Parameters for coagulation properties of direct acidified milk and for the structure of milk gels

Sour milk products are produced for the most part by bacterial acidification. The parameters of acidification and gelation are constrained within a certain range of temperature and period of time due to the activity of the applied culture. If acid is directly added to milk the acidification can be separated from the gelation and therefore, a distinction between parameters of acidification and those of gelation is possible. The aim of the study was to examine the influence of direct acidification on the coagulation properties of caseins and on the structure of the gels in relation to pH and chosen acidulants. Skim milk was cooled to a temperature below 2 degrees C and the pH was adjusted within the range of 4.2 to 5.3 followed by gelation at a certain temperature without stirring. To study the coagulation properties of caseins the onset of gelling was measured in oscillation experiments. Parallel to this the samples that had been filled into a beaker were put into a water bath with gelation temperature. The structure of these gels was examined by measuring the firmness. It was shown that direct acidification will only lead to homogeneous milk gels, if the caseins are prevented from aggregating during the acidification. The colloidal stability of caseins can be described as a linear relationship between the pH of the acidified skimmilk solution and the temperature of gelling. Acidification without coagulation has to be carried out under conditions below that line. The position of this coagulation line can be moved by the chosen acidulant. So, the coagulation line is shifted to pH 0.2 lower values if hydrochloric acid is applied instead of lactic or citric acid. From the measurement of the gel point the range of gelation in which set-type milk gels are formed can be derived. Homogeneous and stable gels are formed within the range of pH 4.5 to 5.0, if skim milk is acidified with lactic or citric acid. The direct acidification with the latter acidulant produced the firmest gels. Acidification with hydrochloric acid formed markedly weaker gels and the range of gelation was between pH 4.3 and 4.8. In order to study the rate of acidification the direct acidification was compared with bacterial acidification in terms of gel firmness. Gels were markedly firmer when direct acidification was applied instead of bacterial acidification.