The influence of PH value on the stability of hydroxytyrosol

The impact of pH value on the stability of hydroxytyrosol is primarily achieved by altering its molecular structural state, reaction activity, and surrounding chemical environment. The specific mechanism can be analyzed through differences in molecular behavior under acidic, neutral, and alkaline conditions:

In an acidic environment (pH < 7), hydroxytyrosol exhibits relatively high stability. The phenolic hydroxyl groups in its molecule are less likely to dissociate, mainly existing in the form of neutral molecules. The electron cloud density of phenolic hydroxyl groups is low, resulting in weak reactivity with oxidants such as oxygen and free radicals, and a slow oxidative degradation rate. Meanwhile, acidic conditions can inhibit the deprotonation of phenolic hydroxyl groups, reducing the probability of structural damage caused by intramolecular rearrangement or interactions with other ions due to charge changes. For example, in fruit juices or acidic beverages with pH 3-5, the oxidation half-life of hydroxytyrosol is significantly longer than in neutral or alkaline systems, with the retention rate increased by 20%-30%.

Under neutral conditions (pH 6-8), the phenolic hydroxyl groups of hydroxytyrosol start to partially dissociate, making the molecule exhibit a certain polarity. Its interactions with water molecules and dissolved oxygen in the environment are enhanced, and the oxidation reaction rate gradually accelerates. At this point, the phenoxide anions formed after phenolic hydroxyl groups lose protons are more easily oxidized to quinone intermediates, which in turn trigger chain degradation reactions, leading to a gradual reduction in the active components of hydroxytyrosol. For instance, when stored for the same period in neutral aqueous solution, its degradation rate is 10%-15% higher than under acidic conditions, which may be accompanied by slight color changes (such as deepening of pale yellow).

Alkaline environments (pH > 8) are the most unfavorable for the stability of hydroxytyrosol. Strongly alkaline conditions promote the complete dissociation of phenolic hydroxyl groups, forming highly active phenoxide anions. The electron cloud density of these anions increases significantly, making them more prone to oxidation reactions with oxygen, hydroxyl radicals, etc., and may also trigger intramolecular cyclization, cleavage, and other structural rearrangements. In addition, under alkaline conditions, hydroxytyrosol may form unstable complexes with metal ions in the solution (such as Fe³⁺, Cu²⁺), accelerating its oxidative degradation. For example, in systems with pH 9-10, its degradation rate can reach 3-5 times that under acidic conditions, with the retention rate dropping to below 50% within a few hours, and the degradation products may produce pungent odors or precipitates.

It is worth noting that the impact of pH value is also related to other components in the system. In systems containing buffers, pH fluctuations are small, and the degradation rate of hydroxytyrosol is relatively stable; in matrices without buffering capacity, its own oxidation reaction may cause local pH changes, further exacerbating its instability. In addition, high temperatures under alkaline conditions (above 80°C) will synergize with pH value, making the degradation rate of hydroxytyrosol increase exponentially, which puts forward higher requirements for stability control in application scenarios such as food processing and pharmaceutical preparations.

Acidic conditions can significantly improve the stability of hydroxytyrosol by inhibiting the dissociation of phenolic hydroxyl groups and oxidation reactions; while neutral to alkaline conditions accelerate its oxidation and structural damage. This mechanism provides a key basis for pH regulation of hydroxytyrosol in different application scenarios.