# Changes in Selected Quality Indices in Microbially Fermented Commercial Almond and Oat Drinks
## 3 Results and Discussion
### Volatile Compound Changes after the Fermentation of PB Beverages  
The odorous compounds in the control and prepared beverages are presented in Table 5. The aroma of the control PB beverages was relatively gentle and mild (preliminary sensory analysis--data not shown) and typical for the used raw plant material. The aroma was created by 25 (almond) and 30 (oat) tentatively identified volatile compounds, with a prevalence of aldehydes, esters, and lactones in the almond drink (5 compounds in each group) and aldehydes (5 compounds), esters (7 compounds), and organic acids (4 compounds) in the oat drink. After fermentation, the highest increase in the number of identified volatiles was found for both drinks fermented by _C. antarctica_ CAN 0001 (36/37 compounds) and _Y. lipolytica_ YLP 0001 (39 compounds). The aroma profiles of both beverages fermented by yeasts were mostly differentiated, without characteristic compounds for the used strains. In contrast, compounds such as: 1-propanol, 2-methyl; propylenglycol; terpinene-4-ol; isoamyl acetate; 4-ethylquaiacol; and thymol in the almond beverages (they were absent in the control sample, although found in all fermented samples) were characteristic for the LAB-fermented beverages. For the oat drinks, LAB also produced propylenglycol; 2-methylproanal; 4-octanolide; and pentanoic acid. It is also worth noting that unique compounds were found only in the almond beverages fermented by both strains of _L. plantarum_ (1-hexanol; propanal, 3-methylbutanal, hexanal, and benzaldehyde). A PCA analysis of the variation between the control and fermented beverages based on volatile compounds is shown in Figure 1. This figure confirms the lack of characteristic compounds (absent/present) in the studied drinks, identifying clear variation between the beverages. The drinks that were most differentiated from the others are three almost drinks: the control (the characteristic compound was 2-heptanone), fermented by _L. bulgaricus_ (the characteristic compounds were furfural and beta-pinene), and fermented by _T. casei_ (the characteristic compounds were (z)-3-hexen-1-ol, ethyl butyrate, ethyl tetradecanoate, benzyl acetate, and butanoic acid). In the case of the used microbiota strains, the highest similarity of oat and almonds beverage aromas was found for those fermented by _T. casei_ TCS 0001 (newly created volatiles were pentanal, 4-octanolide, and pentanoic acid) and _K. marxianus_ KF 0001 (created volatiles were propylenglycol and pentanoic acid). To date, there are still a lack of studies on aromas in plant-based fermented beverages, especially by yeasts. Based on the analogy to dairy products, the aroma of fermented food is mainly dependent on the type of processed food matrix, the microflora species/strains used, and the fermentation conditions. A recent study [61] showed that among the volatiles created by some prominent LAB strains in kefir grain, aldehydes, ketones, alcohols, and acids prevailed. For fermented dairy products produced by LAB strains, acetaldehyde, diacetyl, acetoin, and butanediol are typical aroma compounds as a result of metabolizing citrates [62]. From this catalogue, only acetaldehyde was found in the majority of samples in the current study, but this compound was also found in both control samples. According to [PERSON] and [PERSON] [63], citrate is a compound that plant tissues usually contain in considerable amounts, which can explain the presence of acetaldehyde plant beverages. The presence of this compound is especially valuable since acetaldehyde gives a positive taste and aroma [64]. It is well known that the aroma profile of LAB-fermented food differs between homo- or heterolactic fermentation types [44; 65].