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The effect of shear rate, temperature, sugar and emulsifier on the tempering of cocoa butter
Journal of Food Engineering 77 (2006) 936–942 www.elsevier.com/locate/jfoodeng
The effect of shear rate, temperature, sugar and emulsifier on the tempering of cocoa butter D. Dhonsi, A.G.F. Stapley
*
Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK Received 12 January 2005; accepted 2 August 2005 Available online 22 September 2005
Abstract The influence of shear rate and temperature on the tempering of different mixtures of cocoa butter, sugar and lecithin has been studied using a concentric cylinder viscometer as a shearing device and using viscosity measurement to monitor crystallization. Shear rates ranging from 1 to 50 s 1 were tested at four different isothermal temperatures (13 °C, 17 °C, 20 °C and 23 °C). Three different material compositions were investigated: plain cocoa butter, a cocoa butter/sugar mixture (44 wt% sugar) and a cocoa butter/sugar/lecithin mixture (44 wt% sugar and 0.2% lecithin). In each case a gradual remelt was performed in the viscometer to obtain an indication of the sample onset melting point. The results for cocoa butter show that at lower temperatures induction times were much shorter, unaffected by shear and generally lead to a lower melting point sample. At 23 °C, induction times were shear dependent, with higher shear rates producing higher melting samples, suggestive of higher melting polymorphs. The addition of sugar caused a universal and substantial decrease in induction times, and although tempering at 23 °C was still shear dependent the onset melting point was lower than for the sugar only sample. The addition of lecithin caused a slight delay in the onset of crystallization. The results tend to suggest that sugar crystals provide sites for heterogeneous nucleation, which is slightly weakened by lecithin which coats the sugar surfaces. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Lecithin; Nucleation; Crystallization; Chocolate; Induction time; Fat
1. Introduction The tempering of chocolate is an important stage in the manufacture of chocolate. The tempering process produces seed crystals of the Form V or b polymorph of cocoa butter, which enables this polymorph to be the dominant form in the subsequent moulding step (Talbot, 1994). This is achieved by subjecting the chocolate to a carefully defined shear and temperature history, whereby the chocolate is first melted, then cooled to initiate nucleation of seed crystals of the b crystal form, and finally warmed by a few degrees to melt out crystals of any lower melting polymorphs that may have formed. The application of shear during the cooling and holding stages appears to be a key factor in the success of tempering processes. Indeed, all the different *
Corresponding author. Tel.: +44 1509 222525; fax: +44 1509 223923. E-mail address: [email protected] (A.G.F. Stapley).
0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.08.022
methods of tempering used throughout the history of chocolate moulding involve an element of shear. Even the earliest hand tempering methods subjected the molten chocolate to shear by spreading it across a marble slab with a knife. However it is only in recent years that the importance of shear has become fully appreciated and its influence investigated, although the precise mechanism by which shear aids tempering is still not fully understood (see review by Sato & Koyano, 2001). The earliest published work appears to be by Ziegleder (1985), who sheared cocoa butter in a rotational viscometer at 20 °C. More recently, the effect of shear on chocolate or cocoa butter tempering has been studied in a number of different flow geometries, for example, scraped surface heat exchanger with cocoa butter and chocolate (Bolliger, Zeng, & Windhab, 1999), Couette geometry with milk chocolate (Stapley, Tewkesbury, & Fryer, 1999) and cocoa butter (Mazzanti, Guthrie, Sirota, Marangoni, & Idziak, 2003),
D. Dhonsi, A.G.F. Stapley / Journal of Food Engineering 77 (2006) 936–942
cone and plate system with cocoa butter (MacMillan et al., 2002), parallel plate viscometer with milk chocolate (Briggs & Wang, 2004), and a helical ribbon device with cocoa butter (Toro-Vazquez, Pe´rez-Martinez, Dibildox-Alvarado, Charo-Alonso, & Reyes-Hernandez, 2004). A variety of techniques for measuring the effect of tempering on-line have been employed. A popular method is to monitor the torque applied to the shearing device to provide a measurement of the onset of crystallization. This approach was used by Ziegleder (1985), Briggs and Wang (2004), and Toro-Vazquez et al. (2004). Loisel and co-workers (Loisel, Keller, Lecq, Launay, & Ollivon, 1997, 1998) also used this technique to follow the crystallization of dark chocolate in a scraped surface heat exchanger. Bolliger et al. (1999), however, tracked crystallization using a Near Infra Red probe, the response of which was correlated to the viscosity and melting enthalpy of the chocolate. The most sophisticated and powerful technique is X-ray diffraction, which enables direct identification of polymorphs to be made and has provided some additional insight into the possible mechanism of tempering. For example, MacMillan et al. (2002) showed that shear can accelerate interpolymorphic transformations from Form III to Form V. Mazzanti et al. (2003) then demonstrated using X-ray diffraction that shear has an orientation effect on fat crystals. They argue that the presence of shear forces which are large enough to induce orientation (which is possible in crystallites as small as 70 nm in high shear flow) should be able to provide enough energy to promote a transformation either via a solid–solid transformation or via partial melting of crystallites. When X-ray methods have not been used for on-line measurement many workers have also included post mortem analyses to assess the final state of temper. Differential scanning calorimetry (DSC) is typically employed, and has been used by Ziegleder (1985), Loisel, Lecq, Keller, and Ollivon (1998), Bolliger et al. (1999), Stapley et al. (1999), and Toro-Vazquez et al. (2004). To date, however, little work has been carried out examining the effect of composition on tempering. Loisel et al. (1998) investigated the effect of adding tristearin, distearin, stearic acid and emulsifiers. It was found that tristearin reduced induction times, whereas distearin and stearic acid had a slight delaying effect. The effect of emulsifier (lecithin or PGPR) was difficult to assess as the rheology of the chocolate was significantly altered by their addition, and resulted in a lack of reproducibility of conditions within the complex scraped surface heat exchanger geometry used. The main aim of the work presented here is to examine the effect of sugar and emulsifier on the tempering behaviour of cocoa butter. Both are ingredients of chocolate which greatly affect its rheology and so may affect how shear influences crystallization (Vernier, 1997). A Couette geometry was chosen in which to perform experiments as it offers a well-defined shear pattern which is reasonably reproducible over large variations in rheological behaviour.
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The following specific objectives were pursued: (i) To use a Couette viscometer to temper cocoa butter at a variety of temperatures to assess how shear affects crystallization at four different temperatures ranging between 13 °C and 23 °C. (ii) To investigate the effect of other chocolate components such as sugar and lecithin on the crystallization of cocoa butter. (iii) To trial a method of determining an in situ melting point measurement of the cocoa butter formed using the viscometer. This method of in situ melting point determination has the advantage that material does not need to be transferred to a second instrument (and experience temperature fluctuations), but has the disadvantage of returning only a single melting point value, whereas melting of natural fats always occurs over a range of temperatures. To test the technique, selected samples of cocoa butter were also analysed after the solidification stage by differential scanning calorimetry. 2. Materials and methods 2.1. Materials Cocoa butter and lecithin were supplied by Nestle UK Ltd (York). Silk sugar was supplied by British Sugar PLC. The particle size distribution of the silk sugar as determined by a Coulter LS130 particle size analyser using isobutyl alcohol as the dispersion agent is shown in Fig. 1. Experiments were conducted using the following formulations: (a) Cocoa butter only. (b) Cocoa butter (44 wt%) and sugar (56 wt%). (c) Mixture (b) with 0.2% added lecithin. Formulations (b) and (c) were prepared by heating the cocoa butter to 50 °C to form a liquid melt, adding the required mass of sugar and lecithin and stirring manually with a plastic spatula until a homogeneous mixture was obtained.
Fig. 1. Particle size distribution of the sugar used.
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2.2. Crystallization experiments Experiments were carried out using a Haake VT550 concentric cylinder viscometer using the MVII cup and bob assembly. The inner diameter of the cup was 42 mm and the outer diameter of the bob was 36.8 mm. The cup was surrounded by a water jacket which allowed the sample temperature to be regulated by water from a temperature controlled recirculating water bath. Two water baths (Hetofrig, Heto Birkerod Denmark) at different temperatures were used, and the flow to the jacket could be rapidly switched between the two to allow a ‘‘step-change’’ in temperature of the sample (with a minute or so of thermal lag). The viscometer was controlled by a computer running ThermoHaake RheoWin software, which also recorded shear stress, shear rate and water jacket temperature versus time. The samples were poured into the viscometer in the molten state and the following procedure followed: (i) The sample was heated to 50 °C using the first water bath, where it was held for 10 min, whilst shearing at 50 s 1. This was to remove any crystal history from the sample. The high shear rate was to minimise sedimentation of sugar crystals. (ii) The bath was then cooled down to 35 °C at 3.5 °C min 1, whilst still shearing at 50 s 1. This was achieved by cooling the first water bath, and adding ice to the bath if necessary. The bath was then held at 35 °C for 5 min to allow the sample to equilibrate, and remove heat from the viscometer, but without inducing crystallization. This intermediate stage resulted in a much faster approach to the final isothermal hold temperature than if cooled directly from 50 °C. (iii) The shear rate for the experiment was then set, and the sample crash cooled to the desired isothermal hold temperature, by switching over to the water supply from the second bath, which was preset at the correct temperature. (iv) The shear stress was then recorded until the shear stress reached 50 Pa for plain cocoa butter or 150 Pa (mixtures containing sugar) to avoid damage to the viscometer. This is due to the sample solidifying.
to whether sample was liquid enough for the viscometer to rotate. The procedure was as follows: After shearing was halted in the crystallization experiment the sample was left for 10 min to allow the sample to continue solidifying. A shear rate of 0.1 s 1 was then applied to the sample. If the measured shear stress did not exceed 50 Pa within 10 s of shear being applied, the sample was deemed to have ‘‘melted’’ and the ‘‘melting point’’ was recorded as the current sample temperature. The experiment was then terminated. If the measured shear stress did exceed 50 Pa within 10 s of shear being applied, the shearing was automatically stopped, and the sample was deemed to be ‘‘not melted’’. The sample temperature was then raised by 1 °C, 3 min allowed for the sample to come to thermal equilibrium again, and the procedure repeated. 2.4. Differential scanning calorimetry Selected samples of solidified cocoa butter were also analysed by DSC after the isothermal hold period, in order to compare the DSC melting response to the viscometer melting data. A Perkin Elmer Pyris Diamond DSC (at the Division of Food Sciences, University of Nottingham) was used using aluminium sample pans. The DSC was previously calibrated using indium and cyclohexane at a scan rate of 2 °C/min. After the isothermal hold period the shearing was stopped, 10 min allowed to pass, and the viscometer moved to a cold store room of air temperature 5 °C. A sample of cocoa butter were then dug out from the viscometer annulus and loaded into an aluminium sample which was hermetically sealed with an aluminium lid using a sample press. The sealed pan was then transported from the cold room to the DSC on a copper block covered with a ceramic lid, to maintain the pan temperature. The pan was then quickly loaded into the DSC from the block. The DSC cell temperature was pre-set at 5 °C. The sample weight was obtained by weighing the pans after the experiment along with the ring that is cut-off during the pressing process, and subtracting the initial masses of the pan and lid. The samples were then scanned upwards in the DSC at 2 °C min 1. 3. Results and discussion 3.1. Cocoa butter
Experiments were performed at four different isothermal hold temperatures (13 °C, 17 °C, 20 °C and 23 °C). Seven shear rates were tested for the plain cocoa butter (1 s 1, 5 s 1, 10 s 1, 20 s 1, 30 s 1, 40 s 1, 50 s 1), and four shear rates for the sugar and lecithin samples (20 s 1, 30 s 1, 40 s 1, 50 s 1). 2.3. Melting point determination For each experiment, an in situ test was performed to assess the melting point of the cocoa butter formed according
The viscosity–time curves for plain cocoa butter at the four temperatures are shown in Fig. 2. All show an increase of apparent viscosity with time, which is due to the formation of ice crystals in the annulus. Two types of behaviour are observed depending on the temperature. At 13 °C and 17 °C the viscosity increases smoothly with time and there appears to be little difference in the results at different shear rates. At 20 °C and 23 °C the increase in viscosity is more sudden (characteristic of a true induction time) and is dependent on shear rate, with higher shear rates resulting
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Fig. 2. Viscosity time curves of plain cocoa butter isothermally held at (a) 13 °C, (b) 17 °C, (c) 20 °C and (d) 23 °C and at different shear rates.
Table 1 In situ melting points (°C) of plain cocoa butter samples determined after holding at different isothermal hold temperatures and shear rates, and allowing a further 10 min to set Shear rate (s 1)
50 40 30 20 10 5 1
of melting point with the isothermal shear rate, with the highest melting product forming after shearing at 50 s 1 (melting point 34 °C–35 °C), and progressively reducing to 28 °C at 1 s 1. The DSC measurements from samples sheared at 20 s 1 (Fig. 3) showed very similar melting behaviour between the samples, except for that held at 13 °C, which showed a significant peak at lower temperatures. When comparing the
1.2 After 13 ˚C
1.0 DSC Heat Flow (W/g)
in shorter induction times. Induction times are also strongly dependent on temperature (as would be expected) with very significantly longer induction times at higher temperatures (lower super-cooling). There was some anomalous behaviour at a shear rate of 1 s 1, where an intermediate plateau region is observed (Fig. 2c). Similar results were reported by Loisel et al. (1998) for dark chocolate, who plausibly suggested that this apparent two step crystallization may be a fractionation effect. Melting point data from the in situ viscometer method are shown in Table 1. These indicate that the sample held at 13 °C produced a lower melting product (melting at 23 °C or 24 °C) irrespective of the shear rate used. The 17 °C sample had a higher melting point of 28 °C–29 °C. The 20 °C and 23 °C samples however showed a variation
After 17 ˚C 0.8 After 20 ˚C 0.6
After 23 ˚C
0.4
Isothermal hold temperature 23 °C
20 °C
17 °C
13 °C
35 34 34 32 32 28 28
34 33 32 30 30 29 28
29 29 29 29 28 28 27
24 24 23 23 23 23 23
0.2 0.0
10
20
30
40
50
Temperature (˚C)
Fig. 3. Differential scanning calorimetry heating scans of samples of plain cocoa butter extracted from the viscometer after previously holding at various isothermal temperatures at a shear rate of 20 s 1.
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DSC melting data with the viscometer melting data it should be borne in mind that they are measuring two slightly different effects. The viscometer melting point corresponds to the point where just enough of the sample has melted to create a slip plane of movement in the annulus. This may not require very much of the material to have melted. The viscometer melting points appears to lie near to the beginning of the DSC peaks, which supports this view, and particular explains why the viscometer was sensitive to lower melting forms produced in the 13 °C runs. In summary, the results show that at low hold temperatures (13 °C) crystallization is fast, independent of shear rate and produces more of the low melting polymorph, whereas at higher hold temperatures, crystallization of higher melting polymorphs is hastened by the action of shear. These findings are consistent with previously published results, where shear has been found to promote the formation of higher polymorphs. Both Ziegleder (1985) and MacMillan et al. (2002) sheared cocoa butter at 20 °C. MacMillan et al. (2002) observed using small angle X-ray dffraction that at zero shear Form IV was produced when cocoa butter was held at 20 °C, but that Form V was formed when a shear rate of only 3 s 1 was used, although this was preceded by a period where Form II/III briefly appears before transforming to Form V. Induction times were of the order of 10 min at 6 s 1 reducing to 2 min at 12 s 1. Ziegleder using a Couette system and a DSC for poly-
morph identification after shearing found that shear rates over 12 s 1 the favoured polymorph was Form V compared to Form IV below 12 s 1. Induction times of the order of 20–25 min were generally observed when Form IV was produced and of the order of 10 min when Form V appeared. There is thus reasonable agreement between this work and that of Ziegleder, but the cone and plate system of MacMillan et al. appears to produce shorter induction times, suggesting that the equipment geometry or materials may be influencing nucleation. It should also be noted that cocoa butter is also subject to compositional variations, which can lead to different behaviour being observed under otherwise nominally identical conditions. Stapley et al. (1999) found that when holding milk chocolate at 22 °C for set times, a critical shear rate was required to temper chocolate if the hold time was fixed or a critical hold time was required if the shear rate was fixed. It can be seen in the light of this work that both of those findings can simply be explained by the induction times for crystallization being reduced by an increase in shear rate. 3.2. Cocoa butter/sugar mixture The results for the cocoa butter/sugar mixture are presented in Fig. 4. The main observation is that induction times are greatly reduced compared with the equivalent
Fig. 4. Viscosity time curves of the plain cocoa butter/sugar mixture isothermally held at (a) 13 °C, (b) 17 °C, (c) 20 °C and (d) 23 °C and at different shear rates.
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experiments performed with plain cocoa butter. However, only the samples held at 23 °C showed any effect of shear rate on induction time. These also showed the highest melting samples (see Table 2), although these were significantly below those recorded for the plain cocoa butter experiments. Indeed the melting points from cocoa butter/sugar samples were consistently below those obtained with plain cocoa butter—the exception being samples held at 13 °C which universally produced a low melting product (melting point around 24 °C). The presence of sugar would be expected to increase the local shear rates in the cocoa butter phase for a given Table 2 In situ melting points (°C) of cocoa butter/sugar samples with and without 0.2% lecithin added, determined after holding at different isothermal hold temperatures and shear rates, and allowing a further 10 min to set Mixture
Shear rate (s 1)
Isothermal hold temperature (°C) 23 °C
20 °C
17 °C
13 °C
Without lecithin
50 40 30 20
28 28 29 30
25 25 25 26
27 27 27 25
24 24 24 23
With 0.2% lecithin
50 40 30 20
28 28 28 30
25 25 26 28
26 26 26 25
25 25 25 25
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overall shear rate, as the sugar particles are non-deformable. The impact of sugar on average local shear rate might be roughly estimated as giving a factor of two increase, and thus promote the formation of higher polymorphs, which is shear dependent. However, the effect is also seen at the lower temperatures where shear is of less or no importance, and also favour the production of lower polymorphs. It therefore appears that the presence of the sugar promotes the formation of the lower melting polymorphs, by possibly acting as an aid to heterogeneous nucleation on the sugar crystals. The reason for the higher shear samples yielding lower induction times at 23 °C compared to 20 °C is unclear. 3.3. Cocoa butter/sugar/lecithin mixture The addition of lecithin was observed to produce a slight increase induction times compared to the cocoa butter/ sugar mixtures (see Fig. 5), although possibly to a greater degree at 23 °C. Lecithin thus appears to retard crystallization, but does not change the melting point of the product formed (see Table 2). Lecithin is known to migrate to sugar/fat interfaces and coat sugar crystals, which influences the rheology of chocolate and aids dispersion of the sugar crystals in the fat (Johansson & Bergenstahl, 1992; Vernier, 1997, or see Beckett, 2000). Thus two effects may be occurring. Firstly,
Fig. 5. Viscosity time curves of the plain cocoa butter/sugar/lecithin mixture isothermally held at (a) 13 °C, (b) 17 °C, (c) 20 °C and (d) 23 °C and at different shear rates.
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the lecithin covering the sugar surface may provide a worse surface for heterogeneous nucleation to occur. Secondly, the lecithin might enable greater slip between the fat phase and sugar particles, which would reduce shear rates within the fat phase. The induction times are also similar to those observed for milk chocolate systems by Stapley et al. (1999) (between 200 and 300 s at 22 °C and 53 s 1), and Briggs and Wang (2004) (approximately 400 s at 30 s 1 and 24 °C), although it should be noted that the fat phase of milk chocolate differs from cocoa butter by the addition of milk fat which has a significant proportion of lower molecular weight (and lower melting) triglycerides. 4. Conclusions The tempering of cocoa butter, a cocoa butter/sugar mixture and a cocoa butter/sugar/lecithin mixture has been studied using a rotational viscometer device, and using the viscometer measurement to monitor crystallization. The viscometer also served as a crude device to determine the melting points of the subsequently solidified samples. This produced reasonable results, although it appears to be mainly sensitive to the lower melting fractions in a sample. It was found that at low hold temperatures (13 °C) crystallization was fast, independent of shear rate and produced more of the low melting polymorph, whereas at higher hold temperatures (20 °C and 23 °C) crystallization of a higher melting polymorph was hastened by the action of shear. The reduction in induction times with increased shear rate suggests that shear is important at the nucleation stage. This would tend to rule out interpolymorphic transitions as the key mechanism to tempering in this case. The addition of sugar resulted in much shorter induction times, which were only shear dependent at 23 °C (i.e. not at 20 °C as was the case with plain cocoa butter). Sugar appears to act as a heterogeneous nucleation agent which preferentially promotes the formation of lower polymorphs. The effect of lecithin was to slightly retard induction times compared to the cocoa butter/sugar mixture, which may be caused by lecithin coating the sugar crystals. Acknowledgements We wish to thank Steve Beckett of Nestle UK for providing cocoa butter and lecithin and for useful discussions,
and to British Sugar for providing the silk sugar. We also wish to thank Dr. Bill MacNaughtan of the Division of Food Sciences, University of Nottingham for running the DSC samples. References Beckett, S. T. (2000). The science of chocolate. Cambridge: The Royal Society of Chemistry, pp. 77–82. Bolliger, S., Zeng, Y., & Windhab, E. J. (1999). In-line measurement of tempered cocoa butter and chocolate by means of near-infrared spectroscopy. Journal of the American Oil Chemists Society, 76(6), 659–667. Briggs, J. L., & Wang, T. (2004). Influence of shearing and time on the rheological properties of milk chocolate during tempering. Journal of the American Oil Chemists Society, 81(2), 117–121. Johansson, S., & Bergenstahl, B. (1992). The influence of food emulsifiers on fat and sugar dispersions in oils. II. Rheology, colloidal forces. Journal of the American Oil Chemists Society, 69(8), 718–727. Loisel, C., Keller, G., Lecq, G., Launay, B., & Ollivon, M. (1997). Tempering of chocolate in a scraped surface heat exchanger. Journal of Food Science, 62(4), 773–780. Loisel, C., Lecq, G., Keller, G., & Ollivon, M. (1998). Dynamic crystallization of dark chocolate as affected by temperature and lipid additives. Journal of Food Science, 63(1), 73–79. MacMillan, S. D., Roberts, K. J., Rossi, A., Wells, M. A., Polgreen, M. C., & Smith, I. H. (2002). In situ small angle X-ray scattering (SAXS) studies of polymorphism with the associated crystallization of cocoa butter fat using shearing conditions. Crystal Growth and Design, 2(3), 221–226. Mazzanti, G., Guthrie, S. E., Sirota, E. B., Marangoni, A. G., & Idziak, S. H. J. (2003). Orientation and phase transitions of fat crystals under shear. Crystal Growth and Design, 3(5), 721–725. Sato, K., & Koyano, T. (2001). Crystallization properties of cocoa butter. In N. Garti & K. Sato (Eds.), Crystallization processes in fats and lipid systems (pp. 429–456). New York: Marcel Dekker. Stapley, A. G. F., Tewkesbury, H., & Fryer, P. J. (1999). The effects of shear and temperature history on the crystallization of chocolate. Journal of the American Oil ChemistÕs Society, 76(6), 677–685. Talbot, G. (1994). Chocolate temper. In S. T. Beckett (Ed.), Industrial chocolate manufacture and use (pp. 156–166). London: Blackie Academic and Professional. Toro-Vazquez, J. F., Pe´rez-Martinez, D., Dibildox-Alvarado, E., CharoAlonso, M., & Reyes-Hernandez, J. (2004). Rheometry and polymorphism of cocoa butter during crystallization under static and stirring conditions. Journal of the American Oil Chemists Society, 73(6), 195–202. Vernier, F. (1997). Influence of emulsifiers on the rheology of chocolate and suspensions of cocoa and sugar particles in oil. PhD Thesis. University of Reading. Ziegleder, G. (1985). Improved crystallization behaviour of cocoa butter under shearing. Internationale Zeitschrift fur Lebensmittel Technologie und Verfahrenstechnik, 36, 412–416.
The effect of shear rate, temperature, sugar and emulsifier on the tempering of cocoa butter D. Dhonsi, A.G.F. Stapley
*
Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK Received 12 January 2005; accepted 2 August 2005 Available online 22 September 2005
Abstract The influence of shear rate and temperature on the tempering of different mixtures of cocoa butter, sugar and lecithin has been studied using a concentric cylinder viscometer as a shearing device and using viscosity measurement to monitor crystallization. Shear rates ranging from 1 to 50 s 1 were tested at four different isothermal temperatures (13 °C, 17 °C, 20 °C and 23 °C). Three different material compositions were investigated: plain cocoa butter, a cocoa butter/sugar mixture (44 wt% sugar) and a cocoa butter/sugar/lecithin mixture (44 wt% sugar and 0.2% lecithin). In each case a gradual remelt was performed in the viscometer to obtain an indication of the sample onset melting point. The results for cocoa butter show that at lower temperatures induction times were much shorter, unaffected by shear and generally lead to a lower melting point sample. At 23 °C, induction times were shear dependent, with higher shear rates producing higher melting samples, suggestive of higher melting polymorphs. The addition of sugar caused a universal and substantial decrease in induction times, and although tempering at 23 °C was still shear dependent the onset melting point was lower than for the sugar only sample. The addition of lecithin caused a slight delay in the onset of crystallization. The results tend to suggest that sugar crystals provide sites for heterogeneous nucleation, which is slightly weakened by lecithin which coats the sugar surfaces. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Lecithin; Nucleation; Crystallization; Chocolate; Induction time; Fat
1. Introduction The tempering of chocolate is an important stage in the manufacture of chocolate. The tempering process produces seed crystals of the Form V or b polymorph of cocoa butter, which enables this polymorph to be the dominant form in the subsequent moulding step (Talbot, 1994). This is achieved by subjecting the chocolate to a carefully defined shear and temperature history, whereby the chocolate is first melted, then cooled to initiate nucleation of seed crystals of the b crystal form, and finally warmed by a few degrees to melt out crystals of any lower melting polymorphs that may have formed. The application of shear during the cooling and holding stages appears to be a key factor in the success of tempering processes. Indeed, all the different *
Corresponding author. Tel.: +44 1509 222525; fax: +44 1509 223923. E-mail address: [email protected] (A.G.F. Stapley).
0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.08.022
methods of tempering used throughout the history of chocolate moulding involve an element of shear. Even the earliest hand tempering methods subjected the molten chocolate to shear by spreading it across a marble slab with a knife. However it is only in recent years that the importance of shear has become fully appreciated and its influence investigated, although the precise mechanism by which shear aids tempering is still not fully understood (see review by Sato & Koyano, 2001). The earliest published work appears to be by Ziegleder (1985), who sheared cocoa butter in a rotational viscometer at 20 °C. More recently, the effect of shear on chocolate or cocoa butter tempering has been studied in a number of different flow geometries, for example, scraped surface heat exchanger with cocoa butter and chocolate (Bolliger, Zeng, & Windhab, 1999), Couette geometry with milk chocolate (Stapley, Tewkesbury, & Fryer, 1999) and cocoa butter (Mazzanti, Guthrie, Sirota, Marangoni, & Idziak, 2003),
D. Dhonsi, A.G.F. Stapley / Journal of Food Engineering 77 (2006) 936–942
cone and plate system with cocoa butter (MacMillan et al., 2002), parallel plate viscometer with milk chocolate (Briggs & Wang, 2004), and a helical ribbon device with cocoa butter (Toro-Vazquez, Pe´rez-Martinez, Dibildox-Alvarado, Charo-Alonso, & Reyes-Hernandez, 2004). A variety of techniques for measuring the effect of tempering on-line have been employed. A popular method is to monitor the torque applied to the shearing device to provide a measurement of the onset of crystallization. This approach was used by Ziegleder (1985), Briggs and Wang (2004), and Toro-Vazquez et al. (2004). Loisel and co-workers (Loisel, Keller, Lecq, Launay, & Ollivon, 1997, 1998) also used this technique to follow the crystallization of dark chocolate in a scraped surface heat exchanger. Bolliger et al. (1999), however, tracked crystallization using a Near Infra Red probe, the response of which was correlated to the viscosity and melting enthalpy of the chocolate. The most sophisticated and powerful technique is X-ray diffraction, which enables direct identification of polymorphs to be made and has provided some additional insight into the possible mechanism of tempering. For example, MacMillan et al. (2002) showed that shear can accelerate interpolymorphic transformations from Form III to Form V. Mazzanti et al. (2003) then demonstrated using X-ray diffraction that shear has an orientation effect on fat crystals. They argue that the presence of shear forces which are large enough to induce orientation (which is possible in crystallites as small as 70 nm in high shear flow) should be able to provide enough energy to promote a transformation either via a solid–solid transformation or via partial melting of crystallites. When X-ray methods have not been used for on-line measurement many workers have also included post mortem analyses to assess the final state of temper. Differential scanning calorimetry (DSC) is typically employed, and has been used by Ziegleder (1985), Loisel, Lecq, Keller, and Ollivon (1998), Bolliger et al. (1999), Stapley et al. (1999), and Toro-Vazquez et al. (2004). To date, however, little work has been carried out examining the effect of composition on tempering. Loisel et al. (1998) investigated the effect of adding tristearin, distearin, stearic acid and emulsifiers. It was found that tristearin reduced induction times, whereas distearin and stearic acid had a slight delaying effect. The effect of emulsifier (lecithin or PGPR) was difficult to assess as the rheology of the chocolate was significantly altered by their addition, and resulted in a lack of reproducibility of conditions within the complex scraped surface heat exchanger geometry used. The main aim of the work presented here is to examine the effect of sugar and emulsifier on the tempering behaviour of cocoa butter. Both are ingredients of chocolate which greatly affect its rheology and so may affect how shear influences crystallization (Vernier, 1997). A Couette geometry was chosen in which to perform experiments as it offers a well-defined shear pattern which is reasonably reproducible over large variations in rheological behaviour.
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The following specific objectives were pursued: (i) To use a Couette viscometer to temper cocoa butter at a variety of temperatures to assess how shear affects crystallization at four different temperatures ranging between 13 °C and 23 °C. (ii) To investigate the effect of other chocolate components such as sugar and lecithin on the crystallization of cocoa butter. (iii) To trial a method of determining an in situ melting point measurement of the cocoa butter formed using the viscometer. This method of in situ melting point determination has the advantage that material does not need to be transferred to a second instrument (and experience temperature fluctuations), but has the disadvantage of returning only a single melting point value, whereas melting of natural fats always occurs over a range of temperatures. To test the technique, selected samples of cocoa butter were also analysed after the solidification stage by differential scanning calorimetry. 2. Materials and methods 2.1. Materials Cocoa butter and lecithin were supplied by Nestle UK Ltd (York). Silk sugar was supplied by British Sugar PLC. The particle size distribution of the silk sugar as determined by a Coulter LS130 particle size analyser using isobutyl alcohol as the dispersion agent is shown in Fig. 1. Experiments were conducted using the following formulations: (a) Cocoa butter only. (b) Cocoa butter (44 wt%) and sugar (56 wt%). (c) Mixture (b) with 0.2% added lecithin. Formulations (b) and (c) were prepared by heating the cocoa butter to 50 °C to form a liquid melt, adding the required mass of sugar and lecithin and stirring manually with a plastic spatula until a homogeneous mixture was obtained.
Fig. 1. Particle size distribution of the sugar used.
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2.2. Crystallization experiments Experiments were carried out using a Haake VT550 concentric cylinder viscometer using the MVII cup and bob assembly. The inner diameter of the cup was 42 mm and the outer diameter of the bob was 36.8 mm. The cup was surrounded by a water jacket which allowed the sample temperature to be regulated by water from a temperature controlled recirculating water bath. Two water baths (Hetofrig, Heto Birkerod Denmark) at different temperatures were used, and the flow to the jacket could be rapidly switched between the two to allow a ‘‘step-change’’ in temperature of the sample (with a minute or so of thermal lag). The viscometer was controlled by a computer running ThermoHaake RheoWin software, which also recorded shear stress, shear rate and water jacket temperature versus time. The samples were poured into the viscometer in the molten state and the following procedure followed: (i) The sample was heated to 50 °C using the first water bath, where it was held for 10 min, whilst shearing at 50 s 1. This was to remove any crystal history from the sample. The high shear rate was to minimise sedimentation of sugar crystals. (ii) The bath was then cooled down to 35 °C at 3.5 °C min 1, whilst still shearing at 50 s 1. This was achieved by cooling the first water bath, and adding ice to the bath if necessary. The bath was then held at 35 °C for 5 min to allow the sample to equilibrate, and remove heat from the viscometer, but without inducing crystallization. This intermediate stage resulted in a much faster approach to the final isothermal hold temperature than if cooled directly from 50 °C. (iii) The shear rate for the experiment was then set, and the sample crash cooled to the desired isothermal hold temperature, by switching over to the water supply from the second bath, which was preset at the correct temperature. (iv) The shear stress was then recorded until the shear stress reached 50 Pa for plain cocoa butter or 150 Pa (mixtures containing sugar) to avoid damage to the viscometer. This is due to the sample solidifying.
to whether sample was liquid enough for the viscometer to rotate. The procedure was as follows: After shearing was halted in the crystallization experiment the sample was left for 10 min to allow the sample to continue solidifying. A shear rate of 0.1 s 1 was then applied to the sample. If the measured shear stress did not exceed 50 Pa within 10 s of shear being applied, the sample was deemed to have ‘‘melted’’ and the ‘‘melting point’’ was recorded as the current sample temperature. The experiment was then terminated. If the measured shear stress did exceed 50 Pa within 10 s of shear being applied, the shearing was automatically stopped, and the sample was deemed to be ‘‘not melted’’. The sample temperature was then raised by 1 °C, 3 min allowed for the sample to come to thermal equilibrium again, and the procedure repeated. 2.4. Differential scanning calorimetry Selected samples of solidified cocoa butter were also analysed by DSC after the isothermal hold period, in order to compare the DSC melting response to the viscometer melting data. A Perkin Elmer Pyris Diamond DSC (at the Division of Food Sciences, University of Nottingham) was used using aluminium sample pans. The DSC was previously calibrated using indium and cyclohexane at a scan rate of 2 °C/min. After the isothermal hold period the shearing was stopped, 10 min allowed to pass, and the viscometer moved to a cold store room of air temperature 5 °C. A sample of cocoa butter were then dug out from the viscometer annulus and loaded into an aluminium sample which was hermetically sealed with an aluminium lid using a sample press. The sealed pan was then transported from the cold room to the DSC on a copper block covered with a ceramic lid, to maintain the pan temperature. The pan was then quickly loaded into the DSC from the block. The DSC cell temperature was pre-set at 5 °C. The sample weight was obtained by weighing the pans after the experiment along with the ring that is cut-off during the pressing process, and subtracting the initial masses of the pan and lid. The samples were then scanned upwards in the DSC at 2 °C min 1. 3. Results and discussion 3.1. Cocoa butter
Experiments were performed at four different isothermal hold temperatures (13 °C, 17 °C, 20 °C and 23 °C). Seven shear rates were tested for the plain cocoa butter (1 s 1, 5 s 1, 10 s 1, 20 s 1, 30 s 1, 40 s 1, 50 s 1), and four shear rates for the sugar and lecithin samples (20 s 1, 30 s 1, 40 s 1, 50 s 1). 2.3. Melting point determination For each experiment, an in situ test was performed to assess the melting point of the cocoa butter formed according
The viscosity–time curves for plain cocoa butter at the four temperatures are shown in Fig. 2. All show an increase of apparent viscosity with time, which is due to the formation of ice crystals in the annulus. Two types of behaviour are observed depending on the temperature. At 13 °C and 17 °C the viscosity increases smoothly with time and there appears to be little difference in the results at different shear rates. At 20 °C and 23 °C the increase in viscosity is more sudden (characteristic of a true induction time) and is dependent on shear rate, with higher shear rates resulting
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Fig. 2. Viscosity time curves of plain cocoa butter isothermally held at (a) 13 °C, (b) 17 °C, (c) 20 °C and (d) 23 °C and at different shear rates.
Table 1 In situ melting points (°C) of plain cocoa butter samples determined after holding at different isothermal hold temperatures and shear rates, and allowing a further 10 min to set Shear rate (s 1)
50 40 30 20 10 5 1
of melting point with the isothermal shear rate, with the highest melting product forming after shearing at 50 s 1 (melting point 34 °C–35 °C), and progressively reducing to 28 °C at 1 s 1. The DSC measurements from samples sheared at 20 s 1 (Fig. 3) showed very similar melting behaviour between the samples, except for that held at 13 °C, which showed a significant peak at lower temperatures. When comparing the
1.2 After 13 ˚C
1.0 DSC Heat Flow (W/g)
in shorter induction times. Induction times are also strongly dependent on temperature (as would be expected) with very significantly longer induction times at higher temperatures (lower super-cooling). There was some anomalous behaviour at a shear rate of 1 s 1, where an intermediate plateau region is observed (Fig. 2c). Similar results were reported by Loisel et al. (1998) for dark chocolate, who plausibly suggested that this apparent two step crystallization may be a fractionation effect. Melting point data from the in situ viscometer method are shown in Table 1. These indicate that the sample held at 13 °C produced a lower melting product (melting at 23 °C or 24 °C) irrespective of the shear rate used. The 17 °C sample had a higher melting point of 28 °C–29 °C. The 20 °C and 23 °C samples however showed a variation
After 17 ˚C 0.8 After 20 ˚C 0.6
After 23 ˚C
0.4
Isothermal hold temperature 23 °C
20 °C
17 °C
13 °C
35 34 34 32 32 28 28
34 33 32 30 30 29 28
29 29 29 29 28 28 27
24 24 23 23 23 23 23
0.2 0.0
10
20
30
40
50
Temperature (˚C)
Fig. 3. Differential scanning calorimetry heating scans of samples of plain cocoa butter extracted from the viscometer after previously holding at various isothermal temperatures at a shear rate of 20 s 1.
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DSC melting data with the viscometer melting data it should be borne in mind that they are measuring two slightly different effects. The viscometer melting point corresponds to the point where just enough of the sample has melted to create a slip plane of movement in the annulus. This may not require very much of the material to have melted. The viscometer melting points appears to lie near to the beginning of the DSC peaks, which supports this view, and particular explains why the viscometer was sensitive to lower melting forms produced in the 13 °C runs. In summary, the results show that at low hold temperatures (13 °C) crystallization is fast, independent of shear rate and produces more of the low melting polymorph, whereas at higher hold temperatures, crystallization of higher melting polymorphs is hastened by the action of shear. These findings are consistent with previously published results, where shear has been found to promote the formation of higher polymorphs. Both Ziegleder (1985) and MacMillan et al. (2002) sheared cocoa butter at 20 °C. MacMillan et al. (2002) observed using small angle X-ray dffraction that at zero shear Form IV was produced when cocoa butter was held at 20 °C, but that Form V was formed when a shear rate of only 3 s 1 was used, although this was preceded by a period where Form II/III briefly appears before transforming to Form V. Induction times were of the order of 10 min at 6 s 1 reducing to 2 min at 12 s 1. Ziegleder using a Couette system and a DSC for poly-
morph identification after shearing found that shear rates over 12 s 1 the favoured polymorph was Form V compared to Form IV below 12 s 1. Induction times of the order of 20–25 min were generally observed when Form IV was produced and of the order of 10 min when Form V appeared. There is thus reasonable agreement between this work and that of Ziegleder, but the cone and plate system of MacMillan et al. appears to produce shorter induction times, suggesting that the equipment geometry or materials may be influencing nucleation. It should also be noted that cocoa butter is also subject to compositional variations, which can lead to different behaviour being observed under otherwise nominally identical conditions. Stapley et al. (1999) found that when holding milk chocolate at 22 °C for set times, a critical shear rate was required to temper chocolate if the hold time was fixed or a critical hold time was required if the shear rate was fixed. It can be seen in the light of this work that both of those findings can simply be explained by the induction times for crystallization being reduced by an increase in shear rate. 3.2. Cocoa butter/sugar mixture The results for the cocoa butter/sugar mixture are presented in Fig. 4. The main observation is that induction times are greatly reduced compared with the equivalent
Fig. 4. Viscosity time curves of the plain cocoa butter/sugar mixture isothermally held at (a) 13 °C, (b) 17 °C, (c) 20 °C and (d) 23 °C and at different shear rates.
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experiments performed with plain cocoa butter. However, only the samples held at 23 °C showed any effect of shear rate on induction time. These also showed the highest melting samples (see Table 2), although these were significantly below those recorded for the plain cocoa butter experiments. Indeed the melting points from cocoa butter/sugar samples were consistently below those obtained with plain cocoa butter—the exception being samples held at 13 °C which universally produced a low melting product (melting point around 24 °C). The presence of sugar would be expected to increase the local shear rates in the cocoa butter phase for a given Table 2 In situ melting points (°C) of cocoa butter/sugar samples with and without 0.2% lecithin added, determined after holding at different isothermal hold temperatures and shear rates, and allowing a further 10 min to set Mixture
Shear rate (s 1)
Isothermal hold temperature (°C) 23 °C
20 °C
17 °C
13 °C
Without lecithin
50 40 30 20
28 28 29 30
25 25 25 26
27 27 27 25
24 24 24 23
With 0.2% lecithin
50 40 30 20
28 28 28 30
25 25 26 28
26 26 26 25
25 25 25 25
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overall shear rate, as the sugar particles are non-deformable. The impact of sugar on average local shear rate might be roughly estimated as giving a factor of two increase, and thus promote the formation of higher polymorphs, which is shear dependent. However, the effect is also seen at the lower temperatures where shear is of less or no importance, and also favour the production of lower polymorphs. It therefore appears that the presence of the sugar promotes the formation of the lower melting polymorphs, by possibly acting as an aid to heterogeneous nucleation on the sugar crystals. The reason for the higher shear samples yielding lower induction times at 23 °C compared to 20 °C is unclear. 3.3. Cocoa butter/sugar/lecithin mixture The addition of lecithin was observed to produce a slight increase induction times compared to the cocoa butter/ sugar mixtures (see Fig. 5), although possibly to a greater degree at 23 °C. Lecithin thus appears to retard crystallization, but does not change the melting point of the product formed (see Table 2). Lecithin is known to migrate to sugar/fat interfaces and coat sugar crystals, which influences the rheology of chocolate and aids dispersion of the sugar crystals in the fat (Johansson & Bergenstahl, 1992; Vernier, 1997, or see Beckett, 2000). Thus two effects may be occurring. Firstly,
Fig. 5. Viscosity time curves of the plain cocoa butter/sugar/lecithin mixture isothermally held at (a) 13 °C, (b) 17 °C, (c) 20 °C and (d) 23 °C and at different shear rates.
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the lecithin covering the sugar surface may provide a worse surface for heterogeneous nucleation to occur. Secondly, the lecithin might enable greater slip between the fat phase and sugar particles, which would reduce shear rates within the fat phase. The induction times are also similar to those observed for milk chocolate systems by Stapley et al. (1999) (between 200 and 300 s at 22 °C and 53 s 1), and Briggs and Wang (2004) (approximately 400 s at 30 s 1 and 24 °C), although it should be noted that the fat phase of milk chocolate differs from cocoa butter by the addition of milk fat which has a significant proportion of lower molecular weight (and lower melting) triglycerides. 4. Conclusions The tempering of cocoa butter, a cocoa butter/sugar mixture and a cocoa butter/sugar/lecithin mixture has been studied using a rotational viscometer device, and using the viscometer measurement to monitor crystallization. The viscometer also served as a crude device to determine the melting points of the subsequently solidified samples. This produced reasonable results, although it appears to be mainly sensitive to the lower melting fractions in a sample. It was found that at low hold temperatures (13 °C) crystallization was fast, independent of shear rate and produced more of the low melting polymorph, whereas at higher hold temperatures (20 °C and 23 °C) crystallization of a higher melting polymorph was hastened by the action of shear. The reduction in induction times with increased shear rate suggests that shear is important at the nucleation stage. This would tend to rule out interpolymorphic transitions as the key mechanism to tempering in this case. The addition of sugar resulted in much shorter induction times, which were only shear dependent at 23 °C (i.e. not at 20 °C as was the case with plain cocoa butter). Sugar appears to act as a heterogeneous nucleation agent which preferentially promotes the formation of lower polymorphs. The effect of lecithin was to slightly retard induction times compared to the cocoa butter/sugar mixture, which may be caused by lecithin coating the sugar crystals. Acknowledgements We wish to thank Steve Beckett of Nestle UK for providing cocoa butter and lecithin and for useful discussions,
and to British Sugar for providing the silk sugar. We also wish to thank Dr. Bill MacNaughtan of the Division of Food Sciences, University of Nottingham for running the DSC samples. References Beckett, S. T. (2000). The science of chocolate. Cambridge: The Royal Society of Chemistry, pp. 77–82. Bolliger, S., Zeng, Y., & Windhab, E. J. (1999). In-line measurement of tempered cocoa butter and chocolate by means of near-infrared spectroscopy. Journal of the American Oil Chemists Society, 76(6), 659–667. Briggs, J. L., & Wang, T. (2004). Influence of shearing and time on the rheological properties of milk chocolate during tempering. Journal of the American Oil Chemists Society, 81(2), 117–121. Johansson, S., & Bergenstahl, B. (1992). The influence of food emulsifiers on fat and sugar dispersions in oils. II. Rheology, colloidal forces. Journal of the American Oil Chemists Society, 69(8), 718–727. Loisel, C., Keller, G., Lecq, G., Launay, B., & Ollivon, M. (1997). Tempering of chocolate in a scraped surface heat exchanger. Journal of Food Science, 62(4), 773–780. Loisel, C., Lecq, G., Keller, G., & Ollivon, M. (1998). Dynamic crystallization of dark chocolate as affected by temperature and lipid additives. Journal of Food Science, 63(1), 73–79. MacMillan, S. D., Roberts, K. J., Rossi, A., Wells, M. A., Polgreen, M. C., & Smith, I. H. (2002). In situ small angle X-ray scattering (SAXS) studies of polymorphism with the associated crystallization of cocoa butter fat using shearing conditions. Crystal Growth and Design, 2(3), 221–226. Mazzanti, G., Guthrie, S. E., Sirota, E. B., Marangoni, A. G., & Idziak, S. H. J. (2003). Orientation and phase transitions of fat crystals under shear. Crystal Growth and Design, 3(5), 721–725. Sato, K., & Koyano, T. (2001). Crystallization properties of cocoa butter. In N. Garti & K. Sato (Eds.), Crystallization processes in fats and lipid systems (pp. 429–456). New York: Marcel Dekker. Stapley, A. G. F., Tewkesbury, H., & Fryer, P. J. (1999). The effects of shear and temperature history on the crystallization of chocolate. Journal of the American Oil ChemistÕs Society, 76(6), 677–685. Talbot, G. (1994). Chocolate temper. In S. T. Beckett (Ed.), Industrial chocolate manufacture and use (pp. 156–166). London: Blackie Academic and Professional. Toro-Vazquez, J. F., Pe´rez-Martinez, D., Dibildox-Alvarado, E., CharoAlonso, M., & Reyes-Hernandez, J. (2004). Rheometry and polymorphism of cocoa butter during crystallization under static and stirring conditions. Journal of the American Oil Chemists Society, 73(6), 195–202. Vernier, F. (1997). Influence of emulsifiers on the rheology of chocolate and suspensions of cocoa and sugar particles in oil. PhD Thesis. University of Reading. Ziegleder, G. (1985). Improved crystallization behaviour of cocoa butter under shearing. Internationale Zeitschrift fur Lebensmittel Technologie und Verfahrenstechnik, 36, 412–416.