الفهرس 
amino nitrogen contentOnly 14 pages are availabe for public view
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Abstract
.. UMMARY AND CONCLU3 ION’:::
These studies were conducted to obtain more information
about the effect of storage temperature at J8°F
and 65°? for one month; on some physical and chemical
properties of potatoes, which may play a role in the
field of non~enzymatic browning of the dehydrated
product. An investigation was also carried out to
determine the effect of reconditioning of the cold
stored potatoes for 7 days at 80°F or 3 days at l00°F.
This work has delt also with the study of potato phosphorylase
as starch degrading enzyme both in vivo, as
affected by storage, and in vitro as affected by
reaction conditions. In addition, a great part of
these studies were devoted to investigate factors
affecting the activity of this enzyme and its possible
role in the accumulation of sugar:3 under cold storage
conditions. Ill ore over, a comparative study for the
dehydrated potatoes as affected by various storage
temperatures was carried out. The effects of drying
operations such as 2 minutes blanching in boiling
wa’ljer or 3 minutes blanching by steam, and 1 minute
of sulfiting by potassium rnetabisulfite; on the dried
product were also investigated. A new attempt for minimizing the accwnuluted sugdr’~ :n ,~ol::! stored potatoes
was followed by steeping the blanched slices in water for
6 or 18 hours; or in C. 3% yeast suspension for the same
~eriods. The obtained results could be summarized as
follows:
1. Storage of potatoes at 65°F resulted in an
increase in tuber weight losses, sprouting, total solids
content and amino nitrogen content, and a decrease in
glucose-l-phosphate and inorganic phosphate content.
An obvious reduction in starch content was also observed;
which may be explained by the enzymic hydrolysis that
took place during sprouting. It was also found that
storage at ~his temperature had no effect on pH value
or phosphorylase activity.
On the other hand, storage at _l8°F. eliminated
sprouting, but caus~d a high increase in glucose-1-
phosphate and sugars content, and a moderate increase
in pH value and phosphorylase activity. It was found
that starch content was also highly reduced ~y storage
at 38°F. This reduction may be a~tributed to the
action of phosphorylase which was proven to be hig.hly;
active at low temperatures, It may be postulated that
this enzyme catalyzes the first step in a cycle of suDs aquen·. r’’ationc; begining ”lith starch breakdown to
produce glucose-1-phosph:J.te and ending with the accumulation
of sugars in the cold stored potatoes, through
other enzymes. However, by reconditioning of these
potatoes; more than 50% of the decomposed starch was
re-formed again with simultaneous decrease in phosphorylase
activity, pH value, glucose- l-phosphate and sugars
content.
2. The potato phosphorylase enzyme was purified
about ll folds with respect to specifie activity of the
crude enzyme in the juice; by fractional precipitation
of protein with ammonium sulfate, adsorption of the
active enzyme by amylose and dialysis of the enzyme
preparation against potassium phosphate buffer pH 6·. ’8.
The recovery was more than 50%. No effect was observed
for sodium fluoride at conc,mtration8 from O.OOto o.oo.
millimole/ml reaction mixture, on the reaction rate of
the .Purified preparation, proving t·hat it wa8 free from
residual phosphatases to a considerable extent. The
enzyme reaction was linear with time up to 300 minutes.
rha reaction rate exhibited a linear response up to 7.0
and 0.8- 1.0 mg protein/ml reaction mixture for crude
and purified preparation re8pectively.Pur i.” ied pr”’para ti on exhibited a relat i VPly sharp
pH peak at 6.7. rhis result suggests that the reaction
of starch degradation by phosphorylase requires natural
pH values.
·rhe approximate Km values for starch and inorganic
phosphate were O.JJ mg starch/ml and 3-92 x lo-3 mole
inorganic phosphate/litre, calculated from the Lineweaver
and Burk PlC!lt. V m&X for these substrates WOJre 178 and
179 millimicromole glucose-1-phosphate/mg protein/minute
respectively.
rhe values for the temperature co-efficient (Qlo)
for the reaction rates, lie in the range 1.28- l.ll
between temperatures from 0°0. to ]0°0. It is also
evident that the enzyme was higbly effective in
catalyzing starch decomposition at very low temperatures
between 0°0. and 10°0. Therefore, it may have the
major responsibility of starch degradation tt’t low
storage tem.<Jeratures in vivo. Furthermore, the activity
of the enzyme at temperatures higher than 30°0. decreased
significantly as evidenced by Q10
which was smaller than
one and intensively decreased at higher temperatures
suggesting a rapid destruction of the enzyme.It couli b-’ ·_cow>l c1”1 l lhc!t t~l1Yer pot·itJ :J torag ·
at 36°F. led to dehydrated slic~s of a ver; dark and undesirable
color which I’’”J.chred ctbout five folds that occur~
in the corresponding slices obtained from potatoes stored
at 65°F. Reconditioning at b0°F resulted also in a
higher degree of the non-enzymatic browning. On the other
hand, d~hydrated slices obtained from tubers reconditioned
at l00°F had the lightest color among the previously
illentioned treatments.
4. Blanching in boiling water resulted in a high
precentage losses in amino nitrogen, sugars and starch
content than blanching by steam. Tlm.s, browning was
more observed in the dehydrated slices treated with the
later method than in th~ corrEEponding slices treated
with the former one.
5, Sulfiting treatment appeared to be beneficial
for improving color, only in potato olices containing
high levels of rcduci0e; c; otgars. This treatment decreased
also the percentage of loss in amino nitrogen
and redllcing sugars cont•;;nt of dehydrated slices.
6. The originated process which was called the
steeping treatments gave a marvellous improving effect on the color of dehydrated alices obtained frolli the cold
stored tubers, especia_:_~y, -:1it<1 the steeping treatment
in yeast sLtspension for 18 hours. This treatment,
generally, led to a very large decrease (:;:: & 99%) in the
amino nitrogen and reducing sugars content which was
exhausted by the yeast for its growth. It is well
established that these compounds essentially cause
non-enzymatic browning. Accordingly, the optical density
of dehydrated slices extract after dehydration without
slufiting treatment, was reduced frcm 1.090 in the
unsteeped slices to 0.192, C.094, 0.129 and 0.031 in
steeped slices in water for 6 or 18 hours, or in yeast
,sus_tJension for the same periods respectively.
7. J’hg rate of browning depends mainly on the level
of reducing sugars content and possibly amino nitrogen
content in the material immediately b~fore the dehydration
process. The probability for redur’:ng sugars to be the
a~tual limiting factor for browning may be reasonable,
since, the presence of large amounts of amino nitrogen
or non-reducing sugars did not result in a proporational
increase in browning. On the other hand, any change of
reducing sugars content was followed by a pro_tJortional c~ha~~e in th~ euler ~e~ree
ing at 1C0°~ or stcepinc 18 hour~ ~~ y-1a~t susp~~s~_on
are recommended.
de
8. The hydrated slices in general, absorbed the J(.•
highest amount of water within t.he first 15 minutes of
the reconstitution process. Also, the sulfited slices
had more ability to absorb water than unsulfi ted ones.
Moreover, s.lices from tubers stored at 38°F or steeped
slices had a better reconstitution ability than other
slices.