The genus Vitis L. contains two subgenera, Euvitis Planch. (bunch grapes) and Muscadinia (muscadine grapes). The Muscadinia subgenera consists of just three species: V. rotundifolia, the common muscadine grape known throughout the southeastern United States, V. munsoniana, a semitropical variant of V. rotundifolia native to southern Florida, and V. popenoei, a tropical native to southern Mexico. The muscadine grape is the only commonly cultivated member of the Muscadinia subgenus. Muscadine grapes are grown throughout the southeastern United States where winter temperatures remain above –12 °C. Muscadine berries are used for both fresh consumption and for processing into juices and wines. Studies showing the favorable phytochemical profile of muscadine grapes, especially the presence of ellagic acid (Lee and Talcott, 2004), are driving renewed interest in processed muscadine products.
Use of muscadines as a wine and juice grape is hampered by the poor color stability of these products during storage. The primary pigments in grape berries are the anthocyanins, which impart brilliant red and purple colors to grape berries. The common anthocyanidins found in grapes in order of decreasing stability are malvidin, peonidin, pelargonidin, petunidin, cyanidin, and delphinidin (He et al., 2010). Blueness is enhanced with an increase of free hydroxyl groups, whereas redness intensifies with the raising of the methylation of the hydroxyl groups; thus, malvidin is the reddest individual anthocyanidin (He et al., 2010). The presence of adjacent hydroxyl groups of o-diphenols are more sensitive to oxidation, making delphinidin, cyanidin, and petunidin less stable (Hrazdina et al., 1970). In contrast, malvidin, peonidin, and pelargonidin have no ortho-positioned hydroxyl groups making them more resistant to oxidation.
In grapes, O-glycosylation occurs for anthocyanins and the sugar moiety is typically glucose. The dominant allele involved in the production of diglucosidic anthocyanins is not present in V. vinifera resulting in the sole production 3-O-monoglucosides (Jànvàry et al., 2009), whereas other grape species can form 3,5-O-diglucosidic anthocyanins. The 3,5-diglucosides may be more resistant to thermal degradation than the monoglucosides, but they have a decreased ability to form polymeric pigments, making them more prone to oxidation and browning (Flora, 1977; Lee and Talcott, 2004; Sims and Morris, 1986). Many anthocyanins have sugar residues, which are acylated with aromatic or aliphatic acids, which increases their chemical stability (He et al., 2010). Only 3,5-diglucosidic anthocyanins in non-acylated forms have been detected in muscadine cultivars (Goldy et al., 1987; Lamikanra, 1998; Lee and Talcott, 2004). However, a survey of native V. rotundifolia germplasm uncovered the presence of vines producing low levels of 3-O-monoglycosidoc anthocyanins (Goldy et al., 1989).
Muscadine juice color quality and stability are affected by the total amount of anthocyanins in the berry, the relative proportion of the individual anthocyanins, and the lack of intramolecular copigmentation (Flora, 1977, 1978; Sims and Bates, 1994;Talcott et al., 2003; Talcott and Lee, 2002). Ballinger et al. (1974) examined the anthocyanin profile of 39 V. rotundifolia clones and found that delphinidin was the predominant anthocyanin in over 90% of the samples. Despite the predominance of delphinidin, delphinidin content was not associated with visual color ratings of the wines. Wines with good red color were strongly associated with high contents of malvidin and to a lesser extent petunidin. Unfortunately, most muscadine cultivars examined had low levels of both malvidin and petunidin. Two notable exceptions were ‘Tarheel’ and ‘Noble’, which contained good amounts of malvidin and petunidin and produced wines of acceptable color. Since that early work, several authors have used HPLC to better examine the anthocyanin profile of muscadine grapes (Goldy et al., 1989; Huang et al., 2009; Lamikanra, 1988; Lee and Talcott, 2004; Sandhu and Gu, 2010). This work largely corroborated earlier work indicating that delphinidin was the predominant anthocyanin and malvidin was relatively uncommon in most clones.
Attractive color is a main sensory characteristic of fruit products. Muscadine berries typically occur as a very dark purple or black color or as an unpigmented or lightly blushed bronze (greenish yellow) color. Marketplaces typically sell both colors and many consumers have a preference for one over the other. Despite the predominance of the bronze and black colors, other colors are available ranging from lavender to purple and pink through red shades. Breeding cultivars with new skin colors may open up new markets for muscadines and is a priority in breeding programs (Conner, 2010). The most popular method of measuring the surface color of a fruit involves instruments that measure the surface reflectance and relates it to CIELAB coordinates of L*, a*, and b*. Total anthocyanin content and relative amounts of individual anthocyanins were found to be significantly correlated to CIELAB coordinates in V. vinifera berries (Liang et al., 2011).
Early work in the anthocyanin profile of muscadine grapes was limited by the resolution of techniques used, whereas most modern studies of the pigment profile of muscadine grapes have made use of a relatively narrow pool of germplasm consisting generally of clones that are predominantly black in color. In addition, there is no known work that measures the pigment profile of V. munsoniana or V. popenoei germplasm. The goal of this study was to examine an array of colored muscadine germplasm, including germplasm containing V. popenoei and V. munsoniana in their background, to determine the pigment profile of variously colored muscadine berries.
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