Pressure
Some of the water in moist foods can evaporate and contribute some additional vapor. Nonetheless, as long as the residual pressure in the package is very low, the improvement in quality and shelf life is significant. Products that are granular or porous are difficult to vacuum-pack properly, because the pores or holes in the product can retain air and make it difficult to remove. Powdered products are another challenge, because when the machine removes the air from the package, the airflow can carry some of the powder granules along for the ride. Adjusting the filling level may be helpful when working with a powder.