Chemical and bioactive changes during fermentation and in vitro digestion of hemp pressed cake using Rhizopus oligosporus

The increasing popularity of hempseed oil consumption contributes to an increased production of hemp-pressed cake (HPC), which is mainly underutilized feedstock. This study aims to valorise hemp-pressed cake, a byproduct of hemp oil production, by employing solid-state fermentation with Rhizopus oligosporus to enhance its nutritional value and bioactivity. The results indicate that fermentation preserved total protein content and released free amino acids, varying with fermentation duration. Also, a gradual drop in oil content by a maximum of 41.8% after 14 days was observed. Eight fatty acids were identified, with most showing a decreasing trend during fermentation, except for C18:1 (cis-9), which increased. Fermentation contributed to an increase in antioxidant activity and total phenolic content (TPC) at varied levels depending on the extract types and fermentation duration varied levels for analysed. Additionally, in the in-vitro digestion method, the samples were then analysed for TPC, iron chelating capacity, antioxidant activity, antihypertensive activity, antidiabetic activity, and total peptide amount. In addition, fermentation can increase the bioavailability of certain compounds, and it is a prospective use of a waste material produced during cold-pressing of hemp seed oil.