Effect of Mode of Drying on Microstructure of Potato

8/10/2019 Effect of Mode of Drying on Microstructure of Potato

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Effect of Mode of Drying on Microstructureof Potato

Piotr P. Lewicki* and Grzegorz Pawlak

Department of Food Engineering and Process Management,Warsaw Agricultural University (SGGW), Warszawa, Poland

Abstract: Potato v. Irga was subjected to blanching, and thereafter was dried byconvection, puff-drying, and freeze-drying. Microstructure of raw, blanched, anddried tissue was analyzed under the light microscope using computer image analy-sis. It was found that the tissue of the investigated variety is built up from cellsmuch smaller than those described in literature for other cultivars. Blanchingcaused starch gelatinization and increase of cross-sectional area of cells. Therewas no evidence of broken cell walls. Convective drying resulted in cell shrinkageand some breaking of cell walls. It was estimated that some 12% of cells lostintegrity during drying. Limited disintegration of the tissue caused by convectivedrying is attributed to small size of cells, large contact area, and high cohesiveforces between cells. Puff-drying damaged the tissue much more than the convec-tive drying. The most devastating to tissue microstructure was freeze-drying,probably freezing per se.

Keywords: Convective drying; Puff-drying; Freeze-drying; Cell size; Feretdiameter; Shape factor; Cell perimeter

INTRODUCTION

Potatoes are processed in many different ways, one of them being drying.Before drying, peeled and cut potatoes are blanched and the processparameters depend on the further processing. Blanching affects tissuestructure and composition, which influence the course of drying and

quality of the final product. Drying followed by blanching causes many

Drying Technology, 23: 847869, 2005Copyright Q 2005 Taylor & Francis, Inc.ISSN: 0737-3937 print/1532-2300 onlineDOI: 10.1081/DRT-200054233


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changes in potato structure, as well. Hence, the quality of the finalproduct depends on both blanching and drying.

Blanching causes chemical and structural changes in potato tissue.Diffusion of solubles to the blanching water occurs and potassium, mag-nesium, calcium, phosphorus, as well as glucose are lost during hot waterblanching.[1] Ascorbic acid loss and texture changes during blanching ofpotatoes were observed by Eshtiaghi and Knorr.[2] Glucose, fructose, cit-ric acid, potassium, calcium, and magnesium concentrations in potatotissue change during hot water blanching. Calcium was better retainedin the tissue than the other solutes. Moreover, ions were better retainedthan glucose, fructose, and citric acid.[3] Kaymak and Kincal[4] monitored

loss of reducing sugars, and losses of ascorbic acid were measured byGarrote et al.[5] Loss of ascorbic acid, thiamin, riboflavin, niacin, andamino acids occurs during blanching in hot water and the losses aresignificant.[6]

Chemical changes caused by blanching are accompanied by physicalmodifications of potato tissue. Starch gelation in hot water begins atabout 60C and the gelled starch in potato tissue is significant to textureof the material.[7] Preheating of potatoes at 50 to 80C has a firming effecton the cooked potato tissue.[8] During gelation, volume of starch granule

may increase 25 to 30-fold but during cooling the gel shrinks, becomesmore rigid, and may exude some moisture.[9] Swelling gelatinizing starchcan develop pressure, which can contribute to firming of potato tissue.Bartolome and Hoff[10] and Hoff[11] questioned the development ofswelling pressure, while Reeve[12,13] presented microscopic evidence forcreation of pressure during starch gelation.

All the above data show that heat treatment preceding drying causesmany changes in potato tissue. Its chemical composition changes due toleaching processes and starch gelatinization affects tissue mechanical

properties. Hence, material subjected to dehydration differs from theraw potato in many aspects.

Drying causes further physical and structural modifications of potatotissue. The most pronounced macroscopic modification is the shrinkageand deformation of food pieces undergoing drying. Shrinkage at earlystages of drying is approximately equal to the volume of evaporatedwater.[14,15] However, for potato it was shown that shrinkage from thevery beginning was smaller than the volume of evaporated water.[16]

Analysis of shrinkage of many food products shows that it increases lin-

early with lowering of water content in materials undergoing drying,[1420]

in some materials a deviation from linearity is observed at low watercontents [14,17,19,21]

848 Lewicki and Pawlak


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In effect, the shrinkage is restrained and shrinkage stresses aredeveloped.[23] The developed stresses cause food deformation and tissue

breakdown. In consequence, a pronounced alteration of tissue structureon microscopic level occurs.

Blanching changes chemical composition and physical properties ofpotato but has little influence on tissue structure. Sterling[24] showed thatneither hot water nor steam treatment of carrot, potato, and apple causedcell wall break down. The walls were intact, but some cell separation andpresence of cell interstices occurred. The cells of potato remained angularand well coherent when heated at 75 and 90C. Some weakening of mid-dle lamella was observed.[25] It was suggested by Reeve[25] that cell separ-

ation was aided by swelling of gelled starch, which distended cell wallsand pushed them apart. Increase in cell size caused by cooking wasobserved by Harada and Paulus.[26] The increase accounted for about18%. In other reports[27] the observed increase of potato cell size wasas large as 40 to 62%.

The published observations on the effect of blanching on potatomicroscopic structure show that starch is gelatinized, cell walls are intact,and the integrity of cells is preserved. Some weakening of middle lamellaoccurs and is accompanied by swelling of cells leads to separation and

rounding off cells. In consequence, blanched tissue is less mechanicallyresistant in comparison to raw potato, hence it is more vulnerable tofurther changes due to processing.

Drying causes many irreversible changes in plant tissue.[28] Transientthermal and moisture gradients develop tensional and compressionalstresses. The tensional stresses are greater than the compressional one,especially at the boundary of dried material.[29] The stresses cause break-age and fracturing of the tissue undergoing drying. In potato, fracturesare minute and occur during the final stages of dehydration. [30] The stres-

ses developed in potato cubes during drying are relaxing when the processis completed and fracturing of the tissue was observed even several hoursafter the drying was finished. The fracture fines cut through cell walls andgelled starch is freed from the cells upon rehydration. Observations madeby Wang and Brennan[19] showed that surface layers of potato slabs driedby convection are severely damaged after a short time, while the innerstructure is apparently intact. Further drying induces formation of cra-cks, the inner tissue is pulled, apart and numerous holes are produced.

Measurement of electrical conductivity of plant tissue showed that

substantial changes in structure occur during drying.[31]

Even at mild con-ditions of drying (45C and air velocity 0.5 m=s) material showed no pres-ence of intact cells at 20% water content Lowering of water content in

Microstructure of Potato 849


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Blanching and drying cause chemical and structural alterations ofpotato. A lot of research was done considering the effect of blanching

on potato properties and microstructure. However, convective dryingwas of little interest as far as its effect on tissue structure is concerned.Hence, the aim of this work was to assay the effect of different modesof drying on microstructure of blanched potato.

MATERIAL AND METHODS

Potato variety Irga was cut into 0.7 cm cubes and blanched in boiling

water for 2 min.

[33]

Then, the cubes were cooled down to a room tempera-ture in water.Potato cubes were dried by three methods

1. Convective drying in hot air at 60C and air velocity 1.5 m=s. A forcedconvection dryer equipped with 4 shelves, each 20 cm long and 12 cmwide, was used in this experiment. The dryer was loaded with 400 gof potato cubes spread on shelves in a single layer. The hot air flowwas parallel to shelves. The drop of air temperature along the shelves

was about 2

C, hence the drying was assumed to proceed at constanttemperature.2. Puff drying was done in three steps.[34] Potato cubes were dried

to 0.32g=g d.m. water content (found in preliminary experiments)according to the method A, then the cubes were loaded to an expan-sion cylinder. The cylinder was pressurized to 0.180.22 MPa andheated to 150160C. After 45 min the cylinder was open to atmos-phere and expanded product was finished drying by convectionaccording to the method A.

3. Freeze-drying was done in lyophilyzator Alpha LOC-1 (Christ,Switzerland) for 24 h at 20C and pressure equal to 56 Pa. Freezingof potato cubes, spread in a single layer on a freeze-dryer shelves,preceding sublimation, was done in a refrigerator set at 18C for 24 h.

Dried potato cubes were stored in hermetic containers for a week inorder to equilibrate water content inside the cubes and between the cubes.Final water content of convective and puff dried cubes was about 8%,and those freeze-dried about 5%. Ten cubes chosen in random were used

to measure volume by a displacement method using toluene.[35]

Another10 cubes were randomly chosen for microscopic examination.

Samples for microscopic examination were fixed in a solution of



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in a series of ethanol concentrations, beginning with 10% and increasingby 10% up to 70% did dehydration of samples. Dehydration in 80, 90,

and 100% ethanol was done for 30 min. Final dehydration for 30 minwas done with anhydrous acetone. Dehydrated material was saturatedwith Epon, polymerized for 18 h, and cut in microtome into 3-mm-thickslices. Slices were dyed with azur and methylene blue, washed with water,and examined under the microscope. This procedure was used for rawand blanched potato. Dried potato was dehydrated beginning with75% vol. ethanol and then following the procedure for raw material (pro-cedure developed by the Department of Botany of Warsaw AgriculturalUniversity).

Fixed and dyed slices were examined under the microscope. Eachslice contained about 120 cells. Cells on the edges of the slice were nottaken into account, hence about 80100 cells were measured on eachslice. Six to ten slices were chosen in random for examination, givingsome 700800 cells. Microscopic examination was done using opticalmicroscope Olympus AX-70 Provis coupled with computer. Image cap-ture and processing was done with the use of software Analysis 3.0(Soft-Imaging Software GmbH) and Mocha 1.2 (Jandel Scientific).

For each cell the following measurements were done: area and per-

imeter of cross-section. The shape factor was calculated as:

sf 4 p A

P2

The Feret diameter is a diameter of a circle with area equivalent to area ofthe measured object.

dF

ffiffiffiffiffiffiffiffiffiffi4 A

p

r

where A is the area of cross section and P is the perimeter.Histograms of measured values were calculated using Excel v. 97

(Microsoft).

RESULTS AND DISCUSSION

Raw Potato

Microstructure of raw potato is presented in Fig. 1. The cross-sectionalarea of cells is irregular and starch granules are clearly visible In some



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The most frequent area is 4, 200 mm2 and the frequency distribution is

log-normal (Fig. 2). Cumulative frequency shows (Fig. 3) that cells withcross-sectional area less than 10,000 mm2 account for more than 65% ofall cells. Only 5% of cells have cross-sectional area larger than

Figure 1. Microphotograph of raw potato tissue. Bar is 300 mm.



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20,000 mm2, and cells with cross-sectional area as large as 28,000 mm2 arerarely found. The ratio between the smallest and the largest cross-sectional area in raw potato is 145.

The shape of the cells is expressed by the shape factor, which is 1.0for a circle, 0.86 for pentagon, and 0 for a line. The most frequent shapefactor is 0.77 (Fig. 4), which corresponds to a square or an ellipse with along axis more than twice as long as the short one. Cells are rather reg-ular in shape since only 25% of then have shape factor smaller than 0.67

and another 25% have shape factor larger than 0.79.The Feret diameter of cross-sectional area is spread between 16 and

191mm (Fig. 5). The most frequent Feret diameter is 92.5 mm. Some 25%of cells have the Feret diameter smaller than 70 mm, and another 25%have the diameter larger than 124 mm. Cells with diameter larger than180mm are rarely seen under the microscope.

Perimeter of the cross-sectional area shows rather a normal distri-bution (Fig. 6). The perimeter of cross-sectional area vary between 50and 744mm. The most frequent perimeter is 360 mm, but 25% of cells

have perimeter smaller than 262mm. Cells with perimeter larger than460mm account for another 25% of cells.

Assuming that all the cross sectional areas are circles the ratio

Figure 3. Effect of drying on cumulative frequency of cell area distribution inpotato tissue.



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Figure 4. Effect of drying on cell shape factor distribution in potato tissue.



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Comparison of these values with the measured ones shows that small cellsare circular in shape, while large cells are rather irregular and their wallsare creased and folded. Hence, the perimeter of large cells is more than20% longer than that for a circle with equivalent cross-sectional area.

In potato tissue analyzed under the microscope, intercellular spacesare not visible. The cells adhere each to others and are tightly packed.

Blanched Potato

Blanching caused gelation of starch, which is seen in cells as blue areas(Fig. 7). Moreover, the cells become larger and their shape is more regular.No broken cell walls were observed in microscope preparations. Blanch-ing moved the cross-sectional area of cells toward larger values. The dom-inating cross-sectional area is 6,000 mm2 (Fig. 2). Cells with cross-sectionalarea less than 10,000 mm2 account for some 50% of the analyzed popu-lation, while cells with the area larger than 18,000mm2 are about 25%.

Cells with the cross-sectional area larger than 45,000 mm2

are also presentin the blanched potato tissue. The ratio between the smallest and the lar-gest cross sectional area is close to 60 and is much smaller than that

Figure 6. Effect of drying on cell perimeter distribution in potato tissue.



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spread of measured values is from 0.23 to 0.90. The frequency distri-bution of the shape factor is similar to that observed for the raw potato.However, only 25% of cells have the shape factor larger than 0.80.

The Feret diameter of the cross-sectional area of the cells varied from30.8 to 240.0mm. The most frequent Feret diameter is 102.5 mm and is bymore than 10% larger than that measured for raw potato cells. The fre-quency distribution is close to normal (Fig. 5) and the cells with Feret diam-eter larger than 150 mm account for 25% of the analyzed cell population.

The most frequent perimeter of the cross-sectional area is 396 mm.(Fig. 6). The frequency distribution of perimeter is moved toward largervalues in comparison to the raw potato. Some 25% of cells have per-imeter smaller than 330mm, and the other 25% of cells have perimeterlarger than 550 mm.

Comparing blanched potato microstructure with that of raw potato itis evident that heating caused many changes in the tissue structure. First ofall, the cells in blanched potato are larger and more regular in shape. Themost frequent cross-sectional area is larger by 43% and the shape factor

moved from 0.77 to 0.81. In raw potato, the ratio between the smallestand the largest cross-sectional area of observed cells is 145, while in theblanched material the ratio is 60 It shows that blanching did homogenize

Figure 7. Microphotograph of blanched potato tissue. Dark area is gelatinizedstarch. Bar is 300 mm.



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from the ratio between the cross-sectional area and perimeter varies from27.7 to 217.6 mm and does not differ from the measured one. It suggests

that the cell walls are smooth without wrinkles and folds. This effect ofblanching is also seen in the microphotographs (Fig. 7).

Increase in size of cells can be in part attributed to the swelling andgelling of starch granules. Increase in volume of cells calculated on thebasis of the most frequent Feret diameter or the most frequent perimeteramounts for about 30%. But when small cells are taken into account,their volume increases from 7 to 10 times. This is rather difficult toaccept, hence the only reasonable explanation is fusion of few small cellsinto one large cavity. Although broken cell walls are not seen in micro-

photographs, the increase in size of cells suggest that some fusion of cellstakes place during blanching of potato.

Convection Dried Potato

Microphotograph of convection dried potato (Fig. 8) shows the effect ofwater evaporation on the tissue structure. In blanched potato, gelledstarch forms a continuous matter filling completely or in part the cells(Fig. 7). During drying the gel is torn apart into small pieces that fillthe cells, which are seen in microphotographs as debris inside the cells.Moreover, cavities or intercellular spaces are present in the dry material.



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Although most of the cells look intact, some broken cell walls can bedetected in dried tissue.

The most frequent cross-sectional area of cells is 5,750 mm2

(Fig. 2)and is close to that of blanched potato. The ratio between the largestand the smallest cross-sectional area is 31 and is much smaller than thatobserved for blanched potato. The smallest cross-sectional area measuredin dry and blanched potato is practically the same, while the largest onediffers more than two times. Convective drying caused shrinkage of thecube by 37%. It means that the cross-sectional area changed on averageby 27%, thus if some cells do not shrink at all, the others must shrink byat least 54%. The observed cross-sectional area of cells implies that small

cells shrink much less than the large ones.Shape factor of cross-sectional area of cells in dry tissue is moved

toward smaller values (Fig. 4). Its dominating value is 0.75 and corre-sponds to an ellipse in which the long axis is 2.5 times longer than theshort one. And indeed, the cells in microphotographs look elongatedand stretched in one direction. About 25% of cells have shape factorlower than 0.63 and another 25% of the population have shape factorfrom 0.78 to 0.93.

The Feret diameter is 15 to 25% smaller than that measured for

blanched potato tissue. Volumetric shrinkage by 37% corresponds to lin-ear change by 15% on average. The most frequent Feret diameter is91.0 mm (Fig. 5). Some 25% of cells have a Feret diameter lower than74.0 mm while another 25% of the population have a diameter exceeding109.0 mm. The Feret diameter calculated on the basis of cross-sectionalarea and its perimeter varies from 25.0 to 135.9 mm and does not differsubstantially from that measured. Hence it can be concluded that convec-tive drying does not cause any pronounced wrinkling or folding of the cellwalls. This is also seen on the microphotographs (Fig. 7).

Perimeter of the cross-sectional area is smaller by 15 to 25% fromthat in blanched potato. The most frequent value is 336.0 mm (Fig. 6)and the frequency distribution is normal. About 25% of cells have per-imeter lower than 280 mm, while another 25% of the cross-sectional areashave perimeter in the range from 413.5 to 634.8 mm.

Changes in the Feret diameter and perimeter of the cross-sectionalareas caused by drying correspond to those values calculated on the basisof the volumetric shrinkage of the potato cube. This suggests that most ofcells are intact and the cell walls are not broken during convective drying.

Calculation of the cross-sectional area of 700 objects seen under themicroscope shows that in convection dry potato there are some 12%fewer cells than in raw potato tissue It means that during blanching



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Puff-Dryed Potato

Three successive stages of drying, convection, expansion, and convection,resulted in reduction of cube volume by 16%. The shrinkage was muchsmaller than that observed in convection drying and was isometric.

Pressure of superheated steam breaks cell walls and increases the vol-ume of the cube during the expansion stage. Broken cell walls are seen onthe microphotographs (Fig. 9). Moreover the cross-sectional areas lookmuch larger than those seen in convection dried potato tissue. It is movedto large values and the most frequent cross-sectional area is 14,000 mm2

(Fig. 2). The ratio between the largest and the smallest cross-sectional

area is 108 and is similar to that in raw potato. In puff-dried potato50% of cavities have cross-sectional area below 12,400mm2 while inraw potato the cells with cross-sectional area less than 12,000mm2

account for 75% of the population. In puff-dried material cavities withcross-sectional area as large as 180,000mm2 are also present. Cross-sectional area of 700 cells in raw potato corresponds to the area of 346cavities in the puff-dried material. Hence, from the 14 cells, 7 cavitiesare formed during blanching and puff-drying of potato cubes.

Cavities formed during puff-drying are irregular in shape. The shape

factor varies from 0.25 to 0.88, and the most frequent value is 0.69. Thisis a shape factor characteristic for two pentagons joint together. Some25% of cavities have shape factor from 0.72 to 0.88, which means that



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the frequency distribution curve is skewed toward larger values (Fig. 4),and 70% of voids have shape factor lower than the most frequent value.

The shift of shape factor toward smaller values shows that the puff-drying forms cavities of shapes more variable than that observed inraw, blanched, and convection-dried potato tissues.

The most frequent Feret diameter is 140 mm (Fig. 5). Some 25% ofcavities have a Feret diameter smaller than 105.4 mm, and another 25%of voids have a diameter larger than 152 mm. Cavities with a Feret diam-eter as large as 492 mm are also found in puff-dried potato. The Feretdiameter calculated on the basis of cross-sectional area and perimeterof the cavity varies from 64 to 344 mm. Comparison with the measured

values implies that small cavities are enclosed in smooth walls, whilethe large ones have perimeters twice as large as those calculated on thebasis of the measured Feret diameters.

The perimeter of the cavities in the puff-dried potato is between from167 to 2204 mm. The most frequent value is 525 mm (Fig. 6) and some25% of voids have a perimeter larger than 603mm. The frequencydistribution is normal.

Freeze-Dried Potato

Examination of microphotograph of freeze-dried potato tissue (Fig. 10)shows the devastating effect of the freezing process. The cavities are very



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large, broken cell walls protrude into empty spaces, and the continuity ofthe structure is lost. Gelled starch is collected at the cell walls, and prob-ably was pushed there by the ice crystals.

Cross-sectional area of cavities in freeze-dried potato varies from10,000 to 750,000 mm2, and the most frequent value is 56,000mm2

(Fig. 11). About 25% of cavities have cross-sectional area smaller than40,000 mm2 and another 25% of voids have the area larger than182,000 mm2. The frequency distribution is log-normal. A cross-sectional

area of 700 cells in raw potato corresponds to the area of 44 voids infreeze-dried material. Hence, some 16 cells form one cavity, on average.

The cavities in freeze-dried potato are very irregular. There is nodominating shape factor (Fig. 4), which is spread between 0.17 and0.84. Some 75% of voids have a shape factor smaller than 0.65. Suchshape factor is characteristic for very elongated ellipses and indicatesthat the growth of ice crystals was unidirectional. This is also seen inmicrophotographs.

The Feret diameter dominating value is 250 mm (Fig. 5). It varies

from 113 to 978 mm, hence some cavities could be seen with the nakedeye. Some 75% of the Feret diameters are smaller than 480 mm. The Feretdiameter calculated on the basis of the cross sectional area and perimeter

Figure 11. Cross-sectional area distribution in freeze-dried potato.



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calculated on the basis of measured Feret diameter. It means that thewalls are wrinkled, folded, and protruding into the empty spaces.

The perimeter of the cross-sectional area of cavities varies from 430to 6990mm. The most frequent value is 1,190 mm (Fig. 12). Some 75% ofperimeters is smaller than 2,280 mm.

DISCUSSION

Potato tubers are built up of areas that differ in cell size and composition.Besides the bud and stem ends, there are pith, perimedullary, and corticaltissues. Perimedullary starchstorage parenchymaaccounts for themost of the tuber. The average size of a starch storage cell in a potatois about 300 mm[30]. However, the cell sizes differ between cultivars, andthe cell size of the mature Kennebec tubes was about 60% of that inmature Russet Burbank tubers.[36] At harvest, the average cell volumein Kennebec tubers varied from 2:40 to 3:10 106 mm3. In other experi-

ments it was found that in Russet Burbank tubers the volume of parench-yma cells varied from 2:06 to 4:85 106 mm3.[37] In 1cm3 of potato tissue,0 5 106 cells are present It gives an average volume of 2 106 mm3 which

Figure 12. Perimeter of cavities distribution in freeze-dried potato.



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The investigated variety of potato has small cells in comparison topublished data. The most frequent Feret diameter was 92.5 mm, and there

were no cells with a diameter larger than 190 mm. The size of the cellsaffects the mechanical resistance and texture of potato tissue. Mealypotatoes contain larger cells than nonmealy potatoes.[11] Volumetricexpansion stress due to heating is proportional to the diameter of the cell.Hence, stress in the middle lamella of small cells is smaller than thatdeveloped in large cells. In consequence, maximum shear will developbetween adjacent layers of small and large cells. In a tissue containingsmall cells, stresses developed by heating can be expected to be smallerthan those created in the presence of large cells. In conclusion, it can

be expected that the investigated variety of potato will be less vulnerableto processing stresses than those varieties, which produce large parench-yma cells.

Heating of potato tissue causes numerous changes. The starchgranule gelatinizes and its volume may increase 25- to 30-fold. The gelshrinks upon cooling and can exude moisture.[9] However, the cell wallis strong and limits expansion of the cytoplasm, and changes of turgorpressure cause a few percent change in the cell volume. [39] It was alsonoted that in pre-cooked potato slices, gelatinized starch did not fill the

cell cavity entirely.[40]

During cooking, there is no cell rupture but cellseparation is evident.[38] Conventional cooking results in fairly rapid dis-solution of middle lamella and cell separation. The cells are rounded offdue to swelling of gelatinized starch.[12] All the above presented resultswould suggest that blanching causes changes in the potato tissue, whichshould lower its resistance to stresses caused by processing. However,there is also evidence that pre-cooking activates native pectin methyles-terase; a number of metal bridges with Mg and Ca is formed and thetissue becomes more resistant to further thermal degradation.[10] In the

presence of calcium, potato tissue becomes firmer[41]

and uptake of fatduring frying is lower than that observed in nontreated material.[42] Sincea major portion of the total calcium in potato tuber is present in starchgranules,[11] gelatinization is necessary to induce the firming process.

Blanching applied in this work caused increase in cell size. The mostfrequent Feret diameter is 102.5 mm and is almost 11% larger than thatobserved in raw potato. It corresponds to the increase of the cross-sectional area by some 23%. And some rounding off of the cells waspresent because the most frequent shape factor was 5% larger than that

measured for cells in raw potato. From the frequency distribution curve(Fig. 2) it is evident that the changes in cell size are concerning mostlylarge cells At the same time the cumulative frequency of small cells



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25% of the population. On the other hand, cells with cross-sectional arealarger than 20,000mm2 occur in 5% of raw potato and in 20% of

blanched potato.Data published on the effect of convection drying on potato micro-

structure is contradictory. Reeve[30] showed that drying causes partialseparation of cells and formation of minute fractures. The fracturesbecome evident at the final stages of drying. Dehydrated potato form aspongy network composed of fine, interconnecting strands of intactand only slightly shrunken cells. Some voids were present, which weredue to expansion of intercellular gases and weakened middle lamella.On the other hand, Wang and Brennan[19] reported that shrinkage caused

cell wall elongation, damage of structure, and formation of numerousholes in dried potato. Results of this work are rather consistent withthe observations of Reeve.[30] The cells are shrunken and the change inthe size follows the shrinkage of the cube. There is evidence of pro-nounced wrinkling and folding of the cell walls, which in a majority lookintact. The cells are elongated what suggests that shrinkage forces areunidirectional. It is theoretically calculated that in dried potato cubethere is some 12% fewer less cells than in raw material. It suggests thatsome cell walls are broken and larger cavities are formed. Moreover,

analysis of collected data shows that small cells shrink much less thanthe large ones.

It seems reasonable to assume that tissue built up of small cells,closely packed, and having large contact areas is resistant to drying stres-ses. Gelled starch filling the cells can also add some strength to the tissue.It is evident from microphotographs that the gel dries in a whole volumeand is not pushed toward the cell wall. Removal of water forms strandsof concentrated gel, which during the final stages of drying break intosmall pieces. But, the strands can limit the deformation of cell walls

and prevent tissue damage.Puff-drying causes extended injury to the potato tissue. The cell walls

are broken and large cavities are formed. On the average the cavities areless regular in shape than the cells in convective-dried potato. The per-imeter is long in relation to the cross-sectional area and it shows thatfolded and wrinkled walls surround cavities. In general it is estimatedthat in puff-dried cubes there is a half of the number of cells in rawpotato. On the average, two cells form one cavity, whose shape is similarto two pentagons joined together. Gelatinized starch behaves during

puff-drying the way it does during convective drying. Small pieces ofdried gel are seen inside the cavities.

Freeze drying does the most severe damage to the potato tissue Ice



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Effectofmodeofdryingonvariab

lescharacterizingprocessed

materialstructure

Apple(40)

Potato

Convective

Puff-dried

Freeze-dried

Convective

Puff-dried

Freeze-dried

ionalarea

equent

7650

17

,210

19,90

0

5750

14

,000

56,0

00

602

219

,740

3105

136

,330

283220

7,6

53

686

21

,570

1748

189

,712

100

,167

51

,958

meter

equent

55

.1

158

142

91

.0

140

.0

250

27

.7

529

.0

62

.9

916

.6

60514

29

.6

165

.7

47

.2

491

.5

113

.0978

.5

equent

260

670

608

336

525

119

0

106

2773

227

2561

2154124

169

635

167

2204

4306

990


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due to the small size of cells in potato tissue. Growing ice crystals caninjure one, or maybe a few large cells, while small cells are destroyed

extensively. Assuming that the most frequent Feret diameter of cavitiesin freeze-dried material corresponds to the size of ice crystals, it can becalculated that its volume is equal to the volume of 20 cells in raw potato.From a cross-sectional area it was calculated that one cavity in freeze-dried material was formed by fusion of 16 cells. The continuity of thetissue structure is destroyed, and its properties must be much differentfrom that observed in convection-dried potato.

Comparison of the effect of mode of drying on the structure ofapple[43] and potato tissues shows that both the kind of the material

and processing parameters are important. Apple cells are much largerthan potato, and their chemical composition is much different. Inconsequence, convective drying affects apple tissue much more than thetissue of potato (Table 1). Probably the cell size and the presence of gelledstarch are mostly responsible for the observed differences.

In puff-drying created pressure breaks the cell walls, and it seems thatthe breaking force is so large that the injury to the tissue is little depen-dent on the kind of the processed material. The most frequently measuredparameters for potato are smaller by 1020% from that characteristic for

apple. However, the ranges of measured values are the same for bothmaterials.

Freeze-drying is more deleterious to potato than it is to apple tissue.Most frequently measured variables are twice as large in potato as inapple. The ranges of these variables are moved to much larger values,and at the upper end differ by as much as 1.6 to 3.6 times. Hence, theeffect of the kind of processed tissue on the extent of injury caused byfreeze-drying is evident.

CONCLUSION

Mode of drying pronouncedly affects microstructure of potato tissue.Blanching, which precedes drying, causes gelatinization of starch andswelling of cells. The tissue is compact, cells are close each to other,and there is no evidence of broken cell walls. Not all the cells are com-pletely filled with gelatinized starch.

Convective drying causes shrinkage of the tissue; however, there is

no severe damage to the cell walls. It is estimated that about 12% of cellsare broken, and other cells simply shrink during dehydration, but theirintegrity is preserved This effect is attributed mostly to the small size



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Application of external force, besides the shrinkage stresses, causedextensive damage to the tissue. Expanding vapor pressure pulls apart cell

walls and creates a porous structure during puff-drying. Comparison ofthe structure of puff-dried apple and potato suggests that the forces act-ing on the tissue are so large that the damage incurred to the moist tissueis little dependent on the kind of processed material.

The most devastating to microstructure of plant tissue is freeze-drying, probably freezing per se. The damage to the tissue is so extensivethat the structure loses its continuity. A very porous and fragile productis formed. Forces tearing tissue into pieces are dependent on the kind ofprocessed material. This process is more severe to potato than to apple

tissue.Results collected in this work show clearly that to obtain dry product

of high quality, drying mode, drying variables, and pre-drying processingmust be carefully chosen. By choosing an appropriate drying method andpre-drying processing, a product with desired structure and propertiescan be obtained.

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Effect of Mode of Drying on Microstructure of Potato