Sabine Grüner, Frank Otto, Bernd Weinreich report on a new high-pressure spraying process which allows the production of free-flowing powders from liquids in a gentle way which helps prevent the degradation of sensitive components.
In food, cosmetic and the pharmaceutical industry, liquid and powder form substances often have to be mixed to obtain the final product. This mixing process is known to be challenging, especially when high viscous liquids are used. The homogenous mixing of pure powder form components is easier to realise and therefore different technologies to pulverise liquids were developed. In this regard fluid bed technology and the spray drying process have to be mentioned.
Due to the use of hot air stream in established technologies the pulverisation of liquids, particularly sensitive liquids, is often accompanied by losses of volatiles and degradation of sensitive components.
CPF technology (concentrated powder form) is a high pressure spraying process which allows us to obtain free-flowing powders from liquids in a gentle way. First of all, a gas, normally carbon dioxide, is dissolved under high pressure in the liquid and a gas-saturated solution is formed. The conditions are mostly selected so that carbon dioxide is in the supercritical state (p>73.83bar, T>31.04°C). The solution of carbon dioxide causes a marked reduction in the viscosity and in the surface tension of the liquid. This relieves the spraying even of high viscous fluids which was hardly possible in the past.
The gas-saturated solution is rapidly depressurised to atmospheric pressure in a nozzle. The gas is set free and very fine droplets of the liquid are formed. In case of carbon dioxide a remarkable lowering of the temperature in the spray tower is caused by the Joule-Thomson-Phenomenon. Simultaneously a powder form carrier is added to the sprayed liquid and intensively mixed with the fluid droplets by the released gas. The liquid is bound to the carrier by adsorption on the surface, agglomeration of carrier particles and binding effects of capillary forces in the pores. A free-flowing powder with liquid concentrations of up to 80per cent is obtained. The powder form product is separated from the gas by sedimentation, filtration, the use of a cyclone or an electrical field.
A principle flow scheme of a continuously operated plant is given in Fig.1. The liquid is pumped from a vessel into a static mixer, where it is infused with carbon dioxide. Normally the mixing device is operated under pressures from 80 to 250bar and temperatures from 20°C to 80°C. The gas-saturated solution is released into a spray tower and the generated fine liquid droplets are brought into contact with fluidised carrier particles, which were dosed concurrently in the spray tower. The resulting powder is removed from the bottom of the spray tower.
The CPF technology was tested with more than a hundred different liquids and as many different powder form carriers. The versatile use of this process was discovered. Beneath low and high viscous liquids the CPF technology can be applied to lipophilic and hydrophilic liquids or aqueous and oily systems. Depending on the application of the product, nearly all powder form carriers are suitable for this process. In numerous experiments starches, celluloses, silicic acids and many more powders were successfully tested.
Regarding other technologies, various advantages are gained. Beside the fact that high viscous liquids can be easily processed, the pulverisation with CPF Technology takes place in an oxygen- and solvent-free atmosphere at very low temperatures (-10°C0°C). Substances which are sensitive to temperature and oxygen can be processed with almost no change to their quality. The liquid remains unaffected by oxidation or thermal stress.
To date, CPF technology has been used for numerous natural and synthetic liquids in order to prove the universal application of the process. In addition to various plant extracts the range of liquids was extended to include flavours and fragrances as well as lipophilic and hydrophilic substances. Besides their different composition the principal difference between the liquids lies in their viscosity and polarity. It was shown in many experiments that the CPF technology is suitable for both single-component and multi-component systems, such as emulsions.
The mildness of the process was proven with sensitive flavours and essential oils. Due to the low temperature the loss of volatiles could be minimised and the inert gas atmosphere inhibits oxidation processes. The use of liquids for pulverisation with the CPF Technology is nearly independent from typical liquid properties. An extract of pulverised liquids is listed in Fig.2.
A wide range of substances has also been tested with the carriers used. Most powder form carriers are suitable for this technology. Carriers like silicic acid, starch, cellulose, salt, sugar, titanium dioxide, polymers, emulsifiers and many more were used in more than 1000 trials. The selection criteria constituted differences in material properties, particle form and size, bulk density, pourability, porosity and wetting behaviour. Typical carriers are given in Fig.3.
As mentioned, the liquid is bound to the carrier by adsorption, agglomeration or capillary forces in porous particles without being changed. Therefore the final product consists of a liquid part and a solid part. The maximum liquid rate achievable in the product depends strongly on the properties of the carrier. A dependency of the maximum liquid loading from the bulk density of carriers was determined. In principle, maximum liquid loading increases with a decreasing bulk density. When flaky or fibrous carriers are used sometimes a deviation to lower maximum liquid rates was determined.
In order to prove the mild process conditions of the CPF technology, some sensitive flavours were selected for pulverisation. To demonstrate the absence of oxygen during the whole process, flavours highly sensitive to oxygen were sprayed. The main compound limonene reacts rapidly to carvone in the presence of oxygen. Therefore an increasing content of carvone is an obvious pointer for oxidation. In Fig. 4. the carvone content of the original flavour(represented in the proportion to the content of citrale) and some CPF products are given. The results verify there is no increase of carvone, meaning no oxidation has occurred. The inert gas atmosphere in the CPF process is sufficient to pulverise oxygen sensitive liquids without degradation.
Besides the development of a new process for pulverisation of liquids to obtain free-flowing products, it is also of interest to influence the properties of the powder form products by using suitable additives. For this antioxidants, flavours and natural colourings were added to give the products a special quality, meaning a better stability, colour, odour or taste. In all cases additives were added directly to the liquid before the pulverisation step was performed.
In many experiments additives were also used to obtain powder form products with special release effects. In this regard the aim was to get powders with temperature and time- controlled release behaviour. Suitable analytical methods were developed to investigate the influence of different additives on the release in liquid or gaseous media.
The release of a vitamin in water was measured. The release effect was achieved by adding a lipophilic thickener to the liquid mixture. The red line in Fig.4 represents the release ratio when the powder is stirred in water at room temperature. About 20per cent of the valuable ingredient was released independent from stirring time. When the water is heated the release ratio increases up to a complete release, shown by the blue line. Depending on the type and amount of additive, release curves with a more or less sharp increase were obtained.
What kind of release should be realised is mostly governed by the product application. For instance the use of lipophilic thickeners increases the stability of powdered flavours by minimising the loss of volatile flavour components during storage and further processing.
enquiry no 21
Sabine Grüner, Frank Otto and Bernd Weinreich are with Raps GmbH, Kulback, Germany. www.raps.de
