ABSTRACT
The objective of this study was to determine the effectiveness of high-pressure processing to inactivate Listeria innocua (a Listeria monocytogenes surrogate) in Queso Fresco, and to study the effects of the high-pressure treatment on cheese-crumbling properties. Queso Fresco was made with pasteurized, homogenized milk, lactic acid bacterial starter culture, chymosin, and flake salt. Cheeses were pressed (0.1 MPa) for 1h before crumbling and inoculation with a cocktail of 3 strains of L. innocua, and then pressed for 12 h (0.1 MPa). High-pressure processing treatments of sliced cheese rounds included pressure from 400 to 600 MPa for 1 to 25 min. Cheese sample temperatures, initially approximately 21°C, increased during pressurization and decreased gradually during the holding time. The highest temperature increase was to 23.6°C at 600 MPa. Greater than 5-log reductions occurred at set-point pressures of 500, 550, or 600 MPa when held for at least 15, 3, or 1 min, respectively. However, because inactivation was neither complete nor permanent and crumbling properties were not maintained under the conditions tested in this study, high-pressure processing is not recommended for Queso Fresco applications.
Subject(s)
Cheese/microbiology , Listeria/metabolism , Pasteurization/methods , Cheese/analysis , Cheese/standards , Food Handling , Food Microbiology , Food Quality , Hydrogen-Ion Concentration , Microscopy, Electron, Scanning , Pressure , TemperatureABSTRACT
Low methoxyl (LM) pectin was combined with 3-kDa molecular weight cut-off permeates from milk subjected to pH 6.7 to 5 and 7 degrees C or 40 degrees C with the objective of studying the effect of solubilized micellar calcium on viscoelastic properties of LM-pectin-milk mixes. Lowering the pH of skim milk with hydrochloric acid during ultrafiltration gradually promoted permeates to exhibit gel-like behavior when combined with LM-pectin. The onset of the gel-like behavior (G' > 1) occurred at a higher pH when permeates were obtained from milk filtered at 7 degrees C compared with 40 degrees C. As pH value during ultrafiltration approached 5 and regardless of temperature, G' for permeate-pectin mixes approached the same values (approximately 70 Pa) as G' for skim milk-pectin mixes. In all cases G' was highly correlated with free calcium concentration (r > 0.95). The gradual acidification of skim milk-LM-pectin using glucono-delta-lactone, promoted a sharp increase in storage modulus as pH approached 5.2 and a maximum G' increment (DeltaG') at pH approximately 4.9. From pH 4.9 to 4, G' continued to increase but at smaller increments. It was concluded that LM-pectin-casein micelle interaction in milk is a 2-step process: 1) solubilized micellar calcium dependent pectin-pectin interaction as pH approaches 5.0 to 4.9, and 2) pectin-casein micelle interaction in the 5.0-4.9 to 4.0 pH range.
