###### Abstract  
Brewer's spent grain (BSG) and spent coffee ground (SCG) are solid sidestreams from beverage production increasingly being upvected into food, feed and other value-added products. These solid sidestreams are prone to microbial spoilage, negatively impacting their upcycling potential. Three samples each of BSG and SCG were obtained from generators and recycling facilities in Singapore, and their chemical, elemental, and microbial composition was characterized. The spoilage mechanisms were investigated during storage under anaerobic and aerobic conditions. Bacterial loads of sidestreams were low from craft brewery and cafe sources (<1 and 3.53 \(\pm\) 0.03 log\({}_{10}\) CFU/g) and high from recycling facilities (>6 log\({}_{10}\) CFU/g). The microbiome of BSG from recycling facilities was dominated by _Bacilli_, and _B. coagul_s was identified as the most prevalent species. SCG from recycling facilities was dominated by lactic acid bacteria, with _L. panis_ being the most prevalent species. Storage up to 14 days under anaerobic conditions led to further bacterial proliferation mainly by _Bacilli_, lactic acid bacteria, and acetic acid bacteria, while aerobic storage led to extensive fungal contamination, including potential aflatoxin-producing _Aspergillus flavus_. The results shed light on the spoilage mechanisms, while highlighting the short shelf-life and food safety risks of BSG and SCG to inform valorization strategies. food waste; brewer's spent grain; spent coffee ground; microbiome; spoilage; shelf-life +
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processed to have functional properties such as emulsifying and foaming capacity suitable for a range of food formulations [9]. A recent market survey identified 125 products containing BSG-derived ingredients, the majority being snacks and bakery products, but also pasta, cereal, meat alternatives, and more [7]. BSG also has potential use in biotechnological processes, such as bioethanol, enzyme production, and other fermentation processes [10]. However, BSG has a high moisture content and is prone to spoilage; therefore, it is typically valorized for animal feed and biogas, while the remainder is landfilled [11; 12]. The shelf-life has variously been reported to be 2 days [13], 3-5 days [11], and 7-10 days on average [14], due to differences in definitions of shelf-life and storage conditions [15]. Spoilage is caused by excessive microbial growth, leading to dry matter and nutritional losses, unpleasant odors, low palatability, and even toxin formation [11]. The culturable microbiome of BSG has previously been investigated by [PERSON] et al. [16] and found to predominantly consist of spores of aerobic thermophilic bacteria capable of surviving the mashing temperatures between 65 \({}^{\circ}\)C and 76 \({}^{\circ}\)C [17], while microaerophilic (e.g., LAB) and anaerobic bacteria (e.g., _Clostridia_) are thought to proliferate post-production. [PERSON] et al. [18] also investigated the microbiome and storage of BSG at different temperatures under aerobic conditions, but without environmental exposure. They found bacteria total plate counts to reach approximately 8 log\({}_{10}\) CFU/g for mesophilic bacteria and 6 log\({}_{10}\) CFU/g for thermophilic bacteria by day 16 at 20 \({}^{\circ}\)C, with most of the growth happening before day 3 and stabilizing by day 7 [18]. Fungal genera such as _Aspergillus_, _Fusarium_, _Penicillium_, _Alternaria_, and _Rhizopus_ capable of mycotoxin production have all been found in barley grains, and the associated mycotoxins have been found in beer, malted barley, and BSG after 30 days of storage in gunnysacks [7; 15; 19]. Now, with the advent of advancements in molecular profiling and metagenomics, additional insights may be gleaned from the microflora biology on these economically important residues. With respect to coffee, SCG predominantly comprises insoluble dietary fiber, proteins, lipids, and a variety of phenolic compounds [20; 21] and is rich in bioactive compounds, including probiotics, which can promote gut health [22].