Pink Discoloration of Canned Pears: Role of Procyanidin Chemical Depolymerization and Procyanidin/Cell Wall Interactions

Abstract

After canning, pear pieces turn occasionally from whitish-beige to pink. Conditions were set up to obtain this discoloration systematically and investigate its mechanism. Canned pears showed a significantly lower L* coordinate compared with fresh pears, and the L* coordinate of canned pears decreased with decreasing pH. The values of the a* and b* coordinates increased significantly after processing, the increase being greater for the more acidic pH values, with corresponding redder colors. After canning, polyphenol concentrations decreased significantly, mainly due to loss of procyanidins. This supported the hypothesis of conversion of procyanidins to anthocyanin-like compounds. However, no soluble product was detected at 520 nm, the characteristic wavelength of anthocyanins. When purified procyanidins were treated at 95 °C at three different pH values (2.7, 3.3, and 4.0), procyanidin concentrations decreased after treatment, the more so as the pH was lower, and a pinkish color also appeared, attributed to tannin-anthocyanidin pigment. The pink color was bound to cell walls. Extraction of the neoformed pink entities was attempted by successive solvent extractions followed by cell wall degrading enzymes. The pink color persisted in the residues, and canned pears gave significantly higher amounts of residues after solvent and enzyme treatments than fresh pears. Procyanidins were the entities responsible for the appearance of pink discoloration. However, it seems that this pink discoloration also involved the formation of strong, probably covalent, bonds to the cell wall.

Effect of maturity on the phenolic compositions of pear juice and cell wall effects on procyanidins transfer

“…Procyanidins were the predominant phenolic class, with between 6 and 8 g/kg FW, higher than reported by Le Bourvellec et al (2013) in William pears (dessert pears) and by in Gieser Wildeman (cooking pears). Perry pear procyanidins had a high degree of polymerization (DPn ¼ 20 for De Cloche and DPn ¼ 33 for Plant de Blanc fruits), as reported by Guyot et al (2002).…”

“…Yields found here were relatively high than those found in other pear cultivars because cell walls were extracted from whole fruits but were comparable to those reported by Raffo, Ponce, Sozzi, Vicente, and Stortz (2011) for Bartlett pears. The AIS yields obtained by Le Bourvellec et al (2013), Martin-Cabrejas, Waldron, & Selvendran (1994), Jermyn and Isherwood (1956) from the flesh of William, Blanquille and Conference pears were close to 30 g/kg and were relatively higher than yields obtained by for Gieser Wildemann pears. The main sugars in the AIS of both pears cultivars at both maturity stages were glucose (192e284 mg/g), xylose (140e176 mg/g) and galacturonic acid (108e118 mg/g).…”

“…Perry pear procyanidins had a high degree of polymerization (DPn ¼ 20 for De Cloche and DPn ¼ 33 for Plant de Blanc fruits), as reported by Guyot et al (2002). The only flavan-3-ols monomer detected was (À)-epicatechin as observed by Le Bourvellec et al (2013). Hydroxycinammic acids were represented by 5-caffeolyquinic acid, which has been reported as the main hydroxycinammic acid in pears (Galvis S anchez, Gil-Izquierdo, & Gil, 2003;Le Bourvellec et al, 2013;Li et al, 2012;Yim & Nam, 2016).…”

“…The only flavan-3-ols monomer detected was (À)-epicatechin as observed by Le Bourvellec et al (2013). Hydroxycinammic acids were represented by 5-caffeolyquinic acid, which has been reported as the main hydroxycinammic acid in pears (Galvis S anchez, Gil-Izquierdo, & Gil, 2003;Le Bourvellec et al, 2013;Li et al, 2012;Yim & Nam, 2016). Flavonols, located in the peel, mainly quercetin and isorhamnetin glucosides, were present in low quantities <40 mg/kg FW for both cultivars.…”

“…phenolic acids (hydroxybenzoic acids and hydroxycinnamic acids), flavonoids (flavonols, flavones, flavanols, flavanones, isoflavones, proanthocyanidins), stilbenes, and lignans (Collin & Crouzet, 2011). Procyanidins are the major polyphenols class in pear (dessert and perry pear cultivars) (Guyot, Marnet, Le Bourvellec, & Drilleau, 2002;Le Bourvellec et al, 2013; where they contribute to astringency of perry. In perry processing, the use of fruit at the overripe stage is the normal practice to decrease the astringency and to increase colloidal stability during storage.…”