## 4 Bioactive Compounds in Acorns  
Acorns contain high levels of bioactive phenolic compounds, including phenolic acids, flavonoids, and tannins. Nevertheless, recent scientific reports have revealed numerous other compounds with biological activity, both hydrophilic and lipophilic compounds (Figure 5). The content of the main hydrophilic and lipophilic bioactive compounds in acorns of various _Quercus_ species are shown in Tables 2 and 3, respectively. \begin{table} \begin{tabular}{c c c c} \hline \hline _Q. rotundifolia_ & & & \\  [mg CE/100 g d.m.] & & & \\  & [mg GAE/100 g d.m.] & & & \\  & [mg GAE/100 g d.m.] & & & \\  & [mg GAE/100 g d.m.] & & & \\ \hline \hline \end{tabular} \begin{tabular}{c c c} \hline \hline _Q. rober_ & [mg GAE/100 g d.m.] & - & 13,290–233,350 & \\  & [mg GAE/100 g d.m.] & - & 13,290–233,350 & \\  & tannins & & [14, 42, 44] \\  & [mg/100 g d.m.] & condensed & 2090 & \\ \hline \hline \end{tabular} \begin{tabular}{c c c} \hline \hline \hline _[PERSON]. [PERSON]_ & [mg GAE/100 g d.m.] & - & 13,290–233,350 & \\  & [amins & & [14, 42, 44] \\  & [mg GAE/100 g d.m.] & condensed & 2090 & \\ \hline \hline \hline \end{tabular}   
\begin{tabular}{c c c} \hline \hline \hline _[PERSON]. [PERSON]_ & [mg GAE/100 g d.m.] & - & 13,290–233,350 & \\  &The phenolic compounds found in acorns are phenolic acids (primarily gallic and ellagic acids and their derivatives), flavonoids (including catechin, quercetin, and naringin), and tannins (mainly ellagitannins and gallotannins). The results presented in the literature mostly concern the total content of selected groups of compounds and qualitative profile assessment [65; 66; 67; 68]. Therefore, future studies should be conducted to indicate the quantification of individual phenolic acids, flavonoids, and tannins. The highest content of total phenolic compounds in acorns (up to 233,350 mg/100 g d.m.) was presented by [PERSON] et al. [42] for the species _Q. suber_ (Table 2). [PERSON] et al. [48] in acorns of the species _Q. rotundifolia_ determined a relatively low content of total phenolic compounds (840-9163 mg/100 g d.m. ), which depended on the period of seed harvesting during the year and over two years, indicating a variation in acorn composition depending on maturity and suggesting a continuous maturation of acorns, both on the tree and after harvesting. The two most common phenolic acids in acorns are gallic acid and ellagic acid [10]. In a study by [PERSON] et al. [69], gallic acid accounted for up to 0.27% of the dry extract of acorns (previously heat-treated). [PERSON] et al. [68] identified 28 different compounds in the phenolic profile of acorns of the species _Q. coccifera_, 14 of which were classified as gallic acid derivatives, including methyl gallate, galloyl derivatives, galloylmethyl gallate, and tetragalloyl-hexoside. The contents of ellagic and gallic acids in acorns are directly related to the content of tannins, from which they are released by processes including, but not limited to, the action of temperature, enzymes, acids, and alkalis [70]. Tannins are polymerized derivatives of polyphenols with high chemical reactivity and biological activity [70; 71]. Tannins cause a bitter taste in the product and show the ability to complex with proteins, polysaccharides, and metal ions, which makes them considered anti-nutrients [2; 70]. In acorns, the predominant group is hydrolyzing tannins, especially ellagitannins and gallotannins. In a study by [PERSON] et al. [64], the content of total tannins in the acorns of _Q. robur_ was 20,400 mg/100 g d.m. [PERSON] et al. [48] reported the content of hydrolyzing tannins in acorns of the species _Q. rotundifolia_ at 950-1150 mg/100 g d.m. (and condensed tannins at 50-140 mg/100 g d.m. ), of which gallotannins accounted for up to 860 mg/100 g d.m. and ellagitannins up to 290 mg/100 g d.m. [PERSON] et al. [10], among the compounds that make up the profile of ellagitannins in acorns, mention vescalagin, castalagin, and roburins.