Air pockets result in air bubbles in the finished sausage. Use the largest stuffing tube that works with the casing being used so that the meat mixture is not heated by forcing it through too small of a tube. Sufficient casing is slipped onto the stuffing tube and a knot tied in the end before the stuffing starts. Push the mixture into the casings at a steady rate so that the sausages are even in shape and texture. Inspect the surface of the individual sausages and puncture a small hole with a very sharp skewer over any air bubbles.
Some recipes call for evenly puncturing the entire surface of the casing.
Always make the puncture holes as small as possible so that not too much fat is exuded during cooking. This involves hanging the sausages in a ventilated cabinet that has both temperature and humidity controls. Since this step is not usually possible at home, it is left out. The result is that the color of the meat is not as intense as it should be and the casing may not be as transparent. My preference is the latter method.