*3.1. Proteolytic Activity of Lactobacillus Strains*

Results of determinations of the initial and overall proteolytic activity of the tested LAB strains are presented in Figure 1. The initial proteolytic activity of all tested LAB strains was similar and did not differ significantly from that determined in the control sample. A balance was maintained at the early stage of fermentation between the consumption of free fatty acids and the degradation of peptides and milk proteins by LABs. In contrast, differences were observed in the overall proteolytic activity measured after 24 h. Among all analyzed LAB strains, the lowest activity was exhibited by *Lb. delbrueckii* 490, while the highest activity was exhibited by the *Lb. casei* 2639; the values of absorbance—being an indicator of the ongoing proteolysis—accounted for 0.262 and 0.512, respectively (Figure 1). The proteolytic activity of the *Lb. casei* 2639 strain was ca. twofold higher than that of *Lb. delbrueckii* 490. Proteolytic activity of the strains used as adjunct by active cultures in cheesemaking that is too high is undesirable, as it may lead to rapid proteolytic transformations during ripening and, consequently, to changes in the organoleptic traits of cheeses (such as a bitter taste and an untypical aroma).

**Figure 1.** Initial and overall proteolytic activity of examined *Lactobacillus* strains (mean values and standard deviations). A: homogenous group initial proteolysis, n = 6; a–d: homogenous groups overall proteolysis, n = 6).

Donkor et al. [20] determined the initial and overall proteolytic activity (after 24 h incubation) of the following probiotic strains: *Lb. acidophilus* LAFTI L10 and *Lb. casei* LAFTI L26, as well as *Lb. acidophilus* 4962 and *Lb. casei* 279. Likewise, in our study we found no differences in the initial proteolytic activity between the tested strains. The lowest overall proteolytic activity was determined for the strain *Lb. casei* Lc 279 (0.290). In turn, very high activities accounting for 0.460, 0.670, and 1.90 were found for *Lb. acidophilus* LAFTI L10, *Lb. acidophilus* La 4962, and *Lb. casei* LAFTI L26, respectively. The proteolytic activity of *Lb. acidophilus* 2499 determined in our study at 0.400 is similar to that reported by Donkor et al. [20] for *Lb. acidophilus* LAFTI L10 (0.460). In turn, the activity of *Lb. casei 2639* determined in our study at 0.512 is considerably higher from the activity assayed by Donkor et al. [20] for *Lb. casei* Lc 279 (0.290) and over threefold lower from that determined for *Lb. casei* LAFTI L26 (1.90).

It was concluded that the analyzed bacterial strains from the genus *Lactobacillus* exhibited various overall proteolytic activities. Thus, large differences in proteolytic activity may result from the various activities of their proteolytic systems, and resultantly, from the various number of amine groups released in RSM during incubation, as well as from their various nutritional needs.
