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Introduction: Lactoperoxidase (LPO) is an enzyme found within the main components of milk, it has an antibacterial function and is a widely used indicator for the heat treatment of milk. Objective: Therefore, the objective of this study was to determine the presence of OLP in five brands of processed commercial milk and a sample of unprocessed milk. Methodology: The methodology used is based on the interaction of milk with the 1% guaiac system and 12% hydrogen peroxide (H2O2), forming the tetraguayan complex that corresponds to the development of a salmon color. The total solids (TS) were also determined by gravimetry, these values were correlated with the presence of LPO, with these data the quality of the milk was established. Results and Discussion: LPO was obtained in raw milk (LC) and in the LGV, LSC and LAl samples, not so in the LA and LT samples. The determination of ST showed not to have a correlation with the LPO, so only the latter is established as an indicator of quality, this when it is negative, indicates that the milk was subjected to processes of overheating (˃77.8 ° C) and destruction of the peroxidase enzyme during pasteurization. Losing with this, characteristics of its natural state and, therefore, the nutritional value has decreased.