Comparison of nutrient concentrations between pink and red tomato fruit
Nutrients were the basis of the flavor and quality of tomato, whose concentrations were shown in Table 1 and Additional file 1. In pink tomato, the soluble solids, fructose and glucose concentrations were more abundant than in red tomato. Pink cherry tomato had more malic acid (0.19%) than red cherry tomato. Citric acid (0.285%) and sugar/acid ratio (4.69) were richer in pink large-fruited tomato than in red large-fruited tomato.
On the other hand, red tomato had more lycopene and ascorbic acid. In cherry tomato, the concentrations of ascorbic acid, soluble solids, fructose, glucose, and citric acid were significantly more than that in large-fruited tomato.
Comparison of volatiles levels between red and pink tomato fruit
A total of 60 volatiles were detected in this study, including 17 aldehydes, 14 alcohols, 11 ketones, 11 esters, 3 phenols, and 4 other volatiles (Table 2, Additional file 1 and Additional file 2).
The average volatiles components were 41, 39.55, 39.8, and 39.3 in pink cherry, pink large-fruited, red cherry, and red large-fruited tomato, respectively. Total concentrations of volatile were 2.51 × 10-5, 2.20 × 10-5, 2.04 × 10-5, and 1.62 × 10-5 kg L-1 in turn. 1-(2,6,6-Trimethyl-1-cyclohexen-1-yl)-2-buten-1-one was not detected in pink cherry tomato and red large-fruited tomato. 2-Methoxyphenol and 4-heptyl isobutyl phthalate were not detected in red cherry tomato. The levels of aldehydes, alcohols, ketones, esters, phenols, and other volatiles accounted for 30.12%, 28.20%, 18.95%, 16.85%, 2.77%, and 3.21%. C6 compounds were abundant. Hexenal, (E)-2-hexenal, 1-hexanol, (Z)-3-hexenol, and (E)-2-hexenol accounted for 31.65%. Among the decteded volatiles, the following volatiles have bigger odor activity values (OAVs); namely, (E)-2-hexenal (46.05), (E,E)-2,4-decadienal (44.88), and 6-methyl-5-heptene-2-one (33.93), hexanal (31.95), 2,6,6-timethyl-1-cyclohexene-1-carboxaldehyde (28.09), 2-isobutylthiazole (23.74), (E)-6,10-dimetyl-5,9-undecadien-2-one (19.94), (E)-2-octenal (19.23), 4-allyl-2-methoxyphenol (17.25), (Z)-3-hexenol (15.96), (Z)-2-heptenal (15.72), methyl salicylate (12.24).
Pink tomato had higher levels of total volatiles, aldehydes, ketones, esters, and phenols than red tomato. The lipids and carotenoids derivatives were significantly higher in pink tomato than in red tomato, such as hexanal, (E)-2-hexenal, (E)-2-octenal, (E)-2-nonenal, decanal, (E)-2-decenal, (E,E)-2,4-decadienal, 2,6,6-timethyl-1-cyclohexene-1-carboxaldehyde, 3-(4-methyl-3-pentenyl)-furan, 6-methyl-5-hepten-2-one, and 4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-buten-2-one. As the OAVs showing, the green and irritant odors in pink tomato were stronger than those in red tomato. In pink cherry tomato, the concentrations of ketones and aldehydes were significantly higher than other categories tomatoes, especially, 1-octen-3-one, (E)-6,10-dimetyl-5,9-undecadien-2-one (1.38 × 10-6 kg L-1), (E,Z)-6,10-dimethyl-3,5,9-undecatrien-2-one, 2,6,6-timethyl-1-cyclohexene-1-carboxaldehyde (2.21 × 10-7 L-1), (2E)-3-(3-pentyl-2-oxiranyl)acrylaldehyde (9.04 × 10-7 kg L-1), and 3,7,11-trimethyl-2,6,10-dodecanetrienal (1.74 × 10-7 kg L-1) were the highest. These volatiles mainly derived from fatty acids and carotenoids. The OAVs of volatiles with fatty odor were significantly more in pink cherry tomato. On the other hand, in pink large-fruited tomato, phenols (9.61 × 10-7 kg L-1) and other volatiles (8.66 × 10-7 kg L-1) levels were the highest, such as 4-allyl-2-methoxyphenol (5.43 × 10-7 kg L-1) and 2,4-bis(1,1-dimethylethyl)-phenol (1.68 × 10-7 kg L-1).
In red cherry tomato, alcohols concentrations were the highest, such as 3-methyl-1-butanol (3.64 × 10-7 kg L-1), 1-pentanol (1.45 × 10-7 kg L-1), hexanol (3.27 × 10-6 kg L-1), (Z)-3-hexenol (1.57 × 10-6 kg L-1), (E)-2-hexenol (3.41 × 10-7 kg L-1), (E)-2-octenen-1-ol (2.48 × 10-7 kg L-1). These compounds were mainly derived from lipids metabolism. The sweet odor was significantly stronger in red cherry tomato than in other categories of tomatoes. However, red large-fruited tomato had lower volatiles levels.
Flavor sensory evaluations of red and pink tomato
The scores of the sensory evaluation are shown in table 3 and Additional file 1. All of the sensory evaluation scores were higher in pink tomato than those in red tomato. Pink cherry tomato had significantly higher sweetness (6.31 points) and sweetness/acidity ratio than other categories of tomatoes. Besides, the sweetness, overall taste, tomato characteristic flavor, sweetness/acidity ratio, and were more 2.09, 1.52, 1.01, and 0.6 points higher in pink cherry tomato than red cherry tomato, respectively. Correspondingly, their scores were higher 0.45, 1.15, 0.5, and 0.12 points in pink large-fruited tomato than in red large-fruited tomato. Whereas the acidity score was more 0.39 point in red cherry tomato than in pink cherry tomato.
Comprehensive flavor evaluation values (Y) of various categories tomato are shown in table 3 and additional file 1, obtained using the membership function method. Three principal components were extracted from sensory evaluation scores and odors activity values, which could be explained 72.814% variance. According to the formula (5), the following formula could be used to calculate comprehensive flavor evaluation value of each tomato variety.
Y = (α1 × F1 + α2 × F2 + α3 × F3) / (α1 + α2 + α3) = (32.742 % F1 + 23.692% F2 + 16.38% F3) / 72.814%
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The values of comprehensive flavor evaluation were 0.49 (pink cherry tomato), 0.24 (red cherry tomato), 0.15 (pink large-fruited tomato), and -0.24 (red large-fruited tomato), respectively. Comprehensive flavor evaluation values can show that pink tomato have better flavor than red tomato, and cherry tomato are more delicious than large-fruited tomato.
Correlation analyses of key flavor factors in tomato fruit
In order to further explore the contribution of nutrients and volatiles to tomato flavor, the pearson correlations were studied between compound concentration and flavor intensity. For the sake of brevity, only the items are shown in Table 4 and Additional file 1 with a significant correlation or very significant correlation.
The concentrations of ascorbic acid, soluble solids, fructose, glucose, citric acid had a significantly positive correlation with the evaluation score of sweetness, sweetness/acidity ratio, tomato characteristic flavor, overall taste, comprehensive flavor evaluation. The sugar/acid ratio was a very significant positively correlation with the overall taste (0.307**). Many volatiles had a significantly positively correlations with comprehensive flavor evaluation, fatty, green, floral and fruity, vegetable-like, and irritant odors. According to metabolic precursors, the carotenoid-derived volatiles had a significantly positively correlation with the floral and fruity odor. There was a significantly positive correlation between the lipid-derived volatiles and green, floral and fruity odors. The branched amino acid derivatives were significantly positively correlated with the green and vegetable odors. The Phe-derived volatiles were significantly positively correlated with the irritant odor. In addition, the overall taste was significantly positively correlated with 4-allyl-2-methoxyphenol (0.393*), (E,E)-2,4-decadienal (0.302*). The tomato characteristic flavor had significantly positively correlated with 3,7-dimethyl-6-octen-1-ol (0.726*), (E,E)-2,4-decadienal (0.247*). Sweetness/acidity ratio had a significantly positive correlation with 2-phenylethanol (0.894**), (Z)-3-hexenol (0.310**), isopropyl palmitate (0.602*), (E)-2-hexenol (0.493*), 1-octen-3-one (0.427*), 2,4-decadien-1-ol (0.349*), methyl hexadecanoate (0.298*), 1-pentanol (0.251*), and 2-phenylethanol (0.235*). Sweetness was significantly positive correlated with (E,E)-2,4-decadienal (0.350**), 6-methyl-5-hepten-2-ol (0.388*), (2E)-3-(3-Pentyl-2-oxiranyl)acrylaldehyde (0.310*), methyl hexadecanoate (0.279*), (Z)-3-hexenol (0.260*), and 1-pentanol (0.245*). Acidity had a significantly positive correlation with 2-methoxyphenol (0.901*).