The Effects of Substrates and Sonication Methods on the Antioxidant Activity of Kefir Postbiotics oleh Gerardo Chávez-Alzaga, Raúl Alberto Reyes-Villagrana, Gerardo Pavel Espino-Solis, Martha María Arévalos-Sánchez, Ana Luisa Rentería-Monterrubio, Rogelio Sánchez-Vega, Eduardo Santellano-Estrada, Norma Angélica Bolivar-Jacobo, Juan Manuel Tirado-Gallegos, América Chávez-Martínez