Preparation Process and Functional Evaluation of Astragalus membranaceus (Fisch.) Bunge Stem-Leaves Extract Based on Pro-Proliferation and Colonization Activities on Lactiplantibacillus plantarum

To achieve the resource utilization of Astragalus membranaceus (Fisch.) Bunge stem-leaves (AMBSL) and explore their potential as a novel feed additive, this study established an efficient ultrasonic extraction process. Taking extract yield, the contents of major active components (reducing sugars, saponins, polyphenols, and flavonoids), together with the proliferation-promoting and auto-aggregation activities of Lactiplantibacillus plantarum (L. plantarum) as comprehensive evaluation indices, single-factor experiments were conducted to optimize key extraction parameters including solid–liquid ratio, ultrasonic power, ultrasonic time, extraction temperature, and system pH. The crude extract was further fractionated by preparative liquid chromatography to screen functional components responsible for promoting L. plantarum proliferation. The results demonstrated that the optimal ultrasonic extraction conditions were as follows: water as the extraction solvent, solid–liquid ratio of 1:40 g/mL, ultrasonic power of 500 W, ultrasonic duration of 25 min, extraction temperature of 50 °C, system pH of 7, and one-time extraction. Under these optimal conditions, the extract yield reached 18%, with the contents of reducing sugars, saponins, polyphenols, and flavonoids being 140 mg/g, 92 mg/g, 44 mg/g, and 2.9 mg/g, respectively. The obtained extract significantly increased the growth rate of L. plantarum by 29% and improved its auto-aggregation capacity by 41%. Nine fractions (F1–F9) were isolated from the crude extract, and the crude extract exhibited superior probiotic proliferation-promoting activity compared with each single fraction. This study clarifies the optimal ultrasonic extraction parameters for AMBSL and verifies their favorable biological activity in facilitating L. plantarum growth. The findings provide a reliable experimental basis for the development of AMBSL as a novel probiotic feed additive, and offer practical support for the resource recycling of Astragalus by-products and industrial chain extension.