Chocolate demoulding and effects of processing conditions

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Journal of Food Engineering 98 (2010) 133–140 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng Chocolate demoulding and effects of processing conditions Esther L. Keijbets a, Jianshe Chen a,*, Joselio Vieira b a b School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT, UK Nestlé Product Technology Centre, York YO91 1XY, UK a r t i c l e i n f o Article history: Received 31 March 2009 Received in revised form 7 December 2009 Accepted 11 December 2009 Available online 16 December 2009 Keywords: Chocolate Adhesion Moulding Demoulding Crystallization a b s t r a c t Traditional chocolate manufacturing relies largely on the experience and skill of the chocolatier. Nowadays, with the replacement of manual processes with automated equipments, it is increasingly important to apply right processing conditions and controlling parameters, such as the time, the temperature, the moisture content, the relative humidity of the surrounding air, etc. This study investigated the influences of these factors during the moulding and cooling stage of chocolate manufacturing process on the ease of demoulding. Adhesion of chocolate to the polycarbonate mould surface was used as a measure for the demoulding properties, and was determined as the force required to separate a flat mould surface from the solidified chocolate sample. The results demonstrated that processing parameters, like temperature, contact time, and the relative humidity of the surrounding environment, have a significant impact on chocolate crystallization and solidification processes and on the adhesion of chocolate to a mould surface. Experimental findings from this work confirmed observations made during commercial chocolate manufacturing. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The knowledge of chocolate manufacturing has evolved hugely since the introduction of plain chocolate tablets in 1847 by Joseph Fry, the first solid milk chocolate in 1876 by Daniel Peter, and the production of smoother and better tasting chocolate in 1880 by Rodolphe Lindt (Beckett, 2008; Nelson, 1999). Since then, the chocolate manufacturing process has undergone various changes for the purposes of either improved oral experience (flavour and texture) or increased productivity to meet increasing demands for chocolate products. With ever increasing product output, it is essentially important to have a good understanding of the influences of chocolate manufacturing process, as well as the processing conditions, on the quality of the final product. An important example in this case is the adhesion of products during processing, which is a substantial ongoing problem in the food industry in general. During chocolate manufacturing the process of demoulding is particularly prone to adhesion problems, leading to surface defects, production losses and increased processing costs due to equipment cleaning. A basic overview of the main chocolate manufacturing process has been given by Beckett (2008). The critical stages of the chocolate manufacturing process are the mixing of the main ingredients to form a dispersion of cocoa solids (particles) and sugar crystals in a continuous fat phase, consisting of fat crystals and liquid fat * Corresponding author. Tel.: +44 113 3432748; fax: +44 113 3432982. E-mail address: j.chen@food.leeds.ac.uk (J. Chen). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.12.019 (Walstra, 1996), and the crystallisation of the cocoa butter in the correct polymorph (Form V). The aim of the tempering process is to develop the correct polymorphic form, and in order to do so the chocolate is cooled from 45 °C to approximately 30 °C (depending on the type of chocolate, e.g. milk or dark chocolate). After the tempering stage, the liquid chocolate is deposited into polycarbonate moulds and cooled, as is visualized in Fig. 1. Objective of this cooling phase is the solidification the fat phase of the tempered chocolate mass with the correct crystallization, as this will lead to contraction of the chocolate and easy removal of the solidified chocolate from the mould during the subsequent demoulding process. The detrimental effects of moisture on chocolate viscosity and the ease of processing have been observed. In order to prevent the chocolate taking up moisture, Beckett (2001) advised to keep the equilibrium relative humidity (ERH) at around 35–40% during manufacturing. During moulding, chocolate is deposited at a temperature of approximately 30 °C and is cooled down to below 20 oC for solidification. Important for the cooling and solidification stage is to prevent the formation of (moisture) condense on the chocolate surface, which induces sugar bloom. A general recommendation is to keep the cooling temperature above the dew point to prevent against moisture condensation (Beckett, 2008). Another important reason to avoid too low cooling temperature is the set of cocoa butter in the wrong crystalline form causing problems in demoulding. The formation of a larger number of polymorphs with a lower melting point results in a chocolate with a lower viscosity (softer) at room temperature and less contraction during Author's personal copy 134 E.L. Keijbets et al. / Journal of Food Engineering 98 (2010) 133–140 Moulding, cooling and demoulding of dark chocolate Tempered dark chocolate 30-32°C Deposition of chocolate mass into polycarbonate moulds Moulds are preconditioned Tmould ≈ 28 - 30°C Vibration, to release air and spread chocolate in moulds Cooling of moulds with dark chocolate mass Tcool ≈ 20°C tcool ≈ 15 min. Demoulding of solidified chocolate tence of a porous structure in chocolate and debated whether fat migrates through its pores due to diffusion or capillary flow. According to them, chocolate has a microstructure of a particulate nature. Therefore, interparticle passages and connected pores exist between the groups of fat-coated particles, in which the liquid cocoa butter migrates under capillary forces. Chocolate deposits on a mould surface after demoulding are believed to be caused by an imbalance between the adhesion force (between the chocolate and the mould) and the cohesion force within the chocolate matrix. The authors previously demonstrated that the surface energy of mould materials had a huge influence on chocolate–mould interactions, and that a mould material with a surface energy less than 30 mN m–1 could improve the easiness of chocolate demoulding (Keijbets et al., 2009). The aim of the current study was to investigate the effect of processing conditions on the level of chocolate adhesion to the mould surface during demoulding, especially, the processing conditions applied during the moulding and cooling stages of the chocolate manufacturing process, e.g. mould and cooling temperature, contact time and relative humidity (RH). It is hoped that findings from this work will enhance our understanding of the interactions taking place at the chocolate–mould interface. Dark chocolate (moulded) product 2. Materials and methods Fig. 1. A schematic overview of the chocolate moulding, cooling and demoulding process. solidification, as a result of which the solidified chocolate does not easily come away from the mould (Tewkesbury et al., 2000). Microstructure and composition in relation to processing and sensory perception have been the predominant topics of chocolate researches. In their review of ingredient interactions, Awad and Marangoni (2006) stressed the importance of processing conditions in relation to the texture of chocolate. It is generally acknowledged that the key determining factors for the physical characters of chocolate, e.g. texture, hardness, smoothness and stability, are the chemical composition and solid-state structure, which refers to the 3-dimensional network created as a result of the crystallization of cocoa butter. The chemical composition of chocolate is mainly determined by the recipe. As described by Afoakwa et al. (2007) the addition of milk fat to milk chocolate results in a lower melting point, a slow setting or solidification and a softened texture. Liang and Hartel (2004) showed that free fat levels present within different milk powders affected the processing behaviour as well as the physical and organoleptic properties of the final milk chocolate product. The addition of emulsifiers to the chocolate recipe is furthermore known to affect chocolate’s rheological properties, sensitivity to moisture and temperature, and tempering behaviour. Emulsifiers are also known to influence the crystallisation behaviour of cocoa butter, and consequently affect the solidification behaviour. For example, Schantz et al. (2005) showed that the use of Polyglycerol Polyricinoleate (PGPR) instead of lecithin as an emulsifier reduced the seed-forming and crystallisation times in both dark and whole milk chocolate. However, the exact microstructure of chocolate is still not exactly understood. Improved knowledge of the microstructure is required to increase the understanding of oil and fat migration mechanisms in chocolate and chocolate coatings. Loisel et al. (1997) used Mercury porosimetry to study the microstructure of dark chocolate, based on the assumption that chocolate has a porous matrix partly filled with liquid cocoa butter fractions. The amount of empty spaces depends on the condition of the chocolate, e.g. well-tempered or over-tempered, as well as on the amount of cocoa butter present. Aguilera et al. (2004) also assumed the exis- 2.1. Materials Polycarbonate (Barkston Plastics Ltd., Leeds, UK) was chosen as solid substrate because of its commercial application as a mould material in chocolate manufacturing. The respective surface finish of the polycarbonate substrate was obtained by abrasion, using a P600 (3 M™, MarineWare, Southampton, UK) wet or dry abrasive paper and an abraded surface plate. The polycarbonate substrates were cleaned with boiling, distilled water (Millipore, Watford, UK) and dried using compressed air prior to adhesion force determination. Dark chocolate samples used for the surface adhesion determinations were supplied by the Nestlé Product Technology Centre, York (UK). Samples were tempered using commercial methods. The final product contained 52% cocoa solids. 2.2. Determination of chocolate-mould adhesion Experimental determination of chocolate adhesion to the mould surface was performed on the TA-XTplus Texture Analyser (Stable Micro Systems, Surrey, UK), using a fixture specifically developed for this work. The surface adhesion force (or stickiness) was measured by pulling the flat polycarbonate probe off a solid chocolate sample, as shown schematically in Fig. 2, imitating the forces involved in the demoulding process of commercial chocolate bars. A more in-depth description of the experimental determination of chocolate adhesion has already been given elsewhere (Keijbets et al., 2009). liquid chocolate pull off at a constant speed mould probe (solid surface) 60 min. 15 oC sample holder solid chocolate Fig. 2. A schematic diagram of the procedure and conditions of the surface adhesion measurements. Author's personal copy E.L. Keijbets et al. / Journal of Food Engineering 98 (2010) 133–140 The experimental adhesion force, Ea, is defined as the force per surface area, Ea ¼ adhesion force ; surface area ð1Þ where the adhesion force is the peak separation force measured by the probe pulling test and the surface area is the area of contact between the polycarbonate probe surface and the chocolate surface. 2.2.1. Chocolate processing conditions In this study effects of different processing conditions on the adhesion force were investigated under varying conditions of cooling temperature, probe or mould temperature, contact time and relative humidity. The experimental set-up outlined in Fig. 3 was developed specifically for this study. A sealed cabinet (9) with a dimension 145 mm (W)  130 mm (D)  105 mm (H) connected with a Peltier heating/cooling unit (7) (Stable Micro Systems, Surrey, UK) made it feasible to have a controlled experimental temperature between 0 and 50 °C. The contact between chocolate (11) and polycarbonate surface (12), i.e. the chocolate–mould interface, was created at a temperature of approximately 30 °C, after which the temperature was normally adjusted to 15 °C using a standardized temperature profile. The set-up was left without disturbance for 60 min, ensuring complete solidification of the chocolate sample. In order to vary the cooling temperature, the temperature of the cabinet was decreased to 20, 15, 10, 5 or 0 °C, respectively. Relative humidity of the air (0% RH) and air flow rate (10 l/min) were kept constant. Experimental adhesion force was measured after 60 min of cooling at each specified cooling temperature. For variations in mould temperature the thermal equilibration step of chocolate and mould probe of 30 min, before creating contact, was omitted. The respective mould surface temperatures of 20, 0, 10, 20, 30 and 50 °C were obtained by cooling the polycarbonate mould probe in a fridge or freezer or heating in an oven, respectively. The contact time refers to the time that the chocolate–mould interface was in place, i.e. from the moment of contact/interface creation, until the time of probe separation controlled by the Texture Analyser (13). A semi-dynamic cooling process was used, with a flow of cold air (0% RH) being supplied at a rate of 10 l/min. The 1. air supply 2. pressure valve 3. dry column 4. water column 5. valve 6. water bath 7. cooling/heating unit 135 temperature of the air was decreased from 30 °C, the moment of interface creation, to 15 °C in 20 min, after which the air temperature stayed unchanged. During this study the evolution of surface adhesion with contact time was followed from 0 to 450 min. The relative humidity was controlled by using compressed air (1 and 2) that was passed through a dry (0% RH) (3 and 5) and/ or wet column (4), where the air bubbling through distil water to gain 100% RH. By using different ratios of wet and dry air, the relative humidity of the air circulation in the Peltier cabinet was controlled. After mixing the air, it was passed through a copper coil fully merged in a water bath (6) to obtain the designed temperature, before entering the Peltier cabinet. Using the water bath the temperature of the air mixture was adjusted according to the cooling profile used to solidify the chocolate. In general, the temperature profiles used were comparable to those used in industry. The experimental set-up consisted of a 30 min pre-conditioning or equilibrium, during which the mould surface was pre-heated to 30 °C at varying RH conditions (range: 0–100% RH). After that the pre-conditioned mould was placed on top of the liquid chocolate surface and the sample was solidified for 60 min using the standard cooling process with a cooling temperature of 15 °C. 2.2.2. Chocolate and mould parameters For all measurements the temperature and relative humidity in the Peltier cabinet were measured at the time of creation of the chocolate–mould interface and just before determining the experimental adhesion force using a digital thermometer/hygrometer (Fisherbrand, Loughborough, UK). Additionally, a set of parameters was measured to determine the effect of different processing conditions on both the chocolate and mould surface:  Hardness: The solidified chocolate samples were analyzed for hardness by a method described by (Liang and Hartel (2004)) using the Texture Analyser (TA-XTplus, Stable Micro Systems). A 2-mm cylindrical stainless steel probe was used to penetrate the solidified chocolate sample at a constant speed of 0.1 mm/ s to a depth of 5 mm. The maximum force (N) obtained during this penetration was taken as a measure of the hardness.  Chocolate residue weight: The chocolate residue weight or the left-over residues at the probe surface, expressed as the amount of chocolate per unit surface area (mg m2), refers to the amount of chocolate left on the mould surface after separation tests.  Contact angle: The contact angle of a water drop placed on the mould surface after the separation tests was assessed using the sessile drop method. For each mould surface the measurement was done in duplicate. The aim of this test was to establish the change in surface character of the mould after contacting with chocolate, e.g. due to the migration or adsorption of fat from chocolate to the mould surface.  Gloss: The gloss or light reflection of both chocolate and mould surfaces was measured at 3 angles, 20, 60 and 85° using a Tri-GLOSSmaster (Sheen Instruments, Surrey, UK). The glossmaster measures specular reflection, which is the capacity of a surface to reflect light. The aim of this test was to determine the deposition of (fat) residues on the mould surface, as well as possible changes in chocolate gloss. 8. ventilator 9. Peltier chamber 2.3. Determination of moisture uptake 10. sample holder 11. chocolate sample cell 12. adhesion probe Fig. 3. Experimental set-up to measure the effect of processing conditions on the surface adhesion force. Moisture uptake by the polycarbonate mould surface at varying RH was determined using a Cisorp water sorption analyser (CI Electronics Ltd., Salisbury, UK). The method of analysis of the Cisorp can be defined as a gravimetric method at an ambient pressure. Basically, it consists of three separate chambers: a weighing chamber, Author's personal copy E.L. Keijbets et al. / Journal of Food Engineering 98 (2010) 133–140 3. Results and discussion 3.1. Contact time In industry, liquid tempered chocolate (28–32 °C) is deposited into pre-heated polycarbonate moulds (25 °C), which are subsequently placed in a cooling tunnel. These cooling tunnels can be divided into different sections, enabling the use of temperature profiles. The first section uses gentle cooling conditions, to quickly set the chocolate. Whilst the chocolate matrix is still liquid, there is the possibility of cocoa butter migrating to the surface, resulting in the presence of fat bloom on the surface of the chocolate bar. Section 2 applies the coldest air flow (12–13 °C), resulting in the largest amount of latent heat being removed. The 3rd and final section applies again a relatively gentle cooling, to limit the temperature difference between the chocolate sample and the air in the packaging area. A cold chocolate surface may otherwise result in the condensation of water vapour, causing the formation of blemishes and/or sugar bloom. On average, it takes 40 min for a chocolate sample to set, if a cooler with a constant air flow and a temperature of 10–15 °C is used. Fig. 4A indicates that, at t0 (zero contact time), the surface adhesion is zero, but the chocolate residue weight is high. Due to the liquid character of the chocolate at this time, the chocolate should be in close contact with the probe or mould surface and may form a bridge during separation. With time the liquid character of the chocolate declines, as crystallization and solidification processes continue, resulting in a clean separation after approximately 60 min. The adhesion force, meanwhile, increases linearly with time until a contact time of about 90 min, after which the adhesion force appears to become stabilised at around 430 kPa (kN m2). In Fig. 4B a plot is shown of the probe separation force together with the hardness of chocolate samples at respective contact times. It is obvious that both parameters follow the same trend. The crystallisation taking place during cooling is basically a transformation of a significant part of the liquid fat into solid fat crystals. Depending on the stage of crystallisation, there are different quantities of solid and liquid fat present, which are responsible for the differences in hardness observed during this study. The results indicate that with the cooling conditions used during this study a contact time of 60 min is required to form a relatively strong crystal net- -2 1600 1400 400 1200 1000 300 800 200 600 400 100 Chocolate residue weight surface adhesion 0 0 100 200 300 400 200 0 500 Contact time [min] B 550 50 500 45 450 40 400 35 350 30 300 25 250 20 200 Hardness [N] where mRH is the amount of moisture uptake after 30 min at a specific RH, and msat is the amount of moisture uptake at saturation. -2 500 ð2Þ chocolate residue / mould surface [g m ] 1800 -2 mRH Moisture uptake ½% ¼  100; msat A Surface adhesion (x 1000) [N m ] a balance chamber and a humidifier. The humidifier is responsible for the temperature conditioning and for supplying a wet air flow, by passing dry gas (nitrogen or air) at reduced pressure through a cavity filled with distilled water. This wet flow is mixed with a dry flow, after which it is fed to the weighing chamber. A calibrated humidity probe present in the weighing chamber is used to monitor the RH. Two microbalances are placed in the balance chamber, with a thin rod being suspended into the weighing chamber, to which a sample holder is connected. A counterweight is used to mechanically balance the weighing arm. A microprocessor is responsible for the conversion of the forces required to hold the balance arms horizontal into weight readings (Mangel, 2007). In order to determine the moisture sorption by a polycarbonate surface during the pre-moulding stage, a polycarbonate mould piece was placed in the Cisorp weighing chamber for 30 min at a specific RH and a temperature of 30 °C, and the change in weight per minute was recorded. The moisture uptake by the polycarbonate mould surface was calculated as a percentage compared to the moisture uptake responsible for saturation, e.g. 100%, Surface adhesion (x 1000) [N m ] 136 15 150 10 100 50 surface adhesion hardness 0 0 100 200 300 400 5 0 500 Contact time [min] Fig. 4. (A). Surface adhesion of dark chocolate (j) and the amount of residues after probe separation (s) as a function of contact time. (B) Surface adhesion of dark chocolate (j) and the hardness of the solidified chocolate samples (s) as a function of contact time. work so that the chocolate will be cohesive enough to withstand the separation force of demoulding. In general, crystallization and solidification are very important factors in the surface adhesion and demoulding. However, one may conclude that there is probably no need to wait for a complete solidification at the time of demoulding. In addition to tempering and the type and thickness of the chocolate coating in the case of enrobing, several other parameters are known to affect the cooling time (Nelson, 1999). If a chocolate sample is properly tempered, the cooling time will depend purely on the type of chocolate. Tscheuschner and Markov (1989) observed significant texture changes of chocolates within the first six weeks of storage, which they described as after-crystallization. According to Liang and Hartel (2004) the packing arrangement of the dispersed phases in chocolate influences the mechanical properties, such as hardness, of the solidified chocolate. As described by Afoakwa et al. (2007) the Form IV polymorph crystallizes in a double chain, whereas the Form V polymorph crystallizes in a triplechain, consequently enabling closer packing and probably a denser and harder end product. During the early stages of the cooling process a limited number of crystals are formed, resulting in a low Author's personal copy E.L. Keijbets et al. / Journal of Food Engineering 98 (2010) 133–140 adhesion force and soft, liquid-like, product. Increasing the contact time at a cooling condition will enable further crystallization, and the formation of an enhanced fat crystal network. The packing arrangement of this network will depend on the cooling temperature, temper of the chocolate and the presence of Form V seeding crystals. These factors, in combination with the amount of liquid cocoa butter fat present, are expected to determine the porosity of the final chocolate product. 3.2. Cooling temperature As described in Section 3.1, the air temperature used in commercial cooling tunnels during chocolate manufacturing is usually 10–15 °C. The provision of cold air flow is to remove both sensible and latent heat from the liquid chocolate sample, so that a solid product could be formed for easy handling during packaging. On average, a temperature decrease of 10 °C is required. Assuming the specific heat capacity of chocolate to be about 1.6 kJ kg–1 °C1, and the latent heat to be 45 kJ kg–1 (Beckett, 2008), a total of 60 J needs to be removed to cool and solidify each gram of chocolate. The time required for cooling and solidification depends on the rate of heat transfer from the chocolate product to the air, which in turn depends on the temperature and flow rate of the cooling air. As described by Nelson (1999), the cooling process should not be too short in order to prevent against poor chocolate quality. At a low air temperature, there is an increased risk of sugar bloom and a dull finish on the chocolate surface, due to moisture condensation on chocolate surface. A low air temperature may also affect the crystallization of the cocoa butter into the correct polymorphic form, consequently decreasing the volume-reduction or contraction desired for easy demoulding (Beckett, 2008). Fig. 5 shows the relationship between the cooling temperature and the surface adhesion force. It should be stated that, at all cooling temperatures, a clean surface separation was observed, indicating that the cohesive force of the chocolate sample exceeded surface adhesion force and no fracture was observed at the surface region. The surface adhesion decreases almost linearly with increasing cooling temperature until 15 °C, after which a sharp drop in surface adhesion was observed for a cooling temperature of 20 °C (room temperature). Similar to the adhesion force, the hardness of the chocolate samples also decreases with increasing cooling temperature. The temperature difference between the tempered chocolate (30 °C) and the cold air is expected to be responsible for the different surface adhesion forces. Due to the relatively large difference in temperature between the liquid chocolate and 50 500 40 400 30 300 20 200 Hardness [N] -2 Surface adhesion (x 1000) [N m ] 600 10 100 hardness surface adhesion 0 0 0 5 10 15 20 Cool temperature [°C] Fig. 5. Surface adhesion of dark chocolate (j) and the hardness of the solidified chocolate samples (s) as a function of cooling temperature. 137 the cooling air at a low cooling temperature, e.g. 0 °C, a much faster heat transfer occurs between the chocolate and the air or a much faster heat removal. Consequently, a larger amount of crystals is formed within the same length of time. This indicates that a large amount of small fat crystals with a dense packing could be formed at low cooling temperatures. On the other hand, higher cooling temperatures promote the formation of significantly larger crystals because of the faster growth of crystals in relation to the rate of crystal nucleation. Due to the dense and more compact packing arrangement of the crystals at low air temperatures, the chocolate sample becomes harder (see Fig. 5). The penetration force was almost four times higher for the chocolate solidified at below 5 oC than that formed at 20 oC. Tewkesbury et al. (2000) developed a model to predict the temperature distribution within a cooling chocolate and applied the model to compare the cooling curve of tempered chocolate versus that of untempered chocolate. They demonstrated that different polymorphic forms formed during the chocolate solidification and crystallization process were a result of different cooling paths. As can be seen from the hardness curve in Fig. 5, the chocolate is relatively less hard when cooled at a temperature of 20 °C. These results indicate that solidification may take a much longer time at higher cooling temperatures. Therefore, the use of appropriate cooling times and temperatures is essential to ensure that the correct crystallization and solidification process are pursued. 3.3. Probe (mould) temperature The manufacturing of tablets of chocolate requires the liquid, tempered chocolate (28–32 °C) to be deposited into pre-conditioned moulds, which are subsequently passed over a vibrator, to spread the chocolate evenly throughout the mould without air bubble incorporation. The main controlling point is the pre-conditioning of the polycarbonate moulds. The pre-conditioning basically consists of a pre-heating of the mould to a temperature of 28–30 °C, in a low RH environment. The tempered chocolate is sensitive to large temperature variations and as a result the temperature of the mould should be as close to the temper temperature as possible when the chocolate is deposited in the moulds. A too high mould temperature will cause the chocolate tablets to stick to the mould upon demoulding. Due to the temperature of the mould being higher than that of the chocolate, seed crystals present within especially the surface or interfacial layer will be melted out. This will generally lead to the formation of less form V crystals during cooling, as the seed crystals are not present, and consequently hinder chocolate setting. On the other hand, a too low mould temperature causes a flash shock of the chocolate. The sudden temperature drop will cause the chocolate to release excessive heat or energy to the mould surface, resulting in immediate crystallization of the fat present in the interfacial layer. Unfortunately, this crystallization often results in the formation of unstable polymorphic forms, rather than the stable form V. Unlike form V crystal, polymorphic forms are much less closely packed and lead to little volume contraction, a common reason causing demoulding defects. In Fig. 6A the surface adhesion of chocolate is plotted against the mould temperature, to investigate the effect of different mould temperatures at the time of moulding on the final demoulding properties as measured by the surface adhesion force. Notable differences are clearly observable. The adhesion force increases almost linearly with the decrease of mould or probe temperature, except for a mould temperature of 20 °C, where the adhesion force drops to a lowest level. The low surface adhesion at 20 °C is assumed to be caused by the formation of a hydrophilic layer at the interface between the mould and chocolate surface. At such a low temperature, there is an increased risk of moisture vapour condensation, as well as the formation of ice crystals during the Author's personal copy 138 E.L. Keijbets et al. / Journal of Food Engineering 98 (2010) 133–140 A -2 450 200 400 350 150 300 250 100 200 150 50 100 50 0 0 -20 -10 0 10 20 30 40 -2 Surface adhesion (x 1000) [N m ] chocolate weight chocolate residue / mould surface [g m ] surface adhesion 500 50 Probe temperature [°C] B 20 45 40 15 [-] 30 25 10 20 ∆ Gloss ∆ Contact angle [°] 35 15 5 10 5 0 0 -20 -10 0 10 20 30 Probe temperature [°C] 40 50 contact angle gloss Fig. 6. (A) Surface adhesion of dark chocolate (j) and the hardness of the solidified ) as a function of probe temperature. (B) chocolate samples (column bar, ) and surface glossiness (j) of the Differences of contact angle (column bar, polycarbonate mould surface before and after chocolate contact as a function of probe temperature. pre-conditioning stage. The decrease in surface adhesion with increasing mould temperature is expected to be related to the crystallization and solidification processes taking place at the interface, especially the formation of unstable polymorphs and the melting out of seed crystals, as described previously. Fig. 6A also shows the amount of chocolate residue present on the mould surface after demoulding. Nearly all surfaces show some degree of cohesiveadhesive failure, indicating that the cohesive strength of the chocolate sample is approximately the same as the interfacial adhesive bonding strength between mould material and chocolate, resulting in a break within the chocolate sample and chocolate residues adhering to the mould surface. This behaviour can be expected, due to the formation of different polymorphic forms at the chocolatemould interface as a result of the variation in mould temperature. A mould temperature of 50 °C results in the melting out of seed crystals, consequently reducing the crystallization and solidification of the chocolate sample. The amount of chocolate residues adhering to this mould surface was large, and is thought to be caused by the fact that there was no extensive crystal network present to give the chocolate the required cohesive strength and hardness. The hardness of the chocolate samples (not shown) followed the same trend as the adhesion force indicating that solidification might not be complete when mould temperature was set at above 30 °C. A significant reduction in hardness and cohesive strength of the chocolate, together with a reduction in the surface adhesion force causes an increase in cohesiveadhesive failure and the amount of chocolate residue present on the mould surface. A closer look at the mould surface after the adhesion tests reveals significant changes in the contact angle of water, whereas the gloss of the different mould surfaces is fairly constant, as can be observed from Fig. 6B. At intermediate mould temperatures, e.g. 10, 20 and 30 °C, an increase in contact angle can be ascribed to the deposition of a thin film of fat on the mould surface, leading to an increased surface hydrophobicity. Wettability experiments by Luengo et al. (1997) on mica surfaces used for thin film tribology of chocolate also confirmed the presence of a hydrophobic monolayer on the solid (mould) surface after contact with chocolate. At a temperature of 50 °C, the chocolate residue at mould surface was so high that contact angle measurements became less relevant to mould surface but more to chocolate. The low contact angles at 0 and 20 °C may indicate the existence of a thin film (monolayer) of water molecules at the interface, due to condensation and/or the formation of ice crystals. This monolayer prevents fat from migration to the mould surface. A frozen cone/plunger method has been used to investigate adhesion and crystal formation (Beckett, 1999), where a frozen plunger (temperature range 5 to 21 °C) is inserted into a liquid chocolate sample for a short time (2–5 s) to solidify a thin shell of chocolate. A better release of the plunger from the chocolate is observed, according to the manufacturer, if the plunger is coated with a very thin film of ice crystals. Under such a circumstance, the fat is expected to set in the unstable crystalline form, due to the shock cooling. Whether this observation is true or not requires further investigation, but results from our investigation have at least confirmed a strong correlation between mould temperature and crystallization behaviour. A mould temperature of 50 °C causes melting of seed crystals, limiting form V crystallization, whereas a temperature of 0 °C or lower leads to the formation of more unstable polymorphic forms rather than form V. One may conclude that the mould temperature affects surface crystallization of chocolate deposits, which in turn determines the ease of demoulding, as measured by the force required to pull a mould probe off the solidified chocolate. 3.4. Relative humidity The effect of moisture at the chocolate–mould interface on the demoulding has also been investigated by exposing the mould probe to varying relative humidity before in contact with the chocolate. Results shown in Fig. 7 indicate that the surface adhesion increases initially with increasing RH. At approximately 25% RH the adhesion force stabilizes forming a constant plateau till ca. 50% RH. The surface adhesion starts to decrease sharply once the RH is higher than 50%. A similar observation has also been made in industry, where the adhesion of chocolate to roll-refiners increased significantly if the air in the factory was 20–25% RH. The assumption is made that these changes observed in surface adhesion are related to the change of surface hydrophilicity due to the adsorption of water vapour on the polycarbonate surface. The initial increase in surface adhesion coincided with an increase in cohesiveadhesive failure, i.e. the amount of chocolate sticking to the mould surface. A high RH resulted in a smaller surface adhesion, but caused a detrimental effect on the chocolate surface gloss (not shown). The moisture present at the mould surface could dissolve some of the sugar molecules present at the chocolate interface, an effect known as sugar bloom. Therefore, an optimal relative humidity that gives minimum surface adhesion and maximum chocolate quality is at an as low as possible RH. However, Author's personal copy -2 Surface adhesion (x 1000) [N m ] 500 -2 chocolate weight surface adhesion 500 400 400 300 300 200 200 100 100 0 0 0 20 40 60 80 chocolate residue / mould surface [g m ] E.L. Keijbets et al. / Journal of Food Engineering 98 (2010) 133–140 100 Relative humidity [%RH] Fig. 7. Surface adhesion of dark chocolate (j) and the amount of residues after probe separation (column bar, ) as a function of relative humidity. such a condition is practically difficult to achieve in an industrial environment. The hypothesis proposed to explain the relationship between relative humidity and chocolate adhesion is based on the adsorption of water vapour onto the mould surface, consequently forming a wet film of water molecules at high RH, which prevents interactions between the chocolate and mould surface. In order to test this hypothesis, the moisture adsorption by the polycarbonate mould surface at varying RH was measured using CISORP (CI Electronics) as described in Section 2.3. A simple linear relationship was observed between the average moisture uptake (%) of the mould surface and the RH of the air (see Fig. 8). What is interesting is that at low RH (<40%) actually desorption of moisture takes place. Water vapour deposited on the mould surface during the cleaning process was removed from the mould surface at a low RH, resulting in a negative moisture uptake. Deposited surface water could be those molecules existing in multi-layer absorption or even in surface micro-roughness. One important experimental observation worth of noting is that the point of changing from desorption to adsorption almost coincides with that of the sharp decrease of surface adhesion as shown in Fig. 7. Contact time, cooling and mould temperature all affect the crystallization and solidification of the cocoa fat, and consequently the 15 Moisture Uptake [%] 10 5 139 ease of demoulding, as measured by the surface adhesion force. During crystallization a three-dimensional amorphous crystal network is formed (Awad and Marangoni, 2006). The network formation is, however, dependent on the presence of nuclei formed either in the bulk phase, i.e. homogeneous nucleation, or at the surface of existing (foreign) particles, i.e. heterogeneous nucleation (Mullin, 2001). The polycarbonate mould surface is expected to act as a foreign body, inducing nucleation and affecting crystal growth. Varying the temperature of the mould surface will change its characteristics, consequently affecting the crystal formation of the fat phase at the chocolate–mould interface. Fluctuations in mould temperature may cause crystal nuclei at the surface of the chocolate sample to either melt or crystallize and solidify in another polymorphic form. Using a specifically developed STEPSCAN DSC, Baicoo et al. (2006) demonstrated that the cooling conditions affect the kinetics of crystal growth. Within a tempered chocolate the formation of low-melting polymorphs is enhanced when using a fast cooling rate, whereas the formation of higher-melting polymorphs was favoured on slow cooling. The formation of different polymorphic forms furthermore influences the crystal packing at the chocolate–mould interface. Schenk and Peschar (2004) discuss 0 the differences in crystal packing of the b structure, where the layers are packed loosely due to the fatty acid chains being organized perpendicular, compared to that of the b structure, where the chains are organised parallel, consequently forming a much more dense and closely packed structure. The varying crystal packing of different polymorphs is responsible for differences in the degree of contraction obtained on solidification, which in turn affects the demoulding of the solidified chocolate bar (Beckett, 2001). 4. Conclusions The processing conditions used especially during the moulding and cooling stages of the chocolate manufacturing have been shown to significantly influence the mechanism of adhesion between chocolate and mould surface, consequently affecting the demoulding process. Different processing conditions mainly affect crystallization and solidification of the fat phase of the chocolate. The melting out of crystal nuclei and the formation of polymorphic forms other than Form V as a result of processing conditions affects the crystal arrangement of the cocoa butter fat. Consequently, limited contraction is acquired, causing difficulties in demoulding. With respect to the effect of relative humidity, the presence of water vapour leads to the formation of a hydrophilic mould surface, which lowers the surface adhesion, due to limited chocolate–mould interactions. The ease of demoulding can be optimised by pre-heating the (polycarbonate) mould under controlled environmental conditions, i.e. an extremely low RH (0%) and 25–30 °C, and using a cooling temperature of approximately 10–15 °C. Acknowledgements 0 We acknowledge the award of an Industrial CASE Studentship to EK from BBSRC (UK) in collaboration with Nestlé (York) PTC. Special thanks go to Prof. E. Dickinson, Drs. M. Fowler and M. Leadbeater for constructive comments and suggestions. -5 -10 References -15 0 20 40 60 80 100 Relative Humidity [%RH] Fig. 8. Moisture uptake by the polycarbonate surface as a function of environmental relative humidity. Afoakwa, E.O., Paterson, A., Fowler, M., 2007. Factors influencing rheological and textural qualities in chocolate – a review. 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