In the master's thesis, we aimed to determine if acid whey, a by-product in the production of cottage cheese, is a nutritionally suitable medium for fermentation with kefir grains (KZ). At the company Arhel d.o.o., the acid whey was first neutralized, microfiltered (S) and pasteurized (G). Smaller quantities of G were then fermented with KZ, which were filtered after fermentation, and the obtained permeate was subsequently used as a liquid mother culture (TMK) for fermenting larger quantities of G. The fermented product was named spent growth medium (IG). The fermentation was conducted in two batches. IG-1 and IG-2 were then mixed, diafiltered, and the retentate was lyophilized – we got our product (LRS) - while the spent growth medium permeate (IGP) was not processed. We were interested in the sensory acceptability of LRS, its chemical composition, and its potential added value (symbiotic, prebiotic, antimicrobial activity). Additionally, we examined the sensory quality, chemical composition, and total microbial count (TMC) of IGP. By determining the number of yeasts and lactic acid bacteria (LAB) in samples of KZ, IG, and TMK, we assessed the microbiological stability, i.e., how many yeasts and LAB transferred from KZ to TMK as an indicator of the success of fermenting G with TMK. To assess repeatability and good hygiene practice, TMC was also determined in samples of S and G. To determine the prebiotic effect of LRS on the growth of probiotic bacteria, LRS was mixed into liquid media MRS or M17 with probiotic bacteria, and the effect of LRS on growth was observed, with the media itself serving as the control. The antimicrobial activity of water dissolved LRS against pathogenic bacteria was determined by the agar disk diffusion method, while its symbiotic activity was determined by the microdilution method using the indicator strain L. sakei NCDO 2714, thereby determining the minimum inhibitory concentration in the end. Sensory analyses of LRS and IGP were conducted by a panel of experts from Institute of Dairy Science & Probiotics (IML-PRO), and the chemical analyses of LRS and IGP were performed by colleagues at IML-PRO. TMC in IGP, S, and G, as well as the number of yeasts and LAB in KZ, IG, and TMK, were determined by culturing on solid media. We found that the chemical composition of LRS does not change significantly, it does not possess antimicrobial properties, but it exhibits prebiotic and symbiotic effects on the growth of individual strains of probiotic bacteria. IGP is sensory acceptable, while LRS is sensory unacceptable. Fermentation of G with TMK proceeds successfully, but the microbial composition varies greatly between trials, as does the microbial composition of IG.
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