# Tea Infusions as a Source of Phenolic Compounds in the Human Diet
###### Abstract  
Phenolic compounds are components with proven beneficial effects on the human body, primarily due to their antioxidant activity. In view of the high consumption of tea and the numerous factors that affect the nutritional value of its infusions, the aim of this study was to identify the effects of tea type and duration of leaf extraction with water on the levels of phenolic compounds and other components that determine biological activity (oxalates, Ca, Na, Cu, and Mn). Based on assays, infusions of red tea prepared for 20 min were found to be the best source of phenolics (202.9 mg/100 mL), whereas the lowest level of these compounds was determined in infusions of black tea extracted from leaves for 30 min (46.9 mg/100 mL). The highest degree of increase in polyphenol content (by approx. 50%) was noted in red and green tea infused for between 10 and 20 min, whereas for black tea, polyphenol levels decreased with time. The biological activity of tea infusions appears to be determined to the greatest extent by the interactions between phenolic compounds and oxalates (r = 0.6209), calcium (r = 0.8516), and sodium (0.8045). A daily intake of three to four mugs (1 L) of tea infusions provides the human body the entire amount of phenolics recommended for health reasons (as regards red tea, this is possible at 1/3 of the volume) and covers the daily requirement for manganese, as well as (partially) copper. tea; infusions; phenolic compounds; oxalates; Ca; Na; Cu; Mn; recommended daily intake +
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Many authors report that the antioxidant activity of phenolic compounds is largely due to the content of minerals, particularly copper and manganese [5; 11; 12]. The interactions occurring between polyphenols and minerals can result in an increase in mutual activity, but they can also reduce such activity by forming insoluble bonds. The direction of interaction is determined by the levels of these compounds and the chemical forms they take, as well as the contents of other components found in foodstuffs, particularly oxalates, phytates, and polyphosphates [13; 14]. Of all the compounds mentioned above, it appears that the interactions between polyphenols (particularly those of the tannin fraction), oxalic acid, and certain minerals are of particular importance in tea infusions. Oxalic acid is a bicarboxylic acid that, by forming insoluble salts with divalent and trivalent metals, reduces their utilization from food [15; 16]. In plant tissues, this occurs in two forms: soluble (including sodium and potassium oxalates) and insoluble (including calcium and magnesium salts). An excessive supply of these compounds can cause many conditions, the most common of which is kidney stone disease [17]. Studies have demonstrated that tea, although containing oxalates, has an inhibitory effect on the formation of kidney stones, as the oxalates found in tea are hardly absorbed from the gastrointestinal tract, and epigallocatechin gallate, which is found in the tea leaves in large quantities, has an inhibitory effect on the formation of kidney stones [18; 19; 20]. Most researchers claim that for an average tea consumption at a level of 1 L per day, the levels of oxalic acid and its salts found in tea infusions pose no hazard to human health, and due to the high content of phenolic compounds and certain elements, tea should be regarded as a product with a very high nutritional value [17; 20; 21; 22; 23]. The contents of components found in the tea leaves are determined by many factors, e.g., the region and method of tea plant cultivation (soil type, fertilization, climatic conditions, etc. ), the time and form of leaf harvest (including the leaf position on the plant), unit operations they undergo, and the duration of storage [12; 23; 24; 25; 26; 27]. The market offers tea leaves that are minimally processed (e.g., white, yellow, or green tea), partially fermented (e.g., red tea), and fully fermented (e.g., black teas).