## 1 Introduction  
Mushroom farming is a fast-growing industry. Mushroom production has increased over the last decade, and its global annual production is estimated to be around 9 million tons [1]. An increase in mushroom production generates a significant amount of mushroom cultivation residue, which is commonly disposed of by non-environmentally friendly methods [2]. Mushroom cultivation residue has several applications, including bioremediation, crop production, reuse in the cultivation of the mushroom, feed for animals and fish, pest management and as a source of renewable energy [3]. For example, its residue can be used for the removal of contaminants in water and soil, the cultivation of some vegetables, and the production of biofetlizer and cattle and rumulant feed [3]. _Pleurotus_ species is one of the most cultivated mushroom species, contributing to more than 19% of global mushroom production [4]. The oyster mushroom (_Pleurotus ostratus_) is an edible mushroom that grows on lignocellulosic materials produced in the forest or generated from agricultural and food industry practices [5]. Although the oyster mushroom can utilise the agricultural and food industry waste as a substrate to grow, its cultivation generates residue resulting in agricultural waste. The management of mushroom cultivation residue can be challenging. This residue, a combination of mycelium and lignocellulosic materials, can be disposed of by landfilling and incineration or recycled as compost, fertilizer and bio-fuel [2]. Oyster mushroom cultivation residue can also be used as livestock feed [6,7]. Some of these management options, such as incineration on the open field, have a negative environmental impact but others, such as composting, are more environmentally friendly [2]. Selecting a simple, feasible and environmentally friendly waste management action can help farmers manage mushroom cultivation residue. On the other hand, if the chosen waste management strategy results in the production of a value-added product and revenue generation, it can contribute to the circular economy and lead to farmers' financial gain. The characteristics of cultivation residue materials determine the waste management action and its potential for contribution to the circular economy. For example, if the mushroom is cultivated in agro-industrial residues, the cultivation residue can be used as animal feed [7]. If it is cultivated in the food industry by-products, it may have the potential to be used in upcycled foods. Upcycled foods utilise ingredients that otherwise would not have gone to human consumption [8; 9]. These ingredients are unmarketable materials (e.g., by-products from the manufacturing of other food products or scraps of food preparation) that commonly exist in the food supply chain [9]. One of the substrates for oyster mushroom cultivation is a mixture of Brewer's Spent Grains (BSG) and wheat bran [10]. BSG is a by-product of word production in the brewing industry [11], and wheat bran is the by-product of the conventional milling of wheat grains [12]. BSG consists of 20% protein and 80% partly lignified cell wall material rich in feruloylated (arabinoxylan polysaccharides) [13]. BSG and wheat bran have several nutritional benefits. BSG is high in protein (18%), essential amino acids, fibre [11] and beta-glucan [14]. Wheat bran is also high in fibre and a good source of beta-glucan [12]. Beta-glucan has several health benefits as it is a soluble fibre with functional and bioactive properties, including prebiotic effects [15; 16]. After mushroom cultivation, the residue will also contain mycelium. The oyster mushroom mycelium has various nutrients and fibre, including beta-glucan [17; 18]. Therefore, oyster mushroom cultivation residue (mixture of BSG, wheat bran and oyster mushroom mycelium) can serve as a nutritious food ingredient. The mushroom cultivation residue containing BSG is currently used as animal feed [6] and has not been considered for human food production.