Unraveling the science of “sinking and dark-colored” superior agarwood through density, color, and metabolomics

Abstract

Agarwood is a highly precious heartwood containing aromatic resin, which can be used both as medicine and incense. The consumption centers of agarwood are distributed worldwide, and many countries place great emphasis on the quality evaluation of agarwood. Throughout history, the appearance and physical characteristics of agarwood, such as sinkability and color, have often been used as indicators for quality assessment and commercial grading. However, the scientific basis of grading agarwood purely on appearance warrants additional exploration. In this study, 24 batches of agarwood were subjected to traditional sinkability tests and density measurements. The results revealed that samples floating wholly on the sea surface had densities ranging from 0.4465 to 0.7135, whereas samples partially or completely submerged had densities ranging from 0.5494 to 0.9075. Samples that sank completely to the bottom had densities above 1.0, confirming the consistency between sinkability and density. Using an intelligent visual analyzer, the appearance color of agarwood was digitally characterized. Combined with density results, it was found that as density increased, the overall L*, b* and E*ab values of agarwood gradually decreased, while the a* values were consistently positive but relatively small. This indicates that agarwood tends to be dark red, and higher density correlates with darker color. UPLC-MS was employed to comprehensively analyze the chemical components of agarwood, leading to the identification of 89 compounds. Among them, 19 differential components were identified between agarwood with densities below 0.7 and those above 0.7. It was observed that agarwood with densities above 0.7 contained more chromone dimers, while those with densities below 0.7 contained more chromone monomers. This study validates the long-held idea that agarwood, which sinks in water and exhibits a darker color is of superior quality, revealing that such dense, sinking-grade agarwood is enriched with chromone dimers, which provide a scientific basis for its quality assessment.