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Influence of product composition and operating conditions on the unsteady behavior of hard candy cooling process Journal of Food Engineering 101 (2010) 409–416 Contents lists available at ScienceDirec[.]
Journal of Food Engineering 101 (2010) 409–416 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng Influence of product composition and operating conditions on the unsteady behavior of hard candy cooling process M Agustina Reinheimer *, Sergio Mussati, Nicolas J Scenna INGAR-CONICET – Instituto de Desarrollo y Diseño – Avellaneda 3657, S3002GJC Santa Fe, Argentina a r t i c l e i n f o Article history: Received April 2010 Received in revised form 21 July 2010 Accepted 22 July 2010 Available online 27 July 2010 Keywords: Unsteady cooling process Hard candy production process Parametric simulations a b s t r a c t On large industrial scales, the cooling stage in the production process of hard candies is one of the most critical unit operations The main problems affecting final hard candies quality as regards the cooling process are: deformation, fragility and aggregation The main operating conditions of the cooling stage are temperature and velocity of cooling air as well as the residence time of candy inside the tunnel The objective of this work is to study the influence of process operating conditions and candy composition on the unsteady behavior of the cooling process of hard candies to improve final product quality The study is conducted by using a simple mathematical model which was implemented and solved by using gPROMS (general Process Modeling System) A detailed discussion of results is presented through several examples Ó 2010 Elsevier Ltd All rights reserved Introduction In many food technological applications, final product quality highly depends on the unsteady convective heat transfer occurring between a fluid and the solid food Specifically, the temperature transient in drying, cooling and heating processes on food products has to be strictly controlled because it causes serious quality problems On the other hand, food composition must be also considered during process design because it affects the process operation mode For instance, in conduction heat transfer processes, water content plays an important role In the last years, the application of mathematical programming techniques, computational fluid dynamics (CFD) and other advanced process modeling tools for simulation and optimization have drawn growing attention in the food industry (Zorrilla and Singh, 2003; Zorrilla et al., 2003; Zorrilla and Rubiolo, 2005; Migliori et al., 2005; Sun, 2007; Bona et al., 2007; Uyar and Erdog˘du, 2009; Chanteloup and Mirade, 2009; Betta et al., 2009; Jimenez Marquez et al., 2009; Shim et al., 2010) Certainly, they are valuable tools for understanding the involved complex physical processes and designing process equipment in order to ensure product safety and quality However, from a CFD perspective, multi-dimensional models are quite complex to be used in many cases, especially for large optimization problems involving a high number of decision variables, which may lead to high computational time and great effort due to the nature of the * Corresponding author Tel.: +54 342 4535568; fax: +54 342 4553439 E-mail address: mareinheimer@santafe-conicet.gov.ar (M.A Reinheimer) 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd All rights reserved doi:10.1016/j.jfoodeng.2010.07.029 iterative resolution algorithm CFD applications are very useful to simulate systems with complex shapes On the other hand, nonlinear regression techniques are being applied to determine diffusion coefficients and kinetic parameters in (un)steady states of heated and cooled food products which are essential for accurate estimates of food processing and safety (Gerla and Rubiolo, 2003; Miconnet et al., 2005; Rodriguez-Nogales, 2006; Dolan et al., 2007; S ß imsßek, 2007; McLeod et al., 2009; Bower, 2009; Gibert et al., 2010) This paper is focused on a practical and industrial application The goal of this research work is to analyze the influence of product composition and operating conditions on the unsteady behavior of the cooling process of hard candies in order to improve the final product quality The main problems affecting hard candies’ final quality as regards the cooling process are: deformation due to excessive temperature at the tunnel exit; fragility to undergo the subsequent wrapping step because of sharp cooling; sticky candies due to inadequate residence time of candy inside the tunnel The effect of temperature and velocity of the cooling air, thermal conductivity and candy size on the cooling efficiency is studied by using a classic model of unsteady state 1-D heat conduction problem (Ozisik, 1994) and parametric simulations Results obtained from different parameter values are presented and discussed The resulting model is implemented and solved by using gPROMS (general Process Modeling System) gPROMS is a general purpose modeling, simulation and optimization system software The study of the transient temperature behavior in hard candies throughout their production had not been addressed until now 410 M.A Reinheimer et al / Journal of Food Engineering 101 (2010) 409–416 Nomenclature Cp D h k R r T DT t v specific heat (J/kg °C) sample diameter (m) heat-transfer coefficient (W/m2 °C) thermal conductivity (W/m °C) sample radius (m) variable radius (m) temperature (°C) temperature variation (°C) time (s) cooling medium velocity (m/s) Dimensionless numbers Bi Biot Nu Nusselt Pr Prandtl Re Reynolds Greek symbols thermal diffusivity (m2/s) density (kg/m3) viscosity (N s/m2) h residence time (s) a q l Subscripts a air c candy inl inlet This work is the first step of a challenging project, which consists of the model-based optimization of a full-scale facility to manufacture hard candies Materials and methods 2.1 Hard candies Hard candies are classic examples of products in the glassy state Apparently, they are solids, but actually, they are supercooled liquids in a non-crystalline state (Cardoso and Abreu, 2004) Hard candies could be considered as a highly viscous liquid This property interferes in the crystal-forming process Crystallization is an undesirable process during manufacture and storage of hard candies, which occurs if a nuclei crystal (crystal seed) is present (Jackson, 1995) A minimum water content level is required to produce hard candies with an appropriate shelf life Final water content level in candies depends on the cooking temperature (higher than 140 °C), vacuum pressure and the ratio of sucrose and corn syrup Water content may range from 1.5 to percent by weight (Frey, 1967; Childs, 1972; Boursier et al., 1985; Vink et al., 1988; Isse et al., 2008) This limited range is justified in terms of quality and processing aspects Certainly, higher water content can improve cooling efficiency but may induce stickiness not only during this unit operation but also at the wrapping and storage stages (Isse et al., 2008) Fig Candies basic production flow sheet After that, the forming process is achieved The hard candy at 85 °C is shaped by cutting up a dough roll in a stamp-forming one-process The formed hard candies are then cooled in a conventional cooling tunnel Finally, hard candies are individually wrapped Temperatures ranging from 28 to 40 °C are recommended for wrapping stage in order to avoid stickiness problems 2.2 Production process of hard candy 2.3 Description of cooling equipment Fig schematically shows the required unit operations for hard candy production process of hard candy During the manufacturing process, granulated sugar, glucose and water are heated in a steam heated vessel for the sugar to be dissolved and then transferred to a batch vacuum cooker and boiled to remove almost all water Thus the syrup is turned into a candy dough The cooker consists of flash and vacuum chambers Here, cooking temperatures are higher than 140 °C at atmospheric pressure Then the cooker pressure is modified by applying a vacuum of at least 700 mm Hg during the last few minutes of the evaporation process in order to drive off excessive water and provide the hard candy with the desired humidity The last cooking substage is the addition and mix of additives (acids, flavoring and coloring agents) The next step is the dough tempering, where the candy mass is driven to a tempering stainless steel belt which is water refrigerated and then it is mixed to homogenize its temperature Fig illustrates the cooling tunnel The tunnel has two air ducts (entrance and exit) The incoming air flow is regulated by a deflector [D] As shown in Fig 2, the tunnel is composed of three conveyor belts [CB] which are mechanically driven by an engine connected to an adjustable frequency drive [AFD] to vary the residence time of candies While the candies are moving along the tunnel, they get in contact with cooling air [CA] which flows parallel to the belts From the product quality point of view, the temperature and velocity of cooling air as well as the residence time of candies inside the tunnel play critical roles in the cooling efficiency For example, high air velocity may lead to a non-uniform temperature profile which increases the product fragility and cause misshapen candies, and their consequent rejection On the other hand, higher candy temperatures at the end of the tunnel and incorrect residence times lead respectively to deformation and aggregation of 411 M.A Reinheimer et al / Journal of Food Engineering 101 (2010) 409–416 Fig Cross section cooling tunnel candies Therefore, the operational mode of the cooling tunnel is crucial for the final candy quality Certainly, higher product quality is obtained when the tunnel is operated in such a way that the difference of temperature between the center and the surface of the candy is minimized as much as possible (uniform transient temperature behavior) ensuring an appropriate temperature for the wrapping stage (28–40 °C) Higher temperatures than 40 °C lead to stickiness or deformation problems (non-conforming products) On the other hand, lower temperatures than 40 °C lead to higher residence time requiring an ‘‘infinite” length of the cooling tunnel Finally, the thermal-physical properties of products and their composition also influence the product quality – – – – 2.4 Problem statement The main objective of this work is to analyze the influence of composition and operating conditions on the cooling behavior of hard candies by parametric simulations Certainly, this study aims to determine the operating process parameter (velocity and temperature of cooling air, residence time and candy size) that lead to uniform unsteady temperature distributions inside the candy In addition, the effect of water content on the transient behavior will also be studied – – with the temperature can be neglected for the temperature range reported in this work Variations of the specific heat capacity and density of candy with the composition are neglected This assumption is based on calculations between 1% and 5% (w/w) of water content Internal temperature variations in the radial direction are contemplated There is no moisture loss Due to the low water content of hard candy (65%), water loss hardly occurs during candies cooling According to this, moisture intake from air is also negligible, due to the fact that air humidity was monitored and profiles could be considered as constant between the values at tunnel entrance and exit Changes in air temperature and humidity are small enough to produce a negligible effect on the thermal-physical properties of the air Based on this assumption, the properties of air flow are calculated at the conditions of the dry air entering the system The external surface of each sphere is supposed to be surrounded by this cold air with constant properties This assumption is based on the [0.5–3.0 m/s] range of air velocity (Zou et al., 2006) The convective heat-transfer coefficient is computed as the area averaged value of the local heat-transfer coefficient (Becker and Fricke, 2004) 2.5 Assumptions and mathematical model In this section, assumptions and the set of equations describing the model of the transient heat transfer in the cooling process of hard candies are presented The following basic hypotheses have been assumed to derive the mathematical model: 2.5.1 Mathematical model Based on the above assumptions, the energy balance of the unsteady heat transfer process in hard candies can be formulated as follows: @T ðr; tÞ @ T @T ẳ 2ỵ @t @r r @r ac – Candies are considered as homogeneous and isotropic spheres – Choi and Okos models (Choi and Okos, 1986) were used for thermo-physical property estimations – Water and carbohydrate contents are considered as the main components of candy Ash content is maintained constant due to its low value – An average temperature (54 °C) between the candy temperature at the tunnel entrance (80 °C) and exit (28 °C) is assumed for calculations In fact, the thermo-physical property variations 0