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This book continues as volume 7 of a multi-compendium on Edible Medicinal and Non-Medicinal Plants. It covers plants with edible flowers whose floral parts including the stalk and flower nectar are eaten as conventional or functional food, as spice, and may provide a source of food colourant, additive or nutraceuticals. According to Health Canada (2002), a functional food is similar in appearance to, or may be, a conventional food that is consumed as part of a usual diet and is demonstrated to have physiological benefits and/or reduce the risk of chronic disease beyond basic nutritional functions, i.e. they contain bioactive compound. A nutraceutical is a product isolated or purified from foods that is generally sold in medicinal forms not usually associated with foods and is demonstrated to have a physiological benefit or provide protection against chronic disease. Biologically active components in functional foods that may impart health benefits or desirable physiological effects include the following: carotenoids (β-carotene, lutein, lycopene), dietary fibres (β-glucans, soluble fibre), fatty acids (omega fatty acids, conjugated linoleic acid), flavonoids (anthocyanins, flavanols, flavanones, flavonols, proanthocyanidins), isothiocyanates, phenolic acids, plant sterols, polyols and prebiotics/probiotics (fructooligosaccharides—inulin), vitamins and phytoestrogens (isoflavones—diadzein, genistein). Many plants with edible flowers contain many of these bioactive components and essential mineral elements (Mlcek and Rop 2011; Rop et al. 2012), carbohydrates and amino acids in the flowers and other plant parts, imparting a wide array of health benefits and pharmacological properties. According to the Global Industry Analyst Inc., global nutraceuticals market is anticipated to exceed US 243 billion by 2015 (GIA 2012). The United States, Europe and Japan dominate the global market, accounting for a combined market share of more than 85 %. Spurred by the growing affluence, rising disposable income and increasing awareness, particularly in China and India, the Asia-Pacific region is projected to see significant growth in the long term. Functional foods that constitute the faster growing segment in the nutraceuticals market are rising in popularity, as the segment offers a cheaper alternative to dietary supplements. Value-added food products that feature edible flowers offer additional marketing opportunities.

This volume covers selected plant species with edible flowers from families Acanthaceae to Facaceae in a tabular form (Table 1) and 75 such species from the families Amaryllidaceae, Apocynaceae, Asclepiadaceae, Asparagaceae, Asteraceae, Balsaminaceae, Begoniaceae, Bignoniaceae, Brassicaceae, Cactaceae, Calophyllaceae, Caprifoliaceae, Caryophyllaceae, Combretaceae, Convolvulaceae, Costaceae, Doryanthaceae and Fabaceae in detail. Some plants with edible flowers, but are better known for their edible fruits, have been covered in earlier volumes and for other non-floral parts will be covered in subsequent volumes. Other plants with edible flowers from the family Geraniaceae to Zygophyllaceae will be covered in volume 8. The edible flower species dealt with in this volume include both lesser-known, wild and underutilized plants and also common and widely grown ornamentals.

Table 1 Plants with edible flowers in the families Acanthaceae to Fagaceae
Full size table

As in the preceding 6 volumes, topics covered include the following: taxonomy (botanical name and synonyms), common English and vernacular names, origin and distribution, agroecological requirements, edible plant part and uses, plant botany, nutritive and medicinal/pharmacological properties with up-to-date research findings and traditional medicinal uses, other nonedible uses and selected/cited references for further reading.

Use of Edible Flowers

Since antiquity right through the Middle Ages and the seventeenth century, flowers have been featured as an integral part of human nutrition in Europe—ancient Rome, medieval France, Victorian England, Middle East—and in Asia particularly in China, India, Thailand and Japan. Flowers have long been used as decorations in food prepared for the nobility. Today, consumption of edible flowers is increasing worldwide (Mlcek and Rop 2011; Rop et al. 2012). Edible flowers are becoming more popular as evidenced by the profusion of edible flower cookbooks, culinary magazine articles, scientific papers on edible flowers and television shows. Flowers are consumed in various forms, colours and flavours to enhance the nutritional and sensory qualities of foods. Its qualities, freshness and safety depend on the care taken in its harvesting and storage. Many of the lesser-known edible flowers are harvested in the wild from plants in the forest, wasteland, disturbed sites, near waterways and roadside often occurring as weeds (Limnocharis, milkweeds, beggarticks, dandelion, Acacia spp.). In contrast many of the commonly known edible flowers (e.g. roses, chrysanthemums, carnations, marigolds, daylilies, cornflower) are harvested from cultivated garden ornamentals or culinary herb garden (e.g. chives, Mentha spp. borage, rosemary, chamomile).

Edible flowers can be used raw or fresh as a garnish or as an integral part of a dish, such as a vegetable or fruit salad. Today, many restaurant chefs and innovative home cooks garnish their entrees with flower blossoms for a touch of elegance. Many flowers can be fried in light batter or cornmeal, e.g. squash, zucchini flowers or in fritters (e.g. Acacia blossoms). Some flowers can be steamed, boiled, grilled or used in soups and curries. Some flowers can be stuffed or used in stir-fry dishes. Edible flowers can be crystallized, candied; frozen in ice cubes and added to beverages; made into jellies and jams; used to make teas or wines; to flavour liquors, vinegar, oil, honey and scented sugars; added to punch, cocktail and other beverages; and minced and added to cheese spreads, herbal butters, pancakes, crepes and waffles. Many flowers can be used to make vinegars for cooking, marinades or dressings for salads.

Some important rules on the use of edible flowers:

  • Flowers have to be accurately identified before eating.

  • Do not eat flowers from florists, nurseries, garden centres, fruit orchards or flowers from plants found on the side of the road and in murky waterways because of possible contamination from pesticide sprays, vehicle carbon emissions and industrial and effluent run-off.

  • Harvest/pick flowers that are free from diseases, insects, insect damage and soil particles.

  • Pick young fresh flowers and buds on dry mornings, before the sun becomes too strong, to retain the bright colours and intense flavours.

  • Use flowers immediately for best results or refrigerate in a plastic bag for a few days. Dried, frozen or freeze-dried flowers are best used in infusions or cooked.

  • For medium and large flowers like hollyhocks, roses, lilies, calendula, chrysanthemum, lavender, rose, tulip, yucca, hibiscus, lavender, tulip and marigolds, use only the petals and discard stamens, pistil and calyx. The bitter ‘heel’ at the base of the petal should be removed.

  • Eat edible flowers in moderation.

  • People with hay fever, asthma or allergies should best avoid eating flowers since many allergies are due to sensitivity to pollen of specific plants.

Nutrients and Bioactive Phytochemicals in Flowers

Nutrients and phytochemicals contained in flowers are not markedly different from those found in other plant organs (leaves, stem fruit). Several thousands of compounds have been identified in flowers including nutrients (proteins, carbohydrates, lipids, fibre, minerals, fatty acids, vitamins and essential amino acids), flavonoids, carotenoids, anthocyanins and other phenolic compounds, waxes, resins in the floral parts (petals, sepals, pollens, etc.), in the floral nectar, as fragrance volatiles, and essential oil components monoterpenes, sesquiterpenes esters, alcohols (monoterpene and sesquiterpene alcohols), aldehydes, ketones, phenols, alkanes, esters, lactones, coumarins, ether, oxides, fatty acids, fatty acid derivatives, benzenoids, phenylpropanoids, isoprenoids and nitrogen- and sulphur-containing compounds (Mookherjee et al. 1990; Knudsen et al. 1993; Dobson et al. 1997; Kim et al. 2000; Falzari and Menary 2003; Kaisoon et al. 2011; Mlcek and Rop 2011; Rop et al. 2012; Diraz et al. 2012). The concentrations of these compounds vary throughout the development and maturation of the flower and also during storage after harvesting. Health benefits attributable to antioxidant capacity have been shown to be highly correlated with phenolic compounds (Kaisoon et al. 2011; Rop et al. 2012).

Flower Pollen

The composition of pollen changes from floral species to species, variation in absolute amounts of the different compounds can be very high. The major components of pollens are proteins and amino acid, lipids (fats, oils or their derivatives) (Manning 2001), and sugars (Crane 1990); the nutrient profile in dried bee-collected pollen and dried hand-collected pollen are as follows: water, 11 %, 10 %; crude protein, 21 %, 20 %; ash, 3 %, 4 %; crude fats, 5 %, 5 %; reducing sugars, 26 %, 3 %; nonreducing sugars, 3 %, 8 %; starch, 3 %, 8 %; and undetermined components, 29 %, 43 %, respectively. The minor components of bee-collected pollens are more diverse (Crane 1990): flavonoids at least 8; carotenoids (at least 11); vitamins C, E B complex (including, niacin, biotin, pantothenic acid, riboflavin (B2) and pyridoxine (B6)); minerals—macro-elements (K, Na, Ca, Mg, P, S) and micro-elements (Al, B, Cl, Cu, I, Fe, Mn, Ni, Si, Ti and Zn); all free amino acids; terpenes; nucleic acids DNA, RNA and others; enzymes >100; growth regulators auxins, brassins, gibberellins and kinins; and growth inhibitors. All amino acids essential to humans (phenylalanine, leucine, valine, isoleucine, arginine, histidine, lysine, methionine, threonine and tryptophan) can be found in pollen and most others as well, with proline being the most abundant. Most simple sugars in pollen comprise fructose, glucose and sucrose come from the nectar or honey of the field forager. The polysaccharides like callose, pectin, cellulose, lignin, sporopollenin and others are predominantly pollen components. Protein contents of above 40 % have been reported, but the typical range is 7.5–35 %: typical sugar content ranges from 15 to 50 %, and starch content is very high (up to 18 %) in some wind-pollinated grasses (Schmidt and Buchmann 1992). Low lipid levels (0.6–1.9 % dry mass) are found in bee-collected pollen of eucalypts (Bell et al. 1983; Manning and Harvey 2002), whereas an high level of 32 % dry mass is found for canola pollen by Evans et al. 1987. Pollen has been added to diets for domestic animals and laboratory insects resulting in improvements of health, growth and food conversion rates (Crane 1990; Schmidt and Buchmann 1992).

Floral Nectar

The nectar is a liquid with a sweet taste, comprising sugars, amino acids, nonprotein amino acids, proteins, minerals, lipids, organic acids, phenolic compounds, alkaloids, coumarins, saponins, terpenoids, etc. (Nicolson and Thornburg 2007). The major sugars in nectar are the disaccharide sucrose and the hexose monosaccharides glucose and fructose (Baker 1975; Baker and Baker 1983). Bernadello et al. (1999) found that the floral nectar of 29 species native to Argentinian Patagonia to be hexose dominant (72.41 %) or hexose rich (17.24 %); a few were sucrose dominant (10.34 %). Though a large majority of floral nectars is dominated by sucrose, glucose and fructose, the pentose sugar xylose is a major nectar sugar in Protea and Faurea, two related genera of the Proteaceae (Nicolson and van Wyk 1998). Other minor sugars present in trace amounts in nectar include monosaccharides (e.g. mannose, arabinose, xylose), disaccharides (maltose, melibiose) or, more rarely, oligosaccharides (raffinose, melezitose, stachyose) (Baker and Baker 1982a, 1983; Nicolson and Thornburg 2007). Sorbitol is also a frequent constituent of Mediterranean nectars (Petanidou 2005). Minerals have been found in floral nectar (Hiebert and Calder 1983; Heinrich 1989). Potassium was found to be the dominant ion with 35–74 % in nectars; the other cations can be listed up according to their decreasing amounts: Na, Ca, Mg, Al, Fe and Mn (Heinrich 1989). Although all ten essential amino acids are commonly present in floral nectars as free amino acids, some nonessential amino acids such as asparagine and glutamine can occur in much higher concentrations (Nicolson and Thornburg 2007). The presence of amino acids in floral nectars was first reported by Ziegler (1956), later by Lüttge (1961, 1962) and Baker and Baker (1973, 1977, 1986) In Erythrina species pollinated by passerine birds, the total amino acid concentrations are far higher than in hummingbird-pollinated species (Baker and Baker 1982b). Few of the nontoxic nonprotein amino acids, including β-alanine, ornithine, homoserine and γ-aminobutyric acid (GABA), are known to accumulate in nectar (Nicolson and Thornburg 2007). The existence of proteins in nectar has been reported long ago (Pryce-Jones 1944; Lüttge 1961). The first enzymatic activity to be identified in nectar was invertase, found in the floral nectar of Tilia sp. (Beutler 1935). Other proteins identified in various floral nectars included the following: trans-glucosidase in Robinia pseudoacacia (Zimmerman 1953); trans- fructosidase in Impatiens holstii (Zimmerman 1954); phosphatase (Cotti 1962); tyrosinase in Lathraea clandestina (Lüttge 1961); mannose-binding lectin and alliinase in Allium porrum (Peumans et al. 1997); and nectarin IV (Naqvi et al. 2005) and nectarin I, II, II and V in Nicotiana sp. (Carter and Thornburg 2000; 2004a, b).

The presence of lipids has been reported in numerous floral nectars (Vogel 1971; Baker and Baker 1975; Bernardello et al. 1999). Some major lipids found in floral nectars of Calceolaria species (Scrophulariaceae) and in the rhattanys (Krameria species, Zygophyllaceae) included β-acetoxy fatty acids of varying chain length between C16 and C20 (Vogel 1971; Seigler et al. 1978). Ascorbic acid (vitamin C) is well known as an antioxidant in floral nectar (Baker and Baker 1975). Phenolic substances are quite widespread in nectars (Baker and Baker 1982a; Gil et al. 1995; Ferreres et al. 1996). European Eucalyptus honeys were found to have the following flavonoids: myricetin, quercetin, tricetin, luteolin and kaempferol (Martos et al. 2000) and Robinia pseudoacacia flowers to have nectar flavonol rhamnosides as floral markers (Truchado et al. 2008). Alkaloids and allelochemicals have been detected in the nectar of a large number of plants (Hazslinsky 1956; Baker and Baker 1975; Galetto and Bernardello 1992; Detzel and Wink 1993; Adler and Wink 2001). Recently, Singaravelan et al. (2005) reported four secondary alkaloid compounds occurring naturally in floral nectar: nicotine, anabasine, caffeine and amygdalin in many plants including Nicotiana spp. and Tilia cordata (Singaravelan et al. 2006). While terpenoids do occur in plant nectars (Detzel and Wink 1993), most are produced by cells with specialized metabolic potential that are dispersed throughout the flower (Bergström et al. 1995; Dudareva et al. 1998; McTavish et al. 2000).