Mint is an industrial plant that is a good source of essential oil and many phenolic compounds that have several positive benefits to human health and can be used to prevent the development of many diseases. The aim of this research was to investigate the possibility of increasing essential oil and flavonoid content, changing the chemical composition of these compounds in different mint cultivars under foliar application with precursors (phenylalanine, tryptophan, and tyrosine) at two concentrations, 100 and 200 mg L−1, to enable the possibilities for wider use of these plants when they are grown in field conditions. Spraying with phenylalanine at 100 mg L−1 concentration increased essential oil content in Mentha piperita ‘Granada’ plants by 0.53 percentage units. Foliar application with tyrosine solutions at 100 mg L−1 concentration most effectively influenced the essential oil odor profile Mentha spicata ‘Crispa’. The highest number of total flavonoids was in Mentha piperita ‘Swiss’ sprayed with tyrosine at 100 mg L−1 concentration. The flavonoid content depended on the mint cultivar, amino acids, and their concentration. The results showed that the effect of amino acid solutions on different secondary metabolites’ quantitative and qualitative composition differed depending on the mint cultivar; therefore, amino acids and their concentrations must be selected based on the cultivar they are targeting.

Phenolic compounds have a number of benefits to human health and can be used as preventive compounds for the development of some chronic diseases. Mentha plants are not only a good source of essential oils, but also contain significant levels of wide range of phenolic compounds. The aim of this research was to investigate the possibility to increase phenols content in Mentha plants under the foliar application with L-phenylalanine, L-tryptophan, L-tyrosine at two concentrations (100 mg L−1 and 200 mg L−1) and to create preconditions for using this plant for even more diverse purposes. Quantitative and qualitative analyses of phenols in mints were performed by HPLC method. Foliar application of amino acids increased the total phenol content from 1.22 to 3.51 times depending on the treatment and mint variety. The most pronounced foliar application to total phenols content was tryptophane especially in Mentha piperita “Swiss”. Mentha piperita “Swiss” was affected most by foliar application and the amount of total phenolic acids depending on the treatment ranged from 159.25 to 664.03 mg 100 g−1 (DW), respectively, non-sprayed and sprayed with tryptophane 100 mg L−1. Our results suggest that the biophenol content varies according to such factors as foliar application and variety, and every single mint variety has individual response to different applications of amino acids.

Mentha species are one of the world oldest and popular herbs, that are used in cosmetic and food industry, as well as tea for their medical properties. Due the consumer perception of ecological production and aromatic plants production waste utilization the objective of this work was to determine differences in chemical content and colour among different varieties and parts of Mentha plants ecologically grown in Lithuania. In mint leaves there were determined: content of essential oils, dry matter, soluble solids, ascorbic acid, crude fibre, crude ash, photosynthetic pigments and colour CIE L*a*b* parameters. In the case of mint plants stems, the same analyses were performed except ascorbic acid, soluble solids and colour. The highest content of essential oils were found in Mentha spicata L. ‘Moroccan’ and Mentha piperita L. ‘Glacialis’ leaves, while the least in Mentha suaveolens Ehrh. ‘Variegata’. The content of essential oils in the stems was minor and did not depend on mint variety. Plant parts differ significantly in respect of chemical composition. Amount of dry matter in mint leaves and stems was very similar, while crude ash in the leaves was twice higher than in the stems. Content of crude fibre in the leaves did not differ significantly in all investigated varieties and was 3.6 times lower comparing with the stems. Average ratio of chlorophyll a to b in the leaves was equal to 2.78, while in stems equal to 2.83. The lightest was Mentha suaveolens ‘Variegata’, the darkest Mentha spicata L. ‘Moroccan’ and the greenest Mentha piperita L. ‘Swiss’ leaves. Mints and their parts differ in chemical content and can have widespread usage not only for production of high-value natural products but in addition as alternative fibrous biomass plants.

The aromatic amino acids, tryptophan, phenylalanine and tyrosine, are not only components of proteins, but also precursors of many compounds in plants that have a significant impact on their growth, development, reproduction and protective function against various abiotic and biotic factors. With the growing demand for plant-derived chemical compounds, much in vitro and in vivo research is being conducted to intensify the synthesis of these compounds or to change their qualitative composition in plants. The aim of the research was to evaluate the influence of aromatic amino acids on the chemical composition of different varieties of Mentha L. plants. The field experiment was conducted at Aleksandras Stulginskis University (ASU) in 2017–2019, since 2019 at the Vytautas Magnus University Agricultural Academy Experimental Station, which is located in Ringaudai Eldership, Kaunas District (coordinates 54 ° 53′ 08.9″ N, 23° 50′ 08.02″ E). The effect of different concentrations of amino acid solutions on the chemical composition of mints depended on the mint variety. Spraying with all amino acids solutions significantly increased the dry matter, crude fiber and crude ash content in M. spicata ‘Crispa’ mints while a positive effect of amino acids solutions on the protein content was found only in M. piperita ‘Granada’ mints sprayed only with 200 mg l–1 tyrosine solution, where the amount of protein increased significantly by 1.41-fold compared to that of non-sprayed plants. The intensity of the synthesis of photosynthetic pigments depended on the amino acid solutions used and the variety of mint. There were both inhibitory and stimulatory effects.

The aim of this work was to determine the influence of different drying methods on the content of biochemical compounds and changes in physical properties in leaves and stems of organically grown mint. The research was conducted at Aleksandras Stulginskis University (since 2019 – Vytautas Magnus University Agriculture Academy) in Lithuania, in 2015–2016. Mints were grown in an organic farm in the Panevėžys District of Lithuania. Mints were harvested on the (60 BBCH) stage of mint development in 2015. After the harvest, mint leaves were lyophilized by a SCANVAC Coolsafe 55–9 lyophilizer for 24 h at –60°C temperature and hot air dried by a Termaks TS-8265 dryer at 30°C temperature. In mint leaves and stems, essential oil content, amount of dry matter, amount of photosynthetic pigments and colour parameters (L*, a*, b*) of mint leaves were determined. The biggest amount of essential oil was determined in fresh M. spicata ‘Moroccan’ mint leaves and stems – 3.83% (DW) and 0.59% (DW). Drying showed a negative influence on the amount of essential oil in mint leaves and stems. The amount of essential oil decreased in mint leaves by 0.42–1.59% and in stems by 0.16–0.49% depending on the variety of mint and drying method. In dried and lyophillized mints, the amount of dry matter increased 3.35–3.78-fold in mint leaves and 2.92– 4.30-fold in mint stems depending on the variety and drying method. The variety and drying methods had an influence on the stability of chlorophylls a and b. Dried and lyophillized mints had a higher whiteness index (WI) compared with that of fresh mints. The browning index (BI) showed that dried M. gracilis ‘Ginger’, M. piperita ‘Glacialis’, M. piperita ‘Swiss’, M. spicata ‘Moroccan’ and lyophillized M. suaveolens ‘Apple’, M. piperita ‘Glacialis’, M. piperita ‘Swiss’, M. spicata ‘Moroccan’ mints were browner compared with fresh mints, while lyophillized M. gracilis ‘Ginger’ and dried M. suaveolens ‘Apple’ mints were not so brown as fresh mints. Yellowest (YI) were fresh mints. The smallest total colour difference (ΔE) was in dried M. piperita ‘Glacialis’ 18.74 while the biggest one in lyophillized M. spicata ‘Moroccan’ 111.19 mints.