The effect of ultrasound treatment on microbial and physicochemical properties of Iranian ultrafiltered feta-type cheese

Pasteurization failures in the dairy industry have been reported in many previous studies. Hence, ultrasound, as a nonthermal alternative to pasteurization, has been studied in recent years. In this research, retentate of ultrafiltered milk was pasteurized, inoculated with Escherichia coli O157:H7, Staphylococcus aureus, Penicillium chrysogenum, or Clostridium sporogenes, and then treated with ultrasound for 20 min at frequencies of 20, 40, and 60 kHz and intensity of 80%. Microbial and physicochemical properties of the subsequently produced ultrafiltered white cheeses were investigated throughout 60 d of ripening. Sonication at 20, 40, and 60 kHz reduced counts of E. coli O157:H7, S. aureus, P. chrysogenum, and Cl. sporogenes by 4.08, 4.17, and 4.28 log; 1.10, 1.03, and 1.95 log; 1.11, 1.07, and 1.11 log; and 2.11, 2.03, and 2.17 log, respectively. Sonication improved the acidity of ripened cheese, and sonicated samples had lower pH values than control samples at the end of storage. Sonication did not affect fat in dry matter or the protein content of cheese during ripening, but it did accelerate lipolysis and proteolysis; the highest rates of lipolysis index (free fatty acid content) and proteolysis index (water-soluble nitrogen) were observed on d 60 of ripening for samples sonicated at 60 kHz. Sonication did not affect cohesiveness or springiness of cheese samples, but hardness and gumminess increased in the first 30 d and then decreased until 60 d of storage. Furthermore, ultrasound treatment improved organoleptic properties of the cheese. In terms of overall acceptance, samples sonicated at 60 kHz received the highest sensorial scores. Results showed that sonication can improve microbial, physicochemical, and sensorial properties of ultrafiltered white cheese.