Taste analysis
The results of the principal component analysis for uradome tips, bamboo shoots collected from six locations in Yamagata Prefecture, and three concentrations of oxalic acid solution are shown in Fig. 2. The contribution rate of the first principal component was 96.79% and that of the second principal component was 2.34%, explaining approximately 99% of the data. The taste of the uradome tips was plotted in the direction where the first and second principal components were smaller than the sample of six bamboo shoot samples (Fig. 2). The taste of the three concentrations of oxalic acid solution became more positive in the direction of the first and second principal components as the concentration of the oxalic acid solution increased (Fig. 2). These results indicate that the uradome tips had a weaker oxalic acid taste than the six bamboo shoots from Yamagata Prefecture (Fig. 2).
Next, the results of the αASTREE analysis plus the sensor output direction of umami are shown in Fig. 3. The cumulative contribution of the first and second principal components was 93.11%, and more than 90% was explained by the first and second principal component. Umami is stronger in the opposite direction of the NMS sensor output; therefore, the stronger the direction in which the first principal component decreases, the stronger the umami taste (Fig. 3). Uradome tips is plotted in the opposite direction of the NMS output, indicating that it has a stronger umami taste than bamboo shoots (Fig. 3).
Figure 4 shows the results of measurement of bitterness (first taste), bitter taste (aftertaste), umami (first taste), and umami and richness (aftertaste) of the uradome tips based on the bamboo shoots from Yuza Town, Yamagata Prefecture, using the TS-5000Z sensor. The uradome tips had a bitterness (first taste) value of − 0.76, and a bitter taste (aftertaste) value of − 0.19. Both the first taste and aftertaste had smaller values and were, therefore, less bitter than bamboo shoots (Fig. 4). The umami (first taste) value was − 0.91, and the umami and richness (aftertaste) value was 0.46, indicating that the umami taste first perceived in the mouth was weaker than that of the bamboo shoots, but that a persistent umami taste remained after swallowing and was stronger than that of the bamboo shoots (Fig. 4).
The results of the two taste sensor analyses showed a similar trend. αASTREE analysis showed that the oxalic acid (related to egumi taste) of uradome tips was weaker than that of the Yamagata bamboo shoots, and TS-5000Z analysis showed that uradome tips were less bitter than the bamboo shoots in both the first and second tastes. Overall, this suggests that the egumi taste of the uradome tips was weaker than that of the bamboo shoots. As mentioned by Furusawa et al. [7], bamboo shoot growers are particularly interested in the degree of egumi taste. A weaker egumi taste in the uradome tips, as demonstrated in this study, would be a great advantage in considering their edibility.
In addition, analysis using the αASTREE sensor showed that the uradome tips had a stronger umami taste than the bamboo shoots from Yamagata Prefecture. On the other hand, TS-5000Z analysis showed that the during the first taste, umami was weaker than that of the bamboo shoots, while the aftertaste of umami was stronger than that of the bamboo shoots. These results suggest that the umami output data produced by αASTREE is likely to be similar to that of the aftertaste data from the TS-5000Z analysis. Umami and richness (aftertaste) indicate the persistent fullness presented by umami substances [13]. The persistent richness of the taste is also considered to be a major characteristic of the uradome tips, as is the weak egumi taste.
Free amino acid analysis
Figure 5 shows the analysis of 18 free amino acids in uradome tips and bamboo shoots collected from the same bamboo forest in Yamanobe Town. Cystine was not detected in any of the samples, but 17 other free amino acids were detected (Fig. 5). Five amino acids (tyrosine, proline, glutamic acid, aspartic acid, and tryptophan) were more abundant in the bamboo shoots than in the uradome tips, methionine abundance was almost the same in both the uradome tips and bamboo shoots, and the other 11 amino acids were more abundant in the uradome tips (Fig. 5).
Of the 18 free amino acids analyzed in this study, nine are essential amino acids that cannot be biosynthesized by the body (leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and histidine) [14]. The nutritional value of protein, which is composed of amino acids linked by peptide bonds, is evaluated by the composition of the constituent amino acids, especially the balance of essential amino acids [14], and a high content of essential amino acids can be an advantage in food sources. The results of this study showed that eight of the nine essential amino acids (excluding tryptophan) were found in the uradome tips and were in similar quantities as those in the bamboo shoots (Fig. 5). Therefore, the edibility potential of uradome tips is increased due to their higher essential amino acid content.
In addition, leucine was detected only in the uradome tips (Fig. 5). Leucine is a commonly known branched-chain amino acid; branched-chain amino acids are considered to have physiological functions in enhancing muscle protein synthesis [14]. Leucine, which is generally recognized as a functional and essential amino acid, can be obtained from uradome tips, and this is considered to be a favorable feature of uradome tips when compared to bamboo shoots.
Glutamate is the most important free amino acid, with respect to umami [8]. The glutamate content was higher in the bamboo shoots than in the uradome tips (Fig. 5). It is assumed that this result is related to the results of the TS-5000Z analysis, which showed that the umami (first taste) value of the uradome tips was smaller than that of bamboo shoots (− 0.91 with bamboo shoots at 0). On the other hand, some tripeptides that are composed of three amino acids have umami and richness characteristics, such as γ-glutamyl-valyl-glycine [15]. The amino acids that make up γ-glutamyl-valyl-glycine are glutamate, valine, and glycine. In this study, glutamate content was lower in the uradome tips, whereas valine and glycine were higher. Thus, it is possible that the umami and richness associated with the continued aftertaste of uradome tips were strong.
Furthermore, in this study, a higher amount of tyrosine was detected in both the bamboo shoots and the uradome tips compared to the other free amino acids. In grasses, there is an additional pathway for lignin biosynthesis via l-tyrosine [16]. In bamboo shoots, the activity of ammonia lyase, which catalyzes the deammonification reaction of l-thyronine to p-coumaric acid, is remarkably high from the tip to the base of the culm [16]. In addition, the pathway through l-tyrosine plays a very important role in moso bamboo, which is a unique pathway not found in other plants [17]. A study of internode growth and tyrosine distribution showed that the greatest amount of tyrosine was contained near the inflection point of the internode growth curve, and a large amount of tyrosine was pooled in the area where the internode was about to be extended by cell division and elongation [17]. The authors have previously reported on the cell wall composition components of the uradome portion and that of early elongating bamboo shoots [5]. In that report, Klason lignin wasn’t detected in the early elongated bamboo shoots, while its composition of uradome portion was 7–10%, suggesting that the uradome stage is the initial stage of lignin polymerization and deposition [5]. Therefore, it is possible that tyrosine is pooled more in bamboo shoots, where lignin polymerization is about to start, than in the tip of uradome, where lignin polymerization is already in progress.