Anthocyanins instability

Anthocyanins display their typical color at low pH values, where the molecules are in the oxonium form (Wrolstad, 2000). The oxonium form is more stable, and anthocyanins thus have higher stability in an acid environment. If pH rises to levels above 4.5, the anthocyanin instability is particularly problematic. The detailed molecular structure of the anthocyanins, i.e., acylation and the position of the glycosidic substitution, affects the exact pH at which the color changes occur. Copigmented anthocyanins are less susceptible to degradation. 

Anthocyanins are poorly absorbed from the gastrointestinal tract and the mechanisms involved remain unclear. These compounds are usually recovered in very small amounts in human serum after oral ingestion (less than 1% of the dose) or in the IN fraction after in vitro digestion (about 5%). ° Unlike other polyphenols, anthocyanins constitute an exception because intact glycosides are recovered in the body (without deglycosylation prior to absorption). - This may be explained by either the instability of the free aglycone form or by a specific mechanism of absorption for anthocyanins. 

Table XVI shows two experiments which compared the same wine stored under different conditions (46). In both cases, the wine richer in anthocyanins is also that which is less colored. The mediation of tannins is not enough to explain the differences in color these can only be explained by a different structuring of the anthocyanin molecules. More specifically, the anthocyanin molecules will be reduced to colorless flavenes (14) during fermentation, which is a reductive process. The reoxidation occurs more rapidly in wooden casks which allow better oxygen penetration than metal storage tanks or large capacity casks. However, the flavenes themselves are relatively instable and can be irreversibly hydrolyzed into dihydrochalcones (16). This explains the lack of relationship between anthocyanin concentration and color, independent, of course, of the eventual appearance of free sulfur dioxide.

All natural anthocyanins suffer from inherent instability, so they may be degraded to form colorless or brown-colored, often insoluble products. It is evident, that depending on the reaction type, different degradation products are formed however, only little analytical data is available. For example, after reaction of anthocyanins with hydrogen peroxide, substances of the benzofu-ran type were detected (139). Studies with grape must like model solutions indicated that anthocyanins are degraded by coupled oxidation and that they form adducts with caffeoyltartaric acid... 

Flavanol oligomers and polymers are also called condensed tannins or proan-thocyanidins. The term tannin refers to their capacity to interact or react with proteins and precipitate them out. When heated under acidic conditions, these molecules release red anthocyanidin pigments, hence the term proanthocyanidins. The term leucoanthocyanidin, also referring to this particular property, is sometimes encountered in the literature. However, this should be restricted to another group of compounds, flavan 3,4-diols, which are intermediates in the biosynthetic pathway leading to flavanols and anthocyanins (Stafford and Lester 1984 Nakajima et al. 2001 Abrahams et al. 2003) but have never been isolated from grapes, presumably due to their instability. 

Anthocyanins have a considerable potential in the food industry as safe and effective food additives. Their annual world production has been estimated to reach 10,000 tons from grapes alone. However, compared with synthetic colorants they have not been extensively used because of their instability towards a variety of chemical and physical factors. Notwithstanding, polyacylated anthocyanins display a marked stability which may prove to be of particular importance for food technology [33],... 

Qu J, Zhang W, Yu X, Jin M (2005) Instability of anthocyanin accumulation in Vitis vinifera L. var. Camay Freaux suspension cultures. Biotechnol Bioprocess Eng 10(2) 155-161. doi 10.1007/bf02932586... 

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