Alicyclobacillus acidoterrestris is an aerobic, Gram-positive, acid-resistant, non-pathogenic and spore-forming microorganism. Due to its sporulation capacity, this microorganism often found in industrialized citrus juices because they survive thermal processing, multiply and deteriorate the product. In this way, it leads to the development of undesirable sensory

Alicyclobacillus spp. they are spore-forming bacteria that deteriorate acidic fruit-based drinks, causing economic losses, with A. acidoterrestris being the most studied, responsible for causing sensory changes, especially in orange juice. The use of natural antimicrobial agents in foods can be an option with great advantages for the industry and the consumer. Therefore, the objective of this work was to investigate the antimicrobial activity of extracts fermented by kefir, from a grape by-product, against A. acidoterrestris. Four extracts were prepared with kefir grains, being 1-grape extract and kefir grains; 2-grape extract, brown sugar and kefir grains; 3-grape extract, ultrasound and kefir grains; and 4-grape extract, ultrasound, brown sugar and kefir grains. The four extracts were fermented at 28 ºC for 7 days. The extracts were centrifuged at 10,000 rpm for 10 min and the supernatant was subjected to membrane filtration (0.22 μm). The minimum inhibitory concentration (MIC) was determined by the serial microdilution technique of extracts of 50 to 0.1% concentration in Bacillus acidoterrestris (BAT) medium. The minimum bactericidal concentration (CBM) was also determined. Structural changes in cells after treatment were evaluated by scanning electron microscopy (SEM). The MIC value capable of inhibiting the visible growth of A. acidoterrestris for all extracts was 1.6%, while the CBM was 50% for extracts 1 and 3, while for extracts 2 and 4 the CBM was 25%. The results show that extracts 2 and 4 obtained better CBM value, possibly because kefir produced more secondary metabolites with the addition of brown sugar, in addition, the use of ultrasound did not interfere. The results of the inhibitory and/or bactericidal concentration indicate that the extracts have activity against A. acidotrrestris.