Processing of Soft Hispanic Cheese ("Queso Fresco") Using Thermo-Sonicated Milk: A Study of Physicochemical Characteristics and Storage Life

Queso fresco is a handmade cheese consumed in Latin America and some regions of the United States. However, deficient milk processing has affected its microbial quality and it has an extremely short shelf life and low yield. The objective of this work was to process queso fresco using thermo-sonicated milk; physicochemical parameters were evaluated, including microbial quality during storage (4 degrees C). An ultrasonic processor (UP400S, 400 W, 24 kHz, 120 mu m) was used to sonicate raw milk. Seven milk systems (500 mL each) were evaluated: 1 untreated, and 6 treated at 63 mu C/30 min; 63 degrees C/10 min + sonication; 63 degrees C/30 min + sonication; 72 degrees C/15 s; 72 degrees C/15 s + sonication; and 72. C/1 min + sonication. A conventional cheese-making process was followed for all systems. The effect of sonication on milk was quite noticeable. Curdling times were reduced considerably, cheese yield (20.6%) was almost doubled, and luminosity of cheese was increased (L*). Textural properties and microstructure images matched very well. Queso fresco processed at 63 degrees C/120 mu m/30 min had the best quality. After storage for 23 d at 4 degrees C mesophilic count was just 4 log; psychrophilic count, 3.5 log; and enterobacteria count, 3 log. The pH and color remained almost constant and a minor degree of syneresis was observed at end of storage. Due to microstructural rearrangement of the milk components such as fat globules and casein micelles, cheese yield was doubled compared to the traditional handmade product. Shelf life was extended considerably and the product had higher quality.