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Mix sensitive thickeners and stabilizers gently

Dispersing without shear using the vacuum expansion method

Thickeners and stabilizers are sensitive to shear. If they are mixed in liquids, agglomerates are initially formed, which then have to be broken down again by dispersion. The solution to this problem is a process in which the primary particles of the polymer powder are separated by vacuum expansion before being introduced into the liquid, completely wetted on first contact with the liquid, dispersed in situ under vacuum conditions and hydrated agglomerate-free under excess pressure.

ystral TDS-Induction Mixer [Translate to Englisch:] Der TDS-Saugmischer erzeugt in seinem Mischkopf ein Vakuum, mit dem er Pulver direkt unter die Flüssigkeitsoberfläche saugt

Thickeners and stabilizers have the task of giving the end product optimum rheology and texture as well as a pleasant mouthfeel for the consumer. They prevent the product from separating and recipe ingredients from settling or floating. They are initially available as a powder and are used in low concentrations due to their strong thickening effect. To achieve their maximum effect, they require colloidal disintegration in the liquid phase. Proteins also have a viscosity-increasing effect, but are much more shear-sensitive than thickeners and stabilizers.

However, the specific properties of these powders also make them difficult to process. They are usually dusty, sticky and form lumps and agglomerates. They are difficult to stir in and float on the surface of the liquid. If they are then pulled down from the surface with increased stirring power, air also always gets into the product via the drum. This air is undesirable because it is difficult to remove. However, external input via injectors or inline blenders is also problematic, as conventional systems of this type also initially only produce agglomerates.

Once lumps and agglomerates have formed, they have to be broken down again. To do this, dispersion is continued until all lumps are destroyed. With most of these powders, however, hydration begins as soon as they first come into contact with water. They unfold shear-sensitive polymer structures and form gels. From this point onwards, any further dispersion reduces their effectiveness.

To compensate for the loss of effectiveness, the thickeners, proteins and stabilizers must be more concentrated. This increases the costs. This does not improve the quality, flavors are masked. The flow behavior and appearance of the product appear unnatural if too much thickener is used, and the appearance and ultimately the image of the product suffer.

The cause of the problem is always powder agglomerates, which are formed when the powder is added to the liquid and whose subsequent dispersion damages the already hydrated gel. A different process must therefore be used to add the powder, in which no agglomerates are formed and no subsequent dispersion is required.

Dispersing using a vacuum
Powders consist of individual particles that come into contact with each other. However, there is air between these particles. Air can be expanded under vacuum, i.e. the air expands. This effect can be used in a powder stream flowing at high speed to separate the particles. In a powder stream flowing under vacuum, the air expands between the particles. The distances between the particles increase. This separates the particles. The distances between the particles increase as the vacuum increases in flight. No additional air is added - only the air that was already present in the powder expands.
The YSTRAL Conti-TDS uses precisely this effect for vacuum powder suction and dispersion. The machine sucks and disperses powder directly into liquids. To do this, it builds up an extremely powerful vacuum in its dispersing zone. The powder is sucked precisely into this area. The closer the powder gets to the dispersing zone, the greater the vacuum, the faster the powder flows and the greater the distances between the individual particles.

In the dispersing zone, the powder particles come into contact with the liquid under maximum turbulence and are completely wetted and colloidally broken down one by one. Agglomerates do not form. Further post-dispersion is generally not necessary.

At the moment of wetting, the powder is not yet hydrated and therefore not yet shear-sensitive. Maximum dispersion is desired and required at this moment, but not afterwards, not in a downstream dispersion stage or machine. The Conti-TDS wets and disperses in situ under vacuum during the passage through the dispersing and wetting zone. The exposure takes only fractions of a second.

Wetting from the inside
Even dry agglomerated or porous powders with capillary structures inside them are completely wetted in this way. This is due to the fact that these internal structures and capillaries are also filled with air in their initial state. The air inside also expands as it approaches the dispersing zone under vacuum - but without the particle or agglomerate disintegrating first. Complete wetting from the outside takes place under maximum vacuum.

The agglomerate or porous particle, which is completely surrounded by liquid, is then conveyed further with the liquid and passes from the zone of maximum vacuum and maximum dispersion into the surrounding zone of maximum overpressure and strong centrifugal action. The air in the inner capillary volume contracts implosively under the now acting overpressure and draws the surrounding liquid into the interior. Agglomerates disintegrate immediately in this way due to the simultaneous dispersion. Porous particles with internal structures are wetted from the inside.

The air previously contained in the powder and now released coagulates under the centrifugal effect of the rotor to form large air bubbles and is transported together with the liquid to the process container, where it escapes via the liquid surface.

The process is characterized on the one hand by particularly gentle dispersion and on the other hand by particularly fast dispersion and maximum utilization of the powdery ingredients. It is used with great success in the production of dressings, dairy products and desserts, in the production of dough, ice cream premix and beverage concentrates, baby food and many other products in the food production sector. Of course, there are also many similar applications in the manufacture of pharmaceutical products, cosmetics and in chemical production.

ystral Fachartikel Verfahrenstechnik

Magazine: Process engineering
Issue: 09/2017
Author: Dr. Hans-Joachim Jacob

About the author

Dr. Jacob is Senior Expert Process and Applications at ystral. Dr. Jacob, who studied mechanical engineering, joined the company in 1990 as a process engineer and has since been responsible for our key accounts worldwide. His professional passion is the mixing and dispersion of powders in liquids. During his long career, he has gained experience in handling thousands of powders from a wide range of industries and is happy to share his expertise in various technical articles, online seminars and lectures.

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