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

More documents

Recommendations

Info

644 Eur Food Res Technol (2011) 233:637–646increase in the relative content of Val and Ala was alsoobserved in traditional, GH1 and GH2 processed fruits.Detection of carboxymethyllysine (CML),carboxyethyllysine (CEL) and furosine in pearsIn order to observe whether the browning of sun-driedpears also results from the contribution of Maillard reactionproducts, the analysis of carboxymethyllysine (CML),carboxyethyllysine (CEL) and furosine in dried pears wassearched and fresh pears were also analysed to be used ascontrol. CML quantification was performed together withCEL due to the fact that the applied methodology does notallow the separation between these two compounds.Although literature does not report the occurrence offurosine, CML and CEL in unprocessed materials usingthis methodology, it was noticed their occurrence in freshpears. In fact, furosine content detected for fresh pears was871 mg per 100 g of protein present in the sample and theamount of CEL ? CML detected was 275 mg 100 g -1protein. Some authors [29, 30] reported that this kind ofcompounds might partially be formed from degradationproducts derived from lipids or sugars during samplehydrolysis. As these fruits have not been processed, theoccurrence of these compounds may be resultant of anartefact of the methodology used, namely from the acidhydrolysis to which the samples were submitted. In order toavoid this fact, a preliminary protein extraction would berequired to remove all interferences in the analysis methodology.Nevertheless, the amount of furosine andCML ? CEL was higher in dried samples. Using freshpears as control of the methodology applied, it was possibleto determine the amount of these compounds producedafter the different drying technologies used (Fig. 4).For dried pears, furosine amount ranged between247 mg 100 g -1 protein for traditional and 80 mg 100 g -1protein for HAT. For GH1 and GH2, the furosine amountwas 182 and 136 mg 100 g -1 proteins, respectively. Theamount of furosine detected for dried pears was similar tothat described in literature for products to which moderateContent (mg / 100 g Protein)300250200150100500Fur CML+CEL Fur CML+CEL Fur CML+CEL Fur CML+CELTraditional GH1 GH2 HATFig. 4 Furosine (Fur) and carboxymethyllysine (CML) ? carboxyethyllysine(CEL) content (mg 100 g -1 protein) in dried pearsfrom 2009 harvestheat treatment is applied, such as UHT milk products [31,32] (35–269 mg 100 g -1 protein), jams and fruit-basedinfant foods [18] (44–448 mg 100 g -1 protein) and cookedbeef and salmon [29] (100–200 mg 100 g -1 protein). ForCML ? CEL quantification in dried pears, the amount ofthese compounds ranged between 96 mg 100 g -1 proteinfor traditional and 37 mg 100 g -1 protein for HAT. ForGH1 and GH2, the furosine amount was 94 and 71 mg100 g -1 proteins, respectively. The values determined forthe content of CML ? CEL in dried pears were, asexpected, higher compared to those described in literaturefor CML even in severely heat-treated foods, such assterilised evaporated milk [17] (43–52 mg 100 g -1 protein)and hypoallergenic powdered infant formulas [30](13–32 mg 100 g -1 protein). This fact may be due to CELinterference in CML quantification.In both analysis (furosine and CML ? CEL), there wasa tendency for the occurrence of higher levels of thesecompounds in traditional pears and lower contents in HATdried pears, while GH1 and GH2 dried pears presentedintermediate levels (Fig. 4), showing that the traditionalwas the most severe processing of all studied and the HATwas the less severe.Taking into account the amount of the decrease in thecontent of each amino acid with the different treatments(Table 2) and the amounts of Maillard reaction productsquantified (Fig. 4), it is possible to observe a positivecorrelation between the contents of these compounds andthe decrease in the relative amount of Lys and Phe. On thecontrary Val, Ala, Ile and Glx showed a negative correlation.The decrease in the relative content of Pro does notshow any relation with the occurrence of these compounds.The Lys degradation observed in Traditional and GH1pears is comparable to that occurring in condensed milk butis lower than that of chocolate, processed breakfast cerealsand bread [33] and fruits and vegetables in baby foods [34],which allows to include the pears in moderate thermallyprocessed foods if processed according to the traditionalprocess or with GH1. Using this parameter as marker of thethermal damage [11, 13, 35], it can be stated that the pearsproduced by HAT do not lose the nutritional value concerningthe stability of Lys. However, Maillard reactionproducts, when in a moderate amount as observed in traditionaland GH1 pears, may contribute to beneficial antioxidantproperties [36, 37].The differences observed between the colour of traditionalpears (reddish brown colour) and HAT dried pears(yellow-orange colour) are also in accordance with thehigher amount of Maillard reaction products in traditionalprocessed fruits. For GH1 and GH2 dried pears, the colourationwas similar to the traditional dried pears, althoughGH2 fruits presented a not so intense colour as traditionaland GH1 ones (Fig. 1). These results allow to infer that the123
Eur Food Res Technol (2011) 233:637–646 645drying process promotes the occurrence of Maillard reactionsin the S. Bartolomeu pears and that in the traditional,GH1, and GH2 drying processes, they contribute to thereddish brown colour of the dried pears. For HAT drying, itis possible that the heating of the fruits has not been highenough to promote the occurrence of Maillard reactions inthe most advanced steps, when the brown colouration isproduced.Influence of pH, temperature and moisture on driedpear colour formationAs the Maillard reaction is known to be favoured duringthe dehydration process systems [13] under conditions ofpH ranging 5–7, with intermediate moisture content andtemperatures over 50 °C using long processing times, a fewsets of simple experiments were performed on pear tissuesto relate the occurrence of Maillard compounds with thereddish brown colouration.In order to test the influence of pH on the developmentof the colour of the pears, pieces of freeze-dried pulp of afresh pear were immersed in buffer solutions in the rangeof the pH scale. It was observed that the tissues immersedin the solutions with higher pH became darker than theother ones. The major colour variations were observed inthe interval of pH 4–6, whereas at pH 5.0, the colour of thetissue approached the reddish brown colour characteristicof the traditional product (Fig. 5a).abclowmoisturepH 4 pH 5 pH 6sunlightdarkness30 °C 50 °ChighmoisturehighmoistureFig. 5 Effect of a pH variation, b sunlight exposure and c moistureand temperature in the development of the colour of pear tissuesThe freeze-dried pear tissues that have been immersed inthe buffer solution at pH 5.0 were placed (in a wet form) intwo plastic containers, one exposed to sunlight and theother one was placed in the dark. This experiment allowedto observe that the sample exposed to sunlight presented anincrease in the intensity of the colour when compared withthe sample placed in the dark (Fig. 5b). Because watercondensation was observed over the walls of the containerswhere the tissues were exposed to the light and not in thecontrol ones, it was hypothesised that the increase intemperature of the wet tissues could be an important factorfor pear tissues colouration. To test this hypothesis, piecesof freeze-dried pear tissues were immersed in the buffersolution at pH 5.0 and the wet material was cut in twohalves. One part was placed in a Petri dish that was closedto preserve the moisture and the other one was left in directcontact with the air. Both samples were placed in an ovenat 30 and 50 °C. It was observed that the tissues that weremaintained closed, keeping the moisture, presented moreintense colourations and the colour intensity was higher for50 °C than for 30 °C (Fig. 5c).These results allow concluding that the pH, the dryingtemperature and the tissues moisture are importantparameters for the development of the characteristic colourof the traditional product.ConclusionThe results obtained allowed concluding that drying ofS. Bartolomeu pears affects the free amino acid profile,increasing Pro content and decreasing Asx and Glx.Although the proteic amino acid profile was not significantlymodified with the drying process, a loss of therelative content of Lys is observed. This loss is related withthe increase in the amount of furosine, CML and CELformed during the drying process.Although all drying methods tested promoted theincrease in the amount of CML, CEL and furosine, thisincrease was higher in the case of the traditional sun-dryingand greenhouse processing, where the fruits presentedreddish brown colour and lower for the hot air tunnelprocessing, where the fruits became yellow-orange. Theseresults allow to conclude that the drying processing ofS. Bartolomeu pears promotes Maillard reactions, possiblycontributing to the characteristic reddish brown colour ofthis product. These reactions are formed under conditionsof pH, temperature and moisture that promote the developmentof this characteristic colour.Acknowledgments We thank FCT for financial support throughproject PTDC/AGR-ALI/74587/2006, Research Unit 62/94-QOPNA(PEst-C/QUI/UI0062/2011) and Cláudia Nunes Post-Doc grantSFRH/BPD/46584/2008.123
Page 3 and 4: Eur Food Res Technol (2011) 233:637
Page 6: 642 Eur Food Res Technol (2011) 233

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?