Characterization and Comparison of β-galactosidase Enzyme Activity in Yogurt Bacteria Isolated from Sheep's Milk Based on Lactose Hydrolysis

Milk consumption in Indonesia remains low, partly due to lactose intolerance caused by limited lactase activity. Although microbial β-galactosidase from lactic acid bacteria may improve lactose digestion, its activity in bacteria isolated from sheep milk yoghurt remains poorly characterized. This study aimed to isolate and assess β galactosidase-producing bacteria from sheep milk yoghurt and determine their ability to hydrolyze lactose. Sheep milk was fermented using a commercial starter, followed by bacterial isolation through serial dilution and colony purification methods. Isolates were characterized by colony morphology, Gram staining and biochemical tests. Selected isolates were then tested for β-galactosidase activity using lactose-containing media, followed by cell permeabilization using ethanol and enzyme activity measurement using the GOD-PAP method based on the amount of glucose produced. Four bacterial isolates (BK1–BK4) were obtained, exhibiting lactic acid bacteria characteristics and presumed to belong to the genus Lactobacillus based on rod-shaped morphology, Gram-positive reaction, and homofermentative properties. All isolates were able to hydrolyze lactose, although at relatively low levels. The highest enzyme activity was observed in isolate BK1 at 0.034 U/mL with a hydrolysis percentage of 1.417%, followed by BK4, BK3 and BK2. Statistical analysis showed no significant differences among isolates (p > 0.05), indicating similar β-galactosidase production abilities. The low enzyme activity was presumed to be influenced by suboptimal permeabilization methods and reaction conditions that did not yet allow optimal substrate access to intracellular enzymes. Sheep milk yoghurt isolates show potential as β-galactosidase producers; however, extraction and reaction conditions require optimization to enhance activity.