## 1 Introduction  
Currently, with the promotion of environmental sustainability and health awareness, nutritious plant-based protein beverages, such as legume milk, have attracted the increasing attention of consumers [1, 2]. These plant-based dairy replacements possess a reputation for \"health ingredients\" such as increased food safety, reduced allergens, an enhanced nutrition profile, and decreased lactose intolerance compared with their dairy counterparts [2, 3, 4]. Plant-based yogurts produced by the traditional yogurt-making process still have some defects, including unpleasant odor and insufficient phytochemical nutrients. However, they could be modified to improve their acceptability [5]. Hence, the formulas of conventional legume milk products may not be optimal for plant-based systems and should be further explored to improve flavor and quality. As a substitute to conventional soymilk, chickpea drink is a novel type of nutritious plant-based milk with a high amount of resistant starch and protein, less lipid, and no allergenicity compared with other plant-based beverages [2, 6]. With respect to chickpea milk, some research on quality has been conducted into its physicochemical properties, such as viscosity, color, pH, total polysaccharides, etc., [7]. Previous studies found that chickpea drink has a lower appearance rating than its soy counterpart [2, 6]. Fermented chickpeamilk is a type of yogurt-like drink produced by chickpea milk via lactic acid bacterial fermentation [2]. Lactic acid fermentation may be adopted as an approach to reduce the beauty odors in legume foods [8]. It also produces a special fragrance of fermentation, which gives the product a novel and unique flavor. Fermented legume milks possess improved textures and scents and also have certain health-promoting benefits, containing probiotics, high bioactivity, absence of antinutrients, and improved digestibility [9; 10; 11]. Although fermented legume milk is beneficial to the health of consumers, it has not yet been widely accepted because of its off-flavors [12]. The flavor of food is an important indicator of the quality of products. The main ingredients that affect the flavor of legume milk include undesirable beauty flavor substances, e.g., hexanal, 1-ocene-3-alcohol, 1-hexanol, trans-2-ocene aldehyde, and pelargonic aldehyde [13; 14; 15]. Methods for decreasing the unpleasant-flavor-related odorants in legume milk, such as modified pulse with loss of lip-oxygenase (LOX) genes and improved breeding technology, have been used to prepare legume milk with decreased off-flavor [12; 13; 16]. Few studies have used electronic nose (E-nose) and gas chromatography-mass spectrometry (GC-MS) to investigate the aroma characteristics of different formulas of chickpea milks before and after fermentation. Limited data are available regarding the influence of nutrient substance addition and enzymatic treatment on the volatile odor constituents of chickpea milk and the flavor variation before and after fermentation. Accordingly, it is necessary to discuss the effect of using probiotic fermentation and explore the feasibility of enzymolysis and yam addition on the volatile odor constituents of chickpea milk. Chinese yam is a well-known homology of medicine and food and is favored by consumers for its soft and delicate taste [7]. Previous studies reported that the addition of nutrient substances could improve the flavor of legume milk [17; 18; 19]. However, the effect of yam addition on the flavor of legume milk has not yet been reported. A number of investigations reported that enzymolysis affects the volatile components and corresponding flavor of plant-based systems. For example, aroma substances were released after enzymolysis. Meanwhile, undesirable odorants were reduced due to the liberation of conjugate glycoside bonds of odorants [2; 20; 21]. As an odor modification enzyme, papain plays a crucial role in increasing the quality of legume milk [2].