Low-cost permeate powder process

Traditional permeate powder production includes evaporation with flash cooling followed by a time-consuming crystallisation before the concentrate is finally spray dried, a process requiring a lot of energy, expensive processing equipment and building costs. Niro has developed the 'Tixotherm process' which eliminates the drawbacks of the traditional process. It is now possible to produce permeate powder without crystallisation of the concentrate and without spray drying.

Whey proteins can be separated from the whey from cheese production by membrane filtra-tion. The remaining permeate contains lactose, salts and a lot of water. Until recently this low-value product was dumped. However, new environmental legislation has effectively outlawed this method of disposal and many manufacturers have resorted to spray drying the permeate. This is an expensive process for a low-value product. Current practice is evaporation to around 60% total solids followed by flash cooling and a batch-wise crystallisation process, which takes 16-20 hours prior to the final spray drying.

As the price for permeate powder is low, many attempts have been made to develop a process involving low capital investment, low energy consumption and reduced space requirements. The Tixotherm process, developed by Niro, is a solution that meets these requirements.

In the Tixotherm process the permeate is evaporated in a conventional falling film evaporator with either thermal or mechanical vapour recompression to about 60% total solids (TS). To ensure a trouble-free 20 hour operation of the evaporator, a decalcification process can be applied upstream to the membrane filtration.

After evaporation there is a simple three-step process consisting of concentration, curing and final drying and cooling.

The concentration is conducted in a Rosinaire Paddle-Processor, manufactured by Barr-Rosin. The uncrystallised 60% TS product from the evaporator is concentrated as a thin film on the heated side walls to 86% solids. The high shear-rates utilise the thixotropic nature of permeate and almost instantaneous crystallisation is achieved by optimising moisture and temperature of the paste. The evaporated moisture is removed by counter-currently drawing air through the annular space. This saturated air is cleansed in the scrubber and exhausted to the atmosphere.

The Rosinaire paddle dryer consists of a double jacketed horizontal tube heated by means of steam. A centrally located shaft with paddles moves the concentrate slowly forward while the evaporation takes place. The counter-current air stream and the evaporation itself keeps the temperature of the product low, avoiding discolouration.

As the lactose is super-saturated a spontaneous crystallisation occurs. The product viscosity increases, partly due to the crystallisation and increase in the solids content and also due to the thixotropic nature of the product. The vigorous mechanical treatment in the Rosinaire keeps the product fluid and when it is discharged at about 86%TS it is like a paste.

The holding, curing and stabilisation step is conducted in a screw conveyor with two augers. The screw conveyor is double jacketed and chilled water is circulated to cool the product. The lactose becomes super-saturated again so that further crystallisation is obtained and, with sufficient holding time, it is obtained almost 100% crystallisation while browning of the product is avoided.

At the discharge from the screw conveyor the product exhibits a texture suitable for fluid bed drying. The product is then further processed on a combined back-mix/plug-flow fluid bed, similar to the lactose fluid bed, operating with a powder depth of about 1 m. The fluid bed is non-vibrating, reducing mainten-ance costs, and is compact in comparison with vibrating systems. The outlet air from the fluid bed is cleaned in a Sanicip bag filter which is filter is fully CIP'able and provides effective powder removal from the exhaust air.

Milling of the final product is recommended.

The Tixotherm process (Figure 1) offers many advantages in comparison with traditional processes. It requires only 25% of the building space needed for the wet process and it uses 30% less energy.

To summarise all the advantages:

• No pre-crystallisation tanks are needed;
• The high-concentration step takes place at atmospheric pressure;
• Spray drying is not necessary;
• Building cost savings;
• Savings of energy;
• Attractive investment costs.

The Tixotherm process has been tested in a pilot plant and an industrial size plant has already been designed.