Graduate Thesis Or Dissertation
 

Variation of sugars and acids during ripening of pears and in the production and storage of pear concentrate

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  • Two methods of separating sugars and acids of Bartlett pears were compared to decide upon a working method for later studies involving identification and quantitation of sugars and acids during ripening of pears and in the production and storage of pear concentrate. The first method involved precipitation of acids with saturated lead acetate followed by several washings of the precipitate with 85% ethanol for complete removal of sugars. In the second method acids were bound to an anion exchange resin column (Dowex 1-X8), sugars washed off with deionized water, and the acids later eluted with strong formic acid. Gas Chromatographic and Mass Spectrometric (GC-MS) analyses of Trimethylsilyl (TMS) ethers of sugars separated by either method resulted in the identification of fructose, glucose, sorbitol, sucrose, mesoinositol and xylose. Inaddition, galactose, reported as a trace sugar in pears, was identified by gas chromatography, however, confirmation by mass spectrometry could not be obtained. Similar analyses of TMS derivatives of acids separated by lead salts precipitation indicated that a complete separation of sugars from acids was not achieved, resulting in co-chromatography of fructose with citric acid and presence of other unidentified peaks with characteristic ions of TMS sugars in their mass spectra. GC-MS analyses of TMS derivatives of acids separated by ion exchange, on the other hand, indicated complete separation of sugars from acids and the following acids were identified: glycolic, succinic, phosphoric, malic, citric, quinic, mucic, and chlorogenic. Notably, phosphoric acid, which has not been reported in pears, was separated by both lead salt precipitation and ion exchange and identified by GC-MS. However, some of the trace acids which have been reported in pear juice or pear must were not detected in Bartlett pears. The internal standard method was used for gas chromatographic quantitation of four major sugars (fructose, glucose, sorbitol, and sucrose) and four major acids (citric, malic, phosphoric, and quinic) of pears in the following studies. Bartlett pears attained their highest sugar content (13.5 g/100 g fruit) on the fourth day of ripening (66 [greater than or equal to] 2°F ) and at a penetrometer reading of 6 lbs. Sucrose and fructose rose by 0.75% and 1.3% respectively and then decreased, however, sucrose declined more rapidly. Sorbitol and glucose, on the other hand, both decreased steadily by about 0.5%. The total acidity rose by about 1.2 meq/100 g fruit until the fourth day and then followed a near linear decline. The decrease in malic and citric acids accounted for the decline in total acidity, phosphoric and quinic acids undergoing little change. Apart from the hydrolysis of a small amount of sucrose at the depectinization and concentration steps, little change was found to occur in the sugar and acid levels during the production of a concentrate from the juice of Bosc and Cornice pears. Accelerated storage (98°F) of pear concentrate for sixteen weeks resulted in about 6% loss of total sugars. About 2.5% of this loss was accounted for by the loss of sucrose. The remainder was due to the loss of fructose, glucose and, to a much lesser extent, of sorbitol. Fructose and glucose levels underwent erratic fluctuations during this study. This is thought to be due to hydrolysis of sucrose and Maillard browning reactions of reducing sugars occurring simultaneously.
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