Amino acid profile and Maillard compounds of sun-dried pears ...

642 Eur Food Res Technol (2011) 233:637–646Table 2 Protein amino acidscontent (mg 100 g -1 ) of 2009fresh and dried pear fruitsValues are mean ± standarderror (3 replicates)Protein amino acids content (mg 100 g -1 )Amino acid Fresh 09 Traditional 09 GH1 09 GH2 09 HAT 09Ala 15.21 ± 1.22 15.01 ± 0.62 18.94 ± 5.89 17.77 ± 2.55 11.10 ± 2.45Gly 9.11 ± 0.19 8.52 ± 0.06 10.24 ± 2.07 8.02 ± 2.71 8.47 ± 0.79Val 22.32 ± 2.24 21.58 ± 0.57 26.09 ± 0.34 23.30 ± 0.06 19.81 ± 3.05Thr 12.92 ± 1.17 10.50 ± 1.86 11.95 ± 0.61 9.78 ± 3.32 13.32 ± 0.61Leu 64.16 ± 11.64 58.52 ± 5.51 60.25 ± 12.11 66.22 ± 11.93 55.25 ± 4.44Ile 33.28 ± 10.71 29.82 ± 3.64 28.87 ± 0.69 38.28 ± 3.82 34.98 ± 4.74Pro 19.77 ± 6.82 9.22 ± 2.05 14.89 ± 3.12 4.76 ± 2.04 12.45 ± 0.65Asx 44.72 ± 12.23 41.48 ± 2.80 51.99 ± 8.96 49.99 ± 4.64 44.65 ± 2.32Phe 11.48 ± 2.68 9.01 ± 4.37 11.87 ± 0.11 12.45 ± 0.97 11.99 ± 1.02Glx 79.58 ± 13.87 93.48 ± 2.12 90.92 ± 12.39 83.84 ± 6.69 89.92 ± 8.16Lys 24.88 ± 1.13 19.34 ± 2.52 18.99 ± 6.59 23.53 ± 0.78 24.02 ± 3.25Total 337.43 ± 63.89 316.46 ± 13.47 345.98 ± 10.66 336.92 ± 25.21 325.97 ± 30.27amino acid in higher amount, followed by Glx. Consideringtheir relative molar percentages, Pro accounts for39–25% of the free amino acids, whereas Glx accounts for24–15%. Although no data are available for free aminoacids of dried pears, the results are comparable with thoseobtained for the dehydration of grapes where it wasobserved the decrease in the total free amino acid contentas well as a decrease in Asx [11]. These authors attributedthe decrease in the free amino acid content to the low wateractivity of samples, favouring the Maillard reaction atroom temperature. Also, the higher relative amount of freePro has been observed in pear juice [14]. In fact, theincrease in the synthesis of Pro has been reported as aresponse of the fruits to stress conditions [19, 26, 27],namely due to the modulation of expression of proline-richglycoproteins (extensins).Eleven amino acids were found in proteins from freshand traditional sun-dried pears (Table 2; Fig. 2b). In total,they account for 4.23 mg per g of dry weight of the pulp inthe samples from fresh fruits in 2008 season and3.37 mg g -1 in 2009. A value of 2.31 mg g -1 (dry weight)for fresh pear (Pyrus communis L.) is reported in literature[7], although from Beurré variety, which is lower than thevalues for protein found for S. Bartolomeu fruits from bothharvests. In traditional sun-dried fruits, the amount ofprotein amino acids was higher (18%) than in fresh in 2008(5.00 mg g -1 ) and lower (6%) in 2009 (3.17 mg g -1 ).For the fresh fruits, the most abundant protein aminoacids are Glx and Leu (20–19 mol%, each) and Asx(17–13 mol%) (Fig. 2b). Also in relevant amount occur Ile(10–7 mol%), Ala (8–7 mol%) and Val (7 mol%). Asx wasreported by Pilipenko et al. [7] as the predominant aminoacid in Pyrus communis L. fruits of Beurré variety(35 mol%), showing that S. Bartolomeu pears present lowerrelative amounts, although similar amounts are observedwhen expressed in mg g -1 . All amino acids found as constituentsof S. Bartolomeu pears have been also found inBeurré variety. The exception is Hyp, not found in amountsto be detected in S. Bartolomeu pears. Hyp is an amino acidcharacteristic of extensins and arabinogalactan-rich glycoproteins.Pyrus communis L. arabinogalactan-rich glycoproteinsfrom suspension cultures are composed mainly byGlx (16 mol%) [28]. In Chinese species, reporting eightpear varieties, Chen et al. [8] showed large variations in theamino acid profile for the different pear varieties. Anyway,Asn accounted for near 20% of total amino acids. In thesepears, Ser is the main constituent, although not always,which is not the case of Pyrus communis L., where no Serwas found.No significant changes were observed in the amino acidsprofile of proteins from traditional sun-dried pears whencompared to the fresh samples (Fig. 2b). However, inrelation to the same mass of protein, a decrease in thecontent of Pro (-16% in 2008 and -50% in 2009), Phe(-15 and -16%), Lys (-9 and -17%) and Thr (-10 and-13%) was observed in both harvests, contrasting with theincrease in Glx (?17 and ?25%). The higher differencewas Pro, in accordance with the higher increase in Pro foundas free amino acid. Also, the increase in the content in Glxin proteins is in accordance with the lower amounts found asfree amino acid. As no data are available concerning thecomposition of the amino acids of proteins of dried fruits,the origin of this phenomenon remains unknown, althoughit can be suggested that modulation in Pro-rich glycoproteinsynthesis and/or degradation seems to be involved. Concerningthe decrease in Phe, Lys and Thr, it is possible that itis related with the occurrence of Maillard reactions. Thishypothesis will be verified taking into account the analysisof other independent samples, namely the pears driedaccording to different methodologies, which present123