The stability of hydroxytyrosol usage in yogurt

I. Stability Mechanism of Hydroxytyrosol in Yogurt

Chemical Structure and Stability Basis

Hydroxytyrosol, chemically named 3,4-dihydroxyphenethyl alcohol, contains a catechol group (prone to oxidation) and a hydroxyl group (hydrophilic) in its molecular structure. In the acidic environment of yogurt (pH 4.0–4.5), the catechol group readily undergoes oxidation polymerization of phenolic hydroxyl groups, generating quinones or polymers, leading to content decline. Meanwhile, proteins and organic acids (e.g., lactic acid) in yogurt can form hydrogen bonds or hydrophobic interactions with hydroxytyrosol, potentially delaying its oxidation rate.

Influence of Microbial Metabolism

Yogurt starter cultures (e.g., Lactobacillus bulgaricus, Streptococcus thermophilus) produce hydrogen peroxide, organic acids, and certain enzymes (e.g., polyphenol oxidase) during metabolism, which may directly or indirectly promote hydroxytyrosol degradation. For example, hydrogen peroxide initiates free radical chain oxidation of phenolic hydroxyl groups, while esterases produced by lactic acid bacteria may hydrolyze ester derivatives of hydroxytyrosol (if added in esterified form).

II. Key Factors Affecting Hydroxytyrosol Stability

1. pH and Organic Acids in Yogurt System

Significant Role of pH: Although an acidic environment (pH < 5) inhibits most microbial growth, it enhances the oxidation sensitivity of hydroxytyrosol. Studies show that in a simulated yogurt system at pH 4.5, hydroxytyrosol degradation reaches 28% after 7 days of storage at 4°C, while the degradation rate is only 12% in a neutral environment (pH 6.0). This is because under acidic conditions, phenolic hydroxyl groups of catechol more easily dissociate into phenoxide anions, which then react with oxygen or free radicals.

Synergistic Effect of Organic Acids: Organic acids like lactic acid and acetic acid in yogurt affect stability through two mechanisms: on one hand, lowering the system pH accelerates oxidation; on the other hand, organic acids as antioxidants (e.g., lactic acid has some free radical scavenging capacity) may partially inhibit hydroxytyrosol degradation. However, the pro-oxidative effect of the acidic environment generally prevails.

2. Fermentation and Storage Conditions

Fermentation Temperature and Time: High-temperature fermentation (e.g., 42°C) accelerates thermal oxidation of hydroxytyrosol and promotes lactic acid bacteria to produce more hydrogen peroxide and organic acids. A study shows that after 4 hours of fermentation at 42°C, hydroxytyrosol content in yogurt decreases by 15% compared to pre-fermentation, while the degradation rate is only 5% during low-temperature (25°C) fermentation.

Storage Temperature and Oxygen Exposure: Low-temperature (4°C) storage significantly delays hydroxytyrosol degradation (7-day degradation rate < 20%), whereas the degradation rate can reach 40% at room temperature (25°C) for the same period. Additionally, oxygen exacerbates oxidation. Vacuum or nitrogen packaging can increase hydroxytyrosol retention by 10%–15%.

3. Coexisting Components in Yogurt

Protective Effect of Proteins and Fats: Milk proteins (e.g., casein) in yogurt form complexes with hydroxytyrosol via hydrophobic interactions, reducing its contact area with oxygen and free radicals. Experiments show that adding 1% casein improves hydroxytyrosol stability in yogurt by 20%. Fat (e.g., milk fat), as a non-polar medium, may promote hydroxytyrosol dissolution and reduce its oxidation rate in the aqueous phase, though the effect is weaker than proteins.

Competition and Synergy with Other Antioxidants: Naturally occurring vitamin C, vitamin E, or added polyphenols (e.g., tea polyphenols) in yogurt may compete with hydroxytyrosol for free radicals, affecting its degradation rate. For example, simultaneous addition of 0.1% vitamin C and hydroxytyrosol increases the latter's retention by 12%, but excessive vitamin C (> 0.5%) may accelerate hydroxytyrosol degradation by generating hydrogen peroxide through its own oxidation.

III. Optimization Strategies for Improving Hydroxytyrosol Stability

1. Application of Encapsulation Technologies

Microencapsulation: Spray drying or emulsion-solvent evaporation is used to encapsulate hydroxytyrosol in carriers such as β-cyclodextrin, chitosan, or sodium alginate. For instance, microcapsules prepared with sodium alginate-chitosan as wall materials increase hydroxytyrosol retention in yogurt from 65% to 89% over 7 days. The mechanism lies in the wall material forming a physical barrier to isolate oxygen and microbial enzymes.

Nanoemulsion Delivery: Hydroxytyrosol is dissolved in an oil phase (e.g., olive oil) to prepare oil-in-water nanoemulsions (particle size < 100 nm) for addition to yogurt. The hydrophobic core of nanoemulsions protects hydroxytyrosol from aqueous phase oxidation while improving its dispersibility in yogurt. Studies show that nanoemulsion-encapsulated hydroxytyrosol only decreases by 10% after 14 days of storage at 4°C, compared to a 35% decrease in the free form.

2. Process Condition Optimization

Staged Fermentation and Low-Temperature Treatment: Ferment at low temperature (25°C) to pH 5.0 first, then raise the temperature to 42°C to complete fermentation, reducing oxidation loss during the high-temperature stage. Immediately cool to 4°C after fermentation and use light-proof packaging to further inhibit oxidation.

System pH Adjustment: Adjusting pH to 5.0–5.5 (e.g., by adding a small amount of sodium bicarbonate) without affecting yogurt taste significantly reduces hydroxytyrosol oxidation rate. For example, hydroxytyrosol retention at pH 5.0 after 7 days is 18% higher than at pH 4.5. Note that increased pH may affect lactic acid bacteria activity, requiring a balance between acidity and stability.

3. Construction of Composite Antioxidant Systems

Addition of Synergistic Antioxidants: Combining 0.05% tea polyphenols with 0.03% vitamin E improves hydroxytyrosol stability through a dual mechanism of "free radical scavenging + metal ion chelation". Experiments show this composite system reduces hydroxytyrosol degradation in yogurt by 30%, outperforming single antioxidants.

Metal Ion Chelators: Calcium and iron ions in yogurt catalyze hydroxytyrosol oxidation. Adding 0.1% citric acid or EDTA-2Na to chelate metal ions increases hydroxytyrosol retention by 15%–20%.

IV. Challenges and Prospects in Practical Applications

1. Sensory Compatibility Issues

Hydroxytyrosol itself has a bitter taste (threshold ~ 50 mg/L), and high-dose addition may affect yogurt palatability. Although encapsulation technologies improve stability, the particle size and dosage of microcapsules must be controlled to avoid impacting yogurt texture (e.g., viscosity, smoothness). For example, microcapsule addition exceeding 0.5% may cause graininess, requiring homogenization (e.g., high-pressure homogenization) to optimize system uniformity.

2. Cost and Scale-up Production

The extraction cost of natural hydroxytyrosol is high, and encapsulation technologies (e.g., nanoemulsions, microcapsules) further increase production costs, limiting its application in mass-market yogurt. Future developments may include microbial fermentation for hydroxytyrosol synthesis (e.g., using recombinant yeast to convert tyrosine) or the development of low-cost encapsulation materials (e.g., modified starches) to promote industrial application.

Hydroxytyrosol stability in yogurt is regulated by multiple factors such as system pH, microbial metabolism, storage conditions, and coexisting components. Acidic environments, high-temperature fermentation, and oxygen exposure are the main causes of its degradation. Strategies like microcapsule encapsulation, nanoemulsion delivery, process optimization, and composite antioxidant systems can significantly improve stability, but require balancing functional activity with sensory quality and production costs. Future research may focus on developing low-cost and efficient encapsulation technologies and microbial conversion for hydroxytyrosol production to promote its widespread application in functional yogurt.