. Regresson coeffcents of the expermental behavor model and sgnfcance levels at 95% (P-values) for responses of MCL and DT a. Intercepton Lnear Interacton Quadratc Response β 0 β 1 β 2 β 3 β 12 β 13 β 23 β 11 β 22 β 33 DT E E E-4 P-value < * < * < * * * * * * MCL E E E-5

ICR 2011, August 21-26 - Prague, Czech Republc P-value <0.0001 * 0.

034 * 0.4307 0.2329 0.9571 0.8658 <0.0001 * 0.

5 ICR 2011, August Prague, Czech Republc P-value < * < * * < * a MCL = Mean Chord Length; DT = Draw temperature; Coeffcents (β) subndex : 1 = Mx flow rate; 2 = Evaporaton temperature R22; 3 = Dasher rotatonal speed; * = sgnfcant nfluence at 95% confdence nterval. The nfluence of operatng condtons on the draw temperature of sorbet s shown n Fgure 2, where we can observe the contour plots of the draw temperature behavour as a functon of the evaporaton temperature, mx flow rate and dasher speed. It can be seen n fgure 2 that the use of lower refrgerant s flud temperatures lead to lower product s temperatures and thus to hgher ce mass fractons (cf. Table 2, runs 10, 13 and 14), whch effect s undoubtedly due to the hgher amount of supercoolng appled to the product. Fgure 2. Influence of operatng condtons on the draw temperature of sorbet (n ºC). (A) Dasher speed set at rad.s -1. (B) Evaporaton temperature set at C. (C) Mx flow rate set at kg.s -1. We can also observe n fgure 2, that the relatonshp between the mx flow rate and the draw temperature of sorbet s drectly proportonal (cf. table 4, DT β1; table 2, ponts 9, 10 and 11). In other words, at a gven refrgerant's flud temperature, a decrease of the mx flow rate would lead to a reducton of the product s ext temperature. It s our opnon that ths effect s due to the longer resdence tme of the product wthn the freezer, when operatng at lower mx flow rates, snce the sorbet wll reman longer tme n contact wth the freezer wall, allowng more tme for heat removal, and therefore leadng to the decrease of the draw (ext) temperature, as well as to the ncrease of the ce mass fracton. Ben Lakhdar et al. (2005) also reported durng the freezng of 30% sucrose/water solutons, that low product's temperatures (hgh ce mass fractons) were obtaned by a reducton of the product s flow rate at a gven refrgerant's flud temperature. Also n fgure 2 (cf. plots B and C; table 4, DT β 3 ) we can see that the relatonshp between the draw temperature and the dasher speed s drectly proportonal. Therefore, an ncrease of the dasher speed would lead to a very slght ncrease of the draw temperature (cf. table 2, runs 10, 12 and 15). We consder that ths slght nfluence s due to a compensatory effect, between the frctonal energy produced by the scrapng acton of the dasher (whch s dsspated nto the product and ncreases ts temperature), and the mprovng of the heat transfer coeffcent between the product and the freezer wall (whch would reduce the draw temperature and compensate the warmng of the product). Russell et al. (1999) reported a more mportant

ICR 2011, August 21-26 - Prague, Czech Republc warmng of the ce cream, whch effect was attrbuted to the ncrease of the mechancal dsspaton, caused by the ncrease of the dasher speed.

6 ICR 2011, August Prague, Czech Republc warmng of the ce cream, whch effect was attrbuted to the ncrease of the mechancal dsspaton, caused by the ncrease of the dasher speed. On the other hand, Ben Lakhdar et al. (2005) reported that an ncrease of the dasher speed mproves the heat transfer coeffcent, whch led to lower product temperatures. Fgure 3. Influence of operatng condtons on the mean chord length of sorbet (n µm). (A) Dasher speed set at rad.s -1. (B) Evaporaton temperature set at C. (C) Mx flow rate set at kg.s -1. Fgure 3 shows the contour plots of the mean ce crystal chord length response as a functon of the evaporaton temperature, mx flow rate and dasher speed. We can observe n fgure 3 (plot A and C), that the sgnfcant nfluence of TR22 s drectly proportonal to the MCL (cf. table 4, MCL β 2 and β 22 ). Ths means that the use of lower R22 evaporaton temperatures would lead to the reducton of the mean ce crystal chord length (cf. table 2, runs 10, 13 and 14). We beleve ths effect s due to the larger level of supercoolng to whch the sorbet s exposed to, when low refrgerant flud temperatures are used. Ths would mprove the heat removal rate, and enhance the ce nucleaton rate wthn the freezer, reducng thus the ce crystal sze. Koxholt et al. (2000) also found smaller values of ce CSD when lower refrgerant flud temperatures were employed. It s generally thought that shorter resdence tmes (hgher mx flow rates) lead to smaller ce CSD (Koxholt et al., 2000; Russell et al., 1999). However, as we can see n fgure 3 and table 4 (plots A and B; table 4, MCL β 1 ), the mx flow rate dd not show a sgnfcant nfluence on the mean ce crystal chord length. We are of the opnon that ths result s due to a compensatory effect that s produced wthn the freezer, ths s, when low mx flow rates are used (long resdence tmes), the draw temperature of sorbet decreases and smultaneously, ts ce mass fracton ncreases, as well as ts apparent vscosty (cf. table 4, runs 9, 10 and 11). Ths effect would lead to a hgher shear wthn the product, and enhance the attrton of the larger ce crystals, resultng n the producton of new ce nucle by secondary nucleaton. However, snce the resdence tme of the product s longer, the ce crystal coarsenng would be also enhanced, counteractng thus the ce nucleaton rate wthn the product. We can also observe n fgure 3 (plots B and C; table 4, MCL β 3 ) that an ncrease of the scrapng acton of the dasher slghtly reduces the mean ce crystal chord length (cf. table 2, runs 10, 12 and 15). We beleve that

7 ICR 2011, August Prague, Czech Republc ths nfluence s due to the hgher shear rate of the product that s generated when hgher dasher speeds are used, whch may lead to the producton of new smaller ce nucle by secondary nucleaton, nduced by the ce debrs produced from the attrton of the larger ce crystals. 4. CONCLUSIONS Our research has demonstrated that the FBRM sensor s a convenent tool that allowed us to follow onlne the evoluton of the ce crystal chord length n concentrated ce suspensons sorbets contanng up to 40% of ce. Our results have shown that the use of low evaporaton temperatures leads to the reducton of the mean ce crystal chord length and to the decrease of the draw temperature of sorbet, and whch effects are manly due to the hgher level of supercoolng appled to the product. An ncrease of the dasher speed would slghtly reduce the ce crystal chord length, due to the hgher shear of the product whch leads to the attrton of the ce crystals, and produces new smaller ce nucle by secondary nucleaton. Hgher dasher speeds would lead to a very slght ncrease of the draw temperature, due to the frctonal energy dsspated nto the product, whch effect s n part counteracted by the mprovng of the heat transfer coeffcent between the product and the freezer wall. Long resdence tmes (low mx flow rates) lead to the decreasng of the product s temperature, due to the fact that the product remans longer tme n contact wth the freezer wall, and therefore more heat s removed from the product. However, the mx flow rate dd not show a sgnfcant effect on the ce crystal sze. ACKNOWLEDGEMENTS The research leadng to these results has receved fundng from the European Communty's Seventh Framework Programme (FP7/ ) under grant agreement CAFÉ n KBBE REFERENCES 1. Haddad A., Benkhelfa H., Alvarez G., Flck D. (2010). Study of crystal sze evoluton by focused beam reflectance measurement durng the freezng of sucrose/water solutons n a scraped surface heat exchanger. Process Bochemstry. 2. Sablan S., Rahman S., Datta A. et Mujumdar A Handbook of Food and Boprocess Modellng. Chap. 9. Pages CRC Press. 3. Koxholt M., Esenmann B., Hnrchs, J. (2000). Effect of process parameters on the structure of ce cream. Possble methos of optmzng freezer technology. European Dary Magazne. January Russell A.B., Cheney P.E., Wantlng S.D. (1999). Influence of freezng condtons on ce crystallzaton n ce cream. Journal of Food Engneerng. 39: Ben Lakhdar M., Cerecero R., Alvarez G., Gulpart J., Flck D. Lallemand A. (2005). Heat transfer wth freezng n a scraped surface heat exchanger. Appled Thermal Engneerng. 25:45-60.

Online ice crystal size measurements by the focused beam reflectance method (FBRM) during sorbet freezing.

Online ice crystal size measurements by the focused beam reflectance method (FBRM) during sorbet freezing. Onlne ce crystal sze measurements by the focused beam reflectance method (FBRM) durng sorbet freezng. M. Arellano a,b, J. E. Gonzalez a,b, G. Alvarez a, H. Benkhelfa b, D. Flck b, D. Leducq a a Cemagref.