Valorisation of agri-food waste through the use of colorimetry

The valorisation of agri-food waste as food ingredients has gained attention due to their nutritional and functional importance, as well as their role in sustainability and the circular economy. Pigmented residues like grape pomace, prunings, stems and brewing by-products can enhance visual appeal and attract consumers. This study aimed to characterise the colourimetric properties of flours produced from such waste. Samples were dried at 60 ºC, then at 40 ºC for 24 hours with forced air, milled using a Retsch SR 300 and analysed with the CIELAB colour space. Significant differences were found in luminosity (L*), red–green (a*) and yellow–blue (b*). Grape pomace had the lowest lightness (L* = 21.18 ± 0.45) and the most intense red colour (a* = 12.11 ± 0.13), possibly associated with the presence of anthocyanins and tannins. On the other hand, grape pruning showed the greatest luminosity (L* = 54.55 ± 0.22) and the greatest yellowing (b* = 13.29 ± 0.26), which may be related to the content of lignin and other non-pigmented phenols. The stalks showed intermediate luminosity with reddish and yellowish tones, while the brewery waste showed high luminosity and the highest b* value (14.67 ± 0.13), potentially influenced by the presence of melanoidins, resulting from the Maillard reaction. The differences observed between the waste may reflect variations in their composition of phenolic compounds, lignin and natural pigments such as anthocyanins, flavonoids and betalains, as reported in the literature. Although the quantification of these compounds was not carried out in this study, the colorimetric profiles obtained allow for a useful preliminary characterisation to differentiate the materials based on their visual properties. These data provide initial subsidies for future research into the use of these residues as natural ingredients in foods, supporting strategies for utilising by-products in the context of sustainability and innovation in the food chain.