## 3 Results and Discussion
### The Release of Phenolic Compounds in the Intestinal Digestion  
In the intestinal phase of the simulated digestion of peel and flesh of apples (Tables 1 and 2), some of the compounds were not identified, the amounts of some decreased to zero, or they were not detected at all, and some compounds showed stability and were identified through the whole period of intestinal digestion. Compounds that were not identified were anthocyanins (Tables 1 and 2). Similarly, after the intestinal digestion of jaboticaba fruit peel, only 10% of anthocyanins were found at the end of the digestion, and only 1.3% were able to pass the cellulose dialysis membrane [4]. The reason for not detecting anthocyanins or for the decrease in their concentration could be the change of pH from low values in the gastric digestion (pH 3) to higher values in the intestinal phase (pH 7), at which anthocyanins change their chemical form from the form of a flavylium cation to a colorless carbinol pseudo-base [4]. Another subgroup of phenolic compounds that was not identified in the intestinal phase of digestion was the flavan-3-ol subgroup. Similarly, flavan-3-ols from apples were not identified in the intestinal phase of digestion [2], or their amount was lower in the intestinal phase than in the gastric phase [27]. [PERSON] et al. (2011) [29] also reported the degradation of flavan-3-ols in duodenal juice. The decrease in flavan-3-ol content or their complete disappearance could be the result of their degradation by autooxidation, polymerization, or complex formation in an environment of elevated pH [27]. The products of the autoxidation of various catechins after the in vitro gastrointestinal digestion were identified as different homo and heterocatechin dimers [30]. Other compounds that were not identified or their amount decreased to zero at the end of the intestinal digestion are dihydroprochalcones (Tables 1 and 2). After the intestinal digestion of apple peel, the dihydroprochalcone phloem-2-glucose was present for up to 60 min, but at the end of the digestion, it could not be identified. The same dihydroprochalcone from the flesh was not identified during the whole intestinal digestion. The release of dihydroprochalcones had a somewhat different trend in earlier studies. After the intestinal digestion of apples, the number of total dihydroprochalcones increased, which was the result of the increase in released phloem-2-glucose [27]. Dihydrochalcones were also identified in the intestinal phase of digestion of apples in the study of [PERSON] et al. (2012) [2]. Dihydrochalcones showed stability in the duodenal juice [29]. Compounds that were found throughout the whole period of the intestinal phase of digestion were chlorogenic acid and flavonoids (Tables 1 and 2). Chlorogenic acid from the flesh and peel of apples was present in the first 10 min of the intestinal phase in the amount statistically similar to the amount at the end of the gastric phase. Moreover, although there were some visible fluctuations in its content with time, its amount statistically did not change significantly throughout the whole period of intestinal digestion. On the other hand, chlorogenic acid isomer was not identified in the intestinal phase. In earlier studies, chlorogenic acid was also found after the intestinal digestion of apples [2,27], and its amount was not affected by intestinal digestion compared to gastric digestion [27], similar to this study. Some studies reported a decrease in the chlorogenic acid content in the intestine [2]. Chlorogenic acid can isomerize [2,29] into cryptochlorogenic acid and neochlorogenic acid, which degrade its content by 41-77% [2]. Another group of constantly present phenolic compounds is the group of flavonoids. Amounts of individual and total flavonoids in the first 10 min of the intestinal phase were statistically similar to amounts after the gastric phase. Exceptions were quercetin-3-xyloside and total flavonoids from flesh whose content in this intestinal phase was higher than the content at the end of gastric digestion.