Growth Characteristics and Adaptability of Probiotics Using Almond Hull as a Fermentation Substrate

This study aimed to develop a high-value plant-based probiotic beverage via the co-fermentation of Lactobacillus plantarum (P8) and Bifidobacterium animalis subsp. lactis (V9) with almond hull homogenate as the fermentation substrate. Single-factor experiments combined with Box–Behnken response surface methodology were adopted to optimize the key fermentation parameters (compound bacteria ratio, inoculation amount, temperature, and fermentation time), with the probiotic proliferation multiple set as the response value. Furthermore, the physicochemical properties, active component contents, and antioxidant-related indicators of the fermented product were systematically determined and analyzed. The results showed that the optimal fermentation conditions were as follows: a P8:V9 ratio of 1:1, an inoculation amount of 0.1%, a fermentation temperature of 28 °C, and a fermentation time of 66 h. Under these optimal conditions, the viable probiotic count increased by 320-fold relative to the initial viable count, and the fermentation effectively induced the transformation of the bound bioactive components in the almond hull, with the free-flavonoid content increasing by 20.40% and the total polyphenol content decreasing by 6.16% in the fermented product, which reflected the dynamic conversion of nutrient components during the fermentation process. Meanwhile, the antioxidant capacity of the almond hull fermented product was significantly improved compared with the unfermented control. This study confirms the feasibility of almond hull as a suitable matrix for probiotic fermentation, and the findings provide a scientific basis for the development of plant-based synbiotic products and the high-value resource utilization of almond hull as an agricultural by-product.