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Developments in Continuous Aerating of Chocolate Products and Fat Creams

Continuous mechanical aeration in the food processing industry is a widely accepted procedure nowadays as a logical next step in the sophistication of the batch ‘whipping’ process in the tank. Because of the complete control of the variables like system-pressure, throughput, gas to product ratio and mixing rpm it offers so much in uniform product quality, total product control in terms of density and structure and a continuous flow of fresh aerated product arriving at the filling/depositing end of the production line.

For example, the extruded marshmallow with the low densities of 0.25-0.35 kg/l would not have been possible without the continuous aerator and several cake and cream products owe their stable foam structure to the structure control in a continuous whipping process.

However, there is more!!

For some products, aeration was not even possible without the application of the continuous aeration system. In the absence of aqueous proteins in food products, it becomes difficult to aerate and generate a good foam. By virtue of its long molecular structure and elastic properties the proteins are eminently suitable for encapsulating the finely dispersed gas, and together they form a stable membrane around the gas bubble. This therefore, substantially improves the ‘holding capacity’ of the product. When no proteins are present, the gas holding capacity of the product has to be created by its viscosity. Here we look at an optimum between not too viscous to pump and enough viscosity to hold the gas.

Since fat-based products gain viscosity when fats solidify, the temperature control in the continuous aeration process is of essential importance. Friction heat of the rotor speed in the mixinghead upsets this whole temperature control and consequently fats can separate out of the product.

New design mixing head for temperature sensitive products.

Extensive research over the last couple of years has resulted in a complete redesign of the pre-mix chamber of the mixing head. Prior to entering the mixing head the air (or gas) is injected as larger bubbles in the pumpable liquid. To be able to achieve the fine distribution of the gas throughout the liquid and turn it into a fine foam, the process is divided into a breakdown of the gas bubbles in a gas-liquid mixture and subsequently further diminish the bubble size into micron gas-cells. A conventional mixing head uses one high shear zone, but the new redesigned mixing head separates the 3 operations in: injection of the gas, a ‘low shear’ premix chamber and a &lsquohigh shear’ part, both integrated in the same mixing head with the same drive.

Rotor 
Part of the redesigned mixing head with the ‘low shear’ axial premix chamber and the ‘high shear’ radial part.

This figure shows a part of the mixing head, where the product passes through the premixing chamber in the centrifugal part while the foam finishing is done in the high shear radial part of the stainless steel head.

In the new premix chamber the shear is very low thanks to the size of the pins, the larger distance between the rotary and stationary pins and the axial distance to the mainshaft.

Once the coarse gas-liquid mixture enters the radial part of the mixing head, it is subjected to the conventional high shear, which in this case (thanks to the redesigned premix chamber) does not have to be so ‘high shear’ any more.

Tests have shown that rotor (mixing) speeds can be reduced up to 20% compared to the conventionally designed mixinghead, and therefore reducing friction heat in the product.

Chocolate products and fat creams which are only dependent on the temperature sensitive viscosity for holding the gas, benefit very much from this process. Now for the first time the product can actually leave the mixing head colder than the infeed temperature, without temperature shocks on the chocolate fat crystals. Besides, it opens a whole new application with the ‘aerated chocolate and chocolate spreads’.

In the conventional process, the chocolate spreads had to be enriched with protein additives to be able to make the product less sensitive to temperature control during the aeration.

Not only the chocolate products benefit by the combination of low shear premixing and high shear aeration. Proteins in general do not like the high shear and we can see a better stability of many products that were aerated in the new 2-zone mixing head (figure 2). In premixing in the axial part, there is less shear necessary in the radial part.

Front 
Continuous aerating system with the redesigned, 2-zone mixing head

A typical example is also the continuous whipping of fresh cream, where warm gelatin is injected as a second stream in the mixing head to make the whipped cream defrost stable. With the use of a premix chamber the product comes out whiter with an even micro-cellular structure and dull white appearance. Without the premix chamber the product is more yellowish with somewhat shiny appearance which indicates damage to proteins, and consequently separation of whey. Also products containing eggwhite benefit from the reduced rotorspeed since eggwhite tends to denaturalize when overbeaten.

The new redesigned mixing head in the continuous aeration process offers the ultimate control of shear and temperature for typically heat sensitive products like fatcreams, chocolates and chocolate spreads, whereas many other products like eggwhites etc. benefit from the strongly reduced rotorspeed.

Therefore the typical advantages of this redesigned premixing chamber are:

  • less shear-damage to the product;
  • lower rpm of mixing head and less shear necessary;
  • heat exchange and aeration in one mixing head;
  • lower power (kW) of drive necessary;
  • less friction heat into the product.

The Author:
Robert Luiten has a BSc in Chemical Engineering. He has worked in the Food Processing Industry since 1977 and particularly the last 9 years, he has specialized in many aspects of continuous aeration processes in the Dairy, Bakery and Confectionary Industry.

from: CONSUDEL JANUARI 2002

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