Influence of brine concentration and temperature on composition, microstructure, and yield of feta cheese.

The protein matrix of cheese undergoes changes immediately following cheesemaking in response to salting and cooling. Normally, such changes are limited by the amount of water entrapped in the cheese at the time of block formation but for brined cheeses such as feta cheese brine acts as a reservoir of additional water. Our objective was to determine the extent to which the protein matrix of cheese expands or contracts as a function of salt concentration and temperature, and whether such changes are reversible. Blocks of feta cheese made with overnight fermentation at 20 and 31 degrees C yielded cheese of pH 4.92 and pH 4.83 with 50.8 and 48.9 g/100 g of moisture, respectively. These cheeses were then cut into 100-g pieces and placed in plastic bags containing 100 g of whey brine solutions of 6.5, 8.0, and 9.5% salt, and stored at 3, 6, 10, and 22 degrees C for 10 d. After brining, cheese and whey were reweighed, whey volume measured, and cheese salt, moisture, and pH determined. A second set of cheeses were similarly placed in brine (n = 9) and stored for 10 d at 3 degrees C, followed by 10 d at 22 degrees C, followed by 10 d at 3 degrees C, or the complementary treatments starting at 22 degrees C. Cheese weight and whey volume (n = 3) were measured at 10, 20, and 30 d of brining. Cheese structure was examined using laser scanning confocal microscopy. Brining temperature had the greatest influence on cheese composition (except for salt content), cheese weight, and cheese volume. Salt-in-moisture content of the cheeses approached expected levels based on brine concentration and ratio of brine to cheese (i.e., 4.6, 5.7 and 6.7%). Brining at 3 degrees C increased cheese moisture, especially for cheese with an initial pH of 4.92, producing cheese with moisture up to 58 g/100 g. Cheese weight increased after brining at 3, 6, or 10 degrees C. Cold storage also prevented further fermentation and the pH remained constant, whereas at 22 degrees C the pH dropped as low as pH 4.1. At 3 degrees C, the cheese matrix expanded (20 to 30%), whereas at 22 degrees C there was a contraction and a 13 to 18 g/100 g loss in weight. Expansion of the protein matrix at 3 degrees C was reversed by changing to 22 degrees C. However, contraction of the protein matrix was not reversed by changing to 3 degrees C, and the cheese volume remained less than what it was initially.