Changes in proteins and monoterpenic bioactive components as a result of fermenting milk of different goat breeds with kefir grains were investigated. SDS-PAGE was performed to determine the protein profile. Percentages of aS-casein (aS-CN), b-CN, k-CN, b-lactoglobulin, bovine serum albumin and other peptides were measured. Hair goat milk contained 5.7% aS1-CN, whereas cow milk contained 37% aS1-CN and the kefir fermentation process decreased the aS1-CN content in goat milk. Milk samples from extensively fed goats contained higher b-CN. Proteolysis increased due to the kefir fermentation, and new peptides were formed. Monoterpenic compounds such as carvacrol and thymol were present (0.14 mg 100 g-1 and 0.17 mg 100 g-1, respectively) in the Hair goat milk. After kefir fermentation, carvacrol was present (0.30 mg 100 g-1) in Hair goat kefir. This study is the first to show significant differences in proteolysis and monoterpenic bioactive components in the goat milk and kefir.(c) 2022 Elsevier Ltd. All rights reserved.