Effects of different drying methods on the nutritional components, microstructure, and metabolomic profile of Spirulina maxima

Spirulina maxima (S. maxima) is considered a potential solution to address the issue of human food security. In this study, three drying methods were employed: spray drying (SD), freeze drying (FD), and vacuum drying (VD), to process fresh S. maxima (FS). A comprehensive nutritional evaluation was conducted using principal component analysis and cluster analysis, along with an untargeted metabolomics study. The results indicated that after SD, FD, and VD treatments, the phycocyanin content decreased by 23 %, 9 %, and 80 %, respectively, compared to that of the FS. The polysaccharide content also significantly decreased by 36 %, 33 %, and 67 %, respectively. Comprehensive analysis showed that SD resulted in the least loss of nutritional indicators in S. maxima. Further metabolomics analysis revealed that prolonged exposure to high temperatures can lead to metabolic disruptions in 2-oxocarboxylic acid metabolism, purine metabolism, amino acid biosynthesis, and starch and sucrose metabolism. Additionally, the levels of coproporphyrin I and coproporphyrin III increased which ultimately led to a decrease in chlorophyll and phycocyanin contents. Therefore, VD resulted in the poorest quality of S. maxima. This study provided a theoretical basis for the quality assessment of S. maxima in response to drying.