## 4 Discussion
The authors found that optimal extraction of bioactive compounds for cold-brewed black tea was achieved at ul amplitude of 69.9% with a solvent volume of 25 mL and a sonication time of 30 min at 4 \({}^{\circ}\)C. TPC was found to be 78.97 \(\pm\) 1.85 mg GAE/g for 30 min in the ultrasonic brewing system, compared to 19.51 \(\pm\) 0.76 mg GAE/g for 360 min in the conventional brewing system. The extraction of phenolic compounds increased fourfold compared to conventional cold brewing. However, the brewing time was reduced from 6 h to 30 min using the ultrasonic brewing method. Another example is the work of [PERSON] et al. [83] which studied the effect of ultrasound and conventional hot water extraction on the bioactive compounds of yellow tea. Maximum extraction of TPC was carried out at 67 \({}^{\circ}\)C for 30 min with 75% aqueous ethanol solution using an ultrasonic probe. Another example of extraction is grape tea, consumed by steeping tea leaves in boiling water. A traditional drink in China, grape tea has been treated with ultrasound to extract polyphenols. Maximum polyphenols were extracted from grape tea with 70% ethanol at 70 \({}^{\circ}\)C for 40 min using ultrasound [84]. The results of aqueous extraction are also reflected, thus analogous to the text discussed in [PERSON]'s text [85], where antioxidant capacity was assessed at 25-100 \({}^{\circ}\)C using steeping times from 5 to 720 min. However, there are few examples in research of the use of UAE on biological material to extract ions into solution. An example of the use of assisted extraction in the study of ion extraction intensification is provided by, among others, the research of [PERSON] [76]; the assisting factor was a magnetic field. It was observed that an alternating magnetic field had different effects on the extraction of selected metal ions from dry tea leaves into different black tea infusions. Of all the chemical elements, potassium was found to migrate most rapidly when an alternating magnetic field was applied. There was a more than twofold increase in potassium ion extraction efficiency in two of the three tea brands. Variable magnetic field-assisted extraction also significantly improved Ca, Mg and P ion content in the final infusions of all three tea brands. Another extracted active substance analyzed was caffeine. Its content in conventionally extracted black teas ranged from 209.2 to 221.6 mg/L and in green teas from 243.2 to 244.7 mg/L. For the studies presented in this manuscript and the results of all three teas tested, increasing the steeping time resulted in the extraction of more ions for 10 of the 10 analyzed ions. Of the three teas tested, the ion that it was possible to extract in the highest amount was potassium. This element came from extracts treated with ultrasound for 10 min. Here, it is worth pointing out that it is not necessarily the time of 10 min with UAE, where there is the highest amount of ion that was successfully extracted, that will be the most optimal and economical. For example, the potassium content of tea number 1 contained 220.22 mg/10 g of this ion. The results of the analysis of the ion content of infusion tea No. 1 show that extraction for 1 min assisted by ultrasound yielded nearly 253% of the potassium ion concentration compared to the extraction conducted traditionally for 1 min. Subsequent measurements after extending the extraction time to 5 and 10 min, respectively, yielded results of 283% and 317%. Similar results were obtained in research papers that included the factor of extended tea infusion extraction time and the ultrasound-assisted extraction agent used [86,87]. The most pronounced upward trend was for ions with the highest concentrations. Studies by other authors indicate that ultrasound can be used not only to increase the extracted ions but also the previously mentioned bioactive compounds. The concentrations of nickel and aluminum increased progressively with brewing time; their concentrations were higher in teas extracted using the ultrasound method.