Antifungal activity of protective cultures against the yogurt drink-specific spoilage yeasts.

Fungal agents emerged as post-pasteurization contamination are responsible for the spoilage in yogurt drink. In this work, the antifungal effects of some lactic acid bacteria (LAB) on the spoilage yeasts isolated from yogurt drink (Doogh) were evaluated. First, the microbial growth in the yogurt drink samples during the storage time was investigated, and the isolated microorganisms were identified using biochemical methods and sequencing of the specific amplicons. Yeasts (3-7 log CFU ml-1 ) were found to be the most abundant microorganisms (specific spoilage organisms) in several samples. Using the amplification technique of rDNA by ITS1 and ITS4 primers, the dominant yeasts were identified as Pichia kudriavzevii, Kluyveromyces marxianus, and Candida parapsilosis. Then, the antimicrobial activity of 37 strains of LAB against the isolated yeasts was studied using broth microdilution. Eventually, the strains of Lacticplantibacillus plantarum (245, 24, P6, and P7), Lactiplantibacillus pentosus (20), and Levilactobacillus brevis (30) exhibited significant antifungal activity. In the most effective impacts, lag times of C. parapsilosis, K. marxianus, and P. kudriavzevii were increased by almost 12-19 h, 12-19 h, and 2-6 h, respectively, while the area under the growth curve for these yeasts was reduced to lower than 40%, near 16%, and approximately 67%, in the order given. Overall, these bacteria showed high potential as the substituents for chemical preservatives in yogurt drinks. PRACTICAL APPLICATION: Spoilage yeasts were isolated from yogurt drink and identified by molecular method. Isolated yeasts belonged to Pichia, Kluyveromyces, and Candida genera. Inhibitory effects of 37 strains were evaluated against the spoilage yeasts. Cell-free supernatant was used against the isolated fungi in microdilution method. Several LAB strains showed a significant antimicrobial activity.

Semi-continuous production of xanthan in biofilm reactor using Xanthomonas campestris.

Semi-continuous production of xanthan gum using self-immobilized Xanthomonas campestris cells in biofilm reactors was studied. Fermentation was carried out using two different designs of biofilm reactor equipped with a) stainless-steel support (SSS) and b) polyethylene support (PES). Fermentation was performed in three cycles with refreshing the media at the beginning of each: cycle 1, 0-27 h; cycle 2, 27-54 h; and cycle 3, 54-78.5 h. Results showed that the glucose consumption and the pH reduction in the PES biofilm reactor was faster compared to the SSS biofilm reactor. Scanning electron microscopy showed that the SSS was capable to immobilize more cells during the growth of X. campestris. The maximum concentration of xanthan gum in the SSS biofilm reactor obtained after 27 h (3.47 ± 0.71 g/L), while the maximum concentration of xanthan in the PES biofilm reactor obtained after 78.5 h (3.21 ± 0.68 g/L). Thermal stability analysis of xanthan using differential scanning calorimetry showed the presence of two fractures attributed to dehydration and degradation of polymer. The thermogram represented both endothermal and exothermal behaviour of xanthan polymer. Furthermore, the functional groups and molecular structure of the xanthan produced in this study was evaluated using Fourier transform infrared spectrometry and also proton nuclear magnetic resonance. in addition, the surface tension of (0.2 %, w/v) xanthan gum solution was in a range of 52.16-56.5 mN/m. Rheological analysis of xanthan showed that the G' values were higher than the G″ in all frequencies demonstrating a relatively high elasticity of the produced xanthan gum.