# Changes in Selected Quality Indices in Microbially Fermented Commercial Almond and Oat Drinks
## 3 Results and Discussion
### Viscosity of PB Beverages  
The results of the apparent viscosity of the control and fermented beverages are presented in Table 2. Control samples were only slightly differentiated in this regard, reaching values of 20.53 (oat drink) and 29.60 mPa-s (almond drink). None of the LAB or yeast strains used for the fermentation of oat drinks significantly changed the viscosity of the obtained beverages (the noted variation was from 18.44 to 20.36 mPa-s). Much larger changes were observed in the fermented almond beverages. In the majority of samples, the apparent viscosity significantly increased, with the top levels found for samples fermented by LAB and _Y. lipolytica_ strains (values from 90.63 to 108.84 mPa-s). Only the sample fermented by _C. antarctica_ was thinned after fermentation (23.40 mPa-s). The apparent viscosity results for the almond beverages were highly correlated with the titratable acidity (r = 0.92; \(p<0.05\)) and pH (r = \(-0.89\); \(p<0.05\)).  
Similar results were recently reported by [PERSON] et al. [55] during the fermentation of gruels made of rice and soy milk by selected LABs. In this study, the apparent viscosity of rice milk was significantly reduced while an opposite effect was found for fermented soy milk. According to the cited authors, in addition to the composition of fermented plant material, the viscosity of the final product also depends on several factors such as the kinetics of acidification, the amyloid and proteolytic activities of the fermenting microflora and the exopolysaccharides produced (or not) by them, the degree of protein gelation, and protein-starch interactions. For the beverages prepared in the current study, a key factor was probably the concentration of the protein and pH, which determine the gel creation. For the main storage proteins of oat (avenins, 125 storage globulin) and almond seeds (12S storage globulin), their isoelectric point is close to 6.5 (almond) and 6.0-8.0 (oat) [56; 57]. This indicates that in samples with lower pH values than the isoelectric point, proteins were found to be unfolded, with amino acids ready to form hydrogen or dipole bonds with surrounding water molecules (the almond 12S globulin contains ca 20% of charged residues and additionally ca 45% of polar amino acids without charge--the data for the protein is coded as P14812 in the UniProtKB base). For the almond drink fermented by _C. antarctica_, the final pH was 6.51, which is near the value of the isoelectric point of storage proteins (there was visible protein precipitation in this sample). However, in oat beverages, the protein concentration (0.4 g/100 mL) seems to be too low to form a gel structure.  
In general, each interaction between the amino acid and water immobilizes more water molecules for ion-dipole interaction. [PERSON] et al. [58] concluded that negatively charged amino acids were more beneficial for protein solubility, followed by positively charged amino acids, and, finally, charge-neutral amino acids. This indicates that for stable protein gel formation, there is a need for the calculation of charge-neutral, charge-positive, and charge-negative amino acids in each protein. For almond and oat proteins, almonds are much richer in both charge-positive and charge-negative amino acids, so this characteristic also confirms the possibility of creating a stable gel with enhanced viscosity [56; 57].