Genotype, Environment and Management Practices on Red/ Dark-Colored Fruits Phenolic Composition and Its Impact on Sensory Attributes and Potential Health Benefits

1. Introduction

Phenolic compounds (phenolic acids, flavonoids, and stilbenes) are today among the most important classes of phytochemicals, since they are responsible for disease protection conferred from diets rich in these compounds [1]. Some fruits with high content of phenolic compounds, including flavonols, flavones, anthocyanins, and phenolic acids are grapes, sweet cherries, and blueberries. Polyphenolic compounds form complexes with salivary proteins, playing a role in the sensation of astringency, due to delubrication of oral surfaces. For astringency, the tannin molecular weight seems to be important for its perception and to the interactions with salivary proteins. Flavor and color are also important factors for the selection of fruit by consumers. Sweetness and bitterness are mutually suppressed in mixtures, but astringency and bitterness tend to be perceived as negative attributes. Polyphenols’ sensory properties are related to molecules specific structures, including pigments correlated to fruit color [2]. This richness in phenolic compounds is also directly related with the positive effects on human health. However, the phenolic composition of the red/dark‐colored fruits depends on cultivar, maturity, growing environment, cultural practices, postharvest conditions, and processing techniques [3].

2. Phenolic composition of red/dark‐colored fruits

2.2. Phenolic composition of sweet cherry

Cherries are an excellent source of antioxidants, particularly phenolics, such as flavonoids, flavan‐3‐ols, and flavonols in addition to non‐flavonoid compounds such as hydroxycinnamic and hydroxybenzoic acids, which are concentrated in the epicarp and mesocarp of the fruit [25, 26]. The most abundant phenolic compounds are anthocyanins such as cyanidin‐3‐O‐rutinoside, cyanidin‐3‐O‐glucoside, peonidin‐3‐O‐rutinoside and glucoside, as well as pelargonidin‐3‐O‐rutinoside are the most important anthocyanins in cherries [27]. The total anthocyanin content ranged from 6.21 to 94.20 mg cyanidin‐3‐O‐glucoside equivalents/100 g fresh weight in 24 sweet cherry cultivars grown on the mountain sides of the Etna volcano (Sicily, Italy) [28]. Other phenolics in cherries include neochlorogenic acid, p‐coumaroylquinic acid, and chlorogenic acid as the main hydroxycinnamic acids [26, 29, 30], the flavonol rutin and the flavan‐3‐ols (+)‐catechin and (‐)‐epicatechin (Figure 1) [26, 31]. The total phenol content ranged from 84.96 to 162.21 mg gallic acid equivalents/100 g fresh weight in 24 sweet cherry cultivars grown in Italy [28]. Moreover, several studies reported higher phenolic content [26, 32] and antioxidant activity [32] in ripe cherries than in partially ripe. However, other pre‐ and postharvest factors, such as rootstock, cultivar, climate, soil type, storage conditions, and processing can significantly alter the amounts of bioactive compounds. In fact, levels of chlorogenic acid, neochlorogenic acid, p‐coumaric acid, and quercetin‐3‐rutinoside were higher in fruits grown on Weiroot 13 and PiKu 1 rootstocks compared to MaxMa 14, Weiroot 158, F12/1 and Gisela 5 rootstocks [31]. According to Gonçalves et al. [26], the cherry cultivars have the same phenolic pattern, however, with large variation on content as presented in Table 1. The climatic conditions have great influence on phenolic levels. Indeed, Gonçalves et al. [26] stated that higher temperature and solar irradiation favored the biosynthesis of phenolic acids and decreased the content of anthocyanins. However, the phenolic content tends to reach highest levels in the late stage of final maturity as refereed by Stöhr et al. [33]. In recent research, the preharvest application of several products to improve cherry quality, such as the oxalic acid (2 mM), has been studied, which increased anthocyanins, flavonols, neochlorogenic, and chlorogenic acids [34]. All the phenolic compounds and the antioxidant activity increased in several sweet cherry cultivars during cold storage [26, 27, 32, 35]. Also, the level of phenolics in “Canada Giant” and “Ferrovia” cherries increased during 8 days of shelf life [36]. Nevertheless, Esti et al. [37] detected a total anthocyanin content decrease of 41–52% in two sweet cherry cultivars after 15 days at 1°C and 95% RH. The use of edible coatings has been used to extend the postharvest storage of cherries. Petriccione et al. [38] specified that chitosan‐coated sweet cherries presented higher total phenolic, flavonoid, and anthocyanin levels. Moreover, increasing health‐promoting properties of cherry fruit can be achieved with the addition of methyl salicylate treatment to cherry trees. This compound also delays the fruit postharvest senescence process by increasing the activity of the enzymes involved in ROS scavenging [39].

Cultivar	Hydroxycinnamic acids			Flavan‐3‐ols		Flavonols	Anthocyannis
	NcAc	pCqAC	CAc	Cat	Epi	Rut	cy‐3‐ glu	cy‐3‐ rut	pn‐3‐ glu	plg‐3‐ rut	pn‐3‐ rut
Burlat	23.8	24.7	3.8	7.2	6.7	4.8	23.2	44.6	<1.0	<1.0	2.1
Saco	153.5	12.2	9.8	10.5	10.3	11.8	5.1	38.6	n.d.	<1.0	<1.0
Summit	34.4	27.5	7.2	5.8	8.2	3.1	2.4	26.0	<1.0	<1.0	<1.0
Van	65.6	5.6	4.8	3.5	4.5	4.0	3.4	28.2	<1.0	<1.0	1.5

Table 1.

Content of several phenolic compounds in four sweet cherry cultivars (mg /100g fresh weight).

NcAc, neochlorogenic acid; pCqAC, p‐coumaroylquinic acid; CAc, chlorogenic acid; Cat, catechin; Epi, epicatechin; Rut, Rutin; cy‐3‐glu, cyanidin‐3‐O‐glucoside; cy‐3‐rut, cyanidin‐3‐O‐rutinoside; pn‐3‐glu, peonidin‐3‐O‐glucoside; plg‐3‐rut, pelargonidin‐3‐O‐rutinoside; pn‐3‐rut, peonidin‐3‐O‐rutinoside; n.d., not detected.

Adapted from Gonçalves et al. [26].

Almost all phenolic compounds in sweet cherry show strong antioxidant activity [35, 40, 41]. Adequate consumption of phenolic compounds may offer health benefits that include inhibition of tumor cells growth [41], inhibition of inflammation [42], and protection against neurodegenerative diseases [43]. According to Matias et al. [44], a phenolic‐rich extract derived from sweet cherries could be an attractive candidate to formulate an agent for the prevention of oxidative stress‐induced disorders such as intestinal inflammation disorders. In spite of the large variations in the phenolic compounds content observed among several cherry cultivars, the levels of health‐promoting compounds are relevant to human health. Sweet cherries might therefore be considered as a functional food [41]. In fact, cyanidin‐3‐O‐rutinoside can slow down the absorption of carbohydrates by the inhibition of α‐glucosidase which may therefore be useful as inhibitor to prevent or treat diabetes mellitus [45]. Cyanidin‐3‐O‐glucoside showed cardioprotective effects by reducing blood lipid levels in rats [46]. The oxygen radical absorbance capacity (ORAC) assay indicated that the fruit of all genotypes possessed considerable antioxidant activity [28]. Moreover, several cherry cultivars were effective in inhibiting human cancer cells derived from colon (HT29) and stomach (MKN45) [41]. Finally, cherry phenolic, mainly anthocyanins, also protects neuronal PC 12 cells from cell‐damaging oxidative stress (antineurodegenerative activity). However, this protection is dose‐dependent [43].

3. Impact of fruit phenolic compounds on sensorial characteristics

Regarding fruit's oral sensory characteristics, there are six oral sensory attributes of fruit: sourness, sweetness, bitterness, spiciness, aroma, and astringency. For many people, the oral sensory properties of fruit have a great impact on their choice, acceptability, and consumption. Phenolic compounds, apart from possessing valuable biological properties, impart a high sensory activity to foods [61]. They are closely associated with the sensory and nutritional quality of fresh and processed plant foods and may affect positively or negatively the sensory characteristics of food with impacts on color, flavor, and astringency. This impact becomes important for consumer's acceptance, so that health‐promoting products can be palatable and largely consumed [2]. Fruit preservation also influences the quantity and quality of fruits’ phenolic content. For instances, during thawing of fruits, oxidation of phenolic compounds takes place and is negatively correlated with the acceptance level of fruits [62]. However, in a study comparing different pretreating processes of strawberries, samples with the highest phenolic content were also the most pleasant ones [63]. Specific structures are described to be related to polyphenols’ sensory properties, namely color perception. Color, in fruits, is derived from natural pigments that change through plant ripening. Chlorophylls (green), carotenoids (yellow, orange, and red), anthocyanins (red and blue), flavonoids (yellow), and betalains (red) are the primary pigments responsible for fruit color [64]. Also, water‐soluble brown‐, gray‐, and black‐colored pigments may occur due to enzymatic and non‐enzymatic browning reactions [65]. Many polyphenol pigments in plants are reactive anthocyanins, yellow flavanols, and flavones [66]. Anthocyanins can be used in food industry to color food. The six anthocyanins that can be found in the following red/dark‐colored fruits are cyanidin (cherries, blackcurrants, raspberries, and elderberries), delphinidin (blackcurrants and blueberries), malvidin (grapes), pelargonidin (strawberries and radishes), peonidin (cranberries), and petunidin (blueberries)—Figure 2. Due to their water solubility, anthocyanins are applicable for dyeing low pH systems. Increasing pH leads to a lesser color intensity and a bluer tone appears at pH higher than 4.5, giving its bluish color to blackcurrant. Proanthocyanidins react with anthocyanins to form new red pigments [68]. Loss or stabilization of color and increases in the range of available hues are resulted by the conversion of anthocyanins to other compounds during food processing [2]. The color of fruits is a sensory attribute that can really change consumers’ fruit acceptance. It is considered the most important product‐intrinsic sensory cue leading the sensory expectations that the consumer holds concerning the foods that they may consume [69] and, according to Piqueras‐Fizman et al. [70], humans’ experience of taste/flavor is determined by the expectations that they often generate prior to tasting. Consumers inspect fruits, visually, before deciding on whether or not to buy them. People associate certain colors with certain flavors. For instances, red/dark fruit coloring also appears to be a particularly good inducer of sweetness [71].

Gavrilova et al. [72] studied the phenolic profile of four blueberry varieties (V. corymbosumL., cv. Toro, Legacy, Duke, and Bluecrop) and two varieties (Rosenthal and Rovada) of red currants (Ribes rubrumL.) and black currants (R. nigrumL.) cultivated in Macedonia. They found that anthocyanins comprised the highest content of total phenolic compounds in currants (>85%), namely in the dark (black) currents, and lower and variety dependent in blueberries (35–74%). Hydroxycinnamic acid derivatives comprised 23–56% of total phenolics in blueberries and 1–6% in currants (Table 2). Besides bitterness, astringency, and color, some volatile polyphenols are strong odorants [66]. However, in dark‐colored fruits, phenolic compounds present an almost insignificant role in fruit flavor profile. In raspberry fruit (Rubus idaeusL.), phenolic compounds only represent 1% of the total flavor compounds (Figure 3), whose concentration varies between “trace amount” and 0.3 mg/kg [73]. Nevertheless, in wild berries, several volatile phenolic compounds were identified by Honkanen et al. [73], such as 2‐methoxy‐4‐vinylphenol, 2‐methoxy‐5‐vinylphenol, 3,4‐dimethoxybenzaldehyde, and 4‐vinylsyringol, none of which have been reported in cultivated varieties [74]. An important fact stated by Honkanen et al. [73] is that with the exception of ionones, the amounts of individual volatile compounds in wild raspberries were generally three to four times higher than in the cultivated varieties. Moreover, the higher amounts of volatile compounds, in wild raspberry, may have contributed to their characteristic aroma. Also, the increased berry size, hybridization, and/or fertilization lead to worsening in the aroma profile of cultivated raspberries.

Plant‐based phenol compounds, flavonoids, isoflavones, terpenes, and glucosinolates are almost bitter and astringent [75]. These substances provide defense against predators by making the plants unpalatable [75]. But also humans reject foods that are perceived to be excessively bitter [76]. Flavonoid phenols have been indicated as the main responsible for the taste of bitterness and the mouth‐fell sensation of astringency in several types of fruits and in beverages [2, 77]. Several works suggested that some polyphenols can be responsible for the bitterness of fruits even if they are present in very low concentrations [78]. The bitterness and astringency of red wines and red/dark‐colored fruits are mainly given by the flavanols. The mechanisms through which bitter taste perception occurs are not well understood; however, it is known that these mechanisms involve the activation of distinct human bitter taste receptors [77, 78]. While lower‐molecular‐weight phenolic compounds tend to be likely bitter, higher‐molecular‐weight polymers are perceived as astringent. Astringency or drying/puckering mouth‐feel detectable throughout the oral cavity is due to a complex reaction between polyphenols and proteins of the mouth and saliva [79]. Interaction between tannins and saliva proteins plays an important role in astringency perception in wine [80]; however, the physiological and physicochemical mechanisms for this phenomenon are not fully understood and more studies focusing on this subject must be done in wines and fruits.

Total concentration, mean degree of polymerization [81], subunit composition, and distribution [82] are some of the variables related to tannins, highly correlated with the perception of astringency in fruits. Tannins vary in size, from dimers up to oligomers, with more than 30 subunits [83]. Polymer size affects astringency correlating positively with the perception of astringency [84]. Increased galloylation can be responsible for increased “abrasiveness” while trihydroxylation of the B‐ring can decrease it [85]. As referred by He et al. [86], the synthesis of astringent substances controlled by a variety of structural and regulatory genes must be studied. Moreover, these authors state that “(…) cloning and functional identification of genes, in the astringency metabolic pathway, and their spatio‐temporal expression patterns as well as tannin biosynthesis‐related transcription factor genes must be considered in future work to finally make it possible to control fruit astringent substances quantitatively (…)”[86].

4. Effect of microorganisms on fruit phenolic compounds

After the consumption of fruits, the colon is the main site of microbial fermentation, where high molecular weight phenolic compounds are transformed into low molecular weight phenolic compounds such as phenolic acids or lactone structures by intestinal microbiota. The human healthy adult gut microbiota already identified can be classified into three dominant phyla: Bacteroidetes, Firmicutesand Actinobacteria. This highly complex and diverse bacterial ecosystem is mainly composed by a dominant group(> 109 Colony Forming Units (CFU)/g) of anaerobic bacteria, including genera Bacteroides, Eubacterium, Bifidobacterium, Peptostreptococcus, Ruminococcus, Clostridium and Propionibacterium, and sub-dominant groups (< 109CFU/g), of bacteria of the Enterobacteriaceae family, especially E. coli, and the genera Streptococcus, Enterococcus, Lactobacillus, Fusobacterium, Desulfovibrioand Methanobrevibacter[89]. Thus, the microbial metabolism (Table 3) of most of the phenolic classes such as flavonoids, isoflavonoids, lignans, phenolic acids, and tannins may produce metabolites with biological activity, presenting increased antioxidant activity, with evidence on health benefits for consumers. As most dietary polyphenolic compounds occur in glycosylated form in plants [87], for acquiring bioactivity in human body after being absorbed at enterocytes, these compounds must suffer various intestinal transformations, including the activities of digestive and microbial enzymes [88]. After cleavage of sugar responsible for glycosylation, the final absorbed compounds enter the vein circulation toward liver (Figure 4). Other enzymatic transformations occur from the liver to other organs, including digestive tract or via blood being excreted by urine [88].

5. Final remark

Red/dark‐colored fruits are considered healthy and nutritious, the major potential health benefits being a reduced risk for cardiovascular and neurodegenerative diseases. Phytochemicals from red/dark‐colored fruits are also shown to prevent body weight gain, lower blood cholesterol, and reduce cancer risk. Nevertheless, further rigorous, prospective studies are needed in order to better understand the benefits included in red/dark‐colored fruits in our diet. There is also an emergent interest in the study of red/dark‐colored fruits astringency because of the healthy properties of astringent substances found in red/dark‐colored fruits including antibacterial, antiviral, anti‐inflammatory, antioxidant, anticarcinogenic, antiallergenic, hepatoprotective, and vasodilating. The role of phenolic compounds and their metabolites as prebiotics, contributing to beneficial gastrointestinal health effects by modulating gut microbial balance with the simultaneous inhibition of pathogens and stimulation of beneficial bacteria, should also be highlighted.