Dark chocolate instead has shown a significant loss of polyphenols only in the 16th month of storage. Various studies have already noted the impoverishment of polyphenols in cocoa-based products during their storage. This loss is strictly correlated with their oxidation in the corresponding quinones, as well as the polymerization and formation of insoluble quinones (high molecular weight compounds). At the end of the preservation period (Fig. 3), the residual content of not oxidized polyphenols of the chocolates ranged from 50 to 217 mg of gallic acid equivalents (GAE) per 100 g, highlighting a significant difference between the different types of chocolate, especially dark chocolate that has shown the highest polyphenol contents (fig. 3). Considering the low lipid oxidation level that has been noted at the end of storage (Figs. 1 and 2), polyphenols have therefore continued performing a protective activity against fats, despite the loss found in the fraction of not oxidized polyphenols (fig. 3): the biological effects of catechins, flavonols and proanthocyanidins, are well known. Concerning vitamin E, it has a protective effect against oxidation, with 500 mV reduction potential, comparable to that of epigallocatechin gallate (430 mV). Vitamin E proved to be particularly plentiful in the sample derived by hazelnut, i.e. in gianduja chocolate (fig. 4). The vitamin level explains why gianduja chocolate didn’t show considerable lipid oxidation (Fig. 1), although the content of polyphenols was lower than the other two samples (Fig. 3).
In addition, the results of the measurements of vitamin E indicate that there are no significant changes for the tested chocolates over time (fig. 4), except for a slight decrease in gianduja chocolate in the fourth and 16th month of storage. At last, considering the three chocolates, their acidity is lower in the first four months of storage (fig. 5), while it increases in the same way in all samples after the eighth month, then dropping back at the sixteenth month (t16). As possible, the level of pH (Fig. 6) is inversely related to that of acidity (fig. 5). In fact, the pH slightly increased in all samples from t0 to t4 time, while the pH value was around 6,00 from the fourth to the 16th month. The hydrolysis of triglycerides, first step in the oxidative phenomena of fat rancidity, can lead to an increase in free acidity and therefore to a decrease in pH. At the same time, the oxidation of polyphenols determines a decrease in acidity and an increase in the pH. Since the pH is an expression of dissociated protons, it tends to keep more stable values compared to acidity and to highlight less oscillations during storage of chocolates. These results, obtained under conditions simulating those to whom the products are subjected in a point of sale, are of extreme importance to the confectionery companies both to set the most appropriate shelf-life period and to improve the rotation of products at the stores.
