The vacuum It is a very widespread food conservation technique: it in fact guarantees that foods last longer Because, in the absence of air, the conditions suitable for the slowdown of the natural bacterial proliferation, which instead would have – in normal conditions – more quickly. In the same way, the vacuum is also used to optimize the dimensions of deformable objects and materials. But what happens from a mechanical point of view? The interaction between what acts externally to the container and what remains inside, downstream of the vacuum, is responsible for a mechanical process of deformation which induces the variation of form visually identified by classic crushing effect: All the fault of the pressure difference!
How the vacuum works
Get the vacuum means means Reduce the pressure regime that is established internally to the containerbringing it from one initial condition – equal to external atmospheric pressure – with a final condition, with a lower value than the initial one, as close as possible to zero. In normal conditions, the air present inside the container exerts one pressure on all the surfaces with which it is in contact, food included. This pressure value also depends on air densitywhich in turn is linked to what it is compressed in the available space.
By removing part or all the air present, we are gradually decreasing the density and therefore the pressure simultaneously. It also appears clear that, to maintain this mechanical state, The internal part of the container must not communicate with the external oneotherwise this condition can never be achieved. At this point, the external atmospheric pressure – which instead remained unchanged – It now acts on the container without an equal and contrary force that the scale (completely or in part). What is the result of this imbalance of forces? It is generated a crushing effect Which is well seen on the classics – and often used – bags for vacuum, which closely adapt to the shape of the content due to the action of this pressure imbalance acting on the surfaces of the envelope.
The case of coffee: because the package is rigid
The ground coffee is also packaged through a vacuum procedure. Coffee and wrapping are presented as a sufficiently rigid structure to appear a single solid block. In reality, this effect is indirectly caused by the presence of vacuum: the absence of air internally to the coffee envelope makes it granules can be compact and organized in order to minimize the voids presentprecisely because of the atmospheric external pressure that now acts undisturbed and without an internal counterpart that could balance it. This compaction effect is nothing more than The result of the deformation that undergoes the solid skeleton consisting of these coffee granules. Consequently, the solid granular matrix has a stronger interaction between the various grains, precisely due to this external action, and It therefore indirectly has a greater resistance and rigidity. The result? The coffee pack appears solid and non -deformable to us.
As soon as we make a small hole on the container, the internal-exterior pressure regime is rebalanced and the passive effect on the resistance and stiffness, caused by the previous imbalance of forces, fails: grains lose part of the friction and, macroscopicallyforgive resistance and rigidity. Result? The package does not appear to us more rigid as before, but rather it can now undergo great deformations. In reality, this process of variation of the structural behavior of the coffee box is not immediate but it depends on how big the hole we practice is. In fact, the migration of air from the outside to the interior needs a time to take place which is the greater the small the hole is. That’s why, in this transitional phase, the mechanical behavior of the system changes gradually, worsening as time passes.
Used machines to make the vacuum
The tool used to remove the air from the containers on which to produce this pressure reduction is technically called void pump. There are different types, depending on the field of application and the emptiness level you intend to reach. Indeed, a zero pressure is technically impossible to reachbut a very small value, however not null, is sufficient to meet the needs according to the application cases. A void pump consists of an electric motor, which serves to manage the mechanism that triggers one depression aimed at the aspiration of the air in the area of interest. This depression can be created for example through a flexible membraneor through rotation palettes, which create spaces of air accumulation by dilation, and then remove it in specific times of the cycle by subsequent compression.
