Keywords

1 Introduction

Secondary metabolites are considered those small molecules that are not regularly composed in plant tissues and are not obligatory for the growth, development, and reproduction of plants (Hassan et al. 2012). Their function is focused on plant defense against abiotic and biotic stressors, which differentiates them from primary metabolites that are involved in vital functions of plants and their presence is necessary for life maintenance (Tiwary and Rana 2015). However, this segregation is equivocal since in some occasions the intermediates in the biosynthetic pathways of both metabolites are common, making the distinction very complex (Pichersky and Gang 2000). Moreover, secondary metabolites can be used by plants in primary metabolism functions, while they are associated with multiple functions (Erb and Kliebenstein 2020). Therefore, a new definition was coined and now secondary metabolites are considered those specialized compounds (hence the term “specialized metabolites”) that can be found in specific species, which differentiates them from primary metabolites that are omnipresent in all living organisms and plant hormones that have regulatory functions (Erb and Kliebenstein 2020). The recent review of Erb and Kliebenstein (Erb and Kliebenstein 2020) clearly identifies the obstacles in defining and dividing primary and secondary metabolites, while they also highlight that recent advances in analytical chemistry and genetics made this task more complex. Moreover, the same authors suggest the whole perspective for these compounds have to be revised and considering them as components of dynamic metabolic pathways will improve our understanding regarding the functionality of these valuable compounds (Erb and Kliebenstein 2020).

The biosynthesis of secondary metabolites comes to cover specific needs of plants at specific stages of their growth (e.g., the synthesis of volatiles to attract pollinators during the reproductive stage) or when plants have to overcome exogenous restraints (e.g., pest and pathogens attacks, unfavorable environmental conditions) (Hartmann 2007; Ramakrishna and Ravishankar 2011; Wink 2018). The numerous compounds that have been identified in various plant species so far are classified based on their chemical structures and they are categorized into three main groups, namely terpenes, polyphenols, and nitrogen-containing compounds (e.g., alkaloids and glucosinolates) (Hartmann 2007). Despite their structural similarities there are several individual compounds that are species specific and maybe synthesized through the activity of specific enzymes on different or the same substrate (Pichersky and Gang 2000). Therefore, each species composes a different set of secondary metabolites even when the stimuli are the same, e.g. attraction of the same species of pollinators or protection against the same herbivores (Knudsen and Tollsten 1993; Mello and Silva-Filho 2002).

The modern food industry sectors must come up against several contradictory challenges related to food security, on the one hand, and, the health safety, on the other hand. The increasing demands for food production due to increasing population necessitate the production of larger amounts of food products, whereas the consumers’ concerns regarding the food safety and their demands for functional and healthy foods call for the redesign of food industry sector (Carpentieri et al. 2022). Functionality of plant-based food products is associated with the presence of secondary metabolites which are normally produced in various plant tissues and organs, while modern food research focuses on eliciting the biosynthesis of such compounds or incorporating them in the food products (Isah et al. 2018; Carpentieri et al. 2022; Ceccanti et al. 2022). Therefore, the recovery of bioactive compounds from natural sources such as plants and agro-industry by-products is gaining interest aiming to design new functional and healthy foods (Galanakis 2021).

The present chapter will discuss the main categories of secondary metabolites, as well as their applications in the food industry, focusing mostly on commercially available solutions and secondary metabolites obtained from cultivated species. Moreover, the future perspectives that must be considered in order to improve the valorization of these compounds and increase the added value of crops will be discussed.

2 Secondary Metabolites Categories

The term “secondary” or “specialized metabolites” refers to various compounds that are responsible for plant responses to environmental conditions and biotic factors. They are classified based on their chemical structure, composition, or biosynthetic pathways and include phenolic compounds, terpenes and steroids, and nitrogen-containing compounds (mainly alkaloids and glucosinolates) (Bourgaud et al. 2001). There is a great diversity in the chemical structures and biological functions of these compounds, which are species or genotypes specific, while they can be synthesized in particular plant organs when plants are subjected to specific conditions (Petropoulos et al. 2018; Chiocchio et al. 2021).

Phenolic compounds are one of the most common groups of secondary metabolites which are widely distributed in the plant kingdom and are synthesized via the shikimic acid and phenylpropanoid biosynthetic pathways (Bennett and Wallsgrove 1994). There is a great diversity of phenolic compounds which can be further classified into two distinct groups, namely flavonoids (e.g., anthocyanins, flavanols, flavanones, flavonols, flavonones, and isoflavones) and non-flavonoids (e.g., phenolic acids, xanthones, stilbenes, lignans, and tannins) (Durazzo et al. 2019). The non-flavonoid group includes simple phenols which contain a single benzenic ring, phenolic acids which also contain one benzenic ring with a substituent of the carboxylic group and are further classified into benzoic acid and hydroxycinnamic acid derivatives, and other polyphenols with diversified structures (Chiocchio et al. 2021).

On the other hand, the flavonoid group includes compounds with fifteen atoms of carbon that consist of two benzenic (aromatic A and B) rings linked with a unit of three atoms of carbon that forms or not an extra (C) ring (Panche et al. 2016). They are further divided into several sub-groups (chalcones, stilbenes, aurones, flavanones, flavones, isoflavones, phlobaphenes, dihydroflavonols, flavonols, leucoanthocyanidins, proanthocyanidins, and anthocyanins) based on the presence or absence of C ring between the aromatic ones, as well as on the carbon of C ring which is attached on B ring and its saturation and oxidation degree (Panche et al. 2016; Abbas et al. 2017; Liu et al. 2021). Flavonoids are assigned with several functions in plants, including the coloration of flowers or other plant parts which gave them their name (flavus means yellow in Latin), and are mostly commonly found in the aerial plant parts, whereas non-flavonoid phenolic compounds are mainly distributed but not limited in seeds, leaves, stems, and roots (Robbins 2003; Tuominen et al. 2013).

Terpenes are the most diverse group of secondary metabolites with more than 40,000 compounds identified so far in all forms of life (Caputi and Aprea 2012). They are synthesized via the mevalonate pathway by using mevalonic acid as the intermediate compound and consist of repeating units of isoprenes (2 units in the case of monoterpenes and more than 8 units in polyterpenes) (Habtemariam 2019; Kallscheuer et al. 2019). Terpenes are also the building blocks of steroids which are formed from triterpenes, while along with terpenoids which contain additional oxygen-containing groups they are the most abundant class of volatile compounds commonly found in plant species (Derbassi et al. 2022). Their synthesis follows a spatio-temporal pattern regulated by specific genes which result in the numerous volatile compounds that are found in plant species (Kallscheuer et al. 2019).

Alkaloids are nitrogen-containing compounds which depending on their structure are diversified in non-heterocyclic and heterocyclic alkaloids (Lichman 2021). Recent classification of alkaloids considers the similarities in carbon skeleton as the distinguishing parameter in order to include specific compounds in this group (Ziegler and Facchini 2008). Another distinction refers to the precursor compounds, which can be amino acids in the case of “true alkaloids” or other nitrogen-containing compounds in the case or “pseudoalkaloids” (Lichman 2021). Most alkaloids have medicinal properties, therefore its use is limited to the pharmaceutical and drug industry (Aniszewski 2015).

Glucosinolates are another group of sulfur and nitrogen-containing secondary metabolites which are usually founds in species of the Capparales order especially in the Brassicaceae family (Di Gioia et al. 2020). Their biosynthesis consists of three phases, namely the elongation of the side group (R) attached to the core compound (β-d-glucopyranose + (Z)-N-hydroximinosulfate ester + R) through the addition of methylene groups, the formation of the core structure of glucosinolates, and finally the modification of this side group (Di Gioia and Petropoulos 2021). Glucosinolates are indirectly involved in plant defense mechanisms as deterrents of various pests and pathogens and stress alleviators, through the formation of biologically active compounds (e.g., isothiocyanates) after being hydrolyzed (Vig et al. 2009; Justen et al. 2013; Esfandiari et al. 2017). There are numerous glucosinolates which are highly specified on a species level (Di Gioia et al. 2020), while depending on the precursory amino acids they are classified into three main groups, namely aliphatic, aromatic, and indole glucosinolates (Redovniković et al. 2008).

3 Food Industry Applications of Secondary Metabolites

Modern food industry must fulfill consumers’ demands for healthy/functional foods, on the one hand, and the substitution of synthetic compounds with naturally derived ones, on the other hand. Additionally, these two requirements have to be fulfilled without compromising food security which is under threat considering the increasing population and the ongoing climate crisis (Carpentieri et al. 2022), while the sustainability of the whole chain has to be addressed within the circular economy context (Cole et al. 2018; Knorr et al. 2020). For this purpose, secondary metabolites are attributed with interesting bioactive properties which can be used in the food industry, especially for the design of new functional foods and natural food preservatives (Durazzo et al. 2019). The reduction of food waste due to losses throughout the value chain (e.g. crop production, post-harvesting, and processing) could be partly achieved with the use of novel preservatives extracted from plant matrices that could extend shelf life of food products (Cole et al. 2018). The extraction of secondary metabolites from plant tissues (either the primary crop products or the by-products) is also gaining great interest from the scientific community and the food industry sector since it contributes to the added value of crops and reduces the environmental burden (Hartmann 2007). Moreover, the integration of such molecules in food products could increase their overall bioactive properties and contribute to the prevention of chronic diseases and the improvement of consumers’ well-being (Ahmad et al. 2019; Carpentieri et al. 2022).

3.1 Polyphenols

Among these metabolites, polyphenols are the most widely distributed and can be found in plant species and plant-based foods in various forms (Abbas et al. 2017). Their most common uses include the flavoring, coloring, or texturizing of food products (Kallscheuer et al. 2019), as well as their use as preservatives and food additives due to their antimicrobial properties (Wu and Zhou 2021; Oulahal and Degraeve 2022). The great diversity of polyphenols offers many choices to select in terms of biostability, bioavailability, and functionality. The amount of extracted polyphenols accounted for 16,380 tons in 2015 and is expected to be doubled by 2024, while the majority of these compounds is utilized in the beverages and functional food sectors of the food industry (Adebooye et al. 2018). However, the extraction process of secondary metabolites faces many challenges due to the complexity of the obtained mixtures and similarities in chemical structures between the target compounds (Robbins 2003; Kallscheuer et al. 2019). Moreover, the seasonal fluctuation of these compounds in plant tissues, as well as the impact of growing conditions on the final concentration makes the extraction task more challenging. Therefore, the tailored production of bioactive molecules from engineered microorganisms is gaining ground toward a more sustainable and cost-effective recovery of such compounds (Kallscheuer et al. 2019). The recent advances in food science allowed the microencapsulation of such molecules and their incorporation directly in food products or in food packaging aiming to control food-borne pathogens and increase the shelf life of food products without affecting the sensorial properties of the final product (Hintz et al. 2015).

Several examples of polyphenols utilization highlight their importance in the food industry sector. There is sufficient evidence that polyphenol-rich extracts can inhibit food spoilage from bacteria and fungi infections and oxidation (Olszewska et al. 2020). The use of such extracts either through direct incorporation in healthy/functional food products or in food packaging shows a great potential for the food industry with several benefits such as the consumer acceptance and the lack of microbial resistance (Hintz et al. 2015). There are several paradigms of successful application of polyphenol-rich extracts obtained from plants in various food products. The fortification of food products with “protected” polyphenols via encapsulation may increase their stability to oxidation and thermal processing and improve their bioavailability (Cao et al. 2021). Moreover, binding polyphenols with specific molecules (e.g., lipids, proteins, carbohydrates) may increase their bioavailability through the reduced oxidation in the upper part of the gastrointestinal tract, although these effects depend on the molecules (Jakobek 2015). The polyphenols extracted from cocoa hulls were integrated in bakery products via microencapsulation and improved the total phenolic content of the final product (Papillo et al. 2019). The direct incorporation of grape-derived polyphenols in spaghetti significantly improved the bioavailability of bioactive compounds and decreased the glycemic index (Marinelli et al. 2018), while the integration of carrot pomace into pasta also improved the overall phenolic compounds content (Gull et al. 2015). Melon peels are also a good source of polyphenols and the addition of peels flour in food products could increase the total and individual phenolic compounds content as well as the prebiotic effects and the gut microbiota diversity (Gómez-García et al. 2022). Moreover, the addition of herbal mixtures containing Laurus nobilis, Curcuma longa, and Zingiber officinale Roscoe improved the nutritional value, the total phenolics and flavonoids content, as well as reduced the bacterial content in extruded corn snacks (Amer and Rizk 2022).

Non-dairy products and beverages are another category of food products where the application of phenolic-rich extracts shows great interest, since there is a market demand for alternatives to milk-based products, especially for consumers that suffer from allergies to milk protein, lactose intolerance, or those who have to follow special diets due to obesity or high blood cholesterol levels (Fidelis and Granato 2021). Some examples (see also Table 23.1) include the addition of polyphenol-rich flaxseed oil cake in a vegan analog of Camembert cheese (Łopusiewicz et al. 2020); the carob-fruit extracts in meat products (Macho-González et al. 2020); the production of non-dairy yogurt with nanoemulsions of rice bran oil and soy protein (Sengupta et al. 2019); the preparation of dairy free creams with the addition of grapes (Cropotova et al. 2017); the addition of germinated brown rice in a multifunctional non-dairy yogurt (Cáceres et al. 2019); the production of non-dairy ice creams with riceberry and sesame seeds milk (Kemsawasd and Chaikham 2020). Phenolic compounds obtained from flower extracts are also a promising material to incorporate in beverages as coloring or flavoring agents, e.g. the anthocyanin-rich extracts from cornflower used to create a model beverage (Escher et al. 2018), or the use of aqueous extracts from butterfly pea petals used as a coloring agent (Escher et al. 2020). Moreover, the extracts from acerola by-products were used to produce a kombucha-like beverage (Leonarski et al. 2021), while Migliorini et al. (Migliorini et al. 2019) suggested the use of red chicory leaves extracts as a promising coloring agent for beverages with great stability and bioactive properties. Other by-products that could be used for that purpose include those obtained from maize processing commonly known as nejayote (Buitimea-Cantúa et al. 2019), the brewers’ spent grains produced during beer production (Tan et al. 2020), or purple and red colored potato tubers (Sampaio et al. 2021). Fermented beverages rich in polyphenols have also been suggested by Rodríguez et al. (Rodríguez et al. 2020) who studied the polyphenols composition of two beverages obtained from Prosopis alba pods (namely, Aloja and añapa), Samaniego-Sánchez et al. (Samaniego-Sánchez et al. 2020) who developed a new fermented beverage from Saccharum officinarum L. molasses, and Voss et al. (Voss et al. 2021) who proposed the development of a novel symbiotic beverage produced by okara (by-product of soy-bean processing) hydrolyzed by cardoon enzymes and fermented by the prebiotic bacteria Lactobacillus rhamnosus R11 and Bifidobacterium animalis ssp. lactis Bb12. Moreover, instead of extracts the use of individual or mixtures of pure compounds showed promising results, as for example in the case of green tea flavanols epigallocatechin gallate, epigallocatechin, and epicatechin gallate which were incorporated in a catechin-free model beverage (Xu et al. 2021b).

Table 23.1 Examples of incorporation of phenolic compounds in dairy, non-dairy products, and beverages
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The incorporation of polyphenols has also found applications in baked products, e.g. breads, cookies, muffins, doughnuts, and other products which are consumed more or less on a daily basis. The most common source of polyphenols are fruit and vegetables and their processing by-products which are also a good and underutilized source of bioactive compounds and nutrients. The application methods include the incorporation of raw or processed fruit and vegetable directly in the baked products or the addition of purified polyphenols (Ou 2021). Several examples of practical applications are reported (see also Table 23.2), such as the incorporation of fruit and vegetable pomace obtained from grapes in fortified breads, cookies , muffins (Hayta et al. 2014; Walker et al. 2014); the enrichment of muffins with pomace of various fruit berries (Górnaś et al. 2016) or the addition of tomato pomace in flat bread (Majzoobi et al. 2011). Apart from the rich polyphenols content, the high fiber content of pomace may be capable of partially substituting fat in bakery products and design functional foods with low fat and calories content (Sudha et al. 2015; Quiles et al. 2018). However, due to great differences in polyphenols profile of the various pomaces, further studies are needed to evaluate the effects of incorporation on the sensorial quality of the final baked products (Majerska et al. 2019).

Table 23.2 Examples of incorporation of fruit and vegetable by-products in baked products
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Fruit and vegetable juices, pulps, and jams can also be directly incorporated in baked products, helping to improve the nutritional value and the health beneficial effects while at the same time reduce the fat and energy content of the final products (Ou 2021). The various examples include the use of raspberry juice in muffins (Rosales-Soto et al. 2012); peach pulp in cookies (Blanco Canalis et al. 2020); bergamot and orange juice in cakes (Silva et al. 2006; Lertnirundon and Mahidsanan 2020); or lime and sea-buckthorn pulp in breads (Guo et al. 2019; Scarton et al. 2021). Moreover, the application of dried plant tissues (fruit, leaves), vegetable powders and fruit purees and jams as fillings has been tested as an alternative option to enrich baked products in bioactive compounds and particularly in polyphenols, while at the same time reducing the post-harvest losses of perishable fruit and vegetables. For this purpose, dried fruit and powders from several species have been incorporated in various baked products (e.g., beans, fruit berries, melon, pumpkin, citrus fruit, etc., as recently reviewed by Betoret et al. (Betoret and Rosell 2020) and Salehi et al. (Salehi and Aghajanzadeh 2020).

There is also the option to incorporate individual polyphenols after purification and the most commonly applied compounds include various phenolic acids, e.g. caffeic acid, gallic acid, ferulic acid, chlorogenic acid, and rosmarinic acid (Mildner-Szkudlarz et al. 2019; González-Montemayor et al. 2021; Xu et al. 2021a); flavonoids (catechin, epigallocatechin gallate, naringenin, hesperidin, and quercetin (Goh et al. 2015; Fu et al. 2018; Lin and Zhou 2018; López et al. 2019; Mayneris-Perxachs et al. 2019), phenylethanoids (hydroxytyrosol) (Navarro and Morales 2017), or stilbenoids (resveratrol) (Ou 2021) (see also Table 23.3). The aim of this practice is to increase the content of targeted compounds with specified health benefits rather than improving the overall total phenolic compounds content (Ou 2021). Moreover, it is very common that compounds such as chlorogenic acid are used as food preservatives aiming to increase the shelf life of the final products due to its antimicrobial and antioxidant properties, the inhibition of lipid peroxidation, or to improve its prebiotic properties. Other functions of phenolic compounds related to their antimicrobial properties refer to the inhibition of quorum sensing which is the communication system of bacteria that regulates cell functions and the interactions with the host organism (Grandclément et al. 2015). The recent study of Santos et al. (Santos et al. 2021) evaluated the quorum sensing inhibition properties of various polyphenols such as curcumin, capsaicin, gallic acid, phloridizin, and resveratrol and suggested that curcumin and resveratrol showed quorum sensing inhibition activities against Chromobacterium violaceum and biofilm inhibition properties against Aeromonas hydrophila, Salmonella enterica serovar Montevideo, and Serratia marcescens. Other examples include the incorporation of catechin and nisin in gelatin films used to wrap minced pork meat and extended its shelf life from 1 to 4 days (Kaewprachu et al. 2018); edible coating enriched with gallic acid and chitosan increased shelf life of pork stored at modified atmospheres (Fang et al. 2018).

Table 23.3 Examples of incorporation of phenolic compounds in food products
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3.2 Terpenes

Terpenes are a class of numerous compounds widely distributed in the plant kingdom (Masyita et al. 2022). Terpenes are commonly used in the cosmeceutical industry as fragrances; however, they have also found applications in the food industry as flavoring agents or as natural preservatives in the form of direct incorporation in the food products or through integration in film packaging (Maurya et al. 2021; Wang et al. 2022). Essential oils are usually obtained from medicinal plants such as Salvia officinalis L. (Cutillas et al. 2017), lemongrass (Gao et al. 2020), anise (Topuz et al. 2016), cinnamon (Yildirim et al. 2017), thyme (Abdollahzadeh et al. 2014; Gonçalves et al. 2017), basil (Beier et al. 2014), oregano (Hernández-González et al. 2017), clove (Devi et al. 2010), and several other species (Oussalah et al. 2007). The pure essential oils or their mixtures (oils from different species) have been applied as natural preservatives or as flavoring agents in various food products so far, including baked products (Gonçalves et al. 2017; Ju et al. 2018), fresh-cut vegetables and fruit (Kwon et al. 2017; Park et al. 2019), juices (Sarkar et al. 2017; Lee et al. 2020), meat products, and non-dairy products (Cui et al. 2015). However, despite the promising preservating properties of essential oils or pure terpenes and derivatives, there are several drawbacks that hinder their wide application in the food industry due to their high volatility and lability, the potential toxic effects, the low solubility to water, as well as their strong flavor and aroma that may alter the sensorial characteristics of the final product (Salvia-Trujillo et al. 2015).

In the case of direct incorporation, nanoemulsions of essential oils are recently suggested as an eco-friendly alternative to synthetic antimicrobial agents that improves the shelf life and safety of food products without affecting their flavor and taste (Hintz et al. 2015). For example, the co-encapsulation of essential oils obtained from Pimpinella anisum and Coriandrum sativum showed a great antifungal activity and very promising results as a natural preservative of food products (Das et al. 2022). Moreover, nanoemulsions can be used as ingredients in edible and biodegradable coatings aiming to protect the labile volatile compounds of essential oils from oxidization and allow their gradual release throughout the shelf life of stored products (Arrieta et al. 2013; Das et al. 2021). The direct incorporation of essential oils is also applied in the case of hops (Humulus lupulus L.) which are used in the brewery industry as a flavoring agent of beer products (Lamberti et al. 2021). In the study of Abdollahzadeh et al. (Abdollahzadeh et al. 2014), the application of thyme essential oils at 0.8 and 1.2% on cooked minced fish inhibited the growth of Listeria monocytogenes 6 days after storage, while the combined application of thyme essential oils and nisin showed even better results by inhibiting bacterial growth 2 days after storage. Moreover, apart from antimicrobial effects the application of essential oils could improve the sensorial properties and the quality of the final product, as for example the improvement in quality traits of marinated chicken meat with the addition of a blend of thyme and orange essential oils (Rimini et al. 2014) or the increased organoleptic properties of ground meat after the combined application of Chinese cinnamon and cinnamon bark essential oils (Ghabraie et al. 2016). An integrated approach where essential oils are combined with other compounds such as antibiotics, organic acids, and plant extracts or processing methods (vacuum-packaging, irradiation, etc.) is also suggested for increasing the shelf life of sea-food and meat products (Pateiro et al. 2018; Huang et al. 2021). However, the application of essential oils in food products in various countries is permitted within specific amounts depending on the country and the food product, while the synergistic or antagonistic effects of essential oils with other compounds of the food product should also be considered (Pateiro et al. 2018) (Table 23.4).

Table 23.4 Examples of incorporation of essential oils in food products
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Apart from the use of essential oils per se, the use of pure compounds also shows promising results in the food industry, mostly as flavoring and antimicrobial agents (Burt 2004). For example, (R)-limonene is a monoterpene with several applications in the food industry due to its antimicrobial properties (Ciriminna et al. 2014; Khelissa et al. 2021), while other terpenes include eugenol and isoeugenol (Zhang et al. 2017a), (+)-limonene (Hąc-Wydro et al. 2017; Zahi et al. 2017), thymol (Moon and Rhee 2016), 1–8 cineole (Simsek and Duman 2017), geraniol (Tomadoni et al. 2015; Yegin et al. 2015; Syed and Sarkar 2018), carvacrol (Ultee et al. 1998; Martínez-Hernández et al. 2017), citral (Gao et al. 2020), linalool (Liu et al. 2020), α-terpineol (Cosentino et al. 1999), and several other compounds (Table 23.5).

Table 23.5 Examples of incorporation of terpenes in food products
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3.3 Alkaloids

Alkaloids are naturally found in various food products, the most common are the vegetables of the Solanaceae family, e.g. tomato, potato, pepper, etc., although their presence is usually associated with toxic and anti-nutritional effects. The use of alkaloids in the food industry is not widely applied compared to the pharmaceuticals sector where these compounds usually find applications due to their important medicinal properties. One common alkaloid which is widely used in energy and soft drinks due to its stimulating effects is caffeine, while quinine is also used in tonics as a flavoring agent (Kurek 2012). Coffee tree species (e.g. Coffea arabica and C. canephora) are a rich source of caffeine which can be detected in various plants tissues and coffee beans in particular. Coffee processing generates greats amounts of by-products which can be valorized for caffeine extractions and further application in the food industry via microencapsulation or addition in beverages, yogurt, bread, and other food products (Martinez-Saez et al. 2014; Aguiar et al. 2016; Bertolino et al. 2019; Guglielmetti et al. 2019). The recent review of Klingel et al. (2020) proposes several uses of coffee tree plant parts and coffee processing by-products in the EU food industry for the production of novel foods, although the authors suggest that the approval from authorities is needed before the legal marketing of such products. Other uses include the addition of alkaloid-rich plant tissues in various teas, infusions, and beverages, e.g. lotus seeds and leaves (Ding et al. 2017; Cheng et al. 2021). Mulberry leaves are a good example of an alkaloid-rich natural matrix which has been suggested for various food products, e.g. beverages, yogurts, teas, seasonings, and healthy foods (Ma et al. 2022). Considering the important pharmacological properties of these compounds, further research is needed aiming to design new functional foods and further valorize these valuable compounds. However, extensive trials are needed to eliminate toxicity effects and to establish the risk-free dosages for applications as food additives.

3.4 Glucosinolates

Glucosinolates are also a group of compounds with well evidenced positive health effects that are widely distributed in various plant species that are commonly consumed. The Brassicaceae family is the most common source of glucosinolates and several studies suggest the use of Brassica species crops by-products in food products in various forms. For example, Dominguez-Perles et al. (Dominguez-Perles et al. 2011) suggested the application of minimally processed broccoli by-products in organic green tea for the production of a novel functional beverage, while other authors proposed the use of by-products from the same species in gluten-free sponge cakes (Drabińska et al. 2018), in fortified tortilla chips (Vazquez-Duran et al. 2014), bread (Zambelli et al. 2017), etc. Apart from by-products, there is also great potential in using the raw materials per se for the design of novel functional foods and according to Oliviero et al. (Oliviero et al. 2013) three different drying approaches of broccoli were suggested aiming at final products with diverse composition in glucosinolates and by-products. Moreover, there is high interest in using extracts or essential oils obtained from Brassica species as antimicrobial and food preservatives agents in various food products or in film packaging (Pereira et al. 2015; Salehi et al. 2021), while the hydrolysis of glucosinolates produces compounds with prominent antimicrobial activities (Sanchez Maldonado et al. 2015). Selected examples of applications of Brassica by-products in the food industry are listed in Table 23.6.

Table 23.6 Examples of incorporation of glucosinolate-rich by-products or pure compounds in food products
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Moringa oleifera is another interesting species with great potential in designing new food formulations, since many of its health beneficial effects are associated to the presence of glucosinolates (Giuberti et al. 2021). There are already commercially available Moringa-based products in various categories, e.g. protein shakes, porridge, pasta, bread, juice, and chocolate (Teclegeorgish et al. 2021). Another example is the incorporation of powders of Moringa oleifera leaves in a maize grain non-alcoholic beverage called mahewu (Olusanya et al. 2020) or in snacks (Zungu et al. 2020).

4 Conclusions and Future Remarks

The use of plant-based extracts rich in secondary metabolites is a very promising and challenging task to be addressed by the food industry. Despite the successful paradigms of industrial applications of such extracts, further research and efforts are needed to overcome the hindrances related to: (a) bioavailability issues, since secondary metabolites such as polyphenols may interact with other components in food products (e.g., carbohydrates, proteins, lipids, etc.) and alter the overall nutritional value of the final product, (b) biostability issues, since the increased content of plant-based extracts in targeted secondary metabolites does not ensure the stability of these compounds in the final product, (c) the standardization of extraction protocols due to the complexity in the composition of natural matrices and the use of “green” and non-toxic solvents, (d) the effectiveness of extractability due to fluctuations in the content of secondary metabolites in plant matrices due to crop production, post-harvesting, and processing effects, (e) toxicity issues, since plant matrices may contain toxic substances therefore attention is needed prior to integration in industrial scale, (f) the cost-effectiveness of the process which should be addressed to make the final products affordable, (g) the effects of the addition of extracts on the sensorial quality of the final product, (h) the use of raw material instead of processed and purified ones. Addressing these issues would broaden the use of secondary metabolites in the food industry making the design of new functional products easier and cost effective, while increasing the added value of crops and minimizing the environmental burden. Moreover, the affordability of these newly developed products is of high importance to increase the market penetration and the adoption of consumers with direct impacts on the whole food value chain and the well-being of the general public.