## 1 Introduction  
Cheddar cheese is a hard, dry-salted cheese, one of the most popular and widely consumed cheeses globally. Before being transformed into cheese, raw milk undergoes multiple processing steps, including pasteurization, fermentation, coagulation, and maturation. During production, salt to achieve a 2.0% salt concentration is typically added to enhance the cheese's flavor, improve the taste, and inhibit the growth of spoilage microorganisms, thereby extending its shelf life. However, long-term consumption of high-salt foods is known to pose health risks, leading to the development of cardiovascular and cerebrovascular diseases [1]. Consequently, the WHO recommends that adults restrict their daily salt consumption to less than 5 g [2], while the UK Department of Health and Social Care's target for 2020 aimed to reduce the salt content in cheddar and hard cheeses to below 1.9 g per 100 g [3]. With growing global awareness of health risks, consumer demand for low-salt diets is increasing, impacting the production and consumption patterns of traditional high-salt foods, such as cheddar cheese.  
To address this issue, various strategies have emerged in recent years to reduce the salt content in cheese. These include substituting NaCl with KCl or flavor additives [4], alongside techniques like ultrafiltration and the application of high hydrostatic pressure [5]. Among these approaches, direct salt reduction is the simplest and most cost-effectivemethod. Previous studies have shown that salt reduction leads to enhanced microbial growth and excessive protein hydrolysis, both of which are key factors affecting the taste and texture of theeddar cheese [6]. In addition, some reports suggest that high-quality cheddar cheese can be produced with a low sodium level by controlling and monitoring protein hydrolysis and water activity [7]. Currently, research on low-salt cheese primarily focuses on product development, flavor improvement, and functional enhancement. However, studies on the dynamic changes in microbial communities and metabolic patterns of key flavor compounds in cheddar cheese, resulting from salt reduction, remain scarce. There is no comprehensive discussion on the relationship between microorganisms, metabolites, and salt concentration in cheddar cheese. Given cheese's complex microbial diversity and metabolic mechanisms, variations in salt concentration can lead to significant shifts in the microbial community structure, which in turn affect flavor metabolites. Therefore, studying the relationship between salt concentration, cheese microorganisms, and metabolites is the key to solving the problem of unstable flavor quality, while maintaining the traditional flavor in low-salt cheese. From the unique perspective of the interaction between microorganisms and metabolites, this study explored the effect of the salt concentration on cheddar cheese, aiming to construct the relationship between salt concentration, microorganisms, and metabolites in cheese.  
This study initially evaluated the effects of salt reduction on the physicochemical properties and texture of cheddar cheese. Subsequently, differences in volatile flavor compounds among cheddar cheese samples with varying salt concentrations were characterized using gas chromatography-mass spectrometry technology. Furthermore, 16S rRNA gene amplicon sequencing and non-target metabolomics technology were used to analyze the succession patterns of the microbial communities, differential changes in the metabolites, and the metabolic pathways. Ultimately, the relationship between microbial genera and differential metabolites was investigated. This study provides valuable insights into the impact of salt reduction on cheddar cheese, offering a scientific basis for improving the production of low-salt cheddar cheese.