Influence of the Carboxylic Function on the Degradation of d-Galacturonic Acid and Its Polymers.

Past investigations have shown high browning potential during the caramelization of sugar acids in comparison to reducing sugars. However, no approaches to elucidate the chemical mechanisms have been made. Therefore, this study aims to clarify the reasons for the high browning potential by measuring the mutarotation velocity and the elimination of CO2 during the heat treatment of uronic acids. Performed polarimetric experiments show that the mutarotation velocity of d-galacturonic acid exceeds that of d-galactose by a factor of nearly 4.5. However, the ring opening velocity is not the only parameter that differs between the two carbohydrate structures. Measurements of the release of CO2 of heated d-galacturonic acid at 60 °C show a steady increase, and after 48 h, 6% of degraded d-galacturonic acid has eliminated CO2. CO2 release was also found during the heating of pectin, indicating a decarboxylation reaction during thermal degradation. One of the degradation reactions postulated for the release of CO2 leads to α-ketoglutaraldehyde, which is responsible for the formation of several chromophoric substances.

Formation of Phenolic Compounds from d-Galacturonic Acid.

Aqueous d-galacturonic acid (d-GalA) model systems treated at 130 °C at different pH values show an intense color formation, whereas other reducing sugars, such as d-galactose (d-Gal), scarcely react. GC-MS measurements revealed the presence of several phenolic compounds: e.g., 3,8-dihydroxy-2-methyl-4 H-chromen-4-one (chromone) and 2,3-dihydroxybenzaldehyde (2,3-DHBA). These phenolic compounds, especially 2,3-DHBA, possess an intense browning potential and cannot be found within heated model solutions of reducing sugars. Investigations regarding the formation of these substances show that α-ketoglutaraldehyde plays an important role as an intermediate product. In addition, MS analysis of model systems of norfuraneol in combination with 2,3-DHBA showed the formation of oligomers that could also be detected in d-GalA model systems, leading to the assumption that, in addition to reductic acid, these compounds are jointly responsible for the strong color formation during the heat treatment of d-GalA.