Synbiotic Yogurt with Lactobacillus plantarum and Stevia rebaudiana: Physicochemical, Microbiological, and Functional Evaluation

This study demonstrates that fortifying yogurt with *Lactobacillus plantarum* and *Stevia rebaudiana* extract creates a synbiotic product with enhanced antioxidant activity, improved nutritional profile, and superior sensory attributes compared to control samples.

The Gist

Imagine your gut as a bustling city. Sometimes, this city gets invaded by "bad guys" (harmful bacteria) that cause sickness, while the "good guys" (beneficial bacteria) struggle to survive the harsh journey from your mouth to your intestines.

This paper is about building a super-highway to help the good guys survive the trip and fight off the bad guys, all while making the journey taste delicious.

Here is the story of how the researchers did it, broken down into simple steps:

1. The Search for the "Super Soldier" (Finding the Right Bacteria)

The researchers started by digging through old, fermented foods like pickles and traditional yogurt (called Dahi, Tama, and Gundruk in Nepal). They were looking for a specific type of bacteria called Lactic Acid Bacteria (LAB).

Think of these bacteria as potential soldiers. But not all soldiers are tough enough. To find the best one, they put the candidates through a brutal training camp:

• The Acid Test: They threw them into a stomach-simulating acid bath (pH 3.0). Most soldiers melted away, but one, named PG1 (a Lactobacillus plantarum), stood tall.
• The Bile Test: They threw them into a bile salt bath (like the detergent in your intestines). Again, most failed, but PG1 kept marching.
• The Sticky Test: They checked if the bacteria could stick to surfaces (like a velcro strap). PG1 was incredibly sticky, meaning it could easily grab onto your gut walls and stay there.

The Winner: PG1 was the champion. It was tough, sticky, and could even produce its own antibiotics to fight off bad bacteria like E. coli and Salmonella.

2. The "Fuel Station" (Adding Stevia)

Even the toughest soldier needs fuel. The researchers wanted to give PG1 a special energy drink to help it grow faster. They chose Stevia (Stevia rebaudiana), a natural sweetener from a plant.

Usually, people think of Stevia just as a sugar substitute. But the researchers discovered something cool: Stevia acts like a fertilizer for the good bacteria.

• They mixed the bacteria with different amounts of Stevia.
• The Result: When they added 1% Stevia, the bacteria went wild! They grew 49% faster than they did with regular sugar. It was like giving the soldiers a jetpack.

3. Building the "Fortress" (Making the Yogurt)

Now, they had their Super Soldier (PG1) and their Jetpack Fuel (Stevia). It was time to build the vehicle: Synbiotic Yogurt.

• Synbiotic is a fancy word that just means "Probiotic + Prebiotic" (Good bacteria + Food for the bacteria).
• They made four batches of yogurt:
  1. Regular Yogurt: Just the starter culture.
  2. Probiotic Yogurt: Just the Super Soldier (PG1).
  3. Synbiotic Yogurt (The Star): The Super Soldier + Stevia.
  4. Control Yogurt: The Super Soldier + Inulin (a different kind of fiber).

4. The 10-Day Challenge (Storage and Testing)

They put the yogurt in the fridge for 10 days to see how it held up. Here is what happened:

• The Good Guys Survived: The bacteria in the Stevia yogurt (St-Y) stayed alive and happy better than in the other batches. The Stevia acted like a shield, protecting the bacteria from the cold and the acid.
• The Antioxidant Power-Up: Because Stevia is full of plant chemicals (phenols and flavonoids), the yogurt became a vitamin bomb. The Stevia yogurt had way more antioxidants than the others. Think of antioxidants as "rust removers" that protect your cells from damage.
• The Taste Test: A group of people tasted the yogurt. The Stevia yogurt won hands down! It tasted sweeter, felt creamier, and people loved it more than the plain probiotic yogurt.

The Big Takeaway

This paper proves that you can make a yogurt that is:

1. Tough: It keeps the good bacteria alive so they actually reach your gut.
2. Healthy: It's packed with antioxidants that fight disease.
3. Delicious: It tastes great, so people will actually want to eat it.

In a nutshell: The researchers took a tough bacteria, gave it a Stevia energy boost, and turned it into a super-yogurt that tastes good and fights off bad bacteria in your tummy. It's a win for your gut and your taste buds!

Technical Summary

1. Problem Statement

The functional food industry faces a critical challenge in developing dairy products that simultaneously deliver essential nutrition and therapeutic health benefits. While probiotic-enriched foods (like yogurt) are known to modulate gut microbiota, ensuring the viability of probiotic strains (typically requiring $10^8$–$10^9$ CFU/g) throughout the product's shelf life and during gastrointestinal transit remains difficult. Furthermore, there is a need for natural, non-caloric sweeteners that can act as prebiotics to enhance probiotic survival and metabolic activity. Although Stevia rebaudiana is a known natural sweetener with antioxidant properties, its specific efficacy as a prebiotic in synbiotic yogurt formulations, and its impact on the physicochemical stability and sensory profile of the product, remains underexplored.

2. Methodology

The study employed a multi-stage experimental approach to develop and evaluate a novel synbiotic yogurt:

• Isolation and Screening of LAB:
  • Thirteen Lactic Acid Bacteria (LAB) strains were isolated from traditional fermented foods (Tama, Sinki, Dahi, Gundruk).
  • Probiotic Potential Screening: Isolates were tested for:
    • Stress Tolerance: Acid tolerance (pH 3.0), bile salt tolerance (0.15%–0.5%), and phenol tolerance (0.2%–0.4%).
    • Adhesion Properties: Cell hydrophobicity (using various solvents), auto-aggregation, and co-aggregation with pathogens (E. coli, S. aureus, K. pneumoniae, etc.).
    • Safety & Enzymatic Activity: Hemolytic activity, and amylolytic, lipolytic, and proteolytic capabilities.
    • Metabolic Profiling: Sugar fermentation patterns and antagonistic activity against foodborne pathogens via cell-free supernatant (CFS).
• Prebiotic Selection:
  • An aqueous extract of Stevia rebaudiana was prepared.
  • Growth Stimulation: The top three acid/bile-tolerant isolates were tested for growth stimulation in media containing 0.5%, 1%, and 2% stevia extract.
• Yogurt Formulation:
  • The selected strain (L. plantarum PG1) was used to formulate yogurt.
  • Experimental Groups:
    1. SP-Y: Starter only (Control).
    2. P-Y: Probiotic only (L. plantarum).
    3. St-Y: Synbiotic (Probiotic + 1% Stevia extract).
    4. In-Y: Probiotic + 1% Inulin (Positive control for prebiotic).
• Storage and Analysis:
  • Samples were stored at 4°C for 10 days.
  • Parameters Monitored: pH, titratable acidity (TTA), protein content, microbial viability (TVC), total phenolic content (TPC), total flavonoid content (TFC), DPPH radical scavenging activity, and sensory attributes (9-point hedonic scale).

3. Key Contributions

• Strain Selection: Identification and characterization of a potent probiotic strain, PG1 (Lactobacillus plantarum), isolated from local fermented foods, which demonstrated superior acid/bile tolerance and pathogen antagonism.
• Synbiotic Synergy: Demonstration that Stevia rebaudiana extract acts as an effective prebiotic, significantly stimulating the growth of L. plantarum (highest growth index of 49% at 1% concentration).
• Functional Product Development: Successful formulation of a synbiotic yogurt that enhances antioxidant capacity and sensory appeal without compromising probiotic viability.
• Comprehensive Characterization: A holistic evaluation linking microbiological stability with functional bioactivity (phenolics/flavonoids) and consumer acceptability.

4. Key Results

A. Probiotic Screening

• Selection: Isolate PG1 was selected as the optimal strain. It showed:
  • Acid/Bile Tolerance: Maintained viability at pH 3.0 and grew in 0.3% bile salt.
  • Adhesion: High cell hydrophobicity (86.77% affinity for toluene) and strong auto-aggregation (43.84%) and co-aggregation (up to 61.58% with S. aureus).
  • Safety: Non-hemolytic and negative for lipolytic/proteolytic activity.
  • Antagonism: Strong inhibition zones (15–20 mm) against E. coli, K. pneumoniae, S. aureus, and P. aeruginosa.

B. Growth Stimulation

• Stevia Effect: L. plantarum PG1 showed the highest Prebiotic Growth Stimulation Index (PGSI) of 49% in the presence of 1% stevia extract, outperforming inulin and other isolates.

C. Physicochemical & Microbiological Properties (Day 10 Storage)

• Viability: The synbiotic yogurt (St-Y) maintained the highest viable count (7.65 log CFU/ml) after 10 days, showing the least decline (-2.1%) compared to controls, indicating stevia's protective effect.
• pH & Acidity: St-Y showed moderate post-acidification. Initial pH was 4.55, dropping to 4.31 by Day 10. Titratable acidity increased to 0.75% (lactic acid).
• Protein: St-Y exhibited the highest protein content (3.70%), suggesting stevia may contribute to protein stability or synthesis.

D. Functional (Antioxidant) Properties

• Phenolics & Flavonoids: St-Y had significantly higher initial TPC (16.67 µg GAE/g) and TFC (6.28 µg QE/g) compared to controls (<1.1 µg/g).
• Antioxidant Activity: St-Y demonstrated superior DPPH radical scavenging activity (57.84% initially), which remained high at 49.26% after 10 days. Controls showed activity below 14%.

E. Sensory Evaluation

• Acceptability: St-Y received the highest scores for Taste (9.2), Mouthfeel (9.2), and Overall Acceptability (8.2) on a 9-point scale.
• Statistical Significance: St-Y was rated significantly higher than all other formulations (p < 0.05), indicating that stevia improved palatability without off-flavors.

5. Significance and Conclusion

This study validates the potential of Stevia rebaudiana as a dual-function ingredient: a natural sweetener and a potent prebiotic. The resulting synbiotic yogurt (St-Y) offers a superior functional profile compared to standard probiotic yogurt, characterized by:

1. Enhanced Viability: Improved survival of L. plantarum during refrigerated storage.
2. Bioactivity: Significantly elevated antioxidant capacity due to the retention of stevia-derived phenolics and flavonoids.
3. Consumer Appeal: Superior sensory attributes, particularly taste and mouthfeel.

The findings suggest that fortifying yogurt with L. plantarum and 1% stevia extract creates a stable, nutritious, and antioxidant-rich functional food. While the study confirms in vitro efficacy and stability, the authors note that in vivo clinical trials are necessary to fully validate the gastrointestinal survival and health benefits in humans. This research provides a strong foundation for the development of next-generation synbiotic dairy products in the functional food market.