Comparing the Particle and Rheology of Smoothies Made with Different Blenders

Smoothies have been available for almost a century, although it was not until the development, evolution, and commercialization of the blender between 1920 and 1950 that home-made smoothies were possible.

The health food movement of the 1960s made them even more popular. In recent times a trend towards nutritious and natural foods, and the commercialization of dedicated smoothie makers have also contributed to their growing popularity. In the Oxford English dictionary, a smoothie is defined as a smooth, thick drink produced from pureed fresh fruit and milk, ice cream, or yogurt. However, vegetables, nuts, crushed ice, extra juice, seeds or water can also be used to make smoothies, as there are no standard rules to describe it's ingredients.

Smoothness and easy consumption are the widely accepted characteristics of a smoothie. These characteristics are influenced by the viscosity of the suspension, and the particle size of the blended components. The quantity of the liquid utilized and the selection of ingredients help to control the blend viscosity easily. However, the performance of the blender greatly influences the particle size of the blended ingredients. The quality of a blender will often be evaluated by consumers using this criteria – the taste test. This article discusses the evaluation and comparison of two smoothie makers; determining the viscosity and particle (fiber) size of two smoothie recipes produced with each blenders.

Materials and Methods

This study involved the preparation and analysis of two different smoothies: a pink and a green smoothie. The green smoothie consisted of 10g spinach, 100 g pineapple, 100 g apple, and 70 g water. The pink smoothie consisted of 100 g apple, 60 g berries, 30 g pineapple, 10 g spinach, and 70 g water.

Blender A and B (Figure 1) used to blend the ingredients together for 30 seconds. It was obvious that the smoothies seperated rapidly subsequent to the blending process, due to the buoyancy of the pulp fibers in the blend. It was necessary to maintain the homogeneity of the sample, as well as keep the fibers in their dispersed condition in order to obtain highly accurate viscosity measurements.

Pictures of Blender A (left) and Blender B (right) loaded with fruit and vegetables.

Figure 1. Pictures of Blender A (left) and Blender B (right) loaded with fruit and vegetables.

A dispersion tool or mixer, equipped with a standard lower cup and a Kinexus rotational rheometer was used to achieve this (Figure 2). For the two smoothies, it is possible to report the viscosity-shear rate profiles using a novel method for shear rate and shear stress calculations for non-standard measuring systems like mixers. The Mastersizer 3000, equipped with a HydroSight accessory, was used to measure the particle size. The use of the HydroSight accessory enabled the sample during pulp size measurement to be visualized.

Photographs showing Kinexus mixing geometry (left), green smoothie displaying separation (middle) and Kinexus with cup and mixer configuration.

Figure 2. Photographs showing Kinexus mixing geometry (left), green smoothie displaying separation (middle) and Kinexus with cup and mixer configuration.

Results

Figure 3 displays the particle size results for the two blenders and two smoothies. The particle size distribution of the green smoothie was very similar for both smoothie makers, with a median pulp diameter of 386 and 385 µm for Blender A and Blender B, respectively. A slightly smoother drink was produced by the Blender A in the case of the pink smoothie, with a median particle size of 336 µm in comparison with 380 µm for Blender B.

Particle size results for green smoothie blends (top) and pink smoothie blends (bottom) made with Blender A (Blue) and Blender B. (Green)

Figure 3. Particle size results for green smoothie blends (top) and pink smoothie blends (bottom) made with Blender A (Blue) and Blender B. (Green)

The viscosity profiles of the different blends are depicted in Figure 4 as a function of shear rate. Both smoothies exhibited shear thinning behavior as anticipated for a complicated and concentrated dispersion of this kind. The flow curves were nearly identical for the green smoothie, which is in line with the particle size results that were almost identical for both blenders. The processing conditions and ingredients were the same for both of the blends, the only real variables that would affect the viscosity are particle size and size distribution.

Viscosity vs. shear rate curves for the green smoothie blend (top) and pink smoothie blends (bottom) made with Blender A (Blue) and Blender B. (Green)

Viscosity vs. shear rate curves for the green smoothie blend (top) and pink smoothie blends (bottom) made with Blender A (Blue) and Blender B. (Green)

Figure 4. Viscosity vs. shear rate curves for the green smoothie blend (top) and pink smoothie blends (bottom) made with Blender A (Blue) and Blender B. (Green)

In the case of the pink smoothie, the smoothie produced by Blender A had a slightly higher viscosity, compared to that made by the Blender B. The variation in the pulp size measurements for the two blenders could be the reason, as the smoothie made by the Blender A had a larger proportion of smaller particles. In addition to the presence of the larger pulp fibers, the HydroSight showed the presence of a large number of micro-bubbles, which must be taken into account due to their contribution to texture and viscosity.

Finally, Blender B alone was used to evaluate the variations in particle size and viscosity as a function of blending time (Figure 5). The particle size was reduced with a corresponding viscosity decrease when the blending time was increased. In this case, the amount of free water is most likely increased. The smoothies became extremely lumpy after 5 seconds of blending, needing more force to blend. This resulted in a higher measured viscosity.

Particle size and viscosity data for the green smoothie measured after different blending times using Blender B.

Particle size and viscosity data for the green smoothie measured after different blending times using Blender B.

Particle size and viscosity data for the green smoothie measured after different blending times using Blender B.

Figure 5. Particle size and viscosity data for the green smoothie measured after different blending times using Blender B.

Conclusion

The viscosity and particle (fiber) size of two different smoothie recipes were measured for the evaluation and comparison of two different smoothie makers. The two blenders showed a very little difference regarding the measured viscosity and particle size distribution of the blends. A variation in the particle size was also observed when there was a difference in the viscosity, indicating a fundamental relationship.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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