Here at Malvern Panalytical, we seem to enjoy testing the rheological properties of drinks, particularly those of the alcoholic variety! John Duffy and Steve Carrington looked into the differences between traditional ales and they explored the properties of Guinness. Any excuse to go down the pub…!

smoothieTherefore, it was probably about time to explore the healthier option on the drinks menu, and an ideal hangover cure – smoothies. Additionally, this was a perfect opportunity to demonstrate our latest dispersion geometry using John’s loyal drinking partner – the Kinexus rheometer!

We decided to run an experiment in the lab to investigate the blending ability of two smoothie makers – Blender A (left) and Blender B (right) – by testing the particle (fiber) size and viscosity of two different recipes. Although both affect the drinking and taste experience in different ways, the two are very much related since the viscosity will be greatly influenced by the properties of the dispersed pulp fibers, including their size, shape, and concentration.

For this study, we made two different smoothie types with our two smoothie makers, in each case weighing out equal quantities of ingredients (including additional water) and blending for 30 seconds. The first smoothie (green) was made with apple and spinach and the second (pink) was comprised of mixed berries.

After blending, it was clear the smoothies were separating rather rapidly due to the buoyancy of the pulp fibers in the juice. Therefore, to measure their viscosities as accurately as possible it was necessary to keep the fibers dispersed and the sample as uniform as possible during the measurement.

dispersion paddle.

This was achieved using a dispersion tool (or mixer) coupled to a Kinexus rotational rheometer and a standard lower cup. Furthermore, by employing our own unique method for calculating shear stress and shear rate values for non-standard measuring systems we were able to report viscosity-shear rate profiles for our smoothies.

We also analyzed the particle size of our smoothies using the Mastersizer 3000 coupled to a Hydro Sight accessory, which allowed us to visualize as well as measure the size of our pulp, and ultimately determine whether one blender was more efficient than the other.

For the green smoothie, the mean particle size and particle size distribution was found to be similar for both smoothie makers, corresponding with almost identical viscosity-shear rate profiles (Fig 1)

green smoothie results

The more aesthetically appealing pink smoothie showed a very slight difference between the two blenders, with the Blender A producing a slightly smaller particle size (Fig 2.).

pink smoothie results

This resulted in a smoothie with a slightly higher viscosity than that produced with Blender B, which was attributed to a larger number of smaller fibers.

blending time and viscosity of smoothies

What the Hydro Sight revealed was that in addition to the larger pulp fibers being present there was also a large number of micro-bubbles present, which most likely will contribute to viscosity and texture.

The image to the left and charts below show that increasing the blending time resulted in decreasing particle size (and most likely an increase in the amount of free water) for the green smoothie which corresponded with a decrease in viscosity. After 5 seconds of blending, the smoothies were extremely lumpy requiring greater force to mix, therefore resulting in a higher measured viscosity.

viscosity vs shear rate

Overall, our dispersion paddle did a great job of preventing the smoothies from separating. For these recipes there is little difference between the particle size and viscosity for the smoothies produced by the two blenders; however, we can clearly see that blending time has a large impact on our smoothie consistency.

As for the hangover cure…I wouldn’t like to comment!!

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