Potential Use of Hazelnut Beverage in Water Kefir Fermentation: Effects of Sugar Addition on Physicochemical, Microbiological, Rheological, Bioactive, Volatile, and Allergenic Properties.

This study investigated the potential of hazelnut beverage, obtained from hazelnut cake through high-pressure homogenization (100 MPa), for use in water kefir production. The effects of different sugar concentrations (2.5%, 5%, and 7.5%) on product properties and hazelnut allergenicity were also examined. Sugar addition significantly stimulated microbial growth, resulting in significant increases in Lactobacilli, Lactococci, acetic acid bacteria, and yeast counts compared to the control sample. At the end of fermentation, the pH values of all samples (4.32-4.54) were found to be below the food safety critical limit of pH 4.6. The best water-holding capacity and physical stability were observed in the sample containing 5% sugar. Rheological analyses showed that the samples exhibited pseudoplastic behavior and that sugar addition increased viscosity. The addition of sugar contributed to an increase in the total phenolic content (11.26-15.15 mg GAE/100 mL) and antioxidant activity (DPPH: 16.77%-22.69%; ABTS: 42.68%-46.79%) at the end of fermentation. In the volatile aroma profile evaluation, 47 volatile compounds were tentatively identified based on mass spectral library matching, and increasing sugar concentration resulted in higher relative abundances of ethanol (6.72%-15.51%) and acetic acid (6.29%-13.88%). Allergen levels decreased from 16.6 µg/mL to 8.42-12.97 µg/mL with the addition of sugar. In conclusion, hazelnut cake, a byproduct with low added value, has been transformed into a new, functional, plant-based, low allergenic, and consumer-approved functional beverage. The developed hazelnut-based water kefir offers a significant alternative for individuals with milk protein allergy, lactose intolerance, and vegan/vegetarian consumers. PRACTICAL APPLICATIONS: Hazelnut cake is a low-value by-product from cold-pressed hazelnut oil production. It can be transformed into a better functional beverage using water kefir fermentation. The hazelnut-based water kefir offers a sustainable and eco-friendly way to make use of food industry by-products. It also meets the growing demand for plant-based fermented drinks. Fermentation enhances the beverage's bioactive properties and lowers hazelnut allergen levels, increasing its potential for creating products with better functionality. In addition, the product serves as a great option for vegan and vegetarian consumers, as well as for those with lactose intolerance or milk protein allergies. The findings of this study may aid in creating new non-dairy fermented drinks and help diversify sustainable food products in the beverage industry. Using 5% sugar supplementation could be particularly beneficial for industrial uses because of its positive effects on physical stability, water-holding capacity, and overall product quality.