The oil in the vacuum pump can get discolored for several different reasons. There could be rust forming in the pump, the oil might be old, there may be too much acid or sugar in the food being freeze dried, or the gas ballast valve may need to be opened. For starters, Dairyland oil is the best oil to prevent rust and other discoloration in the pump. The blue color of Dairyland oil is caused by detergents in the oil that helps with water and sugars.

Another option to help with oil discoloration is to use the gas ballast valve. The difficulty with the gas ballast valve is that it should usually only be opened when the unit is already at vacuum. When the gas ballast valve is opened during pump down it can spit oil.

Some acidic foods like pickles and pickled foods can cause rust in the pump very quickly since some of the acid fumes will be ingested into the pump. If you suspect that there is rust in your pump you can take the cover off the pump to expose the pump head and use a brush to clean the rust off. Additionally you can use a product called Evapo-Rust ER012 to clean the pump. To do this, drain the oil, fill the pump with Evapo-Rust all the way full, let it sit for 24 hours, drain the Evapo-Rust, then flush once or twice with clean oil. Discard the flush oil, it should not be used in the pump. Not all rust removing products are safe for the pump. Evapo-Rust has been tested and works well with the Blue Alpine pumps.