The effectiveness of US in food processing depends on the quality of the US equipment, the parameters used and the initial microorganism rate of growth in the product. The main causes of contamination by micro-organisms are: (i) poor hygiene of the personnel due to bad work habits (handling contamination) and lack of hygiene formation, (ii) inadequate or non respect of cleaningprocedures, (iii) cross-contamination, (iiii) micro-organisms growth abnormally high due to bad storage conditions of the raw materials and/or non respect of requirements by the purveyor.