The frequency of testing would be dependent on factors such as the process being used, the kind of food being processed and other factors as determined by a process authority. Foods that are categorized as non-potentially hazardous by a pH and/or water activity test might not have to be tested again until the recipe or process is changed. Other foods such as acidified foods would require testing on every batch to insure that the acidification process is being done correctly or corrective action must be taken.