Pretreatment of spent coffee grounds for edible insect farming: impacts on mealworm (Tenebrio molitor) development and gut microbiome

Spent coffee grounds (SCGs) are among the most widely produced food and beverage residues. Due to their nutritional value, negligible cost, and considerable availability as pre-consumer food waste that does not compete with human food, using SCGs as feed for edible insects is highly attractive for developing a circular food economy. In this study, we investigated both the fundamental and practical aspects of using untreated and decaffeinated SCGs as feed for rearing mealworms (Tenebrio molitor), one of the most established edible insect species. Our results showed that incorporating a decaffeination step using green solvent ethanol as the extractant had a positive impact on mealworm larvae, significantly enhancing their body mass and pupation rates. Additionally, the protein content of mealworm larvae fed a wheat bran/SCG blend were higher compared to those reared on an unblended wheat bran diet. Gut microbiome analysis revealed that larvae fed untreated SCGs exhibited a microbial community structure similar to that of the starvation group, whereas decaffeinated SCGs resulted in a distinctly different community. The beneficial bacterial genus Spiroplasma, which degrades plant biopolymers in SCGs via secretion of carbohydrate-degrading and proteolytic enzymes, was significantly inhibited by phytochemicals such as caffeine present in untreated SCGs. In contrast, Enterococcus displayed strong tolerance to these phytochemicals and was also abundant in the starvation group. These findings provide a scientific basis for phytochemical-directed pretreatment of food waste in insect-mediated upcycling and support the application of decaffeinated SCGs in industrial-scale edible insect production.