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Ever thought of fruit juice as a
solid? It may surprise you to know that we can show it's more solid than liquid for a great period of time.
Solids, under
gentle imposed stresses, deform elastically and bounce back to their original shape on the removal of that stress. The
reason for this “bounce-back-ability” is the presence of a continuous network structure capable of storing energy
on deformation. Conversely, liquids do not possess such a structure and therefore do not exhibit elasticity.
Elasticity can be measured with an oscillatory
rheology test and phase angle is the parameter we use to quantify it. Simply put, an oscillation measurement
on a solid will report a phase angle of 0°: purely elastic behaviour. Conversely, a liquid – completely inelastic (or viscous, as we rheologists
call it) – results in a phase angle of 90°. This gives us a scale of “solidity” and therefore
a measure of the presence of elastic network structure.
Below is the result of an oscillation stress sweep test on mandarin juice - with phase angle plotted as a function
of applied stress:
At low stresses we can clearly
see a plateau at a phase angle of around 18°, telling us that the juice, under these gentle, virtually at-rest conditions,
possesses a very well developed soft-solid structure. In other words, the juice would be more accurately describe
as a solid than a liquid! The reason we don’t see this is that the structure is so weak it is completely
disrupted at the slightest disturbance, leading to viscous, liquid-like behaviour. The graphic below shows a comparison
of the juice alongside a smoothie and a yoghurt:
As you can see, all three show similar “solidity” at lower stresses, the only differences being
in the strength of the structure, which in smoothie and yoghurt are strong enough to make the soft-solid condition noticeable
to the casual observer. So how can we use this information? Well, structure, even extremely delicate
ones such as we see in the fruit juice, contributes to many desirable characteristics inother food and beverages as well as pharmaceuticals,
cosmetics, paints and many other materials, for example: - Suspension stability
- Texture and appearance
- Pouring,
draining and dripping
- Penetration, sagging and slumping
So, tell your friends, when fruit juice is in
the carton on the shelf it is a solid – well, more solid than liquid anyway!
If you’d
like to learn what a structure testing can tell you about your products please contact me.
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