Recently, there is a growing demand for new types of cereal products with a higher nutritional value, which can offer health benefits to consumers due to the content of biologically active substances (Dallagnol et al., 2013). In this respect, considerable attention is focused on the use of pseudocereals, mainly quinoa (Chenopodium quinoa) (Dallagnol et al., 2013; Bhargava, Shukla and Ohri, 2006). Quinoa is regarded one of the best vegetal protein sources (12 – 23%), as its protein levels are similar to those found in milk and higher than in true cereals such as wheat, rice and maize (Gordillo–Bastidas et al., 2016; Kaur and Tanwar, 2016; Nisar et al., 2017; Zannini et al., 2018). Moreover, quinoa contains a significant amount of starch (52 – 60%) with low amylose content (7 – 11%) (Ahmed, Thomas and Arfat, 2019) and the amount of dietary fibre in the quinoa is higher than in the other grains (9 – 16%) (Gordillo–Bastidas et al., 2016). In addition, quinoa is a rich source of bioactive compounds like antioxidants, polyphenols, flavonoids, minerals (magnesium, zinc, iron, potassium, phosphorus) and vitamins (E, B group and C) (Kaur and Tanwar, 2016; Tang and Tsao, 2017; Ahmed, Thomas and Arfat, 2019).

Due to the above mentioned nutritional benefits, the quinoa seeds have recently been incorporated into the functional food products (Lorruso et al., 2018; Ujiroghene et al. 2019). Several authors investigated the potential of quinoa for the production of beverages with using various methods of treatment such as soaking, germinating, cooking, malting (Pineli et al., 2015; Kaur and Tanwar, 2016; Urquizo et al., 2017; Zannini et al., 2018). On the other hand, only a few research works are focused to the production of probiotic quinoa-based fermented beverages (Bianchi et al., 2014; Lorusso et al., 2018). Currently, the study of Lorusso et al. (2018) was oriented to assessment the suitability of quinoa for making yogurt-like beverages fermented by using probiotic lactic acid bacteria strains (Lactobacillus rhamnosus SP1) and Bianchi et al. (2014) developed a potentially synbiotic beverage fermented with Lactobacillus casei LC-1 based on aqueous extracts of soy and quinoa with added fructooligosaccharides. More recently Ujiroghene et al. (2019) reported that quinoa-based fermented beverages can be applied for diabetes mellitus treatment due to their ability to inhibit α-amylase activity and reduction or prevence hyperglycemic conditions associated with increased levels of sugar glucose in the blood.

The fermentation of cereals improves the nutritional properties and sensory characteristics of final products and has a positive impact on human health (Rollán, Gerez and LeBlanc, 2019). Nowadays, many scientific studies are focused on the preparation of new types of cereal-based fermented beverages with probiotic and functional properties (Rathore et al., 2012; Nionelli et al. 2014; Ghosh et al. 2015; Magala et al., 2015). Therefore, considering the nutritional and health benefits of quinoa, in this study, the fermented quinoa-based beverages with probiotic potential were developed.

Saponins are bitter compounds that are naturally present in quinoa seeds in the range from 0.1 to 5.0% (Pytel et al., 2018). The content of saponins in the quinoa seeds used in this study was 0.30 ±0.02% (results are not shown). For remove of saponins, washing or water maceration is recommended (Pineli et al., 2015). Washing with tap water was used for removing of saponins in this study. After desaponification, the amount of saponins in quinoa seeds was present in the non-detectable level. Previously Pytel et al. (2018) demonstrated a reduction number of lactic acid bacteria during fermentation of yogurts with 3 and 5% content of quinoa flour. Moreover, it was reported that saponins present in the quinoa seeds also cause a negative effect on protein solubility (Pineli et al., 2015). For these reasons, the removal of saponins from the quinoa seeds representing an important step before the production of lactic acid fermented quinoa-based beverages. Proximate composition of quinoa-based beverages before and after 6 h fermentation is presented in Table 1.

During the fermentation process, the pH decreased and total acidity increased significantly in the beverages. This effect was previously described by Magala et al. (2015) in rice fermented beverages and Urquizo et al. (2017) in the quinoa fermented beverages prepared from various quinoa varieties. A rapid decrease of the pH at the start of fermentation is important to obtain the final product of high quality (Kohajdová and Karovičová, 2008), as well as the rapid increase of total acidity minimizes the growth of undesirable and pathogenic bacteria (Kohajdová Karovičová and Greifová, 2006).

During the lactic acid fermentation, different organic acids (mainly lactic acid) are produced due to the degradation of some components present in the raw material (Kohajdová, Karovičová and Greifová, 2006; Magala, Kohajdová and Karovičová, 2013; Cho et al., 2015). Determination of organic acid profile represents an important tool for monitoring the metabolic activity of fermentation microorganisms (Cho et al., 2015). In this study, the significant increasing of lactic and acetic acids concentration was observed in the fermented beverages in comparison to unfermented samples. Furthermore, slightly decreasing in citric acid content was recorded. This can be attributed to its usage as a substrate in secondary reactions during fermentation process (Magala, Kohajdová and Karovičová, 2013).

Moreover, it was found that, the lactic acid fermentation no significantly influenced dry matter, ash and crude fat content in the quinoa-based beverages. On the other hand, however, after fermentation a significant increasing in protein content was observed in the beverages (increasing about 36.84%). Tangyu et al. (2019) reported that fermentation can increase protein content by the growth of the fermenting microorganisms and by improving protein solubility. According to Simwaka et al. (2017) the increase in protein content after fermentation has been attributed to the increase in nitrogen content released when microorganisms utilize carbohydrates for energy.

From the results also concluded that the fermentation not significantly influenced the content of TDF in the samples of beverages. Similar trend was also observed by Marko et al. (2014) in different fermented cereal matrices. On the other hand, Jorgensen et al. (2010) reported that the fermentation of cereals substrates can concluded in reduction of TDF content as a consequence the enzymatic degradation of fibre.

Since TDF represent an important functional compound with beneficial health effects, the incorporation of fibre rich raw materials obtained from plant processing by products into the non-dairy beverages can be a good strategy for increasing intake of TDF. In this respect, it seems to be a good alternative application of apple pomace as a fibre rich (more than 50% of TDF) by-product obtained from the juice processing industry that was usefully incorporated to cereal-based products in our previous study (Kohajdová et al., 2014). Moreover, this by product can improve sensory acceptance of non-dairy fermented beverages as a flavouring ingredient due to pleasant fruity odour and taste (Sudha, Baskaran and Leelavathi, 2007).

The phenolic compounds are important for human health and nutrition mainly due to their antioxidant activities (Hole et al., 2012; Tang and Tsao, 2017). As shown in Table 1, the significant increasing of TPC was observed in the quinoa-based fermented beverages. A similar effect was also reported by Hole et al. (2012) and Lorusso et al. (2018) for the barley, oat, and quinoa substrates processed by lactic acid fermentation. Previously Bustos et al. (2017) and Lorusso et al. (2018) reported that, during the fermentation process the esterase activity of lactic acid bacteria and endogenous enzymes of cereal substrates can hydrolyse the complex phenolic compounds and their glycosylated forms into free phenolic acids.

Several studies documented that the quinoa seeds are an excellent source of antioxidants and that, the antioxidant activity is in a good correlation with the content of TPC (Kaur and Tanwar, 2016; Tang and Tsao, 2017). This fact was also confirmed in this study when the increased TPC content in the fermented beverages corresponded to a proportional increase of the antioxidant activity of fermented products.

After 6 h fermentation, the content of lactic acid bacteria cocci and rods reached values 7.21 log CFU.mL^-1 and 6.12 log CFU.mL^-1 respectively. The population of lactic acid bacteria in the fermented beverages was increased significantly by 1.08 (cocci) and 1.65 (rods) log orders. This indicates that quinoa is a suitable substrate for lactic acid fermentation.

Consistency represents an important parameter for the development of new functional fermented non-dairy beverages (Magala et al., 2015). The viscosity of quinoa-based beverages significantly increased during the fermentation process. This agreed with Bianchi et al. (2014) and Lorusso et al. (2018) that also observed increased viscosity in the fermented beverages produced from aqueous extract of quinoa and yogurt-like quinoa beverages. Ndife et al. (2019) reported that the increase in the viscosity of cereal beverages after fermentation could be due to an increase in biomass density of the microorganisms.

No significant differences were found in the WHC of beverages before and after fermentation. This indicates that the WHC of beverages remaining stable during the fermentation process.

Sensory characteristics of quinoa-based beverages are shown in Table 2 and photodocumentation of quinoa-based beverages is presented in Figure 1.

Väkeväinen et al. (2020) newly documented that the saponins present in the quinoa seeds decrease the acceptance of quinoa-based products. Following this recommendation and due to the other fact presented in the above part of this article, the quinoa seeds were desaponificated before the use in beverages manufacture. The results concluded that the overall acceptance of nonfermented beverages was lower than 42%. Lactic acid fermentation significantly enhanced overall appearance, taste and odour, but no significantly influenced the overall acceptability of beverages. Fermented beverages were characterised with creamy light colour and sour odour and taste. Due to the low sensory acceptability of beverages, the raspberry syrup was applied as a supplement to improve their acceptability. This ingredient was sweetened with fructose, a sweetener with a low glycemic index that is usually consumed by persons treated to diabetes. The addition of this supplement significantly improved the overall acceptability of fermented beverages. A similar trend was observed by Urquizo et al. (2017) after the addition of bilberries and chocolate flavouring to lactic acid fermented quinoa beverages.

The shelf life of fermented beverages was also monitored for 21 days of storage at 5 °C (Table 3). During the fermentation, the pH of beverages slightly decreased. Moreover, the significant increasing of total acidity was recorded. A similar trend was previously described by Bianchi et al. (2014) after 28 days of storage of fermented quinoa and soy beverages at 5 °C. Furthermore, it was found that no significant differences were recorded in the content of lactic and acetic acids. The results also concluded that during storage, the citric acid was degraded as the consequence of the metabolic activity of fermenting microflora.

The presumptive count of lactic acid bacteria was also determined in this study. It was observed that lactic acid that presented bacteria cocci and rods showed a high rate of survival percentage during the storage of beverages (84.88 and 80.72% respectively). The viability of bacteria is an important characteristic of the use of probiotics in beverages, once they should survive during the shelf life, with minimal desirable value 6 log CFU.mL^-1 (Georgieva et al., 2009; Gallina et al., 2019). This value corresponds to ingestion an 8 log CFU per serving portion of 100 mL (Pereira and Rodrigues, 2012). It was found that lactic acid bacteria cocci (Streptococcus thermophilus) were present in the beverages at the level above this requirement. On the other hand, the population of lactic acid bacteria rods (Bifidobacterium sp., Lactobacillus acidophilus) reached at the end of storage only counts 4.94 log CFU.mL^-1. Kurman and Rasic (1991) and Samona and Robinson (1994) previously suggested that the minimum level for probiotic microorganisms in fermented milk to produce therapeutic benefits should be a value above 5 log CFU.mL^-1. After 14 days of storage the population of lactic acid bacteria rods remained above this value. From the obtained results can be indicated that experimental beverages had a probiotic character.

The potential of quinoa as a raw material for the production of lactic acid fermented beverages was presented in this study. It was shown that the fermentation process significantly increased proteins and total phenolic content and antioxidation activity in the products. Sensory evaluation showed that the overall acceptability of quinoa-based fermented was lower than 46%. The addition of flavouring agent (raspberry syrup) significantly improved acceptability of beverages (79.58%). Regarding these finding, the application of flavourants could be a useful strategy for enhancing consumers’ acceptance of these types of beverages. Moreover, further studies are needed to select suitable flavouring agents according to the preferences of potential consumers. The functionality of commercial probiotic culture was demonstrated by high viable bacterial counts in the fermented beverages (7.21 log CFU.mL^-1 of lactic acid bacteria cocci and 6.12 log CFU.mL^-1 of lactic acid bacteria rods) and during the storage (above 6 CFU.mL^-1 of lactic acid bacteria cocci).

In conclusion, the quinoa is naturally gluten-free raw material and thus, quinoa-based fermented beverages represent a suitable alternative for people having celiac disease. Moreover, quinoa has a low glycemic index and in combination with appropriate ingredients, these beverages can be incorporated into the diabetic diet.