Significance and Impact of the Study: Yellow wine lees (YWL) are the main co-products in yellow wine industry with unbalanced amino acid (AA) profiles and are not well utilized. A strain combination of Candida utilis and Bacillus subtilis was employed to upgrade YWL. Contents of crude protein and peptides of YWL were greatly increased by microbial fermentation. Essential AAs of YWL were highly improved after the solid-state fermentation and no negative impact was observed in in vitro digestibility. Fermented YWL may be a good feed source for ruminants. AbstractYellow wine lees (YWL) are the main co-products in yellow wine industry with unbalanced amino acid (AA) profiles. Solid-state fermentation was employed in this study to upgrade the YWL for ruminant animals. A 3 9 3 orthogonal design was conducted to optimize the fermentation condition for optimal crude protein (CP) yield as follows: ratio of water to total solid medium at 50 : 100 (v/w), temperature of 30°C, and ratio of Candida utilis to Bacillus subtilis at 2 : 1. The contents of CP, peptides and AA of fermented products were 14Á5, 40Á9 and 26Á1% higher than those of the unfermented respectively. In particular, the essential AA were highly improved, especially for lysine and methionine. The fermentation increased the in vitro microbial protein synthesis with higher CP digestibility and dramatically enhanced the ability of scavenging free radicals of the YWL. It is concluded that the microbial pretreatment can greatly improve the nutritional value of YWL, making these materials more suitable as feeds for animals, including ruminants.Upgrading yellow wine lees K.Y. Yao et al.