## 4 Discussion
It was confirmed that some ACE inhibitors were identified in both hydrolysates; however, some of them were observed either in the ex vivo or in the in vitro salmon protein hydrolysate [19]. To recapitulate, the comparison of in silico and in vitro results also differed in our experiment concerning soybean proteins as the material. Nevertheless, they still can be supportive when predicting the bitter-tasting properties of parent peptides based on the fragmentomic approach. Citing the words of [PERSON] et al. [64], \"the BIOPEP-UWM database is a useful data source that supports the analysis of connections between peptide molecular structures and their sensory properties, and additionally enables users to build a potential sensory profile of peptides\". The BIOPEP-UWM database was applied to predict the umami taste of peptides derived from clam _M. [PERSON]_[64]. [PERSON] et al. [45] applied the hybrid approach to assess the bitter taste properties of novel peptide ACE inhibitors derived from _U. lactuca_ (green algae). To this end, green algae proteins were in vitro (enzyme used: papain) and in silico (tool used: PeptideCutter) hydrolyzed. The bitterness of peptides that were identified in the hydrolysates of _U. lactuca_ was predicted based on the calculation of the Q-value introduced by [PERSON] [65]. The Q-value is empirically associated with the hydrophobicity of each amino acid in a peptide sequence. Such a correlation is defined as the Q-rule, according to which the peptide likely to be bitter should have Q over 1400 cal/mole [66]. Based on the Q-rule, it was found that the following green algae-originating sequences: SAGVLPWK (**GV**, **VL**), GAAPITPSPPATKPSTPPKPTPPT (**PK**, **KP**, **PPP**), IECCLLFLALV (**LL**), PVGCLPK (**PK**), DAVEIWRVK (**VE**, **EI**), DEVIPGAL (**VI**), **PKPALCN (**PK**, **KP**), and PPNPPNPPN, were characterized with Q-values ranging from 1440 to 1743.33 cal/mol, which suggested they could taste bitter [45]. Defining these sequences as \"parent peptides\" and implementing the fragmentomic idea, we found that seven out of eight such peptides contained motifs (in bold) described as bitter-tasting indicators (provided in brackets). Working the opposite way, we calculated Q-values (cal/mole) for all parent peptides that were experimentally identified in SPC hydrolysates. They were as follows: 1611.43 (**LSVISPK**), 1793.75 (**DVLVIPLG**), 1780.00 (**LIVILNG**), 1721.66 (**NPFLFG**), 1358.33 (**ISSTIV**), 1908.33 (**PQMIIV**), 2220.00 (**PFPSIL**), 1760.00 (**DDFFL**), and 1741.25 (**FFEITPEK**). Only one peptide (ISSTIV) had the Q-value below 1400 cal/mole. Based on the Q-value, the other peptides identified in SPC hydrolysates containing bitter-tasting motifs with and without the status of an indicator could be classified as bitter. However, some studies on the bitterness of peptides indicated that some of them with the Q-value lower than 1400 cal/mole also tasted bitter. An example of such a peptide is the EVLN sequence (Q-value = 1162.5 cal/mole) identified in cheddar cheese and matching \(\alpha_{\text{s1}}\)-casein as a protein precursor [67]. Moreover, taking into account our point of view, this peptide contains the **VL** bitter-tasting motif, which may be crucial in defining the taste of the whole sequence. Another aspect related to parent peptides likely to be bitter is their size [68]. According to the literature, peptide fractions of soybean protein hydrolysates with molecular weight ranging from 1.9 to 3.3 kDa were characterized by the highest bitterness, whereas the bitterness of other fractions with lower or higher molecular weight than indicated above was milder [68]. Our parent peptides differed in the molecular weight. Thus, the strategy for the elimination of the unwanted taste of a hydrolysate should also be based on the knowledge concerning the amino acid composition of peptides [49]. This point of view fits the fragmentomic idea of the research presented in this article.