The Effect of Yogurt and Kefir Starter Cultures on Bioactivity of Fermented Industrial By-Product from <i>Cannabis sativa</i> Production—Hemp Press Cake

<i>Cannabis sativa</i> (hemp) is a plant considered to be abundant in bioactive compounds. The increasing production of hemp oil is leaving considerable amounts of hemp press cakes (HPC), which have not been sufficiently managed so far. One of the directions of development of plant-based food is the use of by-products of the agri-food industry in accordance with the idea of zero waste and the circular economy, so the purpose of this study was to determine the possibility of HPC fermentation using yogurt and kefir cultures and to determine the effect of the type of starter on the properties of the products. In the present study, starter cultures of yogurt (YO 122) and kefir (commercial grains) were used for HPC fermentation. Changes in lactic acid bacteria (LAB) and yeast population, pH, acidity, the content of bioactive compounds by spectrophotometric methods (proteins, amino acids, polyphenols, flavonoids, reducing sugars) and antioxidant activity (DDPH, ABTS, FRAP and reducing power) were determined. The results showed that it was possible to develop high-value beverages based on HPC with high fermentation efficiency: survivability of LAB and yeast (>10⁶ CFU/g) and acidification (pH in a range of 4.82–6.36 and 5.34–6.49 for yogurt and kefir culture, respectively). Moreover, the stability of hemp protein, with its variable free amino acid composition, antioxidant potential and presented changes in polyphenolic content, was observed during storage. The presented results show a new way to manage HPC as an oil industry residue by using it as a raw material for the development of a bioactive food product and illustrate the relationship between applied starter culture, the direction of fermentation and changes in the content of bioactive compounds.