pH measures the acidity or alkalinity of a solution, but how does it affect phytochemical stability? We know that the lower the pH, the more acidic the solution, and the higher the pH, the more basic or alkaline the solution. These changes in pH can have a significant effect on plant phytochemicals.
Phytochemicals are naturally occurring compounds that are found in plants,responsible for their organoleptic properties and resistance against bacteria, fungi, pests and environmental stress. Some phytochemicals also have important health benefits and are commonly used as active pharmaceutical ingredients to treat various issues and diseases. For example, some phytochemicals act as antioxidants, protecting cells from damage caused by free radicals.
The stability of phytochemicals during the extraction process is affected by many factors, including pH.
A study published in the Journal of Agricultural and Food Chemistry looked at the effect of pH on the stability of plant phenolic compounds. The study found that this kind of molecules was stable at a pH of 4-7. Above or below this range, the phenolic compounds were more likely to be damaged or destroyed during extraction.
It was highlighted that the changes in the molecular composition of the extract observed at high pH were irreversible also after pH reduction. So, in order to unalter the final product it is extremely important, in some cases, to keep the pH at the right level. 
Another study published in the Journal of Antioxidant Activity looked at the effect of solvent pH on antioxidant and phytochemical activities of Glycyrrhiza glabra L. also known as Liquorice. The highest phenol content was present at pH 7 and the highest flavonoid concentration was at pH 9. Therefore, pH 7 was the best for studying phenolic compounds and pH 9 was an excellent factor for determining flavonoid compounds. 
The effect of pH on phytochemical stability is well-documented in research studies like these. The best pH range to be used during extraction of phytochemicals will depend highly on the specific compound you want to extract. It should be noticed that changes in pH can have a significant effect on the stability of phytochemicals. Producers and manufacturers should keep this in mind when choosing pH settings for different extraction purposes.
References: Friedman, M., et al. Effect of pH on the Stability of Plant Phenolic Compounds. J. Agric. Food Chem. 2000, 48(6):2101-10. DOI:10.1021/jf990489j. [Journal Impact Factor = 5.895] [Times Cited = 66]  Devi, Parvesh., et al. Effect of Solvent pH on Antioxidant and Phytochemical Activities of Mulhatti Aerial Parts (Glycyrrhiza glabra L.). Journal of Antioxidant Activity. DOI 10.14302/issn.2471-2140.jaa-21-4027. 2021. Volume No: 2 Issue No: 3. [Journal Impact Factor = 5.952]
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