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. / Graduation date: 1978
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/27299 |
| Date | 20 September 1977 |
| Creators | Akhavan, Iradj |
| Contributors | Wrolstad, R. E. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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