## 1 Introduction  
In recent years, there has been an increasing consumer demand for rational diets. Fermented soy foods have garnered widespread attention globally due to their nutritional and health benefits [1]. Broad bean paste (BBP), also known as Pixian-douban or Doubanjiang, is a primary traditional fermented soy food in China, recognized as a Chinese geographical indication protected product and a national intangible cultural heritage [2]. The production of BBP mainly involves three stages: (1) chopped chili peppers are naturally fermented to produce moromi, (2) broad beans are inoculated with _Aspergillus oryzae_ and then converted to meju via fermentation, and (3) the moromi-meju mixture is exposed to sunlight, sheltered from rain, regularly stirred during the day and left to ferment through dew at night until mature [3]. During the BBP-meju fermentation, vigorous microbial metabolism converts macromolecules such as proteins and starch into amino acids, peptides, flavonoids, and free sugars, imparting rich flavor and various physiological functions, which significantly impact the final quality of BBP [4, 5]. Reports suggested that BBP exhibited excellent anti-inflammatory, antioxidant, anti-hypertensive, and anti-obesity functions [6, 7]. However, according to the Chinese national standard, the salinity of commercial BBP is 15-22%(\(w/w\)), while the amount of salt added in BBP-meju is 12-13% (\(w/w\)) [8]. Numerous studies indicated that a high-salt diet could induce hypertension, osteoporosis, cardiovascular and cerebrovascular diseases, and kidney diseases [9; 10], and was also a risk factor for Alzheimer's disease [11]. The World Health Organization (WHO) recommends a daily sodium intake of 2 g for adults, equivalent to 5 g of salt per day [12]. Additionally, the WHO released the \"WHO global sodium benchmarks for different food categories (2 nd edition)\" [13] in 2024. It serves as a guide for countries and industries to reduce sodium content in various processed foods, aiming to collectively achieve the WHO's goal of reducing global salt/sodium intake by 30% by 2025 [14]. Moreover, the \"Chinese National Nutrition Plan (2017-2030)\" explicitly states the goal of reducing salt intake among Chinese residents by 20% by 2030 [15]. Therefore, reducing BBP salt concentration is crucial to promote further development in this industry. Currently, strategies used for salt reduction in BBP are mainly based on the replacement of NaCl by other substances, including CaCl\({}_{2}\), MgCl\({}_{2}\), KCl, yeast extracts, and phosphates [16]. Research indicated that increasing potassium intake through diet can alleviate symptoms in hypertensive patients induced by sodium [17]. This was primarily manifested by a reduction in urinary calcium excretion as well as a protective effect on skeletal mass [18]. However, fermented foods produced as substitutes using potassium or calcium salt contained bitter and unpleasant off-flavors that severely restricted their consumer acceptance [19; 20]. Meanwhile, since osmotic pressure under high-salt conditions can inhibit the growth of spoilage microorganisms, most low-salt fermented foods require additional preservatives [21], affecting their safety and health. Therefore, salt reduction in fermented foods must be carried out with great caution. Studies have shown that salt-reduced fermentation accelerates BBP fermentation, promotes the production of organic acids (OAs) and free amino acids (FAAs), and increases the accumulation of flavor compounds and alkaloids [4]. Nevertheless, a decrease in salt concentration causes an increase in undesirable flavor compounds such as acetic acid, butyric acid, 2-phenylfuran, decanal, and (Z)-2-heptnal, which makes BBP exhibit acidity and irritation [22]. Furthermore, salt reduction during BBP fermentation can increase _Klebsiella, Cronobacter_, and _Acinetobacter_[23]. They are opportunistic pathogens that pose a food safety threat to BBP.