## 1 Introduction  
Huangjiu is a Chinese rice wine with a history of over 2500 years. It is produced from glutinous rice, corn, or millet using a multi-step process that involves cooking, saccharifying, fermenting, pressing, filtering, frying, and blending [1]. Huangjiu can be classified into two types based on the production region, namely, Southern Huangjiu from Southern China and Northern Huangjiu from Northern China. The former is typically produced from glutinous rice, whereas the latter is produced from millet [2]. Compared to rice (4.3-7.0%), millet contains a higher concentration of crude protein (11.4-13.7%), which makes it nutritionally valuable and offers unique functional properties for food development and application. Millet has been used in various products, such as yogurts, breads, and supplementary foods for infants, and its high protein content renders it an optimal choice for Northern Huangjiu fermentation [3]. The use of millet in Northern Huangjiu production may enhance its nutritional value and provide distinct functional properties in the final product [4]. Chinese millet Huangjiu is typically characterized by its light color, low alcohol content (11-13%), mellow taste, and smooth mouthfeel. Based on data from the China Alcoholic Drinks Association, the consumption of millet Huangjiu in China has exhibited a steady upward trend in recent years. Specifically, in 2020, the annual sales volume of millet Huangjiu in China was reported to have reached 0.43 billion liters.  
Several studies have shown that the flavor of millet Huangjiu is influenced by the raw materials, particularly the protein profiles of millet [2]. Enzymes secreted by fermented bacteria degrade millet proteins and form short peptides and free amino acids. Besides affecting yeast growth, amino acids also affect the quality of Huangjiu, e.g., higher levels of alcohols and esters. Researchers found that during grape wine fermentation processes, nitrogen utilization was positively correlated with total aromatic substance content [5]. Moreover, a positive correlation was found between the consumption of amino acids (e.g., tyrosine, lysine, and leucine) and the formation of alcohols and acetate esters [5]. A previous study indicated that Japanese sake brewed with added protein exhibited a different amino acid composition compared to the control group. Additionally, sensory analysis revealed that the addition of prolamin negatively affected the flavor quality of sake, probably because prolamin is poorly digested during fermentation, resulting in a prolonged fermentation time. Contrary to this, glutenil could be easily digestible into amino acids and oligopeptides. As a result, glutenil may significantly enhance sake's nutritional content and taste. It was confirmed through a clarification experiment that adding glutenil to white wine enhanced the clarity and preserved the aromatic compounds, as well as improved the flavor [6]. Similarly, due to the excellent thermal stability and surface properties (e.g., emulsifiability, foamability, and solubility) within a broad range of pH levels, from 3.0 to 8.0, albumin may also serve as a food ingredient to develop high-quality wine [7].  
The quality of Huangjiu brewed using different raw materials varies greatly. Limited information is only available on the glutinous rice Huangjiu [8; 9], whereas the effects of millet protein components on the volatiles and physicochemical properties of millet Huangjiu have not been thoroughly studied. Accordingly, the addition of exogenous glutenil and albumin was used to modify the protein composition of the brewing raw materials, aiming to examine how different protein components affect the flavor and quality of millet Huangjiu. The goal of the current study was to establish a correlation between the protein components of brewing materials and the overall quality of millet Huangjiu, as well as to offer some guidelines regarding millet selection for aroma enhancement in Huangjiu brewing.