Impacto do tratamento por alta pressão na qualidade microbiológica e tecnológica do leite para o fabrico do queijo

Abstract

High pressure is recognized as one of the most promising low temperature processing technologies and can be applied across a wide range of pressure/time/temperature conditions trinomials. The processing of raw milk by high pressure technology can cause changes in its microbiological, physicochemical and technological properties. Thus, the objective of this research was to evaluate the impact of non-thermal processing by high-pressure technology on milk, as an alternative to conventional thermal processing, in order to produce microbiological safe milk, but with greater technological aptitude for the manufacture of cheese, expressed in the form of higher yield. Thus, different pressure/processing time (p/t) of milk were tested in order to establish the best p/t combination, to allow a better microbiological quality, and consequently a longer shelf life, as well as curd yield. In a first phase, 3 pressures (300, 450 and 600 MPa) were tested for 6 minutes. In a second phase the pressure was set at 600 MPa and tested different processing times (6, 13 and 20 minutes). In all trials the milk, before and after treatment, and during storage under refrigeration up to 15 days, was analyzed in terms of microbiological parameters (aerobic mesophiles, aerobic psychrophils, lactic bacteria, enterococci, staphylococci, enterobacteria and molds and yeasts), physical-chemical (color, pH and titratable acidity) and technological aptitude (coagulation yield).From the first phase it was observed that pressures higher than 300 MPa allowed a greater inhibition of microbial growth. The most sensitive microbial groups at 300 MPa were enterobacteria and enterococci, but the application of pressures at 450 and 600 MPa resulted in inhibition of all target microbial groups. Regarding physical-chemical parameters, these were positively affected by the increase of pressure. On the other hand, high pressure application in milk processing had a variable impact throughout the storage time; after 7 days, 24 to 46% of coagulation yield losses were observed. Regardless of the order of magnitude of the High Pressure, only slight changes in luminosity were observed, and the values of a * and b * were observed.From the second phase, it was found that the increasing of the processing time from 6 to 20 minutes at 600 MPa allowed maintaining a viable number of lactic acid bacteria, enterococci, staphylococci and enterobacteria less than detection limits over 7 days and staphylococci and enterobacteria up to 15 days of storage at 4 ° C. Slight decrease in pH values accompanied by a titratable acidity increase was observed over 15 days of storage. Milk samples processed by high pressure at 600 MPa and for 13 and 20 minutes allowed higher and stable coagulation yield values over the entire storage time, again without significant impact on color.

Date of Award	10 May 2018
Original language	Portuguese
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Suzana Teixeira e Silva (Supervisor)