Flow Behavior of Milk Chocolate Melt and the Application to Coating Flow
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Flow Behavior of Milk Chocolate Melt and the Application to Coating Flow SUKANYA WICHCHUKIT, MICHAEL J. MCCARTHY, AND KATHRYN L. MCCARTHY ABSTRACT: The rheological properties of chocolate, especially shear viscosity and yield stress, are important control parameters for enrobing processes in confectionery manufacture. The rheological parameters of molten milk chocolate were measured at 42°C during steady pipe flow using a magnetic resonance imaging (MRI) viscometric method. The experimental method combines shear rate values obtained from an MR velocity image and shear stress values obtained from an independent pressure drop measurement. The experimental factors were emulsifier type and emulsifier level. The rheogram data were fit by the Casson model to yield the Casson yield stress and plastic viscosity. The Casson yield stress ranged from 1.9 to 15.0 Pa; the Casson viscosity ranged from 6.0 to 14.6 Pa s as a function of emulsifier content. The rheological parameters were incorporated into a drainage theory model to predict coating thicknesses in the enrobing process. The model was solved numerically and yielded good approximations to the experimental values that were between 1.1 to 2.7 mm. Keywords: chocolate, viscosity, yield stress, enrobing, magnetic resonance imaging (MRI) Introduction ing flow. A considerable body of literature published in the mid- he use of enrobing technology is prevalent in the chocolate 1960s and early 1970s discusses liquid entrained on a flat surface Tmanufacturing industry. The molten chocolate or chocolate as the surface is withdrawn vertically from a bath of liquid. This coating flows in a sheet above a moving belt. The product is coat- type of physical situation is important to coating, cleaning, drain- ed as it moves through the sheet. For good quality and accurate ing, lubrication, and viscosity measurement. Tallmadge and Gut- weight control, the chocolate must have the correct viscosity (Ae- finger (1967) cite early applications and present an approach that schlimann and Beckett 2000). incorporates viscous, gravitational, and surface tension forces in Extensive rheological studies of molten chocolate have been the different regimes. Representative work includes Van Rossum performed using rotational viscometry to describe flow properties. (1958), Gutfinger and Tallmadge (1965), and Groenveld (1970). Prior to 2000, the Casson model was recommended as a standard Drainage was considered a special case of the withdrawal pro- method by the Intl. Office of Cocoa, Chocolate, and Sugar Confec- cess. The unsteady state flow process of a flat plate that has been tionery (IOCCC) to quantify rheological properties of the fluid choc- totally withdrawn from a liquid bath is governed by only viscous and E: Food Engineering & Physical Properties olate. The Casson parameters are the Casson plastic viscosity and gravitational forces. Film thickness as a function of position and the Casson yield stress; these values range from 1 to 20 Pa s and 10 time was presented in a straightforward manner for Newtonian flu- to 200 Pa, respectively (Chevalley 1975, 1991, 1999). ids by Bird and others (2002) as fluid draining from tank sides. In a multi-laboratory study, Aeschlimann and Beckett (2000) Gutfinger and Tallmadge (1965) presented the draining analysis for discuss a sequence of tests and recommendations to improve the non-Newtonian fluids. Specifically, the 3-parameter Ellis model was standard method for measuring chocolate viscosity. These research- evaluated. Depending on the values of the parameters, this mod- ers recommend reporting shear stress values at specified shear rates, el reduced to the Newtonian model, the power law model, or the for example, 2, 5, 10, 20, and 50 s–1. In addition, the chocolate must Bingham plastic model. Insightful comments were made regarding be completely melted prior to viscosity measurements, that is, 40°C. the behavior of power law fluids as compared with Newtonian flu- The IOCCC standard for the measurement of viscosity of chocolate ids. A change in the power law consistency index, K, has a more products continues to recommend rotational viscometry using a pronounced influence on the film thickness than the change of concentric cylinder probe (IOCCC 2000). The Deutsches Instite für viscosity in the Newtonian case. In addition, the change in film Normung (DIN, standard developed by the German Institute for thickness with time and position is more pronounced for a Newto- Standardization) bob is recommended to minimize instrument er- nian fluid than a power law fluid. This difference is primarily due rors. Other instrument errors, such as wall slip, are recognized po- to the power law fluid being more uniform in thickness than the tential reasons for inconsistent measurements, especially at low Newtonian one. This effect becomes more pronounced with de- shear rates (Beckett 2001; IOCCC 2000). creasing values of the flow behavior index, n. The success of the enrobing process is dependent both on the This work utilized a magnetic resonance imaging (MRI) technique rheological characterization of the chocolate and the physics of coat- as an integral part of tube viscometry and extends the technique that has been reported by McCarthy (1994), Powell and others MS 20040648. Submitted 9/23/04, Revised 11/8/04, Accepted 11/20/04. Author (1994), Arola and others (1997), and Yoon and McCarthy (2002, Wichchukit is with Dept. of Biological and Agricultural Engineering. Au- thors McCarthy M.J. and McCarthy K.L. are with Dept. of Food Science and 2003). The objectives of this study were to evaluate the rheological Technology and Dept. of Biological and Agricultural EngineeringOne Shields properties of molten milk chocolate and to incorporate the measured Ave., Univ. of California, Davis, CA 95616. Direct inquiries to author rheological properties into an unsteady state draining analysis to McCarthy (E-mail: [email protected]). predict and control coating thickness during the enrobing process. Flow behavior of milk chocolate melt . Materials and Methods Table 1—Apparatus geometry and MRI experimental parameters Apparatus geometry MRI parameters Test samples Tube radius, R (mm) 4.9 Pulse repetition time, TR (ms) 400 The test material was milk chocolate obtained directly from the Tube length, L (m) 1.25 Echo time, TE (ms) 63 manufacturer in 5 gallon containers (Hershey Foods Corp., Hershey, Reservoir radius (mm) 57.5 Number of phase encoding 32 Pa., U.S.A.). Although chemical analysis was not performed for these Reservoir height (mm) 130 Number of frequency encoding 64 particular samples, typical composition by percent for this product Number of averages 16 Field of view (mm) 60 is cocoa mass 11.8%, whole milk powder 19.1%, sugar 48.7%, added Velocity sweep width (mm/s) 188.9 cocoa butter 20%, for a total fat content of 31.5% (Jackson 1999). The milk chocolate was manufactured without added emulsifier. Five chocolate formulations were used: milk chocolate with no added emulsifier (the control), milk chocolate with 0.1% soy lecithin, milk ity image was acquired at a position 0.50 m downstream from the chocolate with 0.3% soy lecithin, milk chocolate with 0.1% synthetic reservoir, which ensured fully developed flow. The MR velocity lecithin (YN), and milk chocolate with 0.3% YN. The emulsifiers were images were obtained using a SMIS NMR spectrometer (Surrey added by weight during the melting and mixing process. The density Medical Imaging Systems, Surrey, U.K.) connected to a 0.1 Tesla of the milk chocolate melt was 1270 kg/m3. electromagnet, corresponding to 4.2 MHz for 1H resonance frequen- Magnetic resonance imaging viscometry cy. Unshielded gradient coils produced orthogonal gradients with maximum gradient amplitudes of Ϯ 2 Gauss/cm. A pulsed gradient The experimental apparatus consisted of a temperature-con- spin-echo pulse sequence was applied to acquire the velocity pro- trolled capillary viscometer connected to a 1818-mL stainless steel files (McCarthy 1994; Arola and others 1997). The apparatus geom- sample reservoir (Figure 1). During each experimental run, the test etry and MRI experimental parameters are given in Table 1. For the fluid from the reservoir was pressure driven through a straight MR image, 32 phase encoding steps were used to characterize the length of 9.8 mm inner dimater glass tubing (Pyrex®, Corning, Inc., z component of the velocity and 64 frequency encoding steps along Big Flats, N.Y., U.S.A.) by the downward motion of a piston moving the x direction read out the radial position (r) of the volume ele- at 0.2 mm/s. The cross-sectional area of the piston was 1.04 × 104 E: Food Engineering & Physical Properties ments. The resulting matrix was zero filled to 64 × 128 and a two- mm2 and spanned the cross-section of the fluid reservoir. The con- dimensional Fourier transformwas performed. The measurement tact area between the piston and the reservoir surface was a lubri- time was on the order of 1 min and reflects multiple data acquisi- cated O-ring. The piston was connected to an actuator/control sys- tions. Data analysis of the MR velocity image was performed to tem fabricated to provide a 0.30-m stroke length and rated for 2200 characterize flow behavior using MatLab7.0.0 (R14) software (The N (Electric Cylinder Model EC2, IDC, Salem, N.H., U.S.A.). MathWorks Inc., Natick, Mass., U.S.A.). Fluid velocity at each radial A pressure measurement was acquired upstream of the imaging position was determined by the position of the maximum signal region using a pressure transducer (Model PX771-100WD1, Ome- intensity in each row of the image matrix. ga Engineering Inc., Stamford, Conn., U.S.A.). The pressure differ- To evaluate shear rate from the velocity profile, the MRI velocity ence was determined by the difference between measurement and data were fit with an even-order polynomial; the global fitting error ambient, characterizing the pressure drop over the length, L, of 1.25 for the polynomial was less than 2%.