This Doctoral Thesis is framed within the European project CRAFT 512626 (2004-07) which main goals were to study the changes produced by ultra-high pressure homogenization (UHPH) in milk and in different vegetal milks and the potential of this new technology to produce from milk different products. The UHPH is a new technology that enables the microbial inactivation, the modification of the main structures of milk and even changes in enzyme activities. With this potential use was thought interesting to address this thesis work involving the study of the UHPH application in milk as an alternative to conventional heat treatment to obtain liquid milk for consumption. To meet this objective, in first place microbiological studies were conducted to establish the conditions of pressure and temperature needed to achieve an initial reduction of microorganisms according to the limits established for heat treated milk. Milk was treated in a range of pressures from 100 to 300 MPa (single and double stage) with inlet temperature (Ti) from 30 to 40ºC and, on the basis of these tests, it was determined that the most appropriate conditions to be used were 200 and 300 MPa with Ti = 30 y 40ºC. Furthemore, based on records of temperature obtained in the UHPH equipment, a high-pasteurized milk (PA; 90ºC, 15 s) was selected as a control sample. Three repetitions were carried out in which milk was treated under the conditions previously selected and then stored at 4ºC for 21 days. On these samples were studied the microbiological, physico-chemical and enzymatic changes and its effects on the proteolysis, lipolysis, oxidation, formation of volatile compounds and sensory characteristics of milk. In this work it was found that UHPH treatments at 200-300 MPa were as effective as PA treatment in reducing the microbial population, but unlike the heat treatment this technology avoids the overheating of the product as was shown by the thermal indicators studied. The microbiological reduction yielded a product with a shelf-life of about 18 days under refrigeration. On the other hand, due to homogenization the UHPH produced a reduction in the milk particles size. In relation to enzymes, the alkaline phosphatase was completely inactivated by all treatments. However, the lactoperoxidase and plasmin were not completely inactivated by certain treatments. The residual activity of plasmin together with the reduction of particle size in UHPH-treated milk led to greater hydrolysis of casein compared to PA milk. Milk samples treated at 200 MPa were characterized by a lipolytic process that was demonstrated by the increase of free fatty acids during storage and that was also confirmed through the sensory analysis. Whereas lipolysis was produced in samples treated at 200 MPa, oxidation took place in samples treated at 300 MPa. Nevertheless, oxidation was not perceived by the majority of consumers who described milk treated at 300 MPa as a milk without defects and it was the most prized in its overall assessment. The UHPH treatment at 300 MPa with Ti = 30ºC could be used as an alternative to pasteurization to obtain fluid milk because it allows microbiological, physico-chemical and organoleptic characteristics similar to PA milk but avoiding the characteristic cooked flavour of PA milk and the tendency to creaming during storage.
Utilización de la Ultra Alta Presión por Homogenización como alternativa al tratamiento de pasteurización para la obtención de leche de consumo
Pereda , J. (Author). 2 Dec 2008
Student thesis: Doctoral thesis
Student thesis: Doctoral thesis