Vacuum bleaching

Color and peroxide values (PV) have been compared for tallows bleached at atmospheric pressure and under vacuum at several temperatures (67). At 90°C, atmospheric bleaching gave better color than bleaching under vacuum, but peroxide values were lower for the vacuum bleached product (PVs of 2 versus 16, respectively). At higher temperatures, color was better and peroxide values were lower. A bleaching temperature of 90°C to 110°C was recommended for beef tallow. 

Figure 24. Standard continuous vacuum bleaching system. (Diagram Courtesy LFC Lochem B. V.) (This figure is available in full color at http //www.mrw.interscience.wiley.com/biofp.)...

The oil is mixed with Trysil (hydrated silica) under atmospheric conditions for 15 min in the vacuum bleaching vessel. 

In operation, the oil is charged to the kettle and heated to about 70-80°C, the clay is added, and heating continued to about 95-105°C. About 0.5-1.5% clay is required depending on oil quality and product specifications. Contact time of 10-20 min is allowed at the maximum temperature before filtering is started. Filters may be of a variety of designs, but plate-and-frame type filters, rather than tank filters, are often used because they are somewhat easier to manipulate. In vacuum bleaching these operations are performed with the bleaching vessel under reduced pressure. 

Fig. 2. Continuous vacuum bleaching system (LURGI). Adapted from Appelqvist and Ohison (1972).

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