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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. |