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Mixing and tempering effect on the rheological and particle size properties of dark chocolate coatings Efecto del mezclado y temperado sobre las propiedades reológicas y de tamaño de partícula de coberturas de chocolate oscuro a
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T. Quiñones-Muñoz , J. A. Gallegos-Infante , N. E. Rocha-Guzmán , L. A. Ochoaa
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Martinez , J. Morales-Castro , R. F. González-Laredo & L. Medina-Torres
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Departamento de Ingenierías Química y Bioquímica, Instituto Tecnológico de Durango, Blvd., Felipe Pescador 1830 Ote., CP 34080, Durango, Dgo., Mexico b
Facultad de Química, Universidad Nacional Autónoma de México, U.N.A.M., CP 04510, México, D.F. Available online: 23 May 2011
To cite this article: T. Quiñones-Muñoz, J. A. Gallegos-Infante, N. E. Rocha-Guzmán, L. A. Ochoa-Martinez, J. MoralesCastro, R. F. González-Laredo & L. Medina-Torres (2011): Mixing and tempering effect on the rheological and particle size properties of dark chocolate coatings Efecto del mezclado y temperado sobre las propiedades reológicas y de tamaño de partícula de coberturas de chocolate oscuro, CyTA - Journal of Food, 9:2, 109-113 To link to this article: http://dx.doi.org/10.1080/19476337.2010.482748
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CyTA – Journal of Food Vol. 9, No. 2, August 2011, 109–113
Mixing and tempering effect on the rheological and particle size properties of dark chocolate coatings
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Efecto del mezclado y temperado sobre las propiedades reolo´gicas y de taman˜o de partı´ cula de coberturas de chocolate oscuro T. Quin˜ones-Mun˜oza, J.A. Gallegos-Infantea*, N.E. Rocha-Guzma´na, L.A. Ochoa-Martineza, J. Morales-Castroa, R.F. Gonza´lez-Laredoa and L. Medina-Torresb a Departamento de Ingenierı´as Quı´mica y Bioquı´mica, Instituto Tecnolo´gico de Durango, Blvd., Felipe Pescador 1830 Ote., CP 34080 Durango, Dgo., Mexico; bFacultad de Quı´mica, Universidad Nacional Auto´noma de Me´xico, U.N.A.M., CP 04510, Me´xico, D.F.
(Received 6 November 2009; final version received 3 February 2010) Chocolate coatings are semisolid suspensions of fine particles from sugar, cocoa, non fat milk solids in an oily phase. Processing steps of chocolate include mixing, refining, conching, tempering, molding, and packing. Tempering is a directed pre-crystallization that consists of shearing chocolate mass at controlled temperatures. The effect of mixing and tempering process on the particle size distribution and rheological behavior of dark chocolate coatings were evaluated. Each sample was melted (65 8C, 15 min) and tempered following three different procedures usually recommended for chocolate. Proximate composition analysis, specific surface area, mean particle diameter, consistency index (K), flow index (n), G0 , G00 and electron micrographs (40006) were obtained. All samples followed Casson flow model and (n) showed a pseudoplastic behavior. Higher values of K were shown by tempering process 3. Shear increased chocolate storage module (G0 ) and its stability. Samples without tempering and shearing have shown higher values of particle size. Keywords: chocolate coatings; mixing; particle size; rheology; tempering Las coberturas de chocolate son suspensiones semi-so´lidas de partı´ culas pequen˜as de azu´car, cacao, so´lidos no grasos de leche en una fase oleosa. Los pasos del procesamiento de chocolate incluyen mezclado, refinacio´n, conchado, temperado, moldeo y empacado. El temperado es una pre-cristalizacio´n directa que consiste en cizallar la masa de chocolate a temperaturas controladas. Se evaluo´ el efecto del mezclado y del temperado sobre la distribucio´n del taman˜o de partı´ cula y el comportamiento reolo´gico de coberturas de chocolate obscuro. Las muestras fueron fundidas (65 8C, 15 min) y temperadas siguiendo tres procedimientos recomendados. Se determinaron el ana´lisis proximal, a´rea superficial especı´ fica, dia´metro medio de partı´ cula, ı´ ndice de consistencia (K), ı´ ndice de flujo (n), mo´dulos viscoela´sticos (G0 y G00 ), y se uso´ micrografı´ a electro´nica, SEM (40006). Todas las muestras presentaron un comportamiento al flujo ajustado al modelo de Casson. Los cambios de K estuvieron en funcio´n del proceso de temperado usado, pero (n) mostro´ cambios en su comportamiento pseudopla´stico. Los mayores valores de K se obtuvieron para el temperado 3 y los menores para el temperado 1. El corte incremento´ el mo´dulo de almacenamiento (G0 ) y en consecuencia su estabilidad. Las muestras sin temperado y corte mostraron el mayor taman˜o de partı´ cula (9.17 mm). Palabras clave: cobertura de chocolate; mezclado; taman˜o de partı´ cula; temperado; reologı´ a
Introduction Chocolate is a dispersion that during consumption activates the pleasure centers of the human brain (Afoakwa, Paterson, & Fowler, 2007). The quality of chocolate confectionary products is related to the appropriate melting behavior, because the products are solid at room temperature and melted on the tongue surface and ingested undergoing dissolution. Particle size distribution and ingredient composition play important roles in shaping its rheological behavior. Chocolate coatings are semisolid suspensions of fine particles from sugar, cocoa and non fat milk solids
in an oily continuous phase. Critical to physical properties is the continuous oil phase composition, which influences mouth feel and melting properties. According to Attaie, Breischuh, Braun, & Windhab (2003), processing steps of chocolate include mixing, refining, conching, tempering, molding and packing. Tempering involves pre-crystallization of a small proportion of triacylglicerides with crystal forming nuclei. Tempering has four key steps: melting to completion (usually at 50 8C), cooling to a point of crystallization (at 32–34 8C), crystallization (25–27 8C) and
*Corresponding author. Email:
[email protected] ISSN 1947-6337 print/ISSN 1947-6345 online Ó 2011 Taylor & Francis DOI: 10.1080/19476337.2010.482748 http://www.informaworld.com
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conversion of any unstable crystals (29–32 8C)(Afoakwa, Paterson, Fowler, & Vieria, 2008). Tempering method is a function of recipe, equipment and purpose. In small factories hand tempering is still used by chocolatiers. Tempering is a directed precrystallization that consists of shearing chocolate mass at controlled temperatures to promote cocoa butter crystallization in a thermodynamically stable polymorphic form (Afoakwa et al., 2008). The final crystal form depends critically on the shear-temperature-time process, which the material has undergone (Hartel, 2001). A properly tempered chocolate exhibits high gloss, appropriate melting temperature, and fat-bloom stability (Debaste et al., 2008). In addition, the flow behavior of tempered chocolate has implications for the processing of chocolate after tempering (Briggs and Wang, 2004). Factors such as conching temperature, particle size distribution, fat content, type and type of emulsifiers, and tempering conditions determine efficiency of mixing, pumping and transportation of finished products during processing (Dhonsi and Stapley, 2006). The role of PSD in suspension flow properties of chocolate has been described by several authors. Afoakwa, Paterson, & Fowler (2008) indicated that particle size distribution influenced rheology and texture of chocolate. Bolenz, Thiessenhusen, & Schape (2003) evaluated the influence of particle size on the rheological properties of chocolate during conching. Chocolate rheology has been extensively studied. Chevalley (1975) published a very complete review about it. Wilson, Speers, & Tung (1993) indicated that the best description of flow chocolate is done by a Casson model (see equation 1) at low shear rate. Two parameters obtained from Casson equation are usually discussed: yield stress and plastic viscosity or Casson viscosity. 1=2
s1=2 ¼ K1 g1=2 þ k0
ð1Þ
where s is the shear stress, g is the shear rate, k0 is the yield stress and K1 is the plastic viscosity or Casson viscosity. Yield stress is a material property denoting transition between pseudo-solid and pseudo-liquid behaviors related to minimum shear stress at first evidence of flow, or transition from elastic to viscous deformation. Unfortunately, rheological properties of two phase systems under stirring conditions may lead to incorrect measurements due to solid particle sedimentation and their slip at the receptacle wall. Among the alternatives to avoid spurious flow phenomena is the use of mixing impellers. Such impellers provide torque data as a measurement of flow resistance against rotational speed. Particulate systems at stress below glass transition determine the thickness and uniformity of chocolate
coatings. Plastic viscosity determines pumping characteristics, coating properties and sensory character of chocolate. Very few studies of oscillatory test on chocolate samples have been done. Particle size distribution plays a clear role in process fluidity, but is generally restricted to experience-based empirical knowledge. The main objective of this study was to evaluate the effect of mixing and tempering process on the particle size distribution and rheological behavior of dark chocolate coatings. Materials and methods A chocolate coating sample was obtained from a local grocery store AMA (type Ganashe, Brand Chantilly). Sample was analyzed for proximal analysis following the Association of Analytical Communities (AOAC) methods: carbohydrates (AOAC 938.18), ash (AOAC 972.15), fat (AOAC 963.15), moisture (AOAC 977.10), and nitrogen (AOAC 970.22). Each chocolate sample was melted at 65 8C for 15 min and equilibrated (rested) for another 15 min. Chocolate samples were tempered following three different procedures usually recommended for chocolate coatings (see Table 1). Particle size A MasterSizer Laser Diffraction Particle size Analyzer (Malvern Instruments Ltd., Malvern, England) was used to analyze chocolate samples (with and without shear). A refractive index of 1.395 for the chocolate coating was obtained at 25 8C in a refractometer (ColeParmer, Vernon Hills, IL, USA) after 1:100 dissolution. About 0.1 g of chocolate coating was dispersed in water (refractive index of water 1.333) at room temperature (25 8C). The sample was placed under ultrasonic dispersion for 3 min to ensure that the particles were independently dispersed. Size distribution was quantified. PSD parameters were obtained including specific surface area, Sauter mean diameter (D [3,2]) and mean particle diameter (D[4,3]). Three replicates for each experiment were performed.
Table 1. ment.
Tempering procedures used in the present experi-
Tabla 1. Procedimientos de temperado usados en el experimento actual. Tempering. Cooling to Stability point of Melting to completion crystallization temperature (8C) (8C) (8C) Type of coating 1 Dark coating 2 White coating 3 Milk coating
45–50 45 40
27 24 25
31 28 28–30
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CyTA – Journal of Food Rheological measurements The rheological behavior of the chocolate coating was examined on a strain-controlled rheometer Carri-Med CSL2 100 (TA Instruments, New Castle, DE, USA), using cone and plate geometry with 6 cm of diameter and angle of 18, coupled to a temperature control System (TA Instruments Thermal Analysis & Rheology). The solutions were characterized regarding their steady shear viscosity function, Z(g), using a unidirectional steady shear flow, at shear rates ranging from 0.3 to 300 s71. The rheological behavior of chocolate coating was characterized using steady shear measurements at 37 8C. The viscoelastic properties, storage modulus G0 and loss modulus G00 were determined through smallamplitude oscillatory shear flows, at frequencies ranging from 0.1 to 600 rad/s under linear viscoelastic conditions. From strain sweep runs, the upper limit of the linear viscoelastic zone was located at a strain of about 0.05. All samples were tested under various temperatures at least by duplicate. All measurements were carried out using rheological parameters obtained from the Power Law, K (consistency index) and n (flow index). On the other side, the oscillatory rheometry test were allowed and G0 and G00 were obtained. Electronic microscopy Chocolate samples tempering with shear and without shear were placed in aluminum pans and fixed with double face carbon tape (Bal-Tec, Fu¨rstentum Liechtenstein) and coated with gold at 10 mbar for 90 s (Polaron SC-7610, Fisson Instruments, CA, USA). Then, samples were examined and photographed with an electron microscope Leica Stereoscan S420i (Cambridge, England) at 500, 2000 and 40006. Statistical analysis A complete randomized factorial design was used. ANOVA and pair wise analysis test (Tukey a ¼ 0.05) was used. Three replicates for each experimental condition were done.
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The fitting of the experimental results given in Supplementary Figure 1 are illustrated by the continued lines and they represent quite well the experimental data. It must be noted that Equation (2) also represents the linear region of the functionality Z–g. Flow curves of chocolate coating samples are shown in Supplementary Figure 1. All experimental samples showed a Casson flow behavior (Equation (1)) (Table 2), a fact that agrees with the results reported by Sonwai and Mackley (2006). Supplementary Figures 2(A) and (B) were modeled well with the two parameters of the Power law Equation (2). Viscosity curves Z–g typically described by the well-known ‘‘Power-law’’ model: n1 Z¼K g ð2Þ where K is the consistency index (Pa.sn) and n is the fluid behavior index (dimensionless). From these results, the rheological parameters such as consistency index show changes in the function of the tempering process used, but the flow behavior index always showed a pseudoplastic behavior (n 5 1). The best models were chosen on the basis of the statistical parameter (determination coefficient) R2. All values showed values of R2 higher than 0.95. Higher values of consistency index (K) were shown for tempering 3. This process has lower thermodynamic force (lower DT, 15 8C); a lower consistency index was obtained for tempering process 1, which has higher thermodynamic force (higher D T, 23 8C). With regard to the influence of mixing conditions and tempering process, results presented tempering 1 process with the lowest K (22.38 Pa*sn). Higher values of K could be attributed to better structural arrangements or more fluid interactions. Results on the effect of shear in chocolate samples with and without tempering are shown in Supplementary Figure 3(A). Results from this figure showed that shear increased the storage module (G0 ); hence this behavior could be associated with a more solid structure of the sample, a faster solid particle formation (fats, sugar, or both), and thus the sample gets
Results and discussion Chemical analysis showed: 12.5% of carbohydrates, 2.6% of ash, 77.6% of fats, 5% of moisture, and 2.3% of protein. There is a higher amount of fat in chocolate coating samples in comparison with published reports for this type of product (30–45% cocoa butter) (Lida et al., 2007). Typical flow curves are shown in Supplementary Figure 1. In general, the chocolate coating behaves as a shear thinning fluid; this means that their viscosity decreases as the rate of deformation increases. Solid lines in Supplementary Figure 1 represent viscosity predictions from the Casson model given by Equation (1).
Table 2. Casson model flow parameters from chocolate coating samples. Tabla 2. Para´metros del modelo de Casson para las muestras de cobertura de chocolate.
Simple Without tempering Tempering 1 Tempering 2 Tempering 3
Casson viscosity (Pa s)0.5
Yield stress (Pa)
R2
2.133 1.350 1.922 2.400
3.946 3.304 4.474 4.082
0.988 0.989 0.980 0.982
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T. Quin˜ones-Mun˜oz et al. Results for tempering process 2 without shear showed a particle size of 6.91 mm + 0.07, but when shear was applied to the sample, the particle size increased up to 8.65 mm + 0.09. Similarly, for tempering process 3 the particle size increased from 6.96 mm without shear to 9.06 mm with shear, so more differences between sheared and non-sheared tempered chocolate samples were associated with higher thermodynamic forces (i.e. more D T). There is a linear relationship between particle size and the consistency index (r2 ¼ 0.92), thus lower viscosity corresponds to lower particle size. Chocolate may be viewed as a polydisperse suspension (sugar, cocoa, and/or milk solids) in a Newtonian fluid (fat phase) (Servais, Jones, & Roberts, 2002), but the presence of solids in continuous phase could change the material performance following a pseudoplastic behavior. Volumetric size (d[4,3]) of samples are shown in Table 3. Higher volumetric values were associated to samples without tempering and without shear, representing particles of higher volume and size, but when shearing was applied, volumetric size decreased to 10.33 mm. Therefore, shearing affected more volume size than surface size. Results for volumetric sizes from tempering processes 1, 2 and 3 showed a similar behavior than particle sizes, so tempering process affect volume characteristics of particles independently of the thermodynamic and mechanical forces applied. Data for specific surface areas showed opposite results to the behavior shown for particle and volumetric sizes. Therefore, when shear was applied to samples without tempering, the surface area increased, but in chocolate samples with tempering the influence of shear has diminished the surface specific area. Electronic micrographs are shown on Supplementary Figures 4(A) and (B) for samples with and without applied shear. From these figures, it seems clear that shear improves homogeneity of the structure as previous results have indicated (i.e., rheology and PSD data). Samples without shear were more heterogeneous having more particles included in the bulk; although the applied shear decreased the particle size probably by mechanical attrition, improving the final texture of the chocolate.
structured more quickly. The experimental tests are essentially nondestructive and can be interpreted in terms of changes on the molecular structure of the material. The effect of tempering process 1 (Supplementary Figure 3(B)) is a pseudo homogenization of the sample, because of the narrow distance between the curves in the rheogram for G0 and G00 . Similar curves are observed in Supplementary Figures 3(C) and (D); thus, when higher frequency is applied, samples were more homogeneous. Samples with applied shear showed an increase in the moduli G0 and G00 in comparison to the samples without shear. The rheological characterization revealed information on the resistance to deformation of these materials when subjected to shear loading. Nonetheless, the rheological behavior of the tempering processes 2 and 3 are influenced by characteristics of both tempering and shear conditions in chocolate coating. This type of experiment simulates real conditions that could be applied to the sample; thus the applied shear increased the sample structure associated to the liquid part (weakly structure) and to the solid part (more organized structure). This behavior is usually found in systems with high lipid content in liquid state (Toro-Vazquez and Gallegos-Infante, 1996) and in cocoa butter (Briggs and Wang, 2004). Particle size distribution results are shown in Table 2. Chocolate coating sample tempering 1 with static conditions showed more homogeneous distribution (one modal distribution) in comparison with tempering processes 2 and 3. The influence of shear is clear on the particle size. Samples without tempering and shear have higher values of particle size (9.17 mm), and samples with shear (150 rpm) without tempering have relatively lower particle sizes (8.0 + 0.08 mm), so the shear diminished particle size in function of mechanical attrition. In the case of tempering process 1 without shear, particle size was 7.08 + 0.11 mm, but when shear was applied, the particle size increased 10% (7.80 mm), which could be explained as a function of the tempering conditions.
Table 3.
Particle size results for chocolate samples.
Tabla 3.
Taman˜os de partı´ cula para las muestras de chocolate.
Treatment Without shear and tempering With shear (150 rpm) and without tempering Without shear and tempering 1 With shear (150 rpm) and tempering 1 Without shear and tempering 2 With shear (150 rpm) and tempering 2 Without shear and tempering 3 With shear (150 rpm) tempering 3
Mean diameter, d(0,5) (mm) 9.17 8.00 7.08 7.80 6.91 8.65 6.96 9.06
+ + + + + + + +
0.15 0.08 0.11 0.07 0.07 0.09 0.12 0.18
Volumetric diameter distribution, d(4,3) (mm) 17.76 10.33 8.17 10.16 8.24 11.68 8.65 11.43
+ + + + + + + +
1.23 0.95 0.89 0.92 0.73 0.89 1.65 0.67
Specific area (m2/g) 1.3537 1.5446 1.7062 1.5951 1.7629 1.5243 1.7796 1.5082
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CyTA – Journal of Food Conclusions This work reports the effect of the conditions of tempering and applied shear, modified flow parameters, and structure of chocolate coatings. The coatings without shear have shown higher viscoelasticity and elastic character than those with applied shear. Such differences were clearly shown at lower frequencies (o 0.1 rad/s), where the nature of the coating determines the rheological behavior. As expected, the effect of increasing the shear rate in the mixer on the rheological properties of the coating is to decrease the viscoelasticity and complexity of elastic character. The microscopy results indicate that all processes are bi-phase heterogeneous systems with different macromolecules-rich, suggesting that these two phases are compatible to a certain extent. Finally, there is a linear relationship between rheological parameters and particle size diameter of chocolate coatings. Supplementary material The supplementary material for this article is available online at dx.doi.org/10.1080/19476337.2010.482748. References Afoakwa, E.O., Paterson, A., & Fowler, M. (2007). Factors influencing rheological and textural qualities in chocolate – a review. Trends in Food Science and Technology, 18(1), 290–298. Afoakwa, E.O., Paterson, A., & Fowler, M. (2008). Effects of particle size distribution and composition on rheological properties of dark chocolate. European Food Research and Technology, 226(6), 1259–1268. Afoakwa, E.O., Paterson, A., Fowler, M., & Vieria, J. (2008). Modelling tempering behavior of dark chocolates from varying particle size distribution and fat content using response surface methodology. Innovative Food Science and Emerging Technologies, 9(4), 527–533.
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Attaie, H., Breischuh, B., Braun, P., & Windhab, E.J. (2003). The functionality of milk powder and its relationship to chocolate mass processing, in particular the effect of milk powder manufacturing and composition on the physical properties of chocolate masses. International Journal of Food Science & Technology, 38(3), 325–335. Bolenz, S., Thiessenhusen, T., & Schape, R. (2003). Fast conching for milk chocolate. European Food Research and Technology, 218(1), 62–67. 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. Chevalley, J. (1975). Rheology of chocolate. Journal of Texture Studies, 6(1), 177–196. Debaste, F., Kegelaers, Y., Liegeois, S., Ben Amor, H., & Halloin, V. (2008). Contribution to the modeling of chocolate tempering process. Journal of Food Engineering, 88(4), 568–575. Dhonsi, D., & Stapley, A.F.G. (2006). The effect of shear rate, temperature, sugar and emulsifier on the tempering of cocoa butter. Journal of Food Engineering, 77(4), 936–942. Hartel, R.W. (2001). Crystallization in foods. Gaithersburg, MD, USA: Aspen Publishers. Lida, F., Chida, M., Kasai, M., Sakanoshita, N., Sakurai, K., & Kamiwaki, T. (2007). Effect of fat type and content on palatability of chocolate. Journal of Japanese Society for Food Science and Technology, 54(1), 18–25. Servais, C., Jones, R., & Roberts, I. (2002). The Influence of particle size distribution on the processing of food. Journal of Food Engineering, 51(3), 201–208. Sonwai, S., & Mackley, M.R. (2006). The effect of shear on the crystallization of cocoa butter. Journal of The American Oil Chemists’ Society, 83(7), 583–596. Toro-Vazquez, J.F., & Gallegos-Infante, J.A. (1996). Viscosity and its relationship to crystallization in a binary system of saturated triacylglycerides and sesame seed oil. Journal of the American Oil Chemists’ Society, 73(10), 1237–1246. Wilson, L.L., Speers, R.A., & Tung, M.A. (1993). Yield Stress in molten chocolates. Journal of Texture Studies, 24(2), 269–286.
