Black Cumin (Nigella sativa L.) Seed Press Cake as a Novel Material for the Development of New Non-Dairy Beverage Fermented with Kefir Grains

In recent years, there has been a growing interest from the food industry in new products that are increasingly desired by consumers because of the functional ingredients they contain. This category certainly includes fermented plant-based beverages, which combine the properties of plant substrates with the beneficial effects of fermentation on human health. In our study, two trial variants containing 20% and 30% black cumin (Nigella sativa L.) seed press cake (BCPC) were inoculated with kefir grain cultures and then incubated at 25 °C for 24 h. The resulting beverages were stored under refrigeration (6 °C) for 28 days. During storage, pH, total free amino acids, reducing sugars, changes in the microbial population, viscosity, textural parameters, and color were measured on days 1, 5, 7, 14, 21, and 28. Throughout the storage period, the number of lactic acid bacteria, as well as yeasts, exceeded the recommended minimum level. Numerous changes in product parameters were observed in the tested beverages as a result of fermentation compared to non-fermented products. This study indicates the possibility of using BCPC as a valuable matrix for the production of a functional kefir-like beverage.