Incorporating the shell of the Portuguese chestnut (Castanea sativa) into chestnut flour is an innovative and sustainable strategy that enables the full utilization of this valuable agri-food resource. This approach contributes to the reduction in agro-industrial waste while enhancing the nutritional and functional properties of the resulting flour, in alignment with the principles of the circular economy and sustainable food systems. The chestnut shell is a natural source of essential amino acids, such as arginine and leucine, and non-essential ones like glutamic and aspartic acids. Its incorporation increases the protein content and improves the amino acid profile of the flour, potentially supporting muscle maintenance, metabolic processes, and overall nutritional value. Moreover, it contains bioactive compounds—including polyphenols and flavonoids—with known antioxidant and anti-inflammatory effects, offering added functional benefits. Thermal processing methods—drying, roasting, and cooking—play a key role in determining the flour’s final characteristics. Drying preserves heat-sensitive amino acids, thus maintaining protein integrity. Roasting enhances the bioavailability of free amino acids, which may improve digestibility and nutrient absorption. Cooking promotes the extraction of phenolic compounds, increasing the flour's antioxidant capacity. Each method has specific benefits, allowing for tailored processing depending on desired outcomes. When the incorporation of chestnut shells is combined with optimized thermal treatment, the result is a flour that is not only nutritionally superior but also functionally enhanced and environmentally friendly. Such flours offer applications across various food products, appealing to consumers seeking sustainable and health-promoting alternatives.

Overall, this strategy represents a promising innovation for the agri-food sector, promoting the development of value-added products through the upcycling of by-products. The approach not only enhances the sustainability of chestnut processing but also meets the increasing consumer demand for foods that are both health-promoting and environmentally conscious.