Mead is an alcoholic beverage obtained from the fermentation of honey diluted in water (1, 2). Other ingredients such as herbs, spices and/or fruits, are commonly added to modify the chemical and sensory characteristics of this beverage (3). The chemical composition of mead can be altered by different factors, such as the origin of the honey, the yeast strains used, and the additives and other ingredients that are incorporated into the wort, as well as the product preparation steps (4, 5).

Although mead has been produced in various regions of the world since ancient times, mainly in Nordic countries and Eastern Europe, in recent years this beverage has shown an important growth in the global alcoholic beverage market (6). However, mead is still mostly produced empirically and as a handcrafted beverage, with relatively few scientific reports in comparison with other alcoholic beverages (7). Moreover, the production of mead shows some problems related to slow fermentation, mainly due to the variability in the composition of honey and its low buffering capacity, as well as to the limited amount of nutrients in the wort, which are essential for yeast development (6, 8).

The ingredients used for the preparation of the beverage have a significant impact on the fermentation process and can modify the bioactive potential of the mead (9, 10). Studies of the characteristics of meads with added herbs and fruits have been published recently (3, 9, 11). Besides, nutritional additives and supplements, mainly those that are nitrogen-based, minerals, vitamins and acids have been incorporated into the wort in order to stimulate yeast growth and consequently improve the fermentation process (7, 12, 13).

Given the above, this review seeks to investigate how different ingredients and additives can affect the stages of the preparation of mead, as well as its bioactive potential, in order to understand the effects on its chemical composition, and thus add greater commercial value to this ancient yet little researched beverage.

Traditionally, mead is made from three basic ingredients: water, honey and yeast. However, in order to diversify the beverage and improve its bioactive and sensory properties, different types of honey and yeast can be used, besides other additional ingredients. According to Starowicz and Granvogl (14), different meads can be obtained by using different ratios of honey and water, as well as by the addition of fruits, herbs and/or spices or other additives. Thus, there are some denominations regarding the types of mead according to the used ingredients, such as: melomel (addition of fruits or fruit juice) (15), cyser (addition of apple juice or cider) (7), pyment (addition of grape juice or wine) (16), and metheglin (addition of spices and/or herbs) (7).

Among the basic ingredients of mead, water is predominant, and thus it is important to check its quality for the preparation of the beverage. In order for water to be used in the manufacture of alcoholic beverages, it must meet certain requirements, such as not having turbidity or high levels of chlorine, having a controlled pH, and meeting the desired microbiological standards (15, 17).

Honey is a natural food which is produced mainly by the honey bee from the nectar of flowers, secretions from living parts of plants, or excretions from sucking insects (18, 19). It is a saturated solution of sugars, mainly fructose (38 %) and glucose (31 %), as well as minor constituents, such as minerals, proteins, amino acids, enzymes, organic acids, aromatic substances, vitamins, phenolic acids, and flavonoids, among others (20, 21). The chemical composition and the sensory characteristics of honey are affected by its botanical and geographic origin, the climatic conditions, maturation stage, bee species, as well as the processing and storage conditions (20, 22). Honey has been acknowledged as a functional food mainly because of its antioxidant and antimicrobial properties (23-25), and its colour is directly related to its phenolic content (26). Thus, honey can be used as a basic raw material for obtaining different foods and beverages, including alcoholic beverages such as mead, and it directly affects the chemical and sensory characteristics of the product (14). Floral honey types are used the most for the preparation of mead (1, 9, 10). However, honeydew honey has aroused the interest of researchers because of its distinct chemical composition (3).

The choice of fermentative microorganisms is also of great importance in the production of mead as they play a key role in the efficient conversion of sugar into ethanol (5). Yeasts are facultative unicellular and anaerobic microorganisms and are responsible for the fermentation process of mead, especially those of the genus Saccharomyces. The species that are best known for their performance in alcoholic beverage fermentations are S. cerevisiae and S. bayanus (27). However, in recent years, efforts have been made to select different yeast strains and also to use non-Saccharomyces yeasts, whether in mixed or pure cultures (28, 29) aiming at a greater control of the fermentation process and of the characteristics of the product (1, 30).

Some studies have also investigated the effects of additives, nutritional supplements, and vegetables, especially fruits, herbs and spices, in the preparation of mead in order to examine the fermentation kinetics and chemical characteristics of the product (1, 11, 27, 31).

In general, the mead preparation comprises some fundamental stages, which include prefermentation step (preparation of the wort), alcoholic fermentation step and post-fermentation step (clarification/bottling) (Fig. S1 (9, 32)) (7). It is noteworthy that there may be changes in the initial and final stages of mead preparation depending on the available infrastructure and the characteristics that are desired for the product. Moreover, different ingredients and additives can be incorporated into the wort prior to the fermentation, while after racking maturation occurs optionally and can be carried out either before or after bottling.

Mead has been reported as a potentially bioactive beverage, mainly because of its profile of phenolic compounds and its antioxidant activity. The phenolic content of mead is strongly related to the ingredients that are used to produce it (14). It is worth noting that honey is an important source of bioactive compounds for mead since some phenolic compounds are transferred from the plants to the honey by the bees (59). However, the concentration of these compounds in mead depends on the origin and on the type of honey, besides the amount used in the preparation of the beverage (60). Although there are many studies on the phenolic profile and antioxidant activity of different types of honey, little information can be found regarding mead.

Phenolic compounds comprise a distinct class of secondary plant metabolites from different plant sources, such as fruits, cereals and herbs (61), which are rich in flavonoids, flavones, flavanones, isoflavones, anthocyanins, catechins, phenolic acids, phytoestrogens, tannins, stilbenes and curcuminoids (62). Phenolic compounds are potent antioxidants that reduce or even inhibit the propagation of oxidation reactions by scavenging the reactive form of oxygen (60).

As reported by Dhalaria et al. (63), the health benefits associated with the consumption of fruits and vegetables have drawn increasing interest from consumers. Thus, the inclusion of ingredients of vegetable origin in the mead can enhance the beverage’s bioactive properties, since Bednarek and Szwengiel (52) report that the antioxidant potential of mead can be improved by adding fruit juices and herbs. Table 3 (9, 27, 46, 52, 60) shows the phenolic composition of the mead containing different ingredients of vegetable origin.

According to a study by Socha et al. (60), two groups of phenolic acids were identified in ten samples of commercial mead from Poland. These authors found that the plant species used in the preparation of the mead had a significant effect on the profile of phenolic acids, especially in the sample made with honey (in a 1:2 ratio of honey and water) and rowan juice (4.46 mg/L), and also in the sample of ’Kasztelanski’ mead (in a 1:1 ratio of honey and water) with added extract of roots and herbs (10.97 mg/L). Among the hydroxycinnamic acids, the highest measured amounts were of chlorogenic, caffeic, p-coumaric and ferulic acids, and among the hydroxybenzoic acids, the identified acids were gallic and protocatechuic (predominant among the analysed samples), besides vanillic acid. Zahrani et al. (64) reported that gallic acid is commonly found in several plant species, has different biological activities, and is considered a substance that contains antioxidant, antibacterial, antifungal, antiviral, anti-inflammatory and antidiabetic properties. However, in relation to protocatechuic acid, its antioxidant, anti-inflammatory and anti-apoptotic effects on animal tissues (mice) were proven in a study conducted by Habib et al. (65).

Švecová et al. (46) analysed 22 samples of commercial Czech meads and noted a great variability in the profile of phenolic compounds, which showed higher values for the samples containing additional ingredients (fruit juice, nuts, and herbal extracts) than the traditional meads, made only with honey and water. This result was especially evident in the mead with added black currant, which showed the highest content of phenolic compounds, with the emphasis on the concentrations of gallic (3.37 mg/L) and caffeic (2.99 mg/L) acids. It is noteworthy that caffeic acid can play an important role in the protection of different tissues and organs, protecting cell membranes from oxidative damage because of its ability to scavenge free radicals (66).

Adamenko et al. (27) investigated the effect of the addition of different varieties of Cornelian cherry on the polyphenol profile of meads and identified some compounds that belong to the groups of monoterpenes, phenolic acids and flavonoids. Among the monoterpenes, the iridoids, which until then had not been identified in the mead, were quantified. The iridoids from these monoterpenes have been reported to exert positive effects on the biological properties of the Cornelian cherry (67). The predominant phenolic acids were gallic (3.8 mg/mL) and chlorogenic (2.4 mg/mL) acids, showing higher concentrations in the mead fermented with the coral Cornelian cherry variety. Chlorogenic acid is a hydroxycinnamic acid that is widely present in plant species and has functional properties that are related to hypoglycaemic, hepatoprotective, antiviral, antibacterial and anti-inflammatory capacity (68). Besides these compounds, Adamenko et al. (27) also detected a different group of phenolic compounds that affect the biological properties and quality of foods, namely, the Q-3-glucuronide (Q-3-glcr), which was the only representative of the flavonols in the samples. On the other hand, anthocyanins were detected only in the mead with the addition of coral Cornelian cherry juice and red Cornelian cherry juice, with a predominance of those derived from pelargonidin. Anthocyanins have drawn increasing attention because of their preventive effect against some diseases, their ability to react with reactive oxygen species resulting from natural metabolic processes in plants or animals (69). Furthermore, according to Martín-Gómez et al. (70), the main health benefit properties of anthocyanins are their neuroprotective, cardioprotective, nephroprotective and ocular protection potential, as well as their anticarcinogenic and anti-inflammatory activity, among others.

In the study conducted by Kawa-Rygielska et al. (9), aronia syrup, dandelion and powdered grape seeds were separately added to the mead wort, and the phenolic compounds identified in the mead were hydroxycinnamic acids (derived from caffeic acid) and hydroxybenzoic acids (protocatechuic and gallic acid) as well as flavonols, procyanidins and flavanones. These authors noted that the highest concentration of phenolic acids was determined in the samples fermented with aronia syrup (52.9 mg/L of hydroxycinnamic acids and 4.39 mg/L of protocatechuic acid). Meanwhile, the mead with added powdered grape seeds stood out for its concentration of gallic acid (3.98 mg/L) and procyanidins (20.97 mg/L), which derived from the vegetable ingredient added to the wort. It is known that procyanidins have antibacterial, antioxidant and anti-obesity activities (71, 72).

Bednarek and Szwengiel (52) investigated samples of commercial mead from Poland. The samples were saturated (heat treated) and unsaturated (non-heat treated), and also supplemented with fruit juice (raspberry, rowan and rosehip), root extract and herbs. These authors noted that the most abundant phenolic acid in the samples was chlorogenic acid, with average concentration of 4.99 mg/L. Naringenin was the flavonoid that had the highest concentration (4.54 mg/L), and is reported in other studies as the predominant flavonoid in Polish honeys (73). Naringenin is a flavanone usually present in citrus fruits. It has different pharmacological and biological properties regarding its antioxidant activity, which is related to anti-inflammatory and antitumour action, antimicrobial activity, and also fights the development of atherosclerosis (74, 75). According to Smruthi et al. (74), naringenin is also present in glycosidic form, as naringin, which is responsible for the bitterness of the fruits.

Furthermore, the presence of tyrosol in mead was first reported in a study conducted by Bednarek and Szwengiel (52), who found that it was produced at concentration of 19.07 mg/L from tyrosine during fermentation. Tyrosol is a compound that is stable and less subject to autoxidation than other polyphenols, and in vitro and in vivo studies point to its biological potential related to antiatherogenic, cardioprotective and neuroprotective effects (76). This information may be interesting, since the polyphenols present in alcoholic beverages can have high bioaccessibility and are more easily absorbed by the intestine (60). However, so far there are no reports of studies that have evaluated the bioaccessibility of phenolic compounds in the mead.

It is important to emphasize that bioactive compounds from different sources added to mead tend to positively influence its sensory acceptability (9, 32). However, it is important to evaluate the desirable concentrations of these compounds in the beverage.

Among the main challenges of the production of mead, the conditions of production stand out. This beverage is often produced either empirically or homemade, and producers many times come across problems related to the lack of standardization of the produced mead (3, 7). Many of these problems are related to the harsh and adverse growth conditions to which yeasts need to respond and adapt. Another challenge for the productive sector is the standardization of mead in relation to its alcohol content, since mead is a beverage that shows a wide range of alcoholic content worldwide, generally between 8 and 18 % (8). Thus, depending on the ratio of honey and water used to make the wort, the incorporation of additional ingredients and the fermentation time, the obtained products will have completely different chemical and sensory characteristics. These differences reflect mainly on the sweetness parameters and on the perception of alcohol content, which are important characteristics for the consumer acceptance of beverages (9, 41). Furthermore, Simão et al. (77) reported that although the technology patents related to mead are significant in number, there still is a demand for the diversification of this beverage.

Due to the slow scientific progress in this field, and taking into account the small amount of research studies on mead compared to other fermented alcoholic beverages (e.g. wine and beer), there is a possibility for research related to production issues, as well as the chemical quality, sensorial and bioactive aspects of the mead, which can and should be investigated.

Despite being relatively simple, the mead preparation process demands caution in relation to different process parameters in order to obtain a high-quality beverage. The addition of different additives and ingredients greatly affects both the preparation process and the composition of the mead. Thus, there is a need for greater dissemination of technical and scientific knowledge for the production of this beverage on a large scale and under controlled conditions for its preparation. It is also noted that there is little data related to the bioactive potential of mead. This way, this review study serves as a motivation for further investigations aiming at the development of innovative products with potential benefits to human health. Therefore, research studies mainly related to the bioaccessibility of the bioactive compounds in mead have great scientific relevance, raising many questions that still need to be answered.