Effect of pH on the Stability of Plant Phenolic Compounds

Abstract

It is not uncommon to treat plant-derived foods and feeds with alkali. Such exposure to high pH is being used to recover proteins from cereals and legumes, to induce the formation of fiber-forming meat analogue vegetable protein, for preparing peeled fruits and vegetables, and for destroying microorganisms. In addition to their profound effects on functional and nutritional properties in such foods, such treatments may also cause other side reactions, including the destruction of natural polyphenolic compounds. Because plants contain a large number of structurally different antioxidant, anticarcinogenic, and antimicrobial polyphenolic compounds, it is of interest to know whether such compounds are stable to heat and to high pH. In this model study, the stability of the following natural polyphenols to pH in the range 3-11 was studied with the aid of ultraviolet spectroscopy: caffeic acid, (-)-catechin, chlorogenic acid, ferulic acid, gallic acid, (-)-epigallocatechin, rutin, and the nonphenolic compound trans-cinnamic acid. This study demonstrates that caffeic, chlorogenic, and gallic acids are not stable to high pH and that the pH- and time-dependent spectral transformations are not reversible. By contrast, chlorogenic acid is stable to acid pH, to heat, and to storage when added to apple juice. (-)-Catechin, (-)-epigallocatechin, ferulic acid, rutin, and trans-cinnamic acid resisted major pH-induced degradation. The results are rationalized in terms of relative resonance stabilization of phenoxide ions and quinone oxidation intermediates. The possible significance of these findings to food chemistry and microbiology is discussed.

The influence of stereochemistry on the antioxidant properties of catechin epimers

“…EGC concentration at pH 8.5 was about 94.7 %. Literature data on the stability of EGC dissolved in buffers at the pH range from 3 to 11 [33] confirm our results showing that EGC is stable at alkaline pH. It additionally supports the conclusion that the observed decrease in the TEAC antioxidant activity of GC comparing to its epimer EGC at pH above 7 is due to poor GC stability at alkaline medium.…”

Effect of dietary fiber on the bioaccessibility of phenolic compounds of mango, papaya and pineapple fruits by an in vitro digestion model

“…Recent studies revealed that some types of PCs can be unstable to physiologic pH conditions, suffering irreversible structural changes (Friedman & Jürgens, 2000;Krook & Hagerman, 2012). These studies explain how simple PCs in papaya fruit, like caffeic acid, might be unstable to alkaline conditions above pH 7 (Friedman & Jürgens, 2000). Dn the other hand, a recent study performed by Bouayed et al (2011), reveals similar information about the bioaccessibility of PCs within the food matrix of different apple varieties.…”

“…Dn the other hand, although there was up to 50% of the total PCs in papaya fruit released at gastric conditions, the intestinal conditions with pancreatin digestion showed a decrease in the PCs concentration, indicating loss of PCs at these conditions. Recent studies revealed that some types of PCs can be unstable to physiologic pH conditions, suffering irreversible structural changes (Friedman & Jürgens, 2000;Krook & Hagerman, 2012). These studies explain how simple PCs in papaya fruit, like caffeic acid, might be unstable to alkaline conditions above pH 7 (Friedman & Jürgens, 2000).…”

Accelerated chemical stability data of O/W fluid emulsions containing the extract of Trichilia catigua Adr. Juss (and) Ptychopetalum olacoides Bentham

“…The reduction of the content of total flavonoids was 34.16% for the FE samples and 35.12%, for the FEL. t 0 : resting period of 24 h after preparation; values were presented as mean ± standard deviation (n = 3) Flavonoids are phenolic compounds with moderate solubility in water and sensitive to the presence of metal ions, ultraviolet radiation, elevated temperatures and to the hydrolysis, which is accelerated direct and proportionally to an increase of the temperature (Bruneton, 1999;Friedman, Jürgens, 2000). Bilia et al (2001) evaluated the chemical stability of flavonols, a class of flavonoids that rutin is part of, as dry extract added or not with the mixture of ascorbic and citric acid (200:1), as antioxidants, at 2.85%.…”