Transforming ordinary yogurt into functional yogurt using low doses of ethanolic extract of Origanum vulgare L.

Abstract

Background: Medicinal plants are widely known for their antimicrobial properties. However, the concentrations required to enhance the shelf life or enrich food products are often relatively high. When applied during fermentation, these levels may inhibit the growth of starter cultures, thereby negatively affecting the final product's quality. As a result, exploring the influence of low doses of plant extracts on fermentation processes holds both scientific and practical significance.

Objective: This study aimed to investigate the dose-dependent effects of Origanum vulgare ethanolic extract (OVEE) on the growth of lactic acid bacteria (LAB) starters, their proteolytic activity, the fermentation rate of milk, and the resulting yogurt’s functional and sensory characteristics.

Methods: The dose-response relationship was evaluated by measuring the optical density (OD600) of LAB cultures incubated for 6 hours in the presence of serial two-fold dilutions of 50 mg/mL OVEE. Proteolytic activity was assessed using the disk diffusion method on milk agar plates. For yogurt production, OVEE was added at various concentrations to milk pasteurized at 85°C for 15 minutes, then cooled to 45°C. The milk was inoculated with 3% LAB starter culture and fermented at 37°C. After coagulation, the yogurt samples were stored at 5°C for ripening. Analyses were conducted on days 1, 14, and 28 of refrigerated storage. Sensory evaluation was performed by a trained panel of 12 participants using a 10-point hedonic scale.

Results: Lactic acid bacteria, unlike pathogenic microorganisms, are highly resistant to medicinal plant extracts. OVEE exhibited only mild inhibitory activity against yogurt LAB strains (MIC > 100 mg/mL). Moreover, certain concentrations of OVEE in the range of 0.1% to 1.0% stimulate the growth of LAB, with the most effective concentration being 0.3%. The addition of OVEE significantly enhanced the proliferation of Streptococcus thermophilus and Lactobacillus bulgaricus in milk, increased the total proteolytic activity, accelerated acidification and clot formation, and enhanced the antioxidant and antimicrobial properties of yogurt. Additionally, OVEE supplementation led to higher lactic acid production and a corresponding reduction in lactose content. It also improved cold storage tolerance, reduced syneresis, and contributed to a more uniform yogurt texture throughout the 28-day cold storage period. Overall, OVEE-treated samples received higher sensory scores in most evaluated attributes. 

Conclusion: Specific concentrations of OVEE are sufficient to stimulate microbial activity, enhance proteolytic processes, and promote the release of bioactive peptides from milk proteins, including antioxidants. The resulting yogurts exhibit deeper fermentation, reduced lactose levels, improved digestibility, and a broader range of health-promoting bioactive compounds. Therefore, carefully selected doses of plant extracts can play an active role in fermentation and elevate conventional dairy products into functional foods.

Novelty: The main novelty of this work lies in demonstrating that specific doses of Origanum vulgare extract can enhance fermentation and enrich yogurt with bioactive compounds, primarily produced by lactic acid bacteria from milk.

Keywords: Origanum vulgare, low dose ethanolic extract, lactic acid bacteria (LAB), microbial growth enhancement, functional yogurt, proteolytic activity, antioxidant activity of yogurt, bioactive compounds, sensory evaluation of yogurt.