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Yield stress studies on molten chocolate

University of British Columbia

A study of the flow properties of four chocolate samples was conducted. These were commercial semi-sweet (HSS), milk chocolate (HMC) and two experimental samples (H1 and H2). The yield stress, an important quality indicator of the chocolate, was estimated from steady shearing flow data by extrapolating the Casson model equation to zero flow rate and, by allowing stresses to relax after shearing. As well, undisturbed samples were examined in start-up flow using Single Vane and Multiple Vane methods. Proximate and sucrose analyses were carried out to determine the chemical composition of each chocolate sample. The mean particle size and the distribution of sizes contained in the samples was determined to further characterize the chocolates. A multivariate analysis of variance indicated that there was a significant difference in chemical composition among the four test samples. The mean particle sizes ranged from 5.73 to 6.27, 6.98 and 7.15 µm for samples HSS, H1, HMC and H2, respectively. The greatest number of particles were in the size range of 4.0 to 5.0 µm. The Casson model equation was fitted to steady flow data obtained with coaxial cylinder fixtures using a Brookfield HAT viscometer, a Brabender Rheotron viscometer, and a Carri-Med Controlled Stress Rheometer. For the Brookfield viscometer, the Casson equation over the shear rate range used, was found to accurately describe the flow characteristics of chocolate samples HMC, HSS and H2, but not sample H1. For the Brabender viscometer and the Carri-Med rheometer, the Casson equation did not fit the flow data over the entire shear rate range used with each instrument. A deviation in linearity occurred below approximately 0.5 s‾1 in the flow data measured in both instruments, thereby making the yield stress determination somewhat ambiguous. Yield values recalculated using only the linear data points were higher. In addition, for the Brabender viscometer, significant differences (p<0.05) were observed in both the yield and viscosity values measured using two coaxial cylinder fixtures of different annular gap widths. Using the Carri-Med rheometer, a significant difference in viscosity (p<0.05) over consecutive test runs was found and a significant difference (p<0.01) in yield stress when samples were sheared for 12 minutes as compared to 30 minutes. Yield stress estimates obtained using Multiple Vane Method I and Method II were comparable for chocolate samples HMC, HSS, and H2, but were significantly higher for sample H1 when using Method II as compared to Method I. Method II may be a more accurate estimate of the yield value of molten chocolate because the assumption of a uniform shear stress distribution over the ends of the vane fixture could not be proven experimentally for samples HSS and H1 when using Method I. Also, the dependence of the yield value on the rotational speed was evident when the vane data were analyzed using Method I, but was not a significant factor (p>0.05) when Method II was used to estimate yield stress. In addition, the single point measurements used to estimate yield stress agreed more closely with values obtained using Method II as compared to Method I. It is postulated that the Single Vane Method or Multiple Vane Method II may provide more accurate estimates of the yield stress of molten chocolate than using the Casson approximation. For the vane methods, direct measurements were taken under virtually static conditions; whereas, in the Casson extrapolation method, yield stress was estimated indirectly from flow data over a broad shear rate range at stresses well beyond the yield point of the sample. The Single Vane Method was simple and required less time than fitting the Casson flow model to shear stress-shear rate data and, therefore, may be more suitable for routine yield stress measurements of molten chocolate in quality control laboratories.

Text

2010-11-04

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http://hdl.handle.net/2429/29792

Food Science

University of British Columbia

Graduate