**1. Introduction**

Functional food is defined as food that is fortified, enhanced, enriched or as a whole food that has a documented health effect beyond that resulting from the presence of nutrients traditionally considered essential [1,2]. Yoghurt, as a nutrient-rich dairy product, can be classified as functional food [3]. The main factors responsible for the beneficial effects of yoghurt are live cultures (*Streptococcus thermophilus* and *Lactobacillus bulgaricus*), proteins (whey and casein), lipids (bioactive fatty acids), vitamins and minerals (calcium and vitamin D) [4,5]. Of all the fermented products, yoghurt is also the most popular in the world [3,6] and best perceived and accepted by consumers [3,7]. Consumers who purchase

functional products want the product to be safe, healthy, and natural and to have pleasant taste. They take into consideration such quality-related attributes on the labeling as freshness of the product, healthful properties, and nutritional value [8].

Dairy products belong to the most innovative food sector in Europe. The innovation of these products is based on product improvement, new formulations or new technologies that are used to meet the needs of specific consumers. Based on the research undertaken on French consumers, nutrient fortifications coming from plant sources were the most acceptable for them [9]. Several studies were conducted on yoghurts to know how yoghurt fortification with vitamins such as vitamins C, B9, B12, A and D [10] or minerals such us chromium, iron, magnesium, manganese, molybdenum, selenium and zinc [11] influenced their properties. Apart from yoghurt's fortification with vitamins and minerals, it has become more popular to add plant-based functional ingredients to the yoghurt like pomegranate juice powder (1–5%) [12], dried pomegranate seeds (5–20%) [13], freeze-dried apple pomace powder (1–3%) [14], flaxseed (0–4%) [15], coconut-cake (0–30%) [16], spirulina (0.25–1%) [17], aloe vera gel (1–5%) [18], saffron (0.0125%) [19] or tea [20–25] to improve their technological and sensory quality as well as health-promoting properties.

Tea is the most common functional beverage in the world [1,26] usually prepared by infusing leaves of the plant *Camellia sinensis* (L.) in hot water [27,28]. Tea can be classified according to the degree of fermentation into un-fermented green tea, semi-fermented oolong tea, and fully fermented black tea [1,27,28]. Thanks to the content of polyphenols, especially epigallocatechin-3-gallate, theaflavins and thearubigins, tea from *Camelia sinensis* (L.) provides several health-promoting effects [1,26,29]. Infusions of herbs, fruits, roots and flowers are also referred to as tea, and their health benefits are known and used by people around the world [30]. One of them is *Melissa o*ffi*cinalis*, called lemon balm, known for its many therapeutic properties such as antioxidant, antidepressant, anti-inflammatory, and antimicrobial activities. Lemon balm can be used for both prevention and treatment in medicine as well as in dietary supplements and functional food [31]. The addition of various extracts of black, green and white tea enhanced antioxidant properties of yoghurts [23]. The influence of tea on lactic bacteria during yoghurt fermentation was also verified and it was shown that this addition did not interfere with the fermentation process and did not affect the survival of bacteria. It was also shown that lactic acid bacteria present in yoghurt did not have a negative effect on the content of tea pro-health compounds [22]. In most of the available publications, the purpose of using tea addition to yoghurt was health promotion and enrichment of these products with antioxidants and ingredients that had a positive effect on human health [20,22–24,32]. The sensory quality of yoghurts developed with teas has been evaluated so far by researchers using only consumer tests [15,21,25,33]. However, there is a lack of information on the influence of tea on the sensory quality of yoghurt measured both by expert panels and consumers and supported by yoghurt textural properties that are very valuable during the new product development process.

The aim of developing the new product was to maintain the functional character of nutri-rich yoghurt and, in addition, to introduce a health-promoting plant material into natural yoghurt without added sugar. That is the reason why the aim of the research was to develop functional tea-infused yoghurts by assessing the sensory quality and texture properties of yoghurts.

#### **2. Materials and Methods**

#### *2.1. Materials*

The material for the actual tests were yoghurts prepared from microfiltered pasteurised cow's milk with 3.2% fat content (Pi ˛atnica, Poland), using the thermostatic method. Four types of leafy tea (*Camellia sinensis*) available on the Polish market were used for the production of yoghurt, i.e., green tea (BioFix, Tuszyn, Poland); black tea, Darjeeling FTGFOP1 Blend Lucky Hill (Tea Club Marek Brzezicki, Lubin, Poland) and oolong tea Oolong Milky (Herbaty Szlachetne Sp. Z o. o., Szczecin, Poland) and also the lemon balm (*Melissa o*ffi*cinalis* L., Cesarska Perła, Warszawa, Poland). In order to inoculate the

milk, freeze-dried starter culture YO-122 (Serowar, Szczecin, Poland) containing *Streptococcus salivarius* subsp. *thermophilus* and *Lactobacillus delbrueckii* subsp. *bulgaricus* were used.

#### Yoghurt Processing

The technological process of set yoghurt production was developed based on modifications of two methods [22,23]. All ingredients were weighed on an analytical balance (RADWAG PS 1000/C/2, Radom, Poland). The milk was heated to 85 ◦C for 30 min. and poured into the beakers with tea leaves (2 g tea/100 mL of milk). It was steeped under a lid for 10 min, from time to time being stirred. Then the solution was manually filtered using gauze filters and cooled to 43 ◦C. We added 0.1% of starter cultures to milk and stirred thoroughly. Then 100 mL of milk was poured into sterile plastic containers with lids. All samples were thermostated at 43 ◦C in the incubator set (Memmert INE 500, Schwabach, Germany) until they reached the pH value of 4.5–4.6 (Voltcraft PH-100ATC, Wollerau, Switzerland), which took approximately 4.5 h. The samples were then removed and allowed to cool. The samples were stored at 4 ◦C for 15 h till the structure was built and then the sensory evaluation and instrumental analysis were undertaken [34–36]. Plain yoghurt was prepared similarly, only tea was not added to it.
