Calorimetric assessment of microbial growth in milk as affected by different conditions

Milk quality assessment is of paramount importance for dairy facilities. Standard microbiological techniques are time consuming and demand for faster, accurate methods is increasing. On the other hand, inefficient temperature control to handle raw milk, due to lack of appropriate facilities, requires the search for strategies to maximize quality from the microbiological standpoint. Activation of the lactoperoxidase system (LPS) has been established as an easy and safe procedure to attain such results. Good quality milk samples were obtained in sterile containers right after milking. A test temperature of 30°C was chosen based upon preliminary tests. Raw unpasteurized milk was incubated at 30°C for 16 h. Microbial growth was assessed at 4 h intervals and the maximum value attained was 4.59 × 10⁶ CFU ml^-1 after 8 h. Simultaneously, metabolic heat rate from the same samples was measured under isothermal conditions at 30°C. A regression equation was obtained to relate microbial growth and heat output, with a coefficient of determination of 0.99. Milk samples were divided in three equal aliquots for (a) pasteurization at 72°C for 15 s; (b) activation of LPS, and (c) untreated control. LPS activated milk maintained good quality during 8 h, while pasteurized milk showed an initial increase in microbial population after 8 h followed by a decline possibly due to residual activity of its LPS.