## 1 Introduction  
Over the past decade, the food and cosmetic industries have shown significant interest in walnut oil derived from _lunglas regiu_ L, recognizing its richness in bioactive compounds such as polyunsaturated triglycerides and fat-soluble vitamins. However, the oil extraction process results in substantial by-products, specifically walnut press cake (WPC), which retains valuable components including oil (20-36%), protein (30-42%), dietary fiber, phenolic compounds (ellagitannins), and minerals [1, 2, 3, 4]. Given the compelling composition of WPC, there is an increasing interest in using it as a nutritional and functional component in food items or as a base for fermentation [1, 5]. Historically, fermentation played a crucial role in the processing of food for human consumption. Fermentation was used primarily for preservation; However, it not only preserves the food but also improves its quality and functionality [5]. Nowadays, fermentation is often used with the aim to improve the bioavailability of health promoting compounds or to increase their biological activity by microbial transformations. The bioavailability of natural polyphenols in human beings is very low due to their low absorption in the gastrointestinal tract. In a recent publication, [PERSON] et al. showed a significant increase of the total polyphenol content and the antioxidant activity of apple pulp due to fermentation with _Lactobacillus acidophilus_[6]. They also observed an increase in the bioavailability of apple polyphenols. The authors explained their findings by suggesting a transformation of larger molecules into smaller ones, which can be easily digested and absorbed. In contemporary times, advancements in fermentation techniques have significantly improved the capabilities, productivity, and efficiency of industrial fermentation. Presently, two primary processing methods are employed: (I) submerged fermentation and (II) solid-state fermentation (SSF) [2]. The present study employs SSF, involving the use of a solid substrate without liquid water, creating an environment where microorganisms can thrive. This approach maximizes nutrient concentration in the solid substrate [7; 8]. The benefits of SSF encompass high yields at low costs, increased microorganism activity, minimal water consumption, reduced waste production, and heightened resistance to contamination [9]. Nevertheless, industrial applications of SSF pose challenges due to issues such as scalability, heat accumulation, and difficulties in regulating growth parameters [8; 10]. Filamentous fungi are the predominant microorganisms employed in SSF. The fermentation temperature emerges as a critical parameter influencing microorganism growth in SSF. Furthermore, the microbial development is affected by variables including the moisture level, chemical composition, and particle dimensions of the substrate, the height of the substrate layer, accessibility of oxygen, initial spore concentration, distribution of microorganisms in the substrate, and the age of the microorganisms [9; 11]. Ellagic acid (EA) is a natural phenolic compound found in various fruits (especially in raspberries, strawberries and pomergnate), vegetables, and nuts (e.g., pecan nuts and walnuts) and has gained notable interest as one of the prevalent antioxidants found abundantly in the human diet [12]. In plants, ellagic acid is mainly present in the precursor form of ellagitannins. Ellagitannins are part of the hydrolysis tannins, compounds composed of hexahydroylphenoyl units, bound to sugar moieties. These tannins are as stringent compounds, protecting the plant from predators. Ellagic acid has to be liberated from these precursor compounds. The industrial liberation of EA from ellagitannins traditionally involves acidic or alkaline hydrolysis, but these methods, besides their considerable environmental impact, are associated with drawbacks such as high production costs and low yields [13].