Hot and cold drying of edible flowers affect metabolite patterns of extracts and decoctions

The European diet is continuously changing due to emerging consciousness on the value of different foodstuff, not only from a nutritional point of view but also looking at sustainability and health-promoting ingredients that can improve welfare of consumers (Pires et al., 2017). In this scenario, many studies explored the potential of edible flowers in the human diet as food, supplements or additives and revealed several interesting sources of bioactive compounds (Fernandes et al., 2017; Zhao et al., 2019; Takahashi et al., 2020; Demasi et al., 2021a). Different aspects of edible flowers are currently under investigation, for example quality attributes, traditional consumption, processing, chemical and nutritional characterisation or biological activities (Zhao et al., 2019; Takahashi et al., 2020), but most of the studies focused on the flavonoid content (e.g. anthocyanins and phenolic compounds), a class of low-molecular weight phenolic compounds commonly present in fruits and vegetables. They are fundamental components in nutraceutical, cosmetic, medicinal and pharmaceutical applications. Indeed, flavonoids modulate enzyme functions and exert positive effects on health, such as anti-inflammatory, anti-carcinogenic, antioxidant and anti-mutagenic (Panche et al., 2016).

Flowers can also be a source of natural colourants for applications in food, cosmetic and dyeing industries, providing a safer option for human health and the environment, counteracting the disadvantages of synthetic dyes (Sivakumar et al., 2011; Bujak et al., 2022).

Fresh flowers have a short shelf-life after harvest (5–15 days depending on the species), and different techniques can be adopted to extend their duration (Fernandes et al., 2019; Zhao et al., 2019; Demasi et al., 2021b). Among these, industry has been using heat treatment techniques for centuries, such as pasteurisation, sterilisation, blanching and drying (Xu et al., 2021). Dehydration is commonly used to prolong the shelf life and preserve quality because it inhibits enzymatic degradation and limits microbial contamination, as well as reduces the weight of the product for more sustainable transport and storage (Fernandes et al., 2019; Zhao et al., 2019; Lu et al., 2020; Qiu et al., 2020). Many different drying methods can be used, such as fluidised bed drying, spray drying, freeze drying, microwave drying, cool wind drying, osmotic drying and combinations of these methods (Fernandes et al., 2019; Stelluti et al., 2021; Xu et al., 2021). However, the application of heat (hot air drying and sun drying) is the most used technique worldwide because of its simple operation, low cost but long drying time, despite some drawbacks, such as undesirable nutritional and biochemical changes that may affect the product quality (Boyar et al., 2017; Fernandes et al., 2019; Qiu et al., 2020). Indeed, chemical composition, aroma component and appearance of herbs and flowers could be altered by the drying process, as well as changes in the tissue anatomy and cellular ultrastructure (Lu et al., 2020; Marchioni et al., 2022; Vallino et al., 2022). Flowers are extremely sensitive to heat compared to other parts of the plant; thus, drying them at low temperature could be a valid alternative to hot drying (HD) because colour, shape and chemical properties could be better preserved. Low-temperature drying, or cold drying (CD), can be achieved through the combination of airflow drying and a heat pump, which works on the principle of refrigeration: It cools an air stream, which renders the air dry and condenses the water contained in it, by recovering the latent heat of evaporation through water vapour removal, which permits air recirculation (Patel and Kar, 2012; Boyar et al., 2017).

Flower extracts may be used as ingredients, particularly for beverages, or additives, or as source of anthocyanins to improve the colour of cosmetics, textiles and food, for example yogurt (Sivakumar et al., 2011; Pires et al., 2018, 2019; Bujak et al., 2022). Ultrasound-assisted extraction (UAE) is an effective method to obtain plant extract faster and more efficiently than the traditionally used maceration. Indeed, ultrasound-assisted extraction causes the instantaneous formation and collapse of cavitation bubbles, which foster cell wall destruction, improving the solvent penetration and the release of phytocompounds (Medina-Torres et al., 2017; Zhao et al., 2019; Stelluti et al., 2021). Edible flowers can be used to prepare wines, functional beverages (e.g. sport drinks, functional bottled water and energy drinks) and herbal teas (Boyar et al., 2017; Pires et al., 2017; Xu et al., 2021), which are defined as beverages made from plants other than Camellia sinensis L. and are highly consumed worldwide (Pereira et al., 2017). The demand for these products is estimated to be increasingly popular (Males et al., 2022). Some flower species are used to prepare herbal teas, such as rose, chamomile, primula, poppy, marigold, echinacea, lavender, rosemary, gentian and chrysanthemum (Boyar et al., 2017; Lu et al., 2020; Falla et al., 2022).

The aim of this study was to understand the potential of eight different edible flowers to be used in two different forms – either as ultrasound-assisted extracts (UAEs) or as decoctions (DEC) – for industrial applications. The more suitable drying technique between HD and CD was evaluated for both types of products to achieve better quality in terms of anthocyanin and phenolic contents and antioxidant activity.

Data on extracted water confirmed that generally water represents about 80% of the flower composition (Pires et al., 2017). According to Zhang (2017), the final moisture content of edible flowers should be 10%–12% to better preserve the quality and stability of the product. Our results highlighted that 24 h of HD and CD was sufficient to reach this percentage in P. vulgaris, while the other species probably need more time. Particularly, the small percentages of water extracted from C. cyanus and generally also from L. angustifolia could be due to their high amount of green portion (calix and spike, respectively), which is thicker than petals. However, both drying techniques were sufficient to grind the flowers and obtain the powder to prepare UAE and DEC; moreover, flowers remained unaltered for the following two years when stored in glass jars at room temperature in the dark.

The presence of phenolic compounds, which are non-nutritional plant secondary metabolites, is highly demanded in functional food and beverages to quench reactive oxygen species and reactive nitrogen species in the human body. The control of this oxidative stress can counteract several age-related diseases, such as inflammations, cancers and cardiovascular, dysmetabolic and neurodegenerative disorders (Del Rio et al., 2013; Zamora-Ros et al., 2013; Aryee et al., 2019; Durazzo et al., 2019; Fraga et al., 2019). The positive properties of phenolics are also exploited in different sectors, from agriculture to food, pharmaceutical and cosmetic industries (Brodowska, 2017). Anthocyanins are natural food additives regardless of the plant source, authorised in Europe with the code E-163 and considered a group of harmless substances (Santos-Buelga and González-Paramás, 2019).

The lowest values of TAC, TPC and antioxidant activity of dry flowers were mostly recorded in L. angustifolia and C. cyanus considering both UAE and DEC, except for TAC DEC, where the lowest values were found in P. vulgaris and B. perennis, subjected to HD and CD, respectively. Overall, UAE was found to extract a higher amount of anthocyanins (Figure 2) than DEC, while DEC allowed to obtain a higher quantity of phenolic compounds (Figures 3 and 6) and a higher antioxidant activity (Figure 4). Considering the drying method, the effect was different according to the product considered: in UAE, CD was more effective on TAC, HD was more effective on TPC, while the drying technique was irrelevant on ABTS; in DEC, CD was found to be the most efficient drying method to get better results for all the parameters.

The values of TAC recorded in edible flowers, up to 3,284.6 mg C3G · 100 g^–1, were much higher than that found in commonly consumed vegetables (Li et al., 2012), such as red cabbage (199 mg C3G · 100 g^–1) and purple cauliflower (201 mg C3G · 100 g^–1), or fruit, up to 414.95 mg C3G · 100 g^–1 in Vitis vinifera, one of the richest sources of anthocyanins (Ribeiro et al., 2015; Solymosi et al., 2015).

The stability of anthocyanins is negatively affected by several factors, such as heat, light and oxygen (Qiu et al., 2020; Cai et al., 2022). Flower pigments are commonly divided into polyene pigments (lutein, carotene) and phenolic pigments (anthocyanins, flavonoids); unstable glycosides of the latter are easy to hydrolyse and isomerise, resulting in the degradation of the pigments under thermal processing (Xu et al., 2021). In particular, rising the heating temperature during thermal processing has been found to decrease the TAC of edible flowers, causing degradation and discolouration of petals, and our results showed the same behaviour as TAC was much lower in DEC (prepared at boiling temperature, circa 100 °C) than in UAE (prepared at room temperature, circa 23 °C). The higher temperatures were found to cause loss of the anthocyanin content and browning reaction in Rosa rugosa flowers (Qiu et al., 2020), confirming what is recorded in this study, where most of the flowers had browning reaction after HD. The main traits that contribute to sales for processed flowers are appearance, aroma and colour, which are determined and affected by several factors, such as hot air during processing (Zhao et al., 2019). The use of low temperature drying and UAE can thus be effective techniques to preserve the visual appearance and TAC of edible flowers, respectively, raising the quality of the dry product. Generally, the effect of the drying method was less evident probably because of the smaller difference in temperature between HD (50 °C) and CD (22 °C). However, heat pump drying is characterised by several positive features, such as obtaining a good visual quality, which is suitable in the sectors where the aesthetic value is essential, like bakeries. Moreover, this equipment has easy-to-control conditions and has high energy efficiency. On the other hand, other characteristics must be evaluated, such as high initial investments, the presence of the refrigerant and the complexity of the structure compared to traditional airflow drying (Lv and Zhang, 2017).

The values of TPC recorded in edible flowers, up to 9,034 mg · 100 g^–1, were much higher than those found in commonly consumed vegetables (Cai et al., 2004), such as Lactuca sativa (780 mg GAE · 100 g^–1) and Spinacia oleracea (900 mg GAE · 100 g^–1), and infusions of rose, jasmine and osmanthus flowers (375–433 mg GAE · 100 g^–1 (Hussain et al., 2019). Teas of dry rose petals can reach very high values of TPC and antioxidant activity, as found in this study, representing one of the most promising healthy beverages (Vinokur et al., 2006). Results on dry lavender macerates (Duda et al., 2015) or UAE with different solvents (Dorozko et al., 2019) showed a higher content of TPC (1,183–1,816 mg GAE · 100 g^–1) than those found in this study, but we obtained double values when performing DEC on the same species, similar to those obtained with freeze-dried flower DEC (Dobros et al., 2022). DEC was highlighted as the most performant method to extract antioxidant compounds also in aromatic plants Achillea millefolium (Dias et al., 2013) and Melissa officinalis (Sentkowska et al., 2016).

Concerning TPC, Lu et al. (2020) suggested that the increase in phenolic compounds during the heating process might be due to different causes. The high temperature breaks plant tissue, leading to an easier release of phenolics during the extraction process; moreover, heat can inactivate the enzyme polyphenol oxidase, which usually degrades phenolic compounds in fresh tissues, thus leading to a higher content of TPC; finally, fresh flowers can sense water loss during drying, synthesising phenolic compounds as a defence mechanism.

Concerning the phenolic profiles detected by HPLC, UAE allows obtaining more compounds than DEC in terms of frequency, although their concentrations were much higher in DEC. The effect of drying varied mostly depending on the species, and a general effect was observed only for flavanols, where HD was more effective in both UAE and DEC products.

Phenolic compounds are divided into phenolic acids and flavonoids (Pires et al., 2019). The former are the main compounds produced by plants, including benzoic and cinnamic acids and are used to improve the nutritional, organoleptic and antioxidant properties of food, besides being used in pharmaceutical and cosmetic industries (Kumar and Goel, 2019). The latter include anthocyanins, flavonols and flavanols, all water-soluble pigments that confer red, pink-orange and blue colours to fruits and flowers (Brodowska, 2017; Pires et al., 2019).

In this study, however, a different pattern was observed since benzoic and cinnamic acids were not detected in several species, flavonols and flavanols instead being the most frequent and abundant classes of compounds, confirming previous studies on the same species of fresh (Demasi et al., 2021a) and dry dahlia, rose, calendula and centaurea (Pires et al., 2017). Nevertheless, the pattern of phenolic classes identified in the UAE of the same species of fresh and dry flowers cannot be overlapped as some classes increased after drying and some decreased, depending also on the species. This can be due to the different composition of each extract, in which every compound follows its own process according to conditions.

Flower species vary widely in terms of phytochemical characteristics and composition; therefore, testing different processing and extraction methods is highly important to improve the knowledge on the more suitable technique to gain the highest amount of bioactive compounds and pigments for each species. This study explored HD and CD applied to eight edible flowers, followed by ultrasound-assisted extraction or decoction preparation. The phenolic profiles detected by HPLC analysis showed peculiar patterns for each species, and both drying and extraction methods differently affected the yield of phytochemicals. Concerning the extraction method, it was seen that a higher amount of TAC can be obtained with UAE, while DEC gained a higher amount of phenolic compounds and greater antioxidant activity. For DEC, CD was found to be the most efficient drying method for all the parameters. For UAE, CD gave a higher quantity of anthocyanins, while HD a higher quantity of phenolics, particularly flavanols. Overall, the studied flower extracts and DEC can contribute to produce healthy food and beverages and coloured produce. R. pendulina and R. canina are the most promising sources for designing innovative natural products.