In vegetables, taste, more than price and appearance, has an important influence on consumer food choice and is a significant predictor of consumption (Glanz et al., 1998). Taste and aroma both involve chemical sensations and make up the primary components of flavor (Reineccius, 1994). In most fruits and vegetables, taste is usually associated with fewer compounds than the wide array of volatile organic compounds associated with aroma (Whitfield and Last, 1991). In vegetables, sourness is associated with organic acids, bitterness is often the result of phenolic compounds, saltiness is attributable to sodium or potassium, and sweetness is the result of sugars, including fructose, glucose, and sucrose (Sims and Golaszewski, 2003).
Sugars not only affect the flavor and acceptance of vegetables, but they also alter the perception of flavor sensations associated with other organic compounds (Auerswald et al., 1999). Increasing the amount of sugar in tomato (Solanum lycopersicum L.) while maintaining the level of organic acids increases the perceived sweetness and increases preference (Malundo et al., 1995). In broccoli (Brassica oleracea L.), increases in sugar concentration have been suggested as a way to mask the bitter taste of sulfur-containing glucosinolates and promote consumption (Schonhof et al., 2004). In winter squash (Cucurbita spp.), consumer preference depends on the ratio of sugar to starch; varieties having higher sugar-to-starch ratios are considered sweeter and are preferred (Merrow and Hopp, 1961). In general, if sweetness is perceived in a vegetable, even mildly, it may be enough to encourage consumption (Dinehart et al., 2006). Sensory panelists, who preferred dry bean (Phaseolus vulgaris L.) and edamame [Glycine max (L.) Merr.] varieties that were classified as sweet, used sweetness as a primary characteristic to differentiate among cultivars (Mkanda et al., 2007, Wszelaki et al., 2005).
Simple sugars are the primary natural sweetener in vegetables with the most widely distributed being the monosaccharides glucose and fructose and the disaccharide sucrose (Lopez-Hernandez et al., 1994). The perceived sweetness of a vegetable varies with many factors, including pH, solids content, molarity, the presence of other sweeteners, and the type of sweeteners (Hanover and White, 1993). At equal molarity, glucose is only 74% as sweet as sucrose to the human palate. In contrast, fructose is considered 17% sweeter than sucrose and is perceived by the human palate as the sweetest of all naturally occurring carbohydrates (Joesten et al., 2007).
Although all plant tissues contain simple sugars, developmental differences in tissues consumed as vegetables, e.g., immature ovaries, mature ovaries, inflorescence, roots, tubers, and leafy vegetables, affect sugar accumulations and ratios (Hounsome et al., 2008; Lee et al., 1970). Regardless of the tissue type, sugar concentrations are cultivar-dependent and highly variable (Nunes, 2008).
The stage of maturity of tissue can affect sugar accumulation and composition in vegetables. In bell pepper (Capsicum annuum L.), as fruit matures from green to red, total sugar increases from 2.4% to 4.2% of fresh weight. Glucose, fructose, and sucrose accumulate rapidly between the green and turning phase. As peppers mature to the red phase, glucose and fructose continue to accumulate and sucrose rapidly disappears (Luning et al., 1994). In contrast, a significant increase in the amount of sucrose during late-stage maturity has been reported in muskmelon (Cucumis melo L.) (Bianco and Pratt, 1977; Hughes and Yamaguchi, 1983; Lester and Dunlap, 1985; McCollum et al., 1988; Villanueva et al., 2004).
Snap beans, which are consumed as immature ovaries, are a significant source of soluble fiber, carotenoids, flavonoids, and vitamins (Anderson and Bridges, 1988; Bureau and Bushway, 1986; Favell, 1998; Granado et al., 1992; Hertog et al., 1992; Judith and Vollendorf, 1993; U.S. Department of Agriculture, Agricultural Research Services, 2010). Many important aromatic compounds that contribute to the unique flavor of snap beans have been identified, including 1-octen-3-ol, cis-3-hexenol, and trans-2-hexenal (de Lumen et al., 1978). Although sweetness is generally not considered a quality attribute in snap beans, it has been reported that sensory panelists prefer cultivars considered sweeter in both dry beans and edamame (Mkanda et al., 2007; Wszelaki et al., 2005). In snap beans, variation in total sugar ranges from 0.6% to 5.2% of fresh weight with individual ranges for glucose, fructose, and sucrose of 0.23% to 1.38%, 0.25% to 1.78%, and 0.1% to 0.78% of fresh weight, respectively (Lee et al., 1970; U.S. Department of Agriculture, Agricultural Research Services, 2010).
Prior studies characterizing the simple sugars of snap bean pods have generally focused on a single cultivar at a single developmental stage (Lee et al., 1970; Muir et al., 2009; Sánchez-Mata et al., 2002). In legume crops, including Phaseolus vulgaris, the process of simple sugar conversation and complex polysaccharide accumulation in the seed is well understood (Górecki et al., 2001). In contrast, the accumulation of simple sugars in immature ovaries of snap bean pods is not well understood. The objective of this study was to determine sugar content of snap and Roma market classes of green beans as well as a dry bean variety and to characterize the patterns of accumulation of fructose, glucose, and sucrose during bean pod development.
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