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

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1 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. UR Géne des Procédés Frgorfques. Antony, France. (marcela.arellano@cemagref.fr) b AgroParsTech.UMR n 45 Ingénere Procédés Alments. Pars, France. (benkhel@agroparstech.fr) ABSTRACT The ce crystal sze dstrbuton determnes n part the textural propertes of sorbet and ce cream. Durng sorbet and ce cream manufacturng, a narrow ce crystal sze dstrbuton wth a small mean sze s desred, n order to obtan a smooth texture n the fnal product. Ths research studed the nfluence of the mx flow rate, the evaporaton temperature of the refrgerant flud and the dasher speed on the ce crystal sze and the draw temperature durng sorbet freezng, so as to dentfy optmal operatng condtons. The evoluton of the ce crystal sze was followed by the focused beam reflectance method (FBRM), whch uses an n stu sensor that provdes accurate and repeatable nformaton about the chord length dstrbuton (CLD) of ce crystals. Our results showed that the FBRM sensor s a promsng tool whch makes t possble to montor onlne the development of the ce crystals n sorbets contanng up to 40% of ce. Decreasng the refrgerant flud temperature allows us to reduce the ce crystal sze and to lower the product s temperature, due to the ncrease of the supercoolng drvng force. 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. 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. Keywords: Ice crystal sze; Focused beam reflectance method; Freezng; Scraped surface heat exchanger. INTRODUCTION The qualty of sorbet and ce cream s determned n part by the ce crystal sze dstrbuton (CSD) of the product. A narrow ce CSD wth small ce crystals (<50 µm) s desred to confer a smooth texture and good palatablty to the product. Therefore, t s mportant to dentfy the operatng condtons that affect the most drectly the ce crystal sze, so as to mprove the qualty of the fnal product. The mechansm of ce crystallzaton wthn a freezer s affected manly by the process condtons, such as 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 drvng force that trggers ce nucleaton and t determnes the heat removal rate of the system. 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 []. The mx flow rate dctates the resdence tme of the product wthn the freezer, affectng thus the ce nucleaton and growth mechansms of the ce crystals. In order to characterze the ce CSD, many methods have been used n lterature. However, none of these methods has been able to measure drectly the ce crystal sze n the product's stream durng the freezng process. Recently, onlne technques such as the focused beam reflectance method (FBRM) have been developed for n stu montorng of CSD n crystallsaton processes. In the case of ce crystallzaton, Haddad A. et al. [] have successfully used the FBRM technque to 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 partculates. One of the man advantages of ths technque s ts sutablty for n stu measurements of partculates at hgh sold concentratons. However, the FBRM technque gves no nformaton about the partcle s morphology and t measures a CLD rather than a CSD. But ths measure can be useful to follow the evoluton of crystals' sze. The am of ths research s to use the FBRM technque to study n stu the nfluence of the operatng condtons on the ce crystal chord length as well as on the draw temperature durng the freezng of lemon sorbet. MATERIALS & METHODS Expermental setup - Sorbet freezng The freezng of lemon sorbet mx (25.7 % w/w sweeteners solds, 0.5 % w/w locust bean gum/guar gum/ hypromellose stablser blend) was carred out n a contnuous plot freezer at a laboratory scale (WCB MF

2 50) wth a maxmum capacty of 0.02 kg s -. The dasher speed range of the freezer was vared from 57 to 05 rad.s - and the evaporaton temperature of the refrgerant flud R22 (Chlorodfluoromethane), was vared wthn the range of -0 to -20 C accordng to expermental condtons. Expermental desgn and data treatment A central composte expermental desgn was used to assess the nfluence of 3 operatng condtons: mx flow rate (MFR), dasher rotatonal speed (DRS) and evaporaton temperature of R22 (TR22), on the response varables of ce crystal chord length dstrbuton (CLD) and draw temperature (DT) of sorbet. The central composte expermental desgn was composed of a 2 3 factoral desgn wth expermental ponts at ±, a 'composte' desgn wth ponts at the extremes of the expermental regon (±α, wth α =.68) and a common central pont of the two desgns at zero [2].The expermental desgn was performed twce and 5 replcates of the central pont were performed n order to provde enough nformaton to estmate the expermental error. Table shows the coded values of the expermental desgn and the real freezng operatng condtons. Table. Coded values of operatng condtons for the central composte expermental desgn a. Process Condtons Coded Values Factors Coded varables - α α MFR (kg.s - ) X TR22 ( C) X DRS (rad.s - ) X a MFR = mx flow rate; TR22 = evaporaton temperature of R22; DRS = dasher rotatonal speed. Expermental data were analysed usng response surface methodology and the second-order polynomal used to predct the expermental behavour was the followng: y = β = β X β X + = < j= β X j X j () 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. Ice crystal sze measurements The ce crystal chord length was measured onlne by usng a Mettler-Toledo Lasentec FBRM probe (Model S400A-8). 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 throughout the tme t s been scanned (cf. fgure a). 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 00 logarthmc sze classes). Departng from ths nformaton, the mean chord length of the ce crystals was obtaned by the followng equaton: 00 = 00 c = n c / n, (2) mean = where n s the number of partcles for each of the sze classes of chord length c. When performng our experments the FBRM probe was nserted nto the outlet ppe of the freezer wth a 45 angle relatve to the flow (cf. fgure b), makng t possble 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 bar, wth a flow rate of 5 l/mn. Once the steady state of the freezer was

Draw temperature measurements The draw temperature of the product was measured onlne by means of a calbrated Pt00 probe (Baumer, accuracy of 0. C).

3 establshed, the chord length acquston data was synchronzed wth draw temperature and recorded every 5 seconds for a perod of 0 mnutes. Fgure a. Measurement prncple of a partcle's chord length by the FBRM probe (Mettler-Toledo). Fgure b. FBRM probe nserted at the outlet ppe of the freezer. Draw temperature measurements The draw temperature of the product was measured onlne by means of a calbrated Pt00 probe (Baumer, accuracy of 0. C). The Pt00 probe was nserted nto the outlet ppe of the freezer before the product's ext (cf. fgure b). In order to establsh the relatonshp between the draw temperature of sorbet and ts equlbrum ce mass fracton, the sorbet's equlbrum freezng pont curve was prevously determned n our laboratory usng dfferental scannng calormetrc measurements. RESULTS & DISCUSSION Table 2 shows the real operatng condtons under whch the FBRM measurements were taken and the mean values of the obtaned responses. Each condton was performed twce and 5 tmes for run 0. 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 - C rad.s - 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. Results n table 2 show that the use of the FBRM sensor makes t possble to montor onlne the development of the ce crystals n sorbets contanng up to 40% of ce, whch was one of the objectves of ths research.

4 The global ANOVA analyss n table 3 shows for the mean chord length response, a sgnfcant model regresson (P < 0.000) wth a value of R 2 = 0.94, a varaton coeffcent CV = 2.36% and does not show lack of ft (P = 0.62). The draw temperature of sorbet showed also a sgnfcant regresson model (R 2 = 0.99, CV = 2.24%, P < 0.000) and dd not show lack-of-ft (P = 0.83). Therefore, both 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 <0.000 * 0.62 DT <0.000 * 0.83 a MCL = Mean Chord Length; DT = Draw temperature; CV = Coeffcent of varaton. Accordng to the regresson coeffcent analyss n table 4, t appears that the mean chord length was sgnfcantly affected at a 95% confdence nterval by the evaporaton temperature n ts lnear and quadratc terms (P < for β 2 and P < for β 22 ), followed by the dasher speed n ts lnear term (P = for β 3 ). Whlst the mx flow rate dd not show a sgnfcant effect. In the case of the draw temperature, the regresson coeffcent analyss n table 4 shows that the mx flow rate and the evaporaton temperature of the refrgerant flud had the most sgnfcant effect at a 95% confdence nterval, for both ther lnear and quadratc terms (P < for β, P < for β 2, P = for β and P = for β 22 ) as well as ther nteracton (P < for β 2 ). The dasher speed also 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 3 ). 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 β 2 β 3 β 2 β 3 β 23 β β 22 β 33 MCL E E E-5 P-value <0.000 * <0.000 * * <0.000 * DT E E E-4 P-value <0.000 * <0.000 * <0.000 * * * * * * a MCL = Mean Chord Length; DT = Draw temperature; Coeffcents (β) subndex : = Mx flow rate; 2 = Evaporaton temperature R22; 3 = Dasher rotatonal speed; * = sgnfcant nfluence at 95% confdence nterval. Fgure 2 shows the contour plots of the mean ce crystal chord length behavour as a functon of the evaporaton temperature, mx flow rate and dasher speed. As we can see n table 4 (MCL β 2 and β 22 ) and fgure 2 (A, C), the sgnfcant nfluence of the TR22 s drectly proportonal to the MCL, n other words, the mean ce crystal chord length becomes smaller when the evaporaton temperature of the refrgerant flud decreases (cf. table 2, runs 0, 3 and 4). Ths effect can be explaned by the well known fact that at low refrgerant flud temperatures the sorbet s exposed to a larger level of supercoolng, whch enhances a rapd heat removal rate and thus the ce nucleaton mechansm. Smlarly, Koxholt et al. [3] reported that low refrgerant flud temperatures led to smaller values of ce CSD. We can observe as well n table 4 (MCL β 3 ) and fgure 2 (B, C), that the relatonshp between the dasher speed and the MCL s nversely proportonal (cf. table 2, runs 0, 2 and 5). Thus, an ncrease of the scrapng acton of the dasher slghtly decreases the ce crystal chord length. We beleve that ths nfluence s due to the hgher rate of shear that s produced wthn the freezer, whch may lead to a phenomenon of attrton of the bgger ce crystals, whch produces new smaller ce nucle by secondary nucleaton. It s generally thought that hgh mx flow rates (short resdence tmes) produce smaller ce crystals [3, 4]. However, as we can see n our results (table 4, MCL β ; and fgure 2, A and B) the mx flow rate dd not show a sgnfcant effect on the ce crystal chord length. Consequently, t s our opnon that ths result s due to a compensatory effect that s produced wthn the freezer: when low MFR are used, the draw temperature of sorbet decreases (cf. table 4, runs 9, 0 and ), whch enhances the growth of ce crystals and ncreases the MCL. However, at the same tme, when the sorbet's draw temperature s low, the ce mass fracton

ncreases, as well as the vscosty of the product, producng a hgher shear rate whch leads to the attrton of the bgger ce crystals and generates new smaller ce nucle by secondary nucleaton. Fgure 2.

(B) Evaporaton temperature set at -5.3 C. (C) Mx flow rate set at 0.04 kg.s -.

As we can see n fgure 3, the relatonshp between the MFR and the draw temperature s drectly proportonal (cf. table 4, DT β ).

5 ncreases, as well as the vscosty of the product, producng a hgher shear rate whch leads to the attrton of the bgger ce crystals and generates new smaller ce nucle by secondary nucleaton. Fgure 2. Influence of operatng condtons on the mean chord length of sorbet (n µm). (A) Dasher speed set at rad.s -. (B) Evaporaton temperature set at -5.3 C. (C) Mx flow rate set at 0.04 kg.s -. Fgure 3 shows the contour plots of the draw temperature behavour as a functon of the evaporaton temperature, mx flow rate and dasher speed. As we can see n fgure 3, the relatonshp between the MFR and the draw temperature s drectly proportonal (cf. table 4, DT β ). Thus, for a gven refrgerant flud temperature, when lower MFR rates are used, the draw temperature of sorbet decreases. We beleve that ths effect s due to the fact that at a lower MFR, the resdence tme s hgher, and the sorbet s longer n contact wth the freezer wall, so that more heat s extracted, decreasng the draw temperature and thus ncreasng the ce mass fracton (cf. table 2, ponts 9, 0 and ). Lkewse, Ben Lakhdar et al. [5] reported n the case of freezng of 30% sucrose/water solutons that hgh product flow rates lead to smaller ce mass fracton when the refrgerant flud temperature was held constant. We can observe as well (cf. table 4, DT β 3 ; and fgure 3 B and C) that the relatonshp between the draw temperature and the dasher speed s drectly proportonal. Thus, an ncrease of the scrapng acton of the dasher slghtly ncreases the draw temperature of sorbet (cf. table 2, runs 0, 2 and 5). We beleve that ths effect s due to the frctonal energy orgnated between the blade tp and the freezer wall, whch s dsspated to a certan amount nto the product and slghtly ncreases ts temperature. Russell et al. [4] reported a warmng of the ce cream, caused by an ncrease of the dasher speed, at a constant refrgerant flud temperature.

Fgure 3. Influence of operatng condtons on the draw temperature of sorbet (n ºC). (A) Dasher speed set at 78.54 rad.s -.

CONCLUSIONS Our research has shown that the FBRM technque s a convenent tool that allowed us to follow onlne the evoluton of the ce crystal chord length n sorbets contanng up to 40% of ce content.

6 Fgure 3. Influence of operatng condtons on the draw temperature of sorbet (n ºC). (A) Dasher speed set at rad.s -. (B) Evaporaton temperature set at -5.3 C. (C) Mx flow rate set at 0.04 kg.s -. CONCLUSIONS Our research has shown that the FBRM technque s a convenent tool that allowed us to follow onlne the evoluton of the ce crystal chord length n sorbets contanng up to 40% of ce content. Our results demonstrated that the use of low evaporaton temperatures makes t possble to obtan smaller ce crystals and to decrease the product s temperature due to the hgher level of supercoolng appled to the product. An ncrease of the dasher speed slghtly decreases 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. Hgh dasher speeds slghtly ncrease the draw temperature, due to the frctonal energy dsspated nto the product. Low mx flow rates (long resdence tmes) result n lower draw temperatures due to the fact that the product remans longer tme n contact wth the freezer wall, removng more heat from the product. 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 [] Haddad A., Benkhelfa H., Alvarez G., Flck D. (200). 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 [4] Russell A.B., Cheney P.E., Wantlng S.D. (999). Influence of freezng condtons on ce crystallzaton n ce cream. Journal of Food Engneerng. 39:79-9. [5] Ben Lakhdar. M., Cerecero R., Alvarez A., Gulpart J., Flck D. Lallemand A. (2005). Heat transfer wth freezng n a scraped surface heat exchanger. Appled Thermal Engneerng. 25:45-60.

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

ICR 2011, August 21-26 - 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