Effects of <i>Stinging nettle</i> Powder on Probiotics Survival, Physiochemical Properties, and Nutritional Value of Kefir

Kefir is a historic dairy-fermented beverage produced using lactic acid bacteria and yeast as a starter culture and is considered nutritious with a good taste. Many studies have been conducted to incorporate various possible functional materials into kefir to enhance its nutritional value. This study aims to enrich kefir with 0.25% and 0.5% of <i>Stinging nettle</i> (Sn) powder before fermentation to improve its nutritional value. <i>Stinging nettle</i> (<i>Urtica dioica</i>) is a nutritious and multifunctional herb with a variety of healthful components such as fibers and polyphenols; it has significant potential as a useful food functional ingredient. The physicochemical, microbial, and nutritional properties of kefir fortified with Sn were examined weekly during refrigerated storage for 21 days. The results showed that adding <i>Stinging nettle</i> significantly (<i>p</i> < 0.05) increased the probiotic counts from 7.90 ± 0.22 log to 8.46 ± 0.19 log CFU/g, antioxidant activity (4%), and total polyphenol contents (5%) in kefir yogurt after 12 days of refrigerated storage. The addition of Sn also had a positive effect on the acidity of kefir and increased the viscosity and the syneresis to a certain extent. Furthermore, adding Sn increased lactic acid bacteria counts and the production of short-chain fatty acids after in vitro digestion and colonic fermentation. The results of this study indicated the potential use of Sn powder as a functional ingredient in kefir yogurt and other similar products.