In Vitro Assessment of Gut Microbiota Modulation Through Functional Biscuits Enriched with Almond By-Products

Almond skin is an abundant by-product of almond processing and is recognized for its rich content of dietary fiber, polyphenols, and unsaturated fatty acids along with potential health benefits. This study aimed to evaluate the nutritional composition, prebiotic potential, and microbiota modulation properties of dehydrated almond skin, including its use in 3D-printed functional biscuits. Nutritional analysis revealed high dietary fiber (62.6%) and substantial antioxidant capacity linked to polyphenols. Almond skin supplementation with a concentration ranging from 2.5% to 5.0% significantly enhanced the viability of various probiotic strains during storage, extending their shelf life. Two biscuit formulations, with and without almond skin, were produced and subjected to simulated gastrointestinal digestion (INFOGEST protocol) followed by in vitro fermentation using a minimal gut microbiota model (<i>Bifidobacterium longum</i>, <i>Lactobacillus rhamnosus</i>, <i>Bacteroides caccae</i>, <i>Escherichia coli</i>, <i>Segatella copri</i>, and <i>Clostridioides difficile</i>). Results demonstrated that biscuit enriched with almond skin selectively promoted the growth of beneficial bacteria such as <i>B. longum</i> and <i>L. rhamnosus</i> (from 6.9 to 8.5 log cfu/mL and from 7.8 to 9.0 log cfu/mL, respectively) while suppressing pathogens including <i>C. difficile</i> and <i>E. coli</i>. Moreover, enriched biscuits retained higher polyphenol content and exhibited a favorable macronutrient profile. These findings support the valorization of almond skin as a sustainable functional ingredient offering prebiotic effects and probiotic viability protection, with promising applications in personalized nutrition and gut health management. Further in vivo studies and clinical trials are necessary to confirm these effects and optimize formulations for commercial use.