Influence of conching temperature and some bulk sweeteners on physical and rheological properties of prebiotic milk chocolate containing inulin
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- Konar, N. Eur Food Res Technol (2013) 236: 135. doi:10.1007/s00217-012-1873-x
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Changes in food consumption habits and the developments set forth in the area of health and nutrition also change consumer expectations and demands. Sugar-free foodstuffs and products that have prebiotic activity are among the primary features of such expectations and demands. In the present study, the effects of substituting fine sugar with isomalt and maltitol in milk chocolate samples that contain inulin (9.0 % w/w), which is a substance with prebiotic activity, and the use of varying conching temperatures (CT) (50, 55 and 60 °C) in the sample preparation process on their physical (colour, hardness, water activity) and rheological properties were examined. Rheological data were obtained using the Herschel–Bulkley model which showed the best fitting for predicting rheology. It was determined that all properties included within the scope of the study are affected by the use of different bulk sweeteners or varying CT (P < 0.01). While colour properties, such as brightness (L*), hue angle (h°), water activity (aw) and rate index properties varied in a narrow range, it was determined that the yield stress and viscosity properties, which are among the important quality parameters of chocolate and can have determining effects on sensory properties, manifest variations within a broad range, depending on the CT and the bulk sweeteners used. It was concluded that maltitol is a more suitable fine sugar substitute in milk chocolates containing inulin.
Chocolate manufacturing is a very much complex physical and chemical process that requires numerous technological operations and the addition of different additives for obtaining products with suitable physical and chemical attributes and an attractive appearance and taste . During manufacture, refining and conching determine the particle size and suspension consistency and viscosity for specific textural and sensory qualities [2, 3]. Determination of the rheological properties of chocolate is important for manufacturing high-quality products with a well-defined texture . Additionally, the conching time and temperature affect the rheological properties and the production costs [5, 6].
The rise in cardiovascular disease and obesity and in other diet-related illnesses has led consumers to take a greater interest in the ingredients of food products . Inulin, a functional food ingredient, is potentially a prebiotic substance . Prebiotics can be applied to a variety of foods [9, 10]. Inulin has 10 % of the sweetness of sucrose , allowing it to partially replace sucrose in some formulations [12–14]. Inulin has also been used in chocolate as a low-calorie bulk agent without added sugar .
In the past two decades, the interest in sugar-free bulk sweeteners (BS) has grown in the field of ‘tooth-friendly’ and calorie-reduced confectionery, baked goods and pharmaceutical products. Additionally, sucrose-free chocolates have become popular among consumers and manufacturers because they result in low caloric values . Sucrose constitutes more than 40–50 % of the solids dispersed in fat, and thus, its functional properties, including sweetness, stability, particle size distribution, mouth feel and its impact on the rheological properties of the product, are important for chocolate products . BS substituted for sucrose in chocolates should provide these functional properties for an acceptable product. Sugar alcohols, such as isomalt and maltitol, can be used as BS to manufacture sucrose-free chocolates .
Within the past few years, increasing health and nutrition care and public demands have motivated the production of low-calorie, low-fat and reduced sugar products . Combining sweeteners with fibres and prebiotic compounds (e.g. inulin) and their applications in the production of dietetic foods are promising applications .
In the present study, the effects of substituting fine sugar with different BS, such as maltitol and isomalt, on the rheological properties (such as yield stress, viscosity and rate index) and physical properties (such as colour [brightness, hue angle and chroma], water activity and hardness) of milk chocolate samples that contain inulin (9.0 %, w/w) and have a mean particle size distribution (PSD) value of 20 μm (D[4,3]), were examined. By this study, we aimed to determine the results of using different BS in composition and different temperatures during conching process on physical and rheological properties of prebiotic milk chocolate containing inulin.
Materials and methods
For the preparation of milk chocolate samples, cocoa butter, cocoa mass (Altinmarka, Istanbul Turkey), sugar (SMS Kopuz, Istanbul, Turkey), milk powder (Besel, Konya, Turkey), soy lecithin (Brenntag Chemistry, Istanbul, Turkey), polyglycerol polyricinalate (PGPR) (Palsgaard, Zierikzee, the Netherlands), vanillin (Ekin Chemistry, Istanbul, Turkey), inulin (Beneo Orafti, Oreye, Belgium), maltitol (Roquette Frenes, Lestres, France) and isomalt (Beneo Palatinit, Mannheim, Germany) were used. All materials were obtained from Tayas Food Company (Gebze, Kocaeli, Turkey).
A pilot-type chocolate line and temper (Aasted, Farum, Denmark), pilot refiner (Lehmann, Aalen, Germany), pilot conching (BSA Schneider Anlagentechnik, Aachen, Germany) were used for preparing chocolate samples. In addition to the general laboratory equipment, a TA-TXPLUS Texture Analyser (Microstable Systems, UK), LA-300 Laser-Scattering Particle Size Distribution Analyser (Horiba Scientific, USA), CR400 colorimeter (Konica Minolta, Japan), Master aw Water Activity Measurement device (Novasina, Switzerland) and a rheometer (Brookfield R/S Plus, USA) were used for the analysis.
Formulations used for the chocolate samples (%, w/w)
Whole powdered milk
Afterwards, a three-stage tempering process (33–35, 24–25 and 25–26 °C) was implemented (temper index value, measured by temper metre (Chocometer, Aasted Farum, Denmark): 5.5–6.0). Subsequently, the moulding and vibration process (Aasted Farum, Denmark) was conducted at 27–30 °C. After 20 min of cooling (Aasted Farum, Denmark) at 5 °C, the process was completed with a sample output between 13 and 15 °C, and the samples were stored away from light and heat prior to analysis.
BS; fine sugar, maltitol and isomalt
CT; 50, 55 and 60 °C
A method for determining the particle size distribution of each sample was adapted from the method used by Afaokwa et al. . A LA-300 Laser-Scattering Particle Size Distribution Analyser (Horiba, USA) was used. Approximately 0.20 g of each chocolate sample was dispersed in vegetable oil (refractive index, RI = 1.45) at ambient temperature (20 ± 2 °C) until an obscuration of 0.2 was obtained. Ultrasonic dispersion for 2 min to ensure the particles were freely dispersed was maintained by stirring. The size distribution was quantified as the relative volume of particles in size bands presented as size distribution curves. Data analysis is based on the Mie theory. The obtained particle size distribution parameters included the following: D[4, 3] (μm), median; D[3, 2] (μm), 90 % finer than size; D90 (μm), specific surface area; (cm2/cm3), smallest particle size; D10 (μm), Span; [(D90−D10)/D50] (μm) and standard deviation (St Dev.)
The mechanical properties of the chocolates, such as the hardness, were measured using a TA-TXplus Texture Analyser (Stable Micro Systems, UK). A cylindrical, flat-ended, stainless steel probe with a diameter of 2 mm penetrated each sample to a depth of 5 mm at a speed of 1 mm/s at 20 ± 2 °C. The trigger force was set to 0.05 N. Results for the hardness (N) are expressed as the mean value of 5 replicates conducted on different samples of the same lot of each chocolate.
Ten grams of each milk chocolate sample was homogenized, and 2.00 g of the homogenized sample was used to determine the water activity (aw) at 25 °C using a LabMaster aw (Novasina, Switzerland). Aw values of each sample were measured in triplicate at after follow-up day of sample preparation.
Instrumental analyses were performed according to the Instruction Manual of the portable colorimeter, model Chroma Meter CR-400, brand Konica Minolta (Japan), obtaining L*, luminance ranging from 0 (black) to 100 (white), and a* (green to red) and b* (blue to yellow) with values from −120 to +120. All of these values, from the CIELAB System, were obtained at 25 °C. The colour parameters in the study were brightness (L*), hue angle [h° = arctan (b*/a*)] and chroma [c* = [(a*2) + (b*2)]1/2]. All the data were expressed as the mean value of 5 replicates conducted on different samples of the same lot of each chocolate.
Rheological properties of the milk chocolate samples were measured using a rheometer (Brookfield R/S Plus, USA) according to the method used by Sokmen and Gunes  with some modifications. Each chocolate sample was incubated at 50 °C for 75 min, melted, transferred to the rheometer and sheared at a rate of 5.0 s−1 for 10 min at 40 °C in the rheometer before the measurement cycles started. The shear stress was measured at 40 °C with increasing the shear rate from 0.5 to 60 s−1 (ramp up) within 120 s and then decreasing the shear rate from 60 to 0.5 s−1 (ramp down), and during each ramp, 50 measurements were taken. This measurement cycle was repeated 30 times consecutively until thixotropy was eliminated from the samples. The data from the 30th measurement were applied to the Herschel–Bulkley model, which showed the best fitting for predicting rheology, and related rheological parameters, such as yield stress, viscosity and flow behaviour index, were determined.
Quantitative data are expressed as the mean. The results of analysis were analysed using the Tukey’s test (SPSS 15.0, SPSS Inc., Chicago, IL, USA). Values at P < 0.01 were considered significant.
Results and discussion
Particle size distribution (PSD)
Beckett  and Afoakwa et al.  concluded that the largest particle size and specific surface area of solids are the two key parameters: the particle diameter has an impact on the coarseness, and the surface area is important with respect to the fat requirements for obtaining desirable flow properties. As their sizes increase, the particles become more spherical, leading to broadening of the PSD, with a consequential reduction in solid loading as the fat content increases.
PSD values of chocolate samples
Chocolate with fine sugar
Chocolate with maltitol
Chocolate with isomalt
Specific surface area (cm3/cm2)
Largest particle size (D90, μm)
Smallest particle size (D10, μm)
Mean (D[4, 3], μm)
Median (D[3, 2], μm)
Standard deviation (SD)
Food colour influences the expected and perceived sensory characteristics . Colour changes are likely to be specific for each chocolate item . Chroma and hue, two of the perceptual attributes of colour, are defined by converting the orthogonal a* and b* axes into polar coordinates C* and h. Mexis et al.  studied the effect of active and modified atmosphere packaging on the quality retention of dark chocolate with hazelnuts. Aguilera et al.  studied the colour changes in milk chocolate tablets as L*, a*, b*, chroma and hue, and they observed that major changes occurred after day 36 of storage. Afoakwa et al.  studied the particle size distribution and compositional effects on textural properties and the appearance of dark chocolates. In their study, differences in L* (38.25–43.49), C* (11.04–14.36) and h° (38.9–43.9) were found to depend on the particle size distribution (18–50 μm, D90).
Colour, water activity and hardness of chocolate samples prepared by different conching temperature and various bulk sweeteners
Conching temperature (°C)
Hue angle (h°)
Water activity (aw)
38.8 ± 0.67
52.0* ± 0.58
16.3 ± 0.05
0.238* ± 0.004
11.8 ± 0.63
38.4 ± 0.32
51.8*,a ± 0.59
16.4 ± 0.14
0.224* ± 0.002
11.7 ± 0.34
38.4 ± 0.57
51.8*,a ± 0.35
16.3 ± 0.15
0.220* ± 0.003
11.5 ± 022
42.0 ± 0.31
54.9 ± 0.22
18.5 ± 0.18
0.217* ± 0.009
11.7 ± 0.22
40.5 ± 0.31
53.6b ± 0.41
17.4 ± 0.26
0.228* ± 0.007
11.4 ± 0.42
39.5 ± 0.29
52.9b ± 0.58
16.8 ± 0.16
0.239* ± 0.003
11.0 ± 0.22
38.6 ± 0.25
52. 3* ± 0.17
16.0 ± 0.14
0.236 ± 0.010
11.1 ± 0.47
38.8 ± 0.18
52.0*,c ± 0.23
15.4 ± 0.09
0.234 ± 0.003
11.2 ± 0.05
38.0 ± 0.22
51.9*,c ± 0.07
15.2 ± 0.13
0.224 ± 0.007
11.6 ± 0.46
Differences were observed for maltitol, whereas L* similarities were observed in the hue angle values and it was determined that BS and CT can cause variations in the hue angle (P < 0.01). However, although the approximate hue angle values were determined for all three BS types after conching at 55 and 60 °C, differences at 50 °C CT were observed.
As was the case in other colour parameters, as a consequence of the variance analysis conducted for chroma (Table 3), the interaction of CT and BS and the differences between the level averages of the BS and CT temperature factors were found to be statistically significant (P < 0.01). However, despite the fact that the chroma values were in concordance with those reported in previous studies, in the present, hue angles were obtained at higher levels than the values found in those studies. The inclusion of inulin in the samples was considered to have similar effect on the hue angle level. Bolenz et al.  also reported an inulin sample to be darker and different in colour than standard.
In terms of colour properties, it was observed that the use of isomalt and fine sugar in milk chocolate samples containing prebiotic compounds results in sample outputs with closer properties in comparison with samples that contain maltitol. However, the possibility of obtaining higher L* values, or in other words, brighter chocolate output, with the use of maltitol can be considered an advantage. Brighter milk chocolate can also be achieved by simply using less cocoa particles but by this way, its sensory quality may change as negatively.
The hardness of different types and compositions of milk chocolate samples has been studied by various researchers [1–3, 18, 26, 27]. In the study by Kieran Keogh et al. , the hardness of milk chocolate samples (1.97–3.54 N) decreased with increases in the fat and free-fat contents of the powder used. Afoakwa et al. [1, 2] studied the effect of PSD on the hardness of molten and tempered dark chocolate and stated that hardness showed an inverse relationship with particle size, with a significant reduction over all tempering regimes studied and the greatest reduction in under-tempered products.
In the present study, the effects of BS types and CT on hardness were examined (Table 3). Despite the fact that the obtained results varied within the narrow range of 11.0–11.8 N, the variance was found to be statistically significant (P < 0.01). Furthermore, this value range is consistent with the results obtained by Farzanmehr and Abbasi . Farzanmehr and Abbasi  studied milk chocolates containing inulin and bulking agents, such as polydextrose and maltodextrin. They obtained different hardness values (10.1–15.0 N) depending on the composition of the samples. It was also determined that both the BS used in sample preparation and the implemented CT affect chocolate hardness (P < 0.01). The samples that contained maltitol and fine sugar exhibited more similarity in comparison with the samples containing isomalt. Furthermore, while the hardness of the samples containing maltitol and fine sugar decreased with increasing CT that of isomalt-containing samples increased. The obtained results indicate that the use of maltitol as a substitute for fine sugar in inulin-containing chocolates is more suitable than using isomalt in terms of hardness. This finding can be characterized as an indication of the possibility of observing the same similarity in some sensory properties. And also, good chocolate should have a smooth, soft, velvety texture while poor-quality chocolate feels hard, grainy or waxy .
The water activity of chocolate is often between 0.4 and 0.5, but several factors, such as the raw materials used, the surface area of the materials, and the temperature and the humidity of refining and conching, can influence this parameter [29–31]. The fatty surface of chocolates can protect them from external water. However, amorphous sugars can affect the water activity of chocolates. During chocolate processing, amorphous sugar is capable of absorbing water from the environment . Isomalt and maltitol, the bulk sweeteners used in this study, have low and median hygroscopicity, respectively. The water activity for different CTs was found to lie within the range of 0.220–0.238 in samples containing fine sugar, 0.217–0.239 in maltitol-containing samples and 0.224–0.236 in isomalt-containing samples (Table 3). While the water activity was observed to decrease with increasing CT in samples containing isomalt and fine sugar, in maltitol-containing samples, water activity was found to increase as the CT increased. However, an intriguing finding obtained for all three types of sugar was the fact that while the variance in water activity values was not significant in the range of 55–60 °C and 50–55 °C, the variance between 50 and 60 °C was significant (P < 0.01). For this reason, it was concluded that in inulin-containing chocolates, CT is an effective parameter on water activity for varying BS.
Due to its composition, white chocolate is a product with the low water activity  of 4.0 aw, as stated by Rossini et al. . For milk chocolates produced with sugar and inulin (10.45 g/100 g), Farzanmehr and Abbasi  determined a water activity of 0.34. However, these values are different from the findings obtained from the present study, in which lower levels of water activity values were determined. This effect might be the result of the implementation differences in sample preparation procedures, as well as the distinctiveness of the hygroscopic properties of the other compounds included in the samples.
The determined water activity values were convenient for the shelf lives and stability of the samples. In terms of the average values, while maltitol and fine sugar exhibited similarities in terms of water activity, it was observed that isomalt presented with differences.
Rheological measurements of chocolate samples
Conching temperature (°C)
Yield stress (Pa)
Viscosity (Pa s)
Examination of the rate index values of the samples showed similar values for samples with isomalt and maltitol and distinctive values for fine sugar samples. While it is true that almost all the isomalt- and maltitol-containing samples were pseudoplastic (shear-thinning) (0 < n < 1), interestingly, the samples containing fine sugar had a shear-thickening property due to the fact that their rate values were merely above 1. This finding is considered to be a result of the higher crystallinity of fine sugar in comparison with maltitol and isomalt.
Processing parameters, such as conching conditions, particle size distribution, fat content emulsifiers, temper, vibrations and temperature, can have a potential effect on the rheological parameters . In recent years, the composition of chocolate samples, especially those developed for sugar-free chocolate and different sweetener types, has been investigated by different researchers [5, 17, 18, 38, 39]. The duration and temperature parameters implemented in the conching process, which is a very important process of chocolate production, can potentially have a strong effect on the chocolate itself. The findings of our study supported this perspective. Statistically significant variances in yield stress, viscosity and rate index values were determined for all types of sweeteners at different CTs (P < 0.01). However, the direction and degree of such variations changed in connection with the type of sweetener used. While the yield stress values decreased in line with the increasing CT in samples containing fine sugar and maltitol, they exhibited an increase in isomalt-containing samples. Viscosity values, however, exhibited an increase in samples containing fine sugar and a decrease in samples containing maltitol and isomalt with increasing CT. Both changes in maltitol-containing samples were quite more apparent in samples containing maltitol (Table 4).
Samples of low-sugar milk chocolate with prebiotic properties with several different levels of inulin, polydextrose and maltodextrin along with sucralose (0.04 %, w/w) were used instead of sugar by Farzanmehr and Abbasi . The findings show that the use of prebiotic ingredients instead of sugar could lead to the production of low-calorie milk chocolate without the undesirable textural and physiological effects.
The use of different BS types in chocolates containing inulin and differences in temperature during the conching process cause effects of varying directions and degrees on the physical and rheological properties of the products. Maltitol used in the study was notable in terms of both the results it produces in terms of colour, hardness and rheological properties that are more acceptable and closer to those of fine sugar than isomalt and its potential to produce the ‘sugar-free’ and ‘tooth-friendly’ qualities to prebiotic chocolates, the sensory qualities of which are acceptable, by means of its properties, such as caloric value (2.40 kcal/g) and sweetness (nearly 90 % of sucrose). However, the fact that the rheological properties of maltitol-containing samples undergo serious changes with varying CT must be considered. In prebiotic milk chocolate production with maltitol, it is necessary to optimize conching conditions as well as consider other process parameters.