## 1 Introduction  
Yoghurt is strictly defined by the Food and Agriculture Organization (FAO)/World Health Organization (WHO) as a milk product obtained by fermentation through the action of symbiotic starter of _Lactobacillus delbrueckii_ subsp. _bulgaricus_ and _Streptococcus thermophilus_, which shall be viable and active in the final product based on the Codex standard for fermented milk products (adopted in 2003). This dairy product is a globally prevalent functional food with potential health benefits [1]. Although the abovementioned two bacteria are considered as a conventional yoghurt-fermented starter [2], _Lactobacillus_, _Bifidobacterium_, or other probiotic microorganisms are also supplemented to improve the benefits during dairy fermentation in commercial yoghurt products [3]. Emerging studies illustrate that yoghurt is a nutritious food that contains bioactive minerals, proteins, lipids, and carbohydrates and improves immune function through gut-mediated mechanisms directly and indirectly [3; 4]. However, some reports have suggested that the health benefits and textural properties of yoghurt are significantly impacted by the fermentation starter [1; 5; 6], indicating that screening and exploring special lactic acid bacteria (LAB), such as high-yield exopolysaccharide (EPS) strains, possess great potential for application in the yoghurt industry. Meanwhile, in order to make the yoghurt more nutritious and improve the physicochemical properties, minerals, organics (such as polyphenols), fruits, vegetables, and animal products are supplemented or added to enhance the quality of the dairy product [7]. EPS is an important and bioactive organic, and it is not only produced by LAB but also by yeasts, algae, fungi, moulds, plants, and animals [8]. LAB EPS is considered safe and possesses diverse physicochemical properties and biological activities. It is usually regarded as a stabilizer, thickener, emulsifier, and gelling agent to improve the appearance, rheological properties, taste, and texture of fermented food [9; 10]. Meanwhile, LAB EPS is also reported as antioxidant, immunomododulator, anticancer, anticoagulant, anti-inflammatory, antiviral, anticholesterol, and even antidiabetic agents [8; 9; 10]. For yoghurt, gel formation significantly affects its texture. The supplementation of a stabilizer could improve gel formation through the strengthening of the casein network, the delay of milk protein aggregation or precipitation, and a reduction in yoghurt syneresis [11]. Previous report suggested that EPSs produced by _Levilactobacillus brevis_ UCLM-Lb47 and _Leuconostoc_ (_Leu._) _mesenteroides_ subsp. _mesenteroides_ 6F6-12 and 2F6-9 were able to enhance the water-holding capacity (WHC) and improved the viscosity in the mouth of the non-fat set yoghurt [12]. Similar results were also found in the yoghurt with the addition of _Leu. mesenteroides_ XR1 EPS, which showed an improvement of the WHC, an alteration of the yoghurt microstructure, and an enhancement of viscoelastic properties [13; 14]. Another study showed that EPS produced by starter cultures enhanced the sensory quality of goats' milk set yoghurt with reduced syneresis and high apparent viscosity [15]. Meanwhile, EPSs were also reported to play important roles in the stability and flavour characteristics of yoghurt [1; 16]. Consequently, LAB EPS possesses varied functions to improve yoghurt quality during fermentation. RSG7 EPS (RE) with favourable physicochemical properties was previously isolated and purified from _Leu. mesenteroides_ RSG7, which was isolated from pepino [8]. The RE was identified as dextran with a molecular weight (Mw) of 5.47 \(\times\) 10\({}^{6}\) Da and is composed of \(\alpha\)-(1\(\rightarrow\)6) glycosidic linkages as the backbone and \(\alpha\)-(1\(\rightarrow\)2), \(\alpha\)-(1\(\rightarrow\)3), \(\alpha\)-(1\(\rightarrow\)4), and \(\alpha\)-(1\(\rightarrow\)6) glycosidic linkages as the side chains [8]. Further analysis suggested that RE could promote the growth of _Bifidobacterium animalis_ ATCC27673 and _Bif.