Antioxidant and Antiglycation Properties of Seventeen Fruit Teas Obtained from One Manufacturer

The antioxidant activity of teas depends on the type and quality of the ingredients used in the process of tea production, location of the crops, and manner of the raw material processing. Our study is the first to compare the antioxidant and antiglycation properties of seventeen fruit teas obtained from one manufacturer. We evaluated three different brewing times (3, 5, and 10 min) and two brewing temperatures (70 and 100 °C). We demonstrated that infusions with the longest brewing time reveal the highest antiradical activity, while increased brewing temperature does not significantly affect the assessed parameters. The best antioxidant properties were obtained for the teas made from lemon balm with pear, forest fruits, cranberry with pomegranate, raspberry, and raspberry with linden. Fruit teas owe their high antioxidant activity to the presence of polyphenolic compounds in infusions. Extracts from fruit teas also diminish the oxidation and glycation of albumin in vitro, observed as a decrease in the fluorescence of aromatic amino acids and advanced glycation (AGE) and oxidation (AOPP) protein products levels. In conclusion, in order to prepare fruit teas with the best antioxidant properties, a longer extraction time is needed. The health-promoting properties of dried fruit infusions can be modified by changing the qualitative and quantitative composition of the ingredients.


Introduction
Tea is currently one of the most popular beverages in the world [1,2]. Every year, the consumption of teas, especially fruit teas, is increasing as a result of raised public awareness of their health-promoting properties. Fruit teas are a mixture of dried fruit, flowers, or leaves. Contrary to popular belief, these products do not contain black tea leaves. The addition of fruit juice concentrates and acidity regulators (e.g., citric acid) improves the taste and aroma of fruit teas [3]. Indeed, fruit tea infusions are successfully replacing sweetened drinks and juices. Interestingly, studies performed in the USA in the years 2011-2016 demonstrated that the highest tea consumption was observed in elderly people aged 51 to 70 years, non-Latin Asians, white people, and people with higher education and income [2].

Preparation of Samples
The infusions were prepared on the day the fruit teas were provided by the manufacturer.
Using an analytical balance (laboratory weight KERN PLI 510-3M) accurate to 0.001 g, 1 g of dried tea was weighed and transferred to 200 mL beakers. One-hundred milliliters of distilled water was added to every beaker, and then stirred for 30 s with a glass rod. Next, the beakers were covered with watch glasses. We implemented three different times (3,5, and 10 min) and two temperatures (70 • C and 100 • C) of brewing. Extraction conditions were determined based on the manufacturer's instructions and literature analysis-we selected the most frequently applied brewing conditions [16,17,25,26,29]. All the variants were prepared in 3 repetitions. After an appropriate time, the infusions were stirred again for 30 s. The content of beakers was poured through membrane filters with a diameter of 0.45 µm (Biosens). After cooling down, the filtrate was transferred to dark test tubes and used immediately for assays.

Antioxidant Assays
All reagents were purchased from Sigma-Aldrich (Nümbrecht, Germany and/or Saint Louis, MO, USA). The absorbance was measured using Infinite M200 PRO Multimode Microplate Reader Tecan (Tecan Group Ltd., Männedorf, Switzerland). The measurements were performed in triplicate samples and the results were standardized to 1 g of dry weight (DW).

Total Antioxidant Capacity (TAC)
The determination of TAC was based on the oxidation of ferrous ion to ferric ion in the presence of oxidants contained in the sample. Changes in absorbance were determined at 660 nm wavelength, and TAC level was calculated from the calibration curve for Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid). The value was expressed in nmol Trolox/g DW [30].

Radical Scavenging Activity Assay (DPPH)
The antioxidant activity of every sample was measured with DPPH (1,1-diphenyl-2picrylhydrazyl). This compound becomes discolored in the presence of antioxidants, which was the basis for spectrophotometric measurement at 515 nm wavelength. The value was expressed as nmol Trolox/g DW [31].

Ferric Reducing Antioxidant Power (FRAP)
FRAP is based on the reduction of an iron (III) ion to an iron (II) ion, resulting in the formation of a colorful ferrous-tripyridyltriazine complex. The reaction occurs in an acidic environment. Absorbance was measured at 592 nm wavelength by comparing each tested sample to a sample with known ferrous ion concentration. The value was expressed as µmol Fe (II)/g DW [32].

Total Phenolic Content (TPC)
The content of polyphenols in every sample was assayed according to the Folin-Ciocalteu method. It is based on the reduction of phospho-molybdate heteropoly acid Mo(VI) center in the heteropoly complex to Mo(V), which is evidenced by blue coloration of the sample. Absorbance of the samples was measured at 750 nm wavelength. The value was expressed as µg GAE/g DW) [31].

Total Oxidant Status (TOS)
The test was based on the oxidation of a ferrous ion to an iron ion that forms a colored complex with o-cresosulfonphthalein-3,3 -bis(sodium methyliminodiacetate) (xylenol orange) in an acidic medium. The content of oxidants in the sample was proportional to the intensity of color measured according to the spectrophotometric method. The value was presented as µmol H 2 O 2 /g DW [33].

Experimental Model
Bovine serum albumin (BSA, 96% purity) was dissolved in phosphate-buffered saline (PBS, pH 7.4). The concentration of albumin was 40 mg/mL, which reflects its physiological content in human blood. BSA was incubated with chloramine T (20 mM), chloramine T + additivies (0.1/1; v:v), or PBS alone (PBS, pH 7.4) [34]. Vitamin C (ascorbic acid, 10 mM) was used as a known protein oxidation inhibitor. The concentrations of various additives were selected based on other in vitro studies [34,35]. After thorough mixing on vortex and incubation in closed vials (37 • C, 50 rpm shaking, darkness) samples were taken for further analysis. Two incubation times were applied: 30 and 60 min [34]. The experiments were performed in three series, each time in triplicate. Five tea extracts with the best antioxidant properties were selected for analysis: lemon balm with pear, forest fruits, cranberry with pomegranate, raspberry, and raspberry with linden. The results are presented as % of control (BSA + chloramine T).

Advanced oxidation protein products (AOPP)
AOPP concentration was assessed colorimetrically by measuring the oxidative capacity of iodine ion [37]. Fifty milliliters of the sample was incubated with 150 µL of 0.1 M sodium phosphate buffer (pH 7.4), 10 µL of 1.16 M sodium iodide, and 20 µL of glacial acetic acid. Before mixing, the derivatives were measured at 340 nm.

Statistical Analysis
The obtained data were analyzed in the statistical program GraphPad Prism 8.3.0 for MacOS (GraphPad Software, Inc. La Jolla, CA, USA). Normality of distribution was assessed with the Shapiro-Wilk test. The one-way analysis of variance (ANOVA), two-way ANOVA, and Tukey's HSD tests were used to compare quantitative variables. Multiplicity adjusted p value was also calculated. The results were presented as mean ± SD. Correlations between the parameters were evaluated using Pearson correlation coefficient. p < 0.05 was considered statistically significant.

Antioxidant Assays
A number of methods are used to assess the antioxidant properties of food and beverages. However, it is only the total antioxidant activity that informs about interactions between all individual antioxidants. Therefore, in our study we evaluated the total antioxidant capacity (TAC), radical scavenging activity (DPPH), ferric reducing antioxidant power (FRAP), total phenolic content (TPC), and total oxidant status (TOS).

Total Antioxidant Capacity (TAC)
Total antioxidant capacity of the tested teas is contained in the range from 0.79 ± 0.03 nmol Trolox/g DW (Black currant with gooseberry and cherry, T = 70 • C, t = 3 min) to 2.40 ± 0.16 nmol Trolox/g DW (Lemon balm with pear, T = 100 • C, t = 10 min). Moreover, Rosehip with raspberry (2.34 ± 0.33 nmol Trolox/g DW) as well as Forest fruits (2.34 ± 0.43 nmol Trolox/g DW), Cranberry with pomegranate (2.16 ± 0.15 nmol Trolox/g DW), and Raspberry (2.16 ± 0.10 nmol Trolox/g DW) teas were also characterized by high TAC values. All teas (except two) reached their highest TAC at 100 • C after 10 min of brewing, whereas Lemon balm with pear reached its highest TAC at T = 70 • C after 10 min, and Red orange with quince reached its highest TAC at T = 100 • C after 5 min. Generally, the lowest TAC values were found in teas with the lowest temperature and the shortest time (T = 70 • C, t = 3 min) of brewing (Table 2). The two-way analysis of variance (ANOVA) showed that individual infusions differ significantly both in terms of brewing temperature (p = 0.0014) and time (p < 0.0001) ( Table 2).

Radical Scavenging Activity (DPPH) Assay
DPPH values of the teas varied from 129.90 ± 26.91 nmol Trolox/g DW (Cranberry, T = 70 • C, t = 3 min) to 549.10 ± 44.69 nmol Trolox/g DW (Lemon balm with pear, T = 100 • C, t = 10 min). High DPPH values were also achieved by the following teas; Forest fruits (467.70 ± 15.75 nmol Trolox/g DW), Cranberry with pomegranate (453.80 ± 35.46 nmol Trolox/g DW), Raspberry with linden (455.90 ± 43.99 nmol Trolox/g DW), and Raspberry (450.40 ± 41.79 nmol Trolox/g DW). Except the tea Lemon balm with pear, which reached the lowest DPPH value when brewed at 100 • C for 5 min, all teas reached the lowest DPPH values at the lowest brewing temperature (70 • C) and the shortest time (3 min). Eleven teas revealed the highest DPPH values at the brewing temperature of 100 • C and time of 10 min. Raspberry with linden and Cranberry with pomegranate teas demonstrated the highest DPPH values at 70 • C and 10 min of brewing time, Lemon balm with pear and Strawberry with vanilla teas -in T = 70 • C after 5 min of brewing, and Chokeberry with acai berry achieved the highest DPPH values at the brewing temperature of 100 • C after 5 min of brewing (Table 3).
The two-way analysis of variance (ANOVA) demonstrated that individual infusions differ statistically only in terms of brewing time (p < 0.0001) ( Table 3). Table 3. DPPH radical (α-diphenyl-β-picrylhydrazyl) scavenging activity in fruit tea infusions. a p < 0.05 vs. 3 min 70 • C; b p < 0.05 vs. 3 min 100 • C; c p < 0.05 vs. 5 min 70 • C; d p < 0.05 vs. 5 min 100 • C; e p < 0.05 vs. 10 min 70 • C. The highest FRAP value was noted at 100 • C after 10 min of brewing the Raspberry with linden tea (8863.00 ± 536.40 µmol Fe(II)/g DW), and the lowest at the brewing temperature of 70 • C and time of 3 min for the tea Raspberry (1805.00 ± 588.60 µmol Fe(II)/g DW). The following teas also had a high FRAP value; Lemon balm with pear (8699.00 ± 598.30 µmol Fe(II)/g DW), Forest fruits (8651.00 ± 273.50 µmol Fe(II)/g DW), Cranberry with pomegranate (8515.00 ± 115.30 µmol Fe(II)/g DW), and Rosehip with raspberry (8501.00 ± 482.80 µmol Fe(II)/g DW). Except for Cranberry tea, in which the lowest FRAP value was observed when brewed at 100 • C for 3 min, all teas achieved the lowest FRAP values at the lowest temperature and the shortest brewing time (i.e., T = 70 • C, t = 3 min). Ten teas reached the highest FRAP values when brewed at the highest temperature for the longest time (i.e., T = 100 • C, t = 10 min). The teas Chokeberry with acai berry, Forest fruits, Red orange with quince, Rosehip with raspberry, and Strawberry with vanilla achieved the highest FRAP values when brewed at 70 • C for 10 min. The tea Ginger with quince and strawberry had the highest FRAP values at T = 100 • C and brewing time of 5 min (Table 4). The two-way analysis of variance (ANOVA) variance showed that individual infusions differ significantly only in terms of brewing time (p = 0.0001) ( Table 4).

Total Oxidant Status (TOS)
The lowest TOS value was 1.71 ± 0.37 µmol H2 O2/g DW and it was observed for Lemon balm with pear brewed at 100 • C for 10 min. Low TOS values were also achieved by the teas Cranberry with pomegranate (1.80 ± 1.14 µmol H 2 O 2 /g DW), Forest fruits (2.02 ± 0.25 µmol H 2 O 2 /g DW), Raspberry with linden (3.03 ± 0.27µmol H 2 O 2 /g DW), and Raspberry (3.31 ± 0.25 µmol H 2 O 2 /g DW). The highest TOS value, which was 21.49 ± 9.74 µmol H 2 O 2 /g DW, was found in Rosehip with raspberry. The highest TOS values were observed at 100 • C and the shortest brewing time, while the lowest were obtained at the highest brewing temperature and the shortest time (Table 6). The two-way analysis of variance (ANOVA) indicated that individual infusions differ significantly both in terms of brewing time (p < 0.0001) and the brewing temperature-time interaction (p < 0.0001) ( Table 6).

Correlations
Correlations between the analyzed parameters are presented in Figure 1. A positive correlation was observed between TPC concentration and the levels of TAC, DPPH, and FRAP, where the Pearson correlation coefficient for each of the teas was, respectively, 0.7, 0.75, and 0.45 at the statistical significance level of p < 0.0001. Among the parameters determining total antioxidant activity we observed a very strong positive correlation between TAC and DPPH (r = 0.86, p < 0.0001) and a weaker one between TAC and FRAP (r = 0.65, p < 0.0001). As excepted, there was also a negative correlation between TOS concentration and other parameters (p < 0.0001) (Figure 1).

Oxidative Modifications of Albumin
In the next stage of the experiment, we evaluated the effect of selected extracts of fruit teas on protein glycooxidation in vitro. Five tea extracts with the best antioxidant properties were selected for analysis: lemon balm with pear, forest fruits, cranberry with pomegranate, raspberry, and raspberry with linden. The results are presented as % of control (BSA + chloramine T). Vitamin C (ascorbic acid) was used as a known protein oxidation inhibitor.
In general, chloramine T causes the oxidation of albumin, which manifests as an increase in the fluorescence of dityrosine, kynurenine, N-formylkynurenine, and AGE and the content of AOPP. Extracts of fruit teas cause a decrease in albumin glycooxidation in vitro (↓fluorescence of oxidativemodified amino acids, AGE and AOPP; ↑tryptophan content). However, statistically significant differences are observed mainly at the longest time and temperature of brewing. The best antiglycooxidative properties are shown by a lemon balm with pear infusion (Table 7). A positive correlation was observed between TPC concentration and the levels of TAC, DPPH, and FRAP, where the Pearson correlation coefficient for each of the teas was, respectively, 0.7, 0.75, and 0.45 at the statistical significance level of p < 0.0001. Among the parameters determining total antioxidant activity we observed a very strong positive correlation between TAC and DPPH (r = 0.86, p < 0.0001) and a weaker one between TAC and FRAP (r = 0.65, p < 0.0001). As excepted, there was also a negative correlation between TOS concentration and other parameters (p < 0.0001) (Figure 1).

Oxidative Modifications of Albumin
In the next stage of the experiment, we evaluated the effect of selected extracts of fruit teas on protein glycooxidation in vitro. Five tea extracts with the best antioxidant properties were selected for analysis: lemon balm with pear, forest fruits, cranberry with pomegranate, raspberry, and raspberry with linden. The results are presented as % of control (BSA + chloramine T). Vitamin C (ascorbic acid) was used as a known protein oxidation inhibitor.
In general, chloramine T causes the oxidation of albumin, which manifests as an increase in the fluorescence of dityrosine, kynurenine, N-formylkynurenine, and AGE and the content of AOPP. Extracts of fruit teas cause a decrease in albumin glycooxidation in vitro (↓fluorescence of oxidative-modified amino acids, AGE and AOPP; ↑tryptophan content). However, statistically significant differences are observed mainly at the longest time and temperature of brewing. The best anti-glycooxidative properties are shown by a lemon balm with pear infusion (Table 7). Table 7. The effect of selected fruit tea extracts on protein glycooxidation in the model of bovine serum albumin (BSA) oxidized with chloramine T. AGE-advanced glycation end products; AOPP-advanced oxidation protein products; BSA-bovine serum albumin. * p < 0.05 vs. control (BSA + chloramine T, 30 min oxidation); # p < 0.05 vs. control (BSA + chloramine T, 60 min oxidation).

Discussion
The tea industry has been developing rapidly over the last two decades. More and more interest in a healthy lifestyle as well as the discovery of beneficial properties of teas have contributed to increased consumption of these beverages [2,25,38]. Indeed, teas are classified as functional food, i.e., products with documented health-promoting effect [39]. Although fruit teas are very popular, still little is known about their beneficial effects on the human body.
The antioxidant properties of fruit teas depend on the type and quality of the ingredients used in the process of tea production, location of the crops, and manner of the raw material processing. Interestingly, technological processes used in the production of fruit teas as well as conditions of raw material storage significantly affect the quality and health-promoting properties of teas [17,25,28,29]. Numerous studies have demonstrated that during the drying procedures (due to intensive aeration), plant raw material loses even 50% of its initial antioxidant capacity [17,40]. The raw material containing volatile substances (e.g., essential oils) and vitamins (mainly vitamin C) are particularly sensitive to processing [18,40]. Therefore, comparing antioxidant potential of fruit teas from different manufacturers (i.e., produced under different conditions) is of little and questionable value.
Tea brewing conditions (e.g., temperature and time of brewing) also significantly influence the quality of the infusion. Extraction is the simplest method of obtaining individual plant substances from a mixture of solids or liquids. Extracted compounds pass to a properly selected solvent that determines the value of the partition coefficient [40,41]. In case of fruit teas, the only solvent used is water. Therefore, to increase the efficiency of extraction, the temperature of water is raised and/or the brewing time is prolonged. However, a large part of biologically active compounds (mainly polyphenols) contained in fruit teas is susceptible to oxidation, thus high temperature and alkaline environment may be responsible for their degradation [18,40,41]. Consequently, suitable conditions of raw material extraction are crucial for the quality of the obtained infusion [23,41]. Our study is the first to compare the effect of the brewing temperature and time on the antioxidant properties of fruit teas produced and stored under the same conditions.
Based on literature analysis, we chose the most frequently used brewing times (3, 5, and 10 min) and temperatures (70 • C and 100 • C) [16,17,25,26,29]. We demonstrated that infusions with the longest brewing time have the highest antiradical activity.
A number of parameters are used to assess antioxidant properties, starting from the evaluation of individual antioxidants. However, much more information is provided by the resultant free radical scavenging capacity of a given sample, taking into account the synergistic effect of interactions between antioxidants [30,42]. In order to obtain a reliable picture of the antioxidant activity (considering the strengths and weaknesses of a given method and its applicability), it is recommended to apply at least two different methods [30,42]. In our study we evaluated TAC, DPPH, and FRAP. Although individual infusions were characterized by varied antiradical activity, the highest values of TAC, DPPH, and FRAP were observed for the longest time (10 min) and the highest temperature (100 • C) of brewing.
According to the manufacturer's recommendations, the average brewing time for dried tea should be between 3 and 5 min. Our research revealed, however, that this time is not sufficient to obtain an infusion with maximum antioxidant potential. Therefore, we recommend extending the brewing time to~10 min. Longer extraction time enables more antioxidants to pass to the infusion, which results in better health-promoting properties (an increase in TAC, DPPH, and FRAP and a decrease in TOS).
The content of TAC, DPPH, and FRAP correlated positively with total phenolic content (TPC). Thus, increased antioxidant activity of infusions may result from high level of polyphenolic compounds. Indeed, it is believed that polyphenols are the main source of antioxidants in fruit teas. These include flavonoids, tannins, phenolic acids, stilbenoids, lignans, and others [18,40,43]. They present not only antiradical, but also anti-inflammatory or even anticancer effects [18,38]. Therefore, it is not surprising that polyphenols are used in the treatment of some systemic diseases [18,44]. The usage of polyphenols is also indicated in prevention of civilization diseases [18,44]. It is suggested that the health-promoting effects of fruit teas depend on the total phenolic content [44]. Although the scope of TPC in the analyzed infusions is quite wide, it coincides with the data available in literature [3,16,27]. However, the comparison of the obtained results is hindered by the fact that different solvents (water, ethanol, and methanol) were used in the various studies available in literature and the manner of presenting the results is also varied (gallic acid/catechin equivalents or TPC expressed in dry sample mass/solution volume). In our study, the only applied solvent was water, as it is used to prepare the extract by the consumer.
Although we did not directly evaluate the degree of oxidation/degradation of polyphenolic compounds, TPC was highest in teas with the longest brewing time (10 min) and the highest brewing temperature (100 • C). Therefore, it can be assumed that neither an increase in extraction time nor higher temperature of water cause degradation of polyphenols in the analyzed teas. However, it cannot be excluded that polyphenols may undergo partial oxidation during the brewing process. Interestingly, partially oxidized polyphenols are characterized by a boosted ability to bind free radicals compared to their non-oxidized precursors. These changes were observed, inter alia, in catechin subjected to enzymatic oxidation [18,40,45,46]. Indeed, the improved antiradical properties of partially oxidized polyphenols can be explained by an increased ability to release hydrogen atoms of the hydroxyl group at the aromatic ring. Moreover, they may result from keeping the unpaired electrons in the aromatic ring by relocating them in the π coating [18,40,45,46].
In addition to ionizing/ultraviolet radiation and xenobiotics, high temperature is one of the most important boosters of free radicals [4,5]. For that reason, in our study we evaluated the total oxidant status (TOS) determining the total oxidant content of the analyzed sample. This parameter is particularly useful for assessing the oxidation/degradation rate of food ingredients [33]. We observed the highest TOS values at the highest brewing temperature and the shortest brewing time, while the lowest TOS values were noted at 100 • C after 10 min of brewing. It is therefore very likely that the total oxidant status decreases under the influence of antioxidants passing to infusions during the extraction process. Our hypothesis was confirmed by the negative correlation between TOS concentration and the analyzed antioxidant parameters (TAC, DPPH, and FRAP).
There are no available studies comparing the antioxidant properties of fruit tea infusions depending on the brewing temperature and time. With respect to black and green teas, it has been demonstrated that TPC is significantly higher in teas brewed in hot water, as confirmed by the results of our study [29]. On the other hand, Shannon E. et al. [27] proved that the optimum brewing time for black, green, white, and chamomile teas as well as for the blend of berry and hibiscus tea is 5 min. Extending the brewing time to 10 min did not increase the antioxidant properties [27]. Therefore, it is probable that the antiradical activity of an infusion is largely dependent on the qualitative composition of the blend (black/herbal/fruit tea).
The composition of fruit teas is not limited to one raw ingredient. The presence of particular ingredients in tea blends is determined, on the one hand, by the price of the raw material, and on the other hand, by the sensory properties responsible for the flavor, aroma, and color of the obtained infusions [2,27]. The most common ingredients of fruit teas are hibiscus flower and fruits of chokeberry, black currant, rosehip, and raspberry [47]. Although they mainly determine the color and taste of the infusions, these raw materials are a rich source of both organic acids (citric, malic, tartaric, and oxalic) and also anthocyanins and polyphenolic compounds such as protocatechuic acid [48,49]. Therefore, they may be responsible for the antioxidant properties of fruit teas. However, despite numerous studies on the antioxidant properties of green, black, and red teas, still little is known about teas made from dried fruit. Pękal et al. [3] have shown that the antioxidant activity of studied teas increases in the following order; fruit tea, flavored black tea, and premium black tea. Indeed, the antioxidant properties of a particular infusion are strongly influenced by the percentage of individual raw materials in the mass of the entire product, which translates into a different content of biologically active compounds. In our study, the highest antioxidant activity (highest TAC, DPPH, and TPC) was found in the mixture of lemon balm and pear. In this product, 50% of the dry matter is constituted by the lemon balm herb, which is basically an oil-bearing raw material. However, apart from the essential oil, this raw material also has other biologically active compounds, mainly phenolic acids: rosemarinic, caffeic, chlorogenic, and ferulic acids, which are both esters and glycosides. This raw material also contains triterpenic acids, flavonoids, and minerals [50]. On the other hand, pear fruit has a high content of vitamin C, anthocyanins, and other antioxidants, which may affect health-promoting properties of the tea made from this material [51,52]. Indeed, it is believed that the main source of antioxidants are fruits, among which berries are particularly rich in anthocyanins and tannins [53]. Unfortunately, the available literature lacks data on the total antioxidant potential of the raw material. In our study, infusions of forest fruits, cranberry with pomegranate, raspberry and raspberry with linden also showed high antioxidant activity. Indeed, many studies have shown that these raw materials are rich sources of flavonoids (mainly quercetin, kaempferol, and acacetin derivatives) as well as essential oil [16,[53][54][55]. Interestingly, linden, apart from flavonoids (rutin, hyperoside, quercetin, astragalin, and thiroside) and essential oil, also contains mucous compounds, amino acids or tocopherol, which also determines antioxidant properties [56,57]. The addition of these ingredients may intensify the antiradical effect in comparison to tea with rasberry alone. Sahin et al. [16] demonstrated that pomegranate fruit has the highest antioxidant capacity among the evaluated fruit teas. In our study, the cranberry with pomegranate tea also reached high values of TAC, DPPH, FRAP, and TPC, while the cranberry infusion was already characterized by low antioxidant activity. It can therefore be assumed that it is the pomegranate fruit that is responsible for the health benefits of the cranberry-pomegranate tea. Sahin et al. [16] also showed a significant antioxidant capacity of berry teas (blueberry and blackberry), which was also covered by our study. We showed that forest fruit tea is one of the five teas with the best antioxidant properties.
An important part of our study was also the assessment of the effect of fruit tea extracts on the glycoxidative properties of albumin. Albumin is one of the best-known proteins of the human body involved in the maintenance of oncotic pressure, regulation of the acid-base and oxidative-antioxidative balance as well as transport of many endogenous and exogenous substances [58]. The most sensitive to oxidation are alkaline, aromatic, and sulfur-containing amino acids [59]. In the present study, we evaluated the effect of extracts with the best antioxidant properties on albumin glycooxidation in vitro. We have shown that the analyzed fruit teas reduce oxidation and glycation of albumin observed as a decrease in fluorescence of aromatic amino acids (dityrosine, kynurenine, and N-formylkynurenine), reduced AGE and AOPP levels as well as an increase in tryptophan content. However, the ability to counteract glycation and protein oxidation has been demonstrated mainly in tea extracts with the longest extraction time and the highest temperature. This confirms our previous results on the relationship between antioxidant properties and brewing temperature/time. It is well known that the products of protein oxidation and glycation interact with other proteins, disturbing their structure and function. AGE and AOPP bind to specific receptors and activate many signaling pathways (e.g., NF-κB, NJK, and p21 RAS) that intensify the secretion of proinflammatory cytokines and pro-thrombotic factors [60,61]. AGE and AOPP can also accumulate in tissues, which disrupts the functioning of many organs [60,61]. Thus, fruit tea could improve the redox balance in the course of diseases with a proven etiology of oxidative stress. They can also be used to prevent cancer, inflammation, neurodegenerative diseases, or metabolic diseases, where increased protein glycooxidation occurs. Therefore, our study is a starting point for further research on the therapy of reducing protein oxidation/glycation in living systems.
In conclusion, we proved that brewing time has a significant impact on the antioxidant properties of fruit teas. This may be related to a better release of the biologically active substances contained in the dried fruit and their easier transfer to the infusion at a longer extraction time (10 min). Although fruit teas are a rich source of numerous natural antioxidants, it should be remembered that their proper preparation is crucial. Further studies should aim at developing the optimum qualitative-quantitative composition of teas in order to obtain maximum antioxidant properties at acceptable sensory qualities. The evaluation of antioxidant properties of fruit teas may contribute to the consumer's conscious choice of the final product which, apart from its flavor, will offer valuable health-promoting properties. It is also necessary to assess the effect of fruit tea extracts on antioxidant properties in vivo.
Our study also had certain limitations. First of all, we did not assess individual phytochemicals separately, so we do not know which compounds are responsible for the observed antioxidant properties. Moreover, the extraction of dried fruit was performed in only one solvent, which is also a limitation of our work. As teas with the best antioxidant effect are produced by berries, it is possible that phytochemicals such as anthocyanins have the most antiradical activity. Therefore, obtaining a phytochemical/antioxidant profile of the raw material should be the next step in research. Interestingly, Atoui et al. [62] identified over 60 different flavonoids, phenolic acids and their derivatives in leaf tea and herbal infusions.

Conclusions
Fruit teas are a valuable dietary supplement providing natural antioxidants. The antioxidant properties of fruit teas depend mainly on the brewing time, therefore a longer extraction time (10 min) should be implemented during their preparation. Fruit teas owe their high antioxidant activity to the presence of polyphenolic compounds in the infusion. The best antioxidant properties are found in the teas: lemon balm with pear, forest fruits, cranberry with pomegranate, raspberry, and raspberry with linden. Extracts from fruit teas also diminish the oxidation and glycation of albumin in vitro. Health-promoting properties of fruit teas can be modified by changing the qualitative and quantitative composition of the ingredients.