The soluble solid contents of the chocolates samples are determined by using a refractometer, while the moisture content of the chocolates samples are measured by drying the samples in a oven at 105˚C. The ash contents are determined in a muffle furnace at 600˚C.

Rheological properties of the four chocolates are measured using MCR101 (Austria) rheometer with parallel plate measurement system. The diameter is 50 mm. The rheological measurements include steady-state rheology analysis, effects of temperature on viscosity and dynamic rheological experiment. Three replicates are carried out in the experiments.

Double blind are used to reduce errors. The evaluated indexes include taste, color, texture, odor and overall acceptability of the four chocolate samples. In summary, 12 subjects (6 males and 6 females), with an age range between 21 and 35 years (mean age: 24 years) participated in the study. Before evaluating, the stored chocolates at 4˚C are placed for 1 hour at room temperature. Each piece of chocolate for sensory evaluation weight 10 g. The samples are assessed by nine-full-mark criterion (9 = like extremely, 8 = like very much, 7 = like moderately, 6 = like slightly, 5 = neither like nor dislike, 4 = dislike slightly, 3 = dislike moderately, 2 = dislike very much, 1 = dislike extremely) [14] .

The experiment data are analyzed by using SPSS software. The difference analysis is used with the S-N-K method. The different Lowercase letters indicates significant differences between the same column (P < 0.05). The parameters of each equation are obtained by linear regression using SPSS software.

The main compositions of the four chocolate samples are shown in Table 1. It can be seen from Table 1, the four samples all have low moisture content, the reason may be that water molecules can hydrate with the colloid of other materials in chocolate. After water swells, the materials become sticky which make it harder to process chocolate. Leconte not only has low fat content, but also has lower moisture content and ash content than the others. Compared with Dove and Leconte, Toblerone and Cote Dor have higher fat content because westerners prefer high-calorie and high-fat food.

The steady flow curves of the four molten chocolates are shown in Figure 1. The four samples all have yield stress. The stress increases linearly as the shear rate increases. So the flow of molten chocolates belong Bingham’s model [15] . The stress of Dove and Leconte changes rapidly compared with those of Toblerone and Cote Dor.

The experimental values of the shear stress and the shear rate are fitted with Casson equation,. Casson model is the official method for interpretation of flow data on chocolates. The Casson yield stress

Notes: Values in the same row followed by different superscipt letters are significantly different (P < 0.05).

is calculated as the square of the intercept. However, yield stress calculated by this method isn’t the actual value [10] . The rheological parameters τ₀ of four molten chocolates are calculated and shown in Table 2. Dove and Leconte have higher yield stress, while Toblerone and Cote Dor have lower yield stress. This result means that Dove and Leconte need to overcome higher stress before they can flow.

The relationship between temperature and viscosity of the chocolates samples is shown in Figure 2. The temperature dependence of chocolates is well described by Arrhenius relationship. The flow activation energy (Ea) is calculated by the equation logη = logη₀ + Ea/RT. Ea and R² are showed in Table 3. The flow activation energies of the chocolate samples are almost same. Because the sensitivity of viscosity to temperature (SVT) is proportional to the flow activation energies, the four molten chocolates have same SVT.

The viscosity of the chocolate samples decreases with close rate as the temperature increased. The viscosities of Toblerone and Cote Dor chocolates are lower than Dove and Leconte chocolates at the same temperature. The reason may be the imported chocolates have higher fat content. As the material of Chocolate is a kind of dispersion system, the viscosity of chocolate depends on the ratio of the dispersed phase and the dispersing medium [16] . Generally, the viscosity of chocolate will increase obviously if the fat content bellows a certain limit.

Molten chocolate is a suspension with sugar, cocoa and other solid particles in the continuous fat phase. Different from Newtonian flow, molten chocolate contains solid particles and has the properties of non-Newtonian flow. It is shown in Figure 3 that melt samples exhibit shear thinning behavior and thixotropic. The hysteresis loop shows that the structure of chocolate will be damaged after shearing occurs. The hysteresis area which represents the size of thixotropic is the area between two curves [17] . When the shear rate is same, the viscosity of Dove and Leconte are higher than that of Toblerone and Cote Dor. The viscosity of chocolate depends on cocoa butter content at a certain temperature range. The increasing of cocoa butter can cause viscosity decrease. Toblerone and Cote Dor have low viscosity because they contain rich cocoa butter.

The region that complex modulus G^* does not vary with the oscillation changes is called linear viscoelastic region (LVR) [18] . 3% strain at is used to determine the rheological properties of chocolate samples.

It is shown in Figure 4 that the frequency sweeps at 37˚C. Storage modulus G' and loss modulus G" increase gradually with the increase of frequency. At the same frequency, the value of G' is higher than that of G". This result shows that the chocolate samples have more elastic than viscous. The elastic of the chocolate suspension is determined by the degree of particles network structure’s deformation recovery. G' reflects not only the ability of elastic deformation recovery but also the value of the elastic potential energy [19] . The small value of G' indicates that the network structure of chocolate is difficult to be formed. Loss factor, tanδ = G"/G', is a dimension less value that compares the amount of energy lost during a test cycle to the amount of energy stored during the same time. As shown in Figure 4, that the value of tanδ for the chocolate samples is more than 1, which shows that the chocolate samples have a larger proportion of sticky ingredients.

Figure 5 shows the variation of storage modulus G' and loss modulus G" with temperature. It can be seen that as the temperature increases, the value of G' increases while that of G" decreases. And the value of tanδ for the molten chocolates also decreases. The tanδ of Toblerone and Cote Dor are higher than that of Dove and Leconte, which shows that Toblerone and Cote Dor have higher proportion of sticky ingredients than Dove and Leconte.

When time increases, the complex modulus G^* is basically unchanged, this result shows the samples have good stability (Figure 6). The complex modulus G^*s of Dove and Leconte are higher than those of Toblerone and Cote Dor, which show Dove and Leconte chocolates have higher anti-deformation ability.

Sensory evaluation can determine the value and acceptability of a food sample [20] . Thus, it is useful to make a comparison among the four chocolate samples. It is observed that Dove and Leconte have higher score than Toblerone and Cote Dor chocolates (Table 4). The result may be caused by the difference of eating habits between our country and the other country.