Between John 6:53 and 54, there is a shift in the verb used for “to eat”; up until v. 53, the Greek verb used in the discourse was εσθιω, and in John 6:54 onwards, the verb is τρωγω. As noted here, some Catholic apologists have incorrectly latched onto this as definitive evidence of Transubstantiation.
However, an important question has to be addressed—why the shift in verb?
As Udo Schnelle correctly notes, the entire Gospel of John presents a strongly anti-docetic Christology, and much of the language and themes contained therein was used by the author to off-set the Docetic Christology that was permeating certain segments of early Christianity (cf. 1 John 4:1-3). This shift in verb is also anti-docetic:
The drastic τρωγειν must be understood in the sense of “gnaw,” and it clearly has an antidocetic accent: it is not a symbolic “eating” of the bread of heaven or a spirit-filled “eating” of the Son of man that gives eternal life, but only the real eating and drinking of the flesh and blood of Jesus Christ in the Eucharist. Thus τρωγειν “offset[s] any Docetic tendencies to ‘spiritualize’ the concept’ (BAGD, 829), in that it unmistakably emphasizes the reality of incarnation and crucifixion that are present in the Eucharist. The expression καγω αναστησω αυτον τη εσχατη ημερα [and I will raise him/her up on the last day in v. 54b is like a refrain (cf. vv. 39, 40, 44): it is the work of the evangelist. Here also we may suspect an anti-docetic tendency: John, with a view to the raising of the dead at the last day protects the inaccessibility of salvation to human control from the Docetists, who believed exclusively in the fullness of salvation in the present (see also v. 57: ζησει εις τον αιωνα). (Udo Schnelle, Antidocetic Christology in the Gospel of John [trans. Linda M. Maloney; Minneapolis: Fortress Press, 1990], 204-5)
In a recent scholarly commentary on the Gospel of John, one scholar wrote the following on the theological importance of pericope:
Jesus’ flesh, both his life and death, is “true food” and his blood “true drink” in that it accomplishes the ends of food and drink: it gives life (6:54-55). Those who do not eat do not have life within them (en hautois, 6:53). This phrasing echoes earlier statements where Jesus claims that “as the Father has life in himself, so also he has granted the Son to have life in himself” (en heautō, 5;26). Here is a parallel between Jesus and believers: the living Father has life in himself, which he grants to the Son, who may in turn give life to believers (4:14; 7:37). Those who eat the bread of life have taken life into themselves, but they do not become the source of life for others. (Marianne Meye Thompson, John: A Commentary [New Testament Library; Louisville, Ky.:Westminster/John Knox Press, 2015], 155-56)
Here is the entry for τρωγω as it appears in BDAG (A Greek-English Lexicon of the New Testament and Other Early Christian Literature [3d ed.]):
7469 τρώγω
• τρώγω (Hom. et al.; TestJob 12:2; SIG 1171, 9; PGM 7, 177; Sb 5730, 5. Not found in LXX, EpArist, Philo or Joseph. B-D-F §101 s.v. ἐσθίειν; 169, 2; Rob. 351; JHaussleiter, Archiv für lat. Lexikographie 9, 1896, 300-302; GKilpatrick in: Studies and Documents 29, ’67, 153) to bite or chew food, eat (audibly), of animals (Hom. et al. ‘chew, nibble, munch’) B 10:3.—Of human beings (Hdt. et al. and so in Mod. Gk.) τὶ someth. (Hdt. 1, 71, 3 σῦκα; Aristoph., Equ. 1077) B 7:8. ὁ τρώγων μου τὸν ἄρτον as a symbol of close comradeship (Polyb. 31, 23, 9 δύο τρώγομεν ἀδελφοί) J 13:18 (s. Ps 40:10 ὁ ἐσθίων ἄρτους μου, which is the basis for this pass.). W. gen. (Athen. 8, 334b τῶν σύκων) Hs 5, 3, 7. Abs. B 10:2. W. πίνειν (Demosth. 19, 197; Plut., Mor. 613b; 716e) Mt 24:38. J uses it to offset any tendencies to ‘spiritualize’ the concept so that nothing physical remains in it, in what many hold to be the language of the Lord’s Supper ὁ τρώγων τοῦτον τὸν ἄρτον 6:58. ὁ τρώγων με vs. 57. ὁ τρώγων μου τὴν σάρκα (w. πίνων μου τὸ αἷμα) vss. 54, 56.—B. 327. DELG. M-M. TW.
