Effect of Different Bulk Sweeteners on the Dynamic Oscillatory and Shear Rheology of Chocolate

| DOI: 10.3933/APPLRHEOL-27-64123 | WWW.APPLIEDRHEOLOGY.ORG

Maryam Kiumarsi, Ali Rafe*, Samira Yeganehzad

Department of Food Processing, Research Institute of Food Science and Technology (RIFST), PO Box 91735-147, Mashhad, Iran

*Corresponding author: [email protected]

Received: 8.6.2017, Final version: 8.9.2017

Abstract: Effect of different bulk sweeteners including maltitol, isomalt and inulin on the steady and dynamic rheological behaviors as well as chocolate structure was investigated. All the chocolate samples showed shear-thinning behavior, which was mainly affected by the shear rates. Among different rheological models, the power law model was the suitable one (high R2) for the chocolate samples containing bulk sweeteners. As the bulk sweetener has the more solid volume fraction, the more flow behavior index n was obtained. Bulk sweeteners depending on their molecular structures were influenced on the apparent viscosity and yield stress. The mechanical spectra of chocolate showed the liquid-like behavior of inulin and maltitol and sol - id-like behavior of sucrose and isomalt. It was also revealed that the chocolate network structure was completely influenced by temperature and related to the frequency range. During thermal processing, the bulk sweeteners did not have any effect on the chocolate consistency and they cause to reduce the network strength of the chocolate. In addition, the critical stress was sensitive to the type of bulk-sweeteners in the chocolate. Consequently, chocolate can be considered as a weak dispersion which makes aggregates and can be modeled by the weak physical gel model. Current research realized great insight to the chocolate rheology in different processes such as enrobing, shell formation and molding.

Key words: Chocolate, bulk-sweetener, yield stress, oscillatory rheology, flow behavior

1 INTRODUCTION tional properties to obtain an acceptable chocolate. Therefore, many attempts have been made in substi - Chocolate is a multiphase complex system containing tution of sugars in sugar-free chocolates by utilizing cocoa butter and/or milk fat as solid continuous phase, sugar alcohols such as isomalt, maltitol, lactitol, sor - sugar and fat insoluble ingredients as disperse phase [1]. bitol and xilitol; synthetic sweeteners such as aspar - Owning to the high fat and sugar in chocolate, it is an tame, sucralose and acesulfame K and prebiotic com - energy-rich product which comprises about 60 % of the pounds like as inulin [6 – 10]. Among different kinds of confectionery industries. The most common sugar used sweeteners, the bulking sweeteners i.e. maltitol and iso - in chocolates is sucrose providing sweetness and energy malt do not have the limiting outcome such as the cool - (394 kcal/100 g sugar), which constitutes 30 to 60 % of ing effect or laxative properties which other sweeteners chocolate depending on the type of product [2]. In recent have [11]. Moreover, due to the improvement in health decades, there is a growing trend within the food indus - and nutrition safety, functional low calorie sweeteners try in the substitution of sucrose with sugar-free bulk such as inulin compounds have been attracted great in - sweeteners to reduce calorie intake, glycemic index and terest; and more investigations have been carried out dental cavity [3, 4]. over the past decennium [12, 13]. Functional properties of sugar, including sweet - Inulin is a fiber-enriched, prebiotic and poly-dis - ness, stability, particle size distribution, mouth feel and perse fructan which joined by (2-1) links. Short chain its effect on the rheological and textural properties of molecules in inulin are responsibβle for sweetness and fla - the chocolate are critically important [5]. The bulk vor enhancement [14]. It has been successfully applied in sweeteners should afford the aforementioned func - substitution of fat and sugar in various foodstuffs. The

© Appl. Rheol. 27 (2017) 64123 | DOI: 10.3933/ApplRheol-27-64123 | 1 | many works have been performed on the rheology of chocolate, there are only a few studies on the dynamic rheology of chocolate [21, 27 – 29]. Since dynamic rheol - Table 1: Ingredients used in sugar-free and reference choco - ogy is a non-destructive procedure in surveying the ma - late formulations (*All the chocolate samples have the same terial structure, it can be used to determine the yield formula except the sweetener. Control sample is the choco - stress, depending on the yield stress (or strain) defini - late with the generic formula and F1, F2, F3 contain maltitol, tion [27], and it can be exploited to obtain astonishing isomalt and inulin, respectively). information on the viscoelastic characteristics, mi - crostructure and fractal dimension of chocolate [30]. most functionality of inulin in chocolate manufacturing To the best of our knowledge, there is no research is the adjustment of the cooling effect during melting in work on the effect of different bulk sweeteners on the the mouth and taste improvement. It has been found no rheological properties of chocolate as well as its rela - significant difference between the common chocolate tion to the molecular structure. Therefore, the main ob - formula containing sugar and inulin [15]. Isomalt and jective of the current research was to evaluate the ef - maltitol are widely used as common sucrose-substitutes fect of bulk sweeteners (maltitol, isomalt and inulin) on in manufacturing sugar-free chocolates providing tex - the steady and dynamic rheological parameters of tural qualities. Intense sweetener should be used along chocolate and to find correlation between the molecu - with isomalt due to its low sweetness. However, the lar structure and rheology. sweetness of maltitol is close to the sucrose and due to its similarity in rheological properties to sucrose, recom - mended as a good alternative. Therefore, effect of sucro - 2 MATERIALS AND METHODS se replacement with low digestible sugars on rheological properties and the processing condition of chocolate 2.1 MATERIALS have been considered [10]. Rheology is the most important quality attribute in Cocoa powder and cocoa butter were kindly supplied chocolate manufacturing and can provide valuable from Guan Chong cocoa manufacture SDN, BHD, Co. (Jo - knowledge on the interaction of molecules and the mol - hor, Malaysia). Soya lecithin and poly glycerol polyrici - ecular structure. Chocolate formulation as well as pro - noleate (PGPR) were obtained from Palsgaard industry cessing conditions, particularly time and temperature (Juelsminde, Denmark). Inulin was provided from Cosu - can be affected on the rheological behavior [16 – 20]. cra Co. (Warcoimg, Belgium). Isomalt and maltitol were Thermal history of chocolate, specially cooling rates, has purchased from Beneo industry (Mannheim, Germany). a significant effect on the kinetics and physical behavior Sugar was purchased from Iran sugar Co. (Tehran, Iran). of the crystallized systems [21]. It has also revealed that the suitable cooling rates and annealing temperatures 2.2 CHOCOLATE PRODUCTION can produce a spherultic network with small fractal di - mension [22]. Furthermore, it was found particle size Chocolate samples were prepared according to the for - distribution is a key factor in chocolate rheology and mula provided in Table 1. The ingredients were weighed sensory perception [16]. For instance, increase in particle and transferred to the laboratory ball mill (Arman kher - size reduces yield stress, apparent viscosity, firmness, ad Co., Mashhad, Iran) with the capacity of 5 kg contain - hardness and melting of the products [19, 23]. It was also ing stainless steel balls with 9.5 mm diameter. Grinding concluded that small amplitude oscillatory shear mea - and conching was performed simultaneously at 100 surements (SAOS) can be applied in determining fractal rpm and 60 ºC for 6 hours. The refined chocolates were dimensions and the relation between rheological be - tempered using a temper machine (rev2, ChocoVision havior and molecular structure [24]. Corporation Co., Poughkeepsie, USA). Tempered choco - Many investigations have been accomplished on lates were then molded using polycarbonate molds and the rheological behavior of chocolate by using different allowed to cool. The final chocolate bars were wrapped bulk sweeteners such as maltitol, isomalt and xylitol on in aluminum foil and stored at ambient temperature the molten chocolate [10, 25], inulin/polydextrose bulk - for further analysis. ing mixture with stevia and thaumatin [14], maltitol and isomalt on the milk chocolate containing inulin [26], in - 2.3 RHEOLOGICAL MEASUREMENTS ulin, polydextrose and maltodextrin [13]. Moreover, the chocolate rheology was evaluated in terms of Casson The rheological experiments were carried out with a yield stress and Casson plastic viscosity due to non-New - controlled stress rheometer (Physica MCR 301, Anton tonian behavior of molten chocolate [27]. In spite of Paar, Germany) fitted with a parallel plate geometry

© Appl. Rheol. 27 (2017) 64123 | DOI: 10.3933/ApplRheol-27-64123 | 2 | (25 mm in diameter, 1.0 mm gap). The chocolate samples 2.3.2 Stress Sweep were melted overnight in an oven at 35 ºC and poured The stress sweep tests were accomplished at the con - on the bottom plate, until the temperature was adjust - stant temperature 35 ºC and at a frequency of 1 Hz by in - ed to the predetermined temperature. The temperature creasing stress from 0.1 to 100 Pa. The complex modulus was controlled quickly and precisely by a Peltier system G* was plotted as a function of oscillatory shear stress. (Viscotherm VT2, Anton Paar). The dynamic and steady Since, G* is the ratio of stress amplitude and correspond - state measurements were performed to determine the ing strain amplitude, it can be used as a measure of the rheological behavior of the sugar-free chocolates. The rigidity of the system. Therefore, it can endow with in - former was used to obtain the linear viscoelastic region formation on the chocolate microstructure and its mol - (LVR) of the sample by stress sweep test in which LVR ecular behavior. When the applied stress amplitude was considered based on the G* and phase angle δ val - passed over the LVR, the G* values will show variation ues. The latter was used to measure the flow behavior and decrease. Since the structure destruction occurs at of the chocolate samples. In order to avoid any slippage this point, the applied stress is no longer being linearly in the steady state measurements, sandpaper was used proportional to the strain amplitude. The chocolate on the plates. All the experiments were carried out at sample has viscoelastic solid-like behavior below the 35 ºC unless otherwise stated. critical stress τc or the corresponding critical strain γ0 and as the stress passed over the critical stress, the ma - 2.3.1 Flow Behavior terial may possibly rupture, due to the structure destruc - After a conditioning step of 2 minutes, the shear rate tion of the molecules as well as of the adopted kinematic -1 was increased from 0.1 to 1000 s . Then, the flow curve conditions. The τc can be typically obtained from the in - was plotted in shear stress τ versus the applied shear tersection of two asymptotic lines drawn through the rate γ. The apparent shear viscosity ηa was measured as initial and post-breakdown modulus data. The τc can al - functi̇ on of increasing shear rate from 0.1 to 100 s -1 so be obtained from the onset where G' and G'' moduli (ramp up). The chocolate rheological behavior with dif - become non-parallel. It can be considered as an approx - ferent bulk sweeteners were fitted by using the Casson imate measure of the yield stress of the material [33, 34]. model (Equation 1), that is a well-known rheological model for chocolate samples [19, 20]. Although, the 2.3.3 Frequency Sweep Tests method had deviations from the ICA method in two Frequency sweep was used to find out the viscoelastic points which has been described previously by Graef et liquid-like or solid-like behavior over the range of dif - al. [27] other models were also used. Thus, to find a bet - ferent timescales. Mechanical spectra of the chocolate ter fitting of chocolate samples, the power law (Equa - samples were determined by the frequency sweep at tion 2) and Chevalley (1991) models (Equation 3) were 10 -1 to 10 2 Hz at 35 ºC. Since, a sample might only permit used as follow: to explore the material properties over a small range of timescales; the frequency sweep of the control sample of chocolate (sucrose) was also measured at different temperatures 33, 35, and 38 ºC. Whereas the chocolate (1) sample completely liquefied at higher temperatures than 38 ºC and solidified at lower than 33 ºC, lower or (2) higher temperature did not consider in the frequency sweep test. (3) From a structural point of view, chocolate can be con - sidered as a fractal network structure based on the in - where τ0 is the yield stress, η is the apparent viscosity teractions among fat crystals forming spherical do - or the so-called “plastic-viscosity”, k is the consistency mains, flocs, entrapping the liquid phase [20, 35]. De - index and n is the power law index. The determination pending on the strength of the network interactions, a coefficient R2 was determined to show the goodness of strong or weak-link regime can be envisaged [24]. Like -1 the model. As the shear stress at γ = 5 s (τγ=5 ) was more as many foods, chocolate can be considered as a weakly precise, repeatable and reproducible to the true yield three dimensional network, where the crystal flocs are stress than Casson yield value; and the viscosity at bound by weak physical interactions. Hence, they be - -1 γ = 40 s (ηγ=40 ) was more repeatable and reproducible have as “critical gels” [24, 36, 37], indicating a solid-like than the Casson plastic-viscosity [31], the τγ=5 and ηγ=40 behavior under small deformations. The relationship were also evaluated. In addition, the ηγ=40 reflects the between the complex modulus G* and the applied os - microstructure of the sample by taking into account the cillation frequency ω can be described by a power-law presence of aggregates [32]. equation as follows:

Figure 1: Shear stress and apparent viscosity of chocolate samples containing different sweeteners at a range of steady-shear rate 0.1 to 100 s-1 and temperature 35 ºC.

3 RESULTS AND DISCUSSION (4) 3.1 FLOW BEHAVIOR Where z is the network extension related to the number of rheological units interacting within the network and The variations of shear stress vs. shear rate of chocolate A is a measure of the strength of theses interactions. samples with different sweeteners are provided in Fig - The more Aand zvalue show the more interaction forces ure 1a. Since, one of the most challenges in chocolate within the network and a board network extension, re - rheology is the yield stress determination, a double log - spectively. It should be stated that another form of arithmic plot of the apparent viscosity versus shear Equation 4 can also be used to explain the frequency de - stress are depicted in Figure 1b. All the chocolate sam - pendency of G' and G'' [34, 38]. Since G' was greater than ples were shown shear-thinning behavior, which is in G'' , G* is approximately equal to G' and as a consequence agreement with the literature [31, 39]. The shear-thin - n (index of the viscoelastic nature of the material) is ap - ning or pseudoplastic behavior of chocolates can be at - proximately equal to 1/z. tributed to the rupture of the inner structure dispersions in which the molecules orientating along the flow lines 2.3.4 Thermal History [39]. The rheological behavior of chocolate samples pos - Temperature sweep tests were examined to evaluate the sessing varying bulk sweeteners showed three distinct thermal history of the chocolate by small amplitude os - regions. Initially, the appearance of a local maximum in cillatory shear measurements. While, low heating/cool - the apparent shear viscosity ηa at very low shear rates ing rates will permit coexistence of various triglycerides (γ < 1 s-1 ), particularly for sucrose, inulin, and maltitol can polymorphs, producing a more disordered network with be attributed to the fact that the samples did not reach large fractal dimension and the final product has a small to steady state conditions (Figure 1b). On the other side, elastic modulus [21, 22], the scan rate of 1 oC/min was the apparent shear thickening at low shear rates can be used in our experiments. Initially, the chocolate sample completely attributed to non-equilibrium measure - was cooled down from 40 to 35 ºC at the rate of 1 ºC/min ments of the viscosity which have also seen for some and keep at this temperature for 5 minutes. Then, it was other polymer solutions such as xanthan and flaxseed heated up to 40 ºC with the same rate. All the experi - gum [40]. As a result, the apparent viscosities of choco - ments were carried out at a frequency of 1 Hz. late samples were underpriced at the shear rate in ques - tion. The apparent viscosity of the chocolate samples was changed at higher shear rates. At medium shear -1 2.4 STATISTICAL ANALYSIS rates (1 < γ < 10 s ), the ηa values of sucrose and isomalt were higher than the other samples and they have the All the experiments were conducted at least in triplicates. same viscosity at shear rate ~5 s -1 . Although the appar - Rheological data and graphs were analyzed by Rheo plus ent viscosity of the F3 sample (inulin) was less than other Version 3.40 (Anton Paar, Germany) and Sigmaplot Ver - samples at γ = 1 s-1 . Finally, at the third region (10 < γ < sion 12.0 (Jandel Scientific, Corte Madera, CA, USA), re - 100 s-1 ), the apparent viscosity of inulin was more than spectively. One-way analysis of variance (ANOVA) was the other samples and the order of viscosity was altered performed using Sigmaplot at significant level P<0.05. as: Inulin > Isomalt > sucrose > maltitol (Figure 1b). In order to better explain the rheological behavior, the power law, Casson and Chevalley model parameters

© Appl. Rheol. 27 (2017) 64123 | DOI: 10.3933/ApplRheol-27-64123 | 4 | Table 2: The rheological parameters of the chocolae samples (* τγ=5 is the shear stress at shear rate 5 s-1 and ηγ = 40 is the appar - ent viscosity at shear rate 40 s-1. †Statistical significant differences among the sweeteners are explained by the alphabetic order).

were evaluated. The model parameters are presented in that the sucrose and maltitol have the lowest ηγ= 40 . Table 2. Among the different models, the best fitting Maltitol in chocolate has resulted the similar viscosity 2 and high determination coefficients R were obtained to sucrose ( ηγ= 40 ), while the viscosity of chocolate with for the power law model. Although, sucrose and isomalt isomalt was significantly more (Figure 1b). The similar samples showed more consistency index K, the maltitol findings were also observed for theses bulk sweeteners and inulin had the same consistency. The lowest shear- [10]. It has been attributed the higher viscosity of iso - thinning index nwas observed for control sample (0.173) malt due to its higher solid volume fraction in chocolate, which may be related to the hydrophilic surface of su - whereas it has slightly less density (1.5 g/cm 3) than the crose and its interaction with each other and other par - other sweeteners (1.60, 1.63 g/cm 3 for sucrose and malti - ticles which makes a stronger structure. On the other tol, respectively) [10]. Since, the sweeteners were added hand, the presence of void spaces among the cocoa dis - to chocolate mix on a weight basis, therefore, chocolate persions was reduced and the cocoa butter immobilized with isomalt had more solids and thus more surface between aggregates. In contrary, inulin showed the area. Furthermore, the Casson plastic viscosity values highest n value (0.625) than that of the other samples. for dark chocolate have been reported between 2.1 and It may be attributed to the inulin molecular structure 3.9 Pas [41]. Although, by substitution of sucrose with which is a polysaccharide with many branched structure isomalt, the apparent viscosity was increased, maltitol that prevent from filling the void spaces among the dis - replacement was shown a reverse effect (Table 2). persions. Similarly, isomalt and maltitol have the same There fore, it can be concluded the bulk sweeteners have chemical formula, but due to the special conformation the significant effect on the rheological properties of of maltitol, it is possible to locate in the void spaces and chocolate. Consequently, bulk sweeteners depending thus it had lower n value (0.295) than isomalt (0.425). on their molecular structures, due to the decreasing of The phenomenon can also be explained by the solid vol - the contact points between particles, high solid volume ume fraction. fraction, persevering large void spaces, a greater lubri - The Casson and Chevalley model showed the same cating effect and a reduction of particle-particle inter - R2, but the highest determination coefficient was ob - actions [42] led to reduce the apparent viscosity. tained for inulin sample (0.99) among the models. Al - though, it should be reminded that the Casson model 3.2 LINEAR VISCOELASTIC MEASUREMENTS is a good fit to the experimental data for the shear rates from 5 to 60 s -1 [29]. On the other hand, the flow behav - Stress sweep tests were performed on the chocolate ior of chocolate get out of steady sate conditions at low samples to determine the linear viscoelastic region, in shear rate ( γ < ~ 5 s-1 ) and therefore, it is not possible to which measured chocolate functions are independent apply the steady state rheological models like as Casson on the amplitude of applied load. The complex modulus, and Chevalley model. As the yield value arises mostly G* (G' /G" ) and phase angel, as a function of applied stress from the interactions of the solid particles and less af - at the frequency of 1 Hz is plotted in Figure 2. The results fected by the fat content [11, 18], the low yield value of showed that LVR range were between 0.3 to 10 Pa for all the inulin sample can be attributed to fewer interac - the samples except for inulin where a narrower region tions of the solid particles. (approximately 0.1 to 0.4 Pa) was observed. There fore, Moreover, the τγ=5 as the yield stress and ηγ=40 as the lowest LVR value was obtained for the inulin sample. apparent plastic viscosity were obtained according to It can also be seen that the isomalt sample has the most Afoakwa et al. [19]. The highest value of yield stress was extended LVR regime than the other samples. However, obtained for isomalt (179 Pa) and sucrose and inulin there is a little difference between the isomalt sample were showed the similar yield stress (~140 Pa). In con - and the control sample. -1 trast, inulin showed the highest ηa at shear rate of 40 s The stress sweep can afford valuable information (about 15 Pas). It may be attributed to the polysaccharide concerning the structural strength of the material. By structure of inulin which increased the viscosity of increasing the applied stress, the bonds holding the chocolate. If it is supposed the viscosity was affected by structure begin to destroy and a sharp reduction in the the bulk sweeteners’ structure, it would be expected value of G* is occurred. This stress is called as a critical

© Appl. Rheol. 27 (2017) 64123 | DOI: 10.3933/ApplRheol-27-64123 | 5 | Figure 2: Typical LVR of chocolate samples containing different Figure 3: Changes in G' (filled symbols) and G'' (open symbols) bulk sweeteners. Phase angle data are plotted in green with in the frequency sweep test of chocolate with different sweet - the symbol of the sweeteners (f = 1 Hz and temperature 35 ºC). eners (35 ºC, LVR = 6 Pa).

stress τc and it can be considered as a measure of the samples containing inulin and maltitol sweeteners yield stress [33, 34]. The τc and critical G* values are pre - dominate over the storage modulus G' at the frequency sented in the Table 3. It can be easily understood that range of 0.1 to approximately 70 Hz, indicating a liquid- by adding inulin to chocolate, the complex modulus like behavior of a weakly structured system. Similarly, shifted to lower values (Table 3). As the G* is the mea - it has been observed the same behavior in the chocolate sure for the rigidity of the system, it can be concluded sample at the frequency of 0.1 to 10 Hz [21]. The liquid- the chocolate structure weakened by adding inulin pre - like behavior of inulin and maltitol were also confirmed biotic. Therefore, the order of the rigidity of the choco - by the steady state measurement, where the samples late samples were as isomalt > sucrose > maltitol > in - showed low yield stress values (Table 2). On the con - ulin. Furthermore, the length of the LVR becomes short - trary, G' values were more than that of the G'' values for er, as the chocolate samples have bulk sweeteners. It the samples containing isomalt or sucrose. Therefore, can be found that both τc and the yield value (based on the chocolate samples showed solid-like behavior and the Chevalley and Casson models) were decreased as had more elasticity than two other samples. This phe - the bulk sweetener, particularly for inulin, was included nomenon can be attributed to the higher solid volume in the chocolate. The similar findings have been found fraction as well as lower density of isomalt or sucrose for adding fat to the chocolate samples [27]. As a result, in chocolate. Moreover, it may conceivably be stated it can be concluded that τc is sensitive to the type of that the presence of isomalt or sucrose at the temper - bulk-sweeteners in the chocolate. Perhaps, it may be ature of 35 ºC can affect the chocolate crystal network, declared that the bulk-sweeteners have the functions increasing either consistency G* or the solid-like behav - like as yield stress reduction and can be replaced in the ior (loss tan δ). The predominance of the G' over the G' ' formula to develop a low-calorie chocolate. values have also observed for dark and milk chocolates at all process phases including mixing, pre-refining, re - 3.3 MECHANICAL PROPERTIES fining, conching and tempering [39, 43]. In order to determine the weak gel model parame - Mechanical spectra of the chocolate samples contain - ters, the G* data were fitted with Eq. 4 and the parame - ing different bulk sweeteners are shown in Figure 3. It ters A and z are given in Table 3. Since, the inulin sample can be seen that the loss modulus G'' of the chocolate did not show the linear relationship between G* and the frequency range it did not consider. It can be calculated G* values from Figure 3 in which z-values can be extract - ed. The most z-value, as a sign of rheological units inter - acting within the network, was found for isomalt sam - ple (2.09). However, the chocolate sample encompass - ing sucrose has a z- value of 1.57 and the lowest value was seen for maltitol (1.06). It can be concluded that sub - stituting of sucrose with isomalt can improve the phys - ical interactions within the chocolate network; howev - er, maltitol showed a reverse effect. Similar trends were Table 3: Critical storage modulus, critical stress, strength of interactions A and network extension z of chocolate with also perceived for the interaction forces within the net - different bulk sweeteners in the weak gel model (* Statistical work ( A-value). In comparison with sucrose, the A value significant differences among the sweeteners are explained was increased by substitution with isomalt and reduced by the alphabetic order). by replacing maltitol. Although, the similar findings in

© Appl. Rheol. 27 (2017) 64123 | DOI: 10.3933/ApplRheol-27-64123 | 6 | Figure 5: Effect of heating and cooling on the rheological be - havior of chocolate. Samples were cooled down from 40 to Figure 4: Mechanical spectrum of dark chocolate at different 35 ºC at 1 ºC/min, held for 5 min, then heated to 40 ºC at the temperatures (33, 35, and 38 ºC). same rate (f = 1 Hz; LVR = 6Pa). the z value have been observed for the chocolate con - characteristic feature of ‘weak gels’ [45], where molec - taining sugar (1.68 – 1.72) [21], but the A-values of the ular associations survive low-amplitude oscillation, but chocolates with bulk sweeteners were higher than the are broken down under steady shear. As a result, the previous works. It may be related to the different for - chocolate can be considered as a weak physical net - mula, ingredients and processing conditions. Conse - work. Based on the literature, these rheological para - quently, the replacement of sucrose with isomalt, due meters for chocolate did not state anywhere and it is to the increased solid content and a broader particle size not possible to have comparison. On the other side, distribution (more z) as a compromise between the op - comparison this behavior of chocolate with other foods posite phenomenon, can yield an increase in the choco - was misleading and ignored here. late consistency. It has also been reported that the par - ticle size D90 of inulin, isomalt and maltitol were 29.22, 3.4 EFFECT OF SWEETENERS ON THE THERMAL 29.11, and 21.45 micron, respectively [2, 14]. However, the HISTORY solid content was increased by substituting of sucrose with maltitol, but the less particle size distribution (less Effect of heating and cooling on the rheological behav - z- value) induce lower consistency and less solid-like be - ior of chocolate samples with different sweeteners is havior. In fact, combination of the bulk sweeteners will presented in Figure 5. At the initial times of cooling from result in chocolates having large crystals with the dense 40 ºC, a sharp reduction was observed due to the fat smaller particles of sweeteners filling in the void spaces crystal formation and grain boundaries. The lowest in the crystal network structure of chocolate. storage modulus was seen for the inulin samples, and Based on the preliminary experiments, frequency cooling and heating cycles did not have any significant sweeps of chocolate samples were examined at 33, 35 effect on its viscoelastic behavior. Although, the same and 38 ºC and the results are plotted in Figure 4. It can trend was also seen for the isomalt sample, it occurs at be seen that the G' has always been higher than the G'' a higher storage value. The chocolate containing malti - at all of the temperatures over the whole frequency tol and sucrose showed the similar viscoelastic behav - range. This is a characterization of solid-like viscoelastic ior during the thermal history. These phenomena may behavior. As the temperature was increased, the G' and be related to the higher volume fraction of inulin in complex viscosity η* were increased. However, there comparison with other sweeteners which have de - was an exception at 35 ºC, which has the lowest storage scribed in details in flow experiments results. It was modulus at the entire frequency range. The results seen a little increase in the storage modulus during the showed that the chocolate network structure was com - holding chocolate samples at 30 ºC, which may be at - pletely affected by temperature and dependent on the tributed to the structural recovery of the chocolate frequency range. Although, the strength of interactions sample. As it has been determined that thermal ramp A of chocolate was increased by temperature (37.11 to affects only fat crystal formation, whilst it does not af - 38.23), the network extension z was decreased (1.50 to fect on the physical and chemical properties of choco - 1.63). The findings elucidates the nature of weak phys - late ingredients [21]. It has also been shown that sucrose ical dispersion properties which was mentioned at the substitution with the bulk sweeteners produced beginning of this section and are in agreement with lit - changes in crystallinity and melting properties which erature [24, 44]. Moreover, the steady shear viscosity η observed by DSC experiments [2]. Therefore, it can pos - was appreciably lower than the complex viscosity η* at sibly be explained that bulk sweeteners did not have equivalent values of frequency and shear rate. This is a any effect on the G* or the solid-like behavior of the

© Appl. Rheol. 27 (2017) 64123 | DOI: 10.3933/ApplRheol-27-64123 | 7 | chocolate during thermal conditioning. In comparison more mouth feel effect. Thus, they may be replaced in with the milk fats, the bulk sweeteners did not play a the formula to develop a low-calorie chocolate. Me - significant role in the crystallisation phenomena [46], chanical properties of chocolate showed the sucrose and causing an apparent reduction in the network substituted with isomalt, due to the increased solid con - strength of the chocolate. Owning to the higher sweet - tent and a broader particle size distribution can yield an ener content in the present work in comparison with increase in the chocolate consistency. However, the sol - the literature evidence [47], the chocolate samples id content was increased by substituting of sucrose with showed a less structured network as explained by the maltitol, but the less particle size distribution induces weak gel model parameters. lower consistency and less solid-like behavior. It can be Ultimately, the high solids packing of the sugar- concluded the bulk sweeteners have the significant ef - free chocolate results a reduction in total surface area fect on the rheological properties of chocolate. Howev - available for fat to coat the sugar crystals and therefore, er, the strength of interactions of chocolate was in - decreasing the amount of energy needed to complete creased by temperature, the network extension was de - melting. Practically, although the sugar-free chocolates creased. Consequently, chocolate can be considered as will begin to melt quickly than the conventional dark a weak dispersion which makes aggregates and can be chocolate, it will take a much longer time for all the sug - modeled by the weak physical gel model. Overall, Cur - ar-free chocolates than that of control sample to com - rent research achieved great insight and valuable pletely melt. The findings are important as it provides knowledge on the chocolate rheology in the view point information on likely oral melting behavior with an im - of bulk-sweeteners in the processes of chocolate such pact on temporal components of flavor release as well as enrobing, shell formation and molding. as oral epithelial sensation.

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