MICROSCOPY of STARCHES and THEIR MODIFICATIONS OTTO A. SJOSTROM Corn Products Refining Company, Argo, 111.
A series of photomicrographs is presented to show the appearance of the more important starches and their modifications, the thin-boiling starches and, dextrins, in their gelatinization and disintegration forms. The pictures demonstrate the changes which take place in the configuration of the granules and illustrate the similarities and differences between the starches in this respect. They also present interesting cases of micellar structural arrangement which becomes visible under certain conditions. I n the text the essential features are pointed out and briefly discussed. Reference is made to the recent “growth ~ t r u c t u r etheory ’~ and pictures are shown to illustrate and support this theory.
During the last decade the structure and behavior of starch have been the subject of numerous investigations, based on the use of colloid chemical methods and of x-rays. Important sources of reference in this ficld are the monographs of Samec (6) and of Katz (9). As for the results of these investigations it is sufficient to say that Niig di’s conception of the form of the micelles has been confirmed; no indications of trichite structure have been found. In relation to the phenomenon of gelatinization new factors 1 ave come to the fore-among them the proper consideration of the role of the “bound” water in the granule. However, scientific knowledge in this territory is still vague and much of it speculative, leaving many complicated problems to be solved in the future. Most of the papers and rionographs dealing with starch structure are more or less profusely illustrated with drawings. Drawings have merits and limitations. In many cases they are too diagrammatic in chai acter and occasionally imagination has played a role in the vrorking out of the details. Photography, since it becaine available for illustrative purposes, has not been used as an aid to description except to show the appearance of ordinary starch granules, in spite of the possibilities it offers for a more exact reproduction of the microscopic image. As a contribution to the illustrative side of starch investigation tlte writer therefore presents here a series of photomicrographs, selected from reproductions made from time to time duriiig a long and close contact with the starch industry and intonded to show particularly the appearance of the gelatinized forms and disintegrations of the ordinary starches and their riodifications.
T
HE problems involved in a study of the structure of starch granules are primarily of a botanical nature, and the structural theories which have been accepted at one time or another in the past have been developed by eminent botanists on the basis of extensive researches. At the same time, any information about the structure is of great interest to the chemist, in so far as it may aid him in his efforts to unravel the many mysteries in regard to the chemical constitution of starch and its unique behavior in its transformations and modifications, important from an industrial as well as from a scientific point of view. Structural studies have therefore as a rule been closely connected with chemical investigations; those noted botanists of the past did not limit their investigations to the botanical phase of the subject but also made contributions to the chemistry of starch. Up to rather recent times the commonly accepted idea of starch structure was, a t least in principle, based upon the trichite theory of Meyer (4). According to him the granules are spherocrystals, built up of crystalline dendritic units (trichites) consisting alternately of the two main constituents of starch, named by Meyer LLa-amylose”and “p-amylose.” The water in the granule is distributed between the trichites, but the distribution is not uniform in a radial direction. Some of the concentric layers are denser and contain less water than, and their refractiveness is different from, that of the less dense layers. This accounts for the appearance of the concentric striations which in some of the starches are well marked. The trichite theory is really a modification of the older micellar theory of Niigeli (6). h-ageli assumed that the granules were built up of primary crystalline units of molecular aggregates which he called “micelles.” The micelles were rounded or oval in shape and arranged according to certain laws of symmetry.
Value of Photomicrographs Photographic reproductioiis, like drawings, have their limitations and weak points. The size of the grain in the emulsion of plates, films, arid papers sets a limit for the fineness of details, and only o ie plane of the object can really be sharply focused, especidly a t higher magnifications. Since most of the objects with which we are here concerned have definite depth, the select ion of the focus plane becomes a matter of judgment. An additional reason for the need of experience and thorough fam liarity with the material under observation is that a change of focus plane often changes the appearance of the details con:;iderably, owing to the effect of the so-called optical confusioii. Starch objects always lose nore or less of their refractiveness upon hydration, so that the contrast between object and field is reduced, often to ruth an extent that it is next to impossible to obtain a picture of any value. I n fact, the lack of contrast is perhaps the greatest of the difficulties met in this kind of work. Staining sometimes helps but is frequently inadvisable, for various reasons; it may, for example, easily lead to the formation of artifacts. However, this phase of the art deserves more experimenting and will probably be improved in the future. 63
64
INDUSTRIAL AND ENGINEEIIIh-G CHEivIISTRl~
FIGURE 1.
RICE STARCH
(X 400)
GRAXULEB
FlCUHE
2. COMPOUND RICE S,~.%RCH GRANULES( x 900)
FIGURE
3. CORN STARCH G n a n a L E s , AVERAGE 400)
(x
~.'ICURE 4.
FIGURE
VOI.. 2R, S O . 1
5.
CORYSTARCH GRANULES, RCUND( X 400) CORN STARCH, PASTED
( X 400) FIGURE 6. WFTY-FLUIDITY Twrx~ C r L r x a CORN STARCH, PASTED
( X 400)
FIOURE7. S E V E N T Y - F ~ ~ ~ - F ~ . U I D I T ~ CORKSTARCH, PASTED (X400)
THIN-BOILING
FIGURE8. NINETY-FLUIDITY TwIS. BOITJYO CORN ST.&RCR,
PA~TED
( X 400)
FIWKE 9. CHLORINATED THISROILING CORNSTARCH, PAS~D (X
Another possible stiurce of errors i.? t,he diffraction effect. Diffraction seams and nlarkirigs are oiten unavoidable since they appear particr~i~rlr at l o x light, intensities which lnay he necessary in order to briiig out contrast and details. In general, the nraking of a good picture is a niatter oi compomise; experience and kiiovledge of the material have to be the guides. The advaiitages for illustrative purposes of a photomicrograpii skillfully t:tkrll itre obvious.
ml)
The starclies n,ere all pliotographed as tiley appeared $+-hen suspended in water. In tlie case of the past,ed or gelatillized ordinary and modified starches, tire s t l s p e n s i o n llad heen )>reviously heated to the ternperat.iires ilrdicated in the descri1)tion or on the titles of tire figllree. The magnification rvas as a rule -100 tirnes; in sollie case^ a magnification of close to 900 tirnes n.ith irnrllersion leils u.ai used in order to bring out details.
JANUARY, 1936
INDUSTRIAL AND EXGIKEERING CHEMISTRY
65
Rice Starch The rice starch granules (Figure 1) are the smallest of the ordinary food starch granules; their diameter is from 3 to 8 microns. They are definitely polygonal in outline and are frequently found in clusters called “ c o m p o u n d ” granules. These are simply aggregates which have not been completeiy broken up in the mechanical preparation of the starch. In the rice grain the g r a n u l e s a r e enclosed in honeycomb-like structures of & e n , similar to the gluten structures in a corn kernel, and in the aggregates the granules are held t o g e t h e r b y t h e gluten p a r t i t i o n s . Several of these clusters are shown a t bigb m a g n i f i c a t i o n iFimre 2) : in one of these the gluten walls are in plain view, extending in the directions of a pentagon. On account of its small dimensions the rice starch granule is not a suitable object for microscopical emminations, and 110 photomicrographs of the modifications of this starch are included here. ,
Y
Psated ( 6 )
I ,
Corn Starch The average corn starch granules (Figure 3) are of medium size, 10 to 25 microns, but deviations in both directions are not uncommon. Their outline is usually polygonal but many round granules are found. Some varieties of corn have a soft and mealy structure and the granules are nearly all round (Figure 4). From one granule in this picture a sector is missing. The water has apparently free access to the interiar of the granule tlirough the fracture, but there is very little increase in volume due to swelling. This fact is interesting and reference will be made to it in the last part of the paper in connection with the behayior of ground starch. The appearance of pasted or gelatinized granules is shown in Figure 5. The picture has been taken at an early stage of the pasting hecause, as pasting p r o d s , the granules lose their refractiveness to such an extent that the photographic reproduction becomes w r y unsatisfactory. In swelling, the inner layers expand more rapidly than the dense surface layer of the granule (the “skin”) so that usually cracks are fornied in the latter, as shown in the pictures. Wevertheless, the swollen granules retain a resemblance of the outlines of the original granules. The ordinary starch is transformed into thin-boiling starch by a treatment with dilute acid a t temperatures below those of gelatinization. Nothing is known dehitely about the nature of the change caused hy the acid, hut i t is commonly considered to be some form of depolymerization. The amylopectin of the starch is the one component essentially involved in the gelatinization or hydrat,ion, and Samec ascribed this property to the presence of an amylophosphoric acid. The loss of phosphoric acid due to the effect of the dilute acid treatment i~-ouldthen explain the fact that. the tendency to gelatinization is materially rednced in acid-treated starches. I-Ioraewr, later investigations (referred to hy Hanson and
Kstz, f) indicated
nut a corresponding decrease of t h e phosphoric acid con-
4 6
tent. Besides, F~QWE 12. TnIcHIn; STRUCTURE OF the theoryis applied TEE STARCH G ~ N U L EAccoRnrNn . TO A. MEYER to starches such as corn and t a p i o c a , which contain onlya small amount of phosphoric acid, the effect of a possible loss of part of this acid seems too insignificant to explain the marked change in properties and behavior of the thin-boiling modifications. We have, therefore, at the present time no plausible explanation of the reduced gelatinization or hydration and of the display of brittleness in the granules of the modified starches. As for the other constituents of the starch which are usually referred to under the collective name of amylose, they are undoubtedly in some degree acted upon by the acid, but the effect of the possible change in these bodies is much less conspicuous than that of the change in the behavior of the amylopectin. As the name “thin-boiling” indicates, these starches give a paste which is more fluid than that of ordinary starch, and by suitable modification of the treatment varying degrees of fluidity can be obtained; the final member of the series is soluble starch which even in fairly high concentration gives a paste of watery consistency. They are of considerable technical importance, are used in a number of industries, and are put on the market under trade names which refer to certain arbitrary fluidity tests consisting of measurements of the rate of flow of the paste through a narrow tube. The fluidity (or rather, viscosity) of the paste is not viscosity in the Same sense as that of a liornogeneous liquid. The paste is largely a suepension of whole and disintegrated ganules, and the degree of viscosity is a measure of the extent to which the suspended particles emwd each other in passing through the narrow tube. The reduced tendency to hydration means that the granules,
66
INDUSTRIAL AND ENGINEERING CHEMISTRY
when heated with water, increase in volume less than granules from ordinary starch. I n a thin-boiling starch paste the room occupied by the suspended starch is therefore smaller in relation to the total volume than in the ordinary starch paste, and the paste is correspondingly thinner. Other factors affect the difference in fluidity between thick- and thin-boiling starches, but this relation of volumes is without doubt the most important. I n a thin-boiling starch granule the structure is not greatly altered; the micellar arrangement is preserved in approximately its original form, although certain connections may have become weakened and some members of the structure dissolved out from the granules. Starches differ in this respect just as they differ in other properties. I n some of them the structure appears more compact and rigid than in others. The granules of a modified starch still swell and disintegrate in contact with hot water, but on account of the lessened degree of hydration and the increased brittleness the structure becomes more or less evident under the microscope. The manner of disintegration and the extent to which the details are revealed depends upon the intensity of the treatment to which the starch has been subjected. Also, in some starches tke structure is more concealed and the refractive contrasts more faint than in others, but on the whole the thin-boiling modifications offer a convenient material for the study of the starch structure. Figures 6, 7, and 8 show the progressive changes in the appearance of pasted thin-boiling corn starch granules with increasing fluidity. The 50-fluidity granules still retain much of the characteristics of the ordinary pasted starch, but the granule is smaller and the cracking of the outer layer is more pronounced. In the 75-fluidity granules the cracked surface layer is very conspicuous; in the 90-fluidity starch the radial cracks are deeper, the increase in the volume of the granules is much less marked, and disintegration begins to take place along concentric lines of cleavage. For certain technical purposes starch is modified by treatment with chlorine, applied either as elementary chlorine or as hypochlorite. The chlorinated starches have usually more or less thin-boiling properties and show in addition specific technical characteristics due to the chlorine treatment. In the particular chlorinated starch shown in Figure 9, the effect of the treatment has been intense enough to make a microstructure visible in the gelatinized granules, when they are viewed under an immersion lens a t high magnification. Close insDection reveals small units of about 0.3 micron aDparent diameter, arranged concentrically in orderly rows. The structural arrangement is distinct only in the outer parts of the granule, and this may be due to some difference in the degree of disintegration in the same granule. We frequently find local differences of this kind in a granule, just as we note differences between individual granules in their reaction towards modifying treatment in general. Figures 10 and 11 show granules of an interesting modified corn starch, gelatinized a t about 85’ C. This starch is the result of a laboratory experiment and has been treated with acid under special conditions. In technical respect the starch has, on the whole, the properties of an ordinary high-fluidity thin-boiling starch, but inspection under the microscope shows that the treatment has had a specific effect on the granule. The micellar arrangement is plainly visible; in fact, this starch presents the best demonstration of micellar structure which the writer has seen. I n one granule in Figure 11 the outer layers have curled back so that the central part is visible and the same micellar arrangement can be seen here as in the outer parts on the other granules. The micelles, as represented by the small globular bodies arranged in the orderly manner seen in the pictures, are
VOL. 28, NO. 1
arbitrary units. Even with their small dimensions they are probably far too large to represent ultimate crystalline molecular aggregates. Besides, we have to make allowance for the optical errors connected with the viewing and photographic reproduction of such small particles. It is certain, however, that they are re$ objects and not optical illusions, such as might be due to diffraction. Hanson and Kate (1) discuss the question of starch structure on the basis of experiments carried out with Lintnerized wheat starch. Their observations indicate that there is an orderly concentric and radial arrangement of globular units of small dimensions in the granule, and this conclusion is supported by the reproductions shown by the writer. The recent investigations confirm, therefore, the principle of Nageli’s micellar theory. Figure 12 gives a reproduction of the drawing which Meyer made to illustrate his trichite theory of the structure. Although his idea of a radial and concentric arrangement of units is correct in a general way. his assumptiin of a dendritic form of theseunits is nbt supported by the microscopical findings. If starch is roasted with or without the addition of acid, it is transformed into dextrin. Dextrin is more or less soluble in water a t ordinary temperature. The thin-boiling starches and soluble starch are also soluble in cold water to some extent, and the distinction between these bodies and lowsoluble dextrin is rather arbitrary on the border line, and based mainly upon differences in their suitability for various technical uses. As is well known, the commercial dextrins are mostly mixtures of all these modifications, in proportions which depend upon the conditions and the intensity of the roasting. The effect of roasting on the starch structure can be studied in spite of the solubility. I n the case of the low-converted dextrins there is more or less of a time element in the solution of the wetted granules which allows observations to be made and with the high-converted dextrins the action of the water can be retarded to suit the observer’s convenience by the addition of glycerol. The interesting fact will then be noted that in a dextrin the concentric connections are much better preserved than the radial ones. Radial cracks are practically absent; the layers peel off like an onion, more or less along the partitions which are indicated by the striations in some of the starches. This mode of disintegration is so characteristic for dextrin that it can be used for the distinction between dextrin and thin-boiling starch, when observing granules under the microscope. As to the deeper significance of this difference and its connection with the mechanism of the formation of starch modifications nothing is known a t the present time. Figure 13 shows a t high magnification the disintegration of a low-converted dextrin in water of ordinary temperature. In the large granule the faint outlines of the envelopes can be seen as they float away from the central part. Figure 14 shows a high-converted dextrin, also at high magnification. The granule is stained with iodine in order to increase the difference in refraction and make it possible to photograph the objects. In a completely dextrinized granule the layers apparently extend uniformly from the center to the surface, possibly with some variation in thickness and density. This is a strong proof of the assumption that the micellar structure is, in a general way, thesame throughout the granule.
Q
Tapioca Starch The tapioca starch granules (Figure 15) are of about the same average size as corn starch granules but show variations
INDUSTRIAL AND ENGINEERING CHEMISTRY
JANUARY, 1936
67
23 FIGURE13. LOX.-CONVERTED CORN DEXTRIX, DIBINTEGRATING IN WATER ( X 900)
FXQDRE 14. HIOH-CONVZIRTED CORN DEXTRIN,DISINTEORATINO IN WATER AND GLYCEROL, STAINED ( X Sao) FIGURE15. TAPIOCA STARCH GRANULES FIGURE
ULES
( X 400)
TAPIOCA STARCH GRANBETWEEN CROBSED NICOLS 16.
( X 400)
STAGE OF PASTING ( X 400) FIGDELE 17. T A P I O C A STARCH, EARLY FICXTRE 18. THIN-BOILINQ TAPIOCA STARCH, PASTED AT 75' TO 80' c. (x 400) FIGURE19. SWEETPOTATO STARCH GRANULES ( X 400)
FIGURE 20.
SWEET POTATO STARCH,
( X 400)
EAELY STAOEOF
PASTING
FIGGRE 21.
SWEET
POTATO &ARC& 80"
PA6TlNQ &AOE
c. ( x 400)
AT
ABOUT
FIGURE 22. THEN-BOXLINQ SWEE ~i POTATO STARCH, PASTTNC STAQE AT A ~ o 80° a C. ( x 400)
FIGORE23WATER SWEET POTATO DEXTRIN, DX6INTEORATINQ IN AND GLYCEROL: STAINED ( X 400)
68
INDUSTHIAL AND ENGINEERlNG CHEMISTRY PIGORE
VOI.. 28, NO. I 24.
WHEAT
STARCH
GRANUI~ES
(X 400)
FICURE 25. WHEATSTARCH,P.L~TED AT ABOUT65' C. ( X 400) FIGURE 26. WHEAT STARCH, PASTED AT 95' G., STAINED (x 400) FIQURE 27. THIN-BOILINR WHEATSTARCH, PASTINO STAGEAT ABOWP$5- C., STAINED ( X 400)
starches of the same kind, presumably prepared in the sane \r.ny, but perhaps with a more or less subtle and a t times concealed difference in the conditions of preparation. Here the niicroscope leaves us in the dark. However, continued work m a y b r i n g improvements in the technic anti added knowledge.
Sweet Potato Starch
in this respect, depending upon the origin of the starch. They are of rounded shape and have on one side an indentation which is characteristic of this starch. It has the form of a conical pit, extending approximately to the well-marked hilum. Figure 16 shows the appearance of the granules in polarized iight. Thiis starch gives a r e ~ l a and r well-defined cross. The granules of the pasted starch (Figure 17) are similar to those of corn starch but are smoother and more translucent. Generally, tapioca granules are softer than corn granules and their structure is much less rigid and compact. In one of the granules the beginning hydration has made the conical pit visible through optical contrast. There is also an indication of micellar graininess in this granule. The outer concentric line is a diffraction seam. Figure 18 shows thin-boiling tapioca starch pasted a t 75' to 80" C., lightly stained with iodine. The cracking of the surface layer and also disintegration in radial directions are seen. In one corner is a granule with indications of the concentric micellar arrangement. As just mentioned, tho tapioca structure is decidedly looscr than that of corn starch, and preparations for microscopic study are greatly lacking in contrast and detail, so that this starch is not a satisfactory material for such inrestigations. Starch users in general are familiar with the fact that. there is a great difference between individual starches in regard to tho character of the paste made from each starch and its modifications and that these individual characteristics determine whether a starch is suitable for specific purposes. This phase of starch behavior will not be discussed in the present paper, hut in regard to such differencesmicroscopic investigation has PO far given very little information. In other words, w cannot make any predictions along these lines from our observations, although ~e might make occasional comments This limitation applies particularly to the differii IIC sometimes find in the technical properties of
Sweet potato starch granules (Figure 19) are similar to corn starch granules in appearance (polygonal forms prevailing), hilt they are larger, the average sine being about one and one-half to two times that of the corn starch granules. The striations, if any, arc indistinct and the granules are refractive, as Figure 19 shows. Figures 20 and 21 show the pasted starcli a t differentstages of gelatinization. The granules are stained lightly u-itlr iodine. In Figure 20 the different stages of swelling are represented in the same granule. In half of the granule the hydration has been almost explosive; the other half is lagging behind but has increased in volume, and fortunately the picture was taken just a t a stage where a chain of micelles is plainly visible close to the outline of tho granule. This latter half shows a mass of round micelles over the whole surface, probably part of the micellar layer of which a section is visible along the edge of the granule. With increased tenrpcrature such features are completely obliterated (Figure 211, and near the boiling point we have only large masses of irregiilar outline. Figtire 22 shows thin-boiling sweet potato starch a t about 8O0 C. This starch is a difficult object for the microscopist for the same reasons as were given in connection with tapioca starch. There are no indications of structural arrangement but the large granule shows definite graininess. The swelling is markedly reduced, compared to that of the ordinary starch. The white glistening points on chis granule are parts, mainly from the outer layer, which have so far resisted gelatinization. Figure 23 shows sweet potato dextrin disintegrating in cold water. The granule is stained lightly with iodine to give contrast, and the concentric layers are gradually floating away and dissolving. The phenomenon is similar to that noted with corn dextrin, and the comments concerning the lat.ter also apply here. The properties of the paste from sweet potato starch and it.3 modifications are similar to those of tapioca starch preparations. Because of the interest of the U. S. Department of Agriculture, the manufacture of this starch will be developed on a commercial scale, and probably in the future the products rvill be used as a partial substitute for tapioca.
JANUAHY, 1936
I \ D I STRIAL A \ D EhCIhEERfNG CHEMlSTHY
69
FIGURE 28. RYE STARCII GHaNULEs ( X 400) FIGURE 29. RYESTARCH, PAWED AT FIGURE
30. R1.E *'ARCH,
A ~ U 65' P
PASTED AT 95'
G. ( X 400)
c.,STAINED ( X 400)
RYE STARCH AT 56" c.; EAnLlEUT STAGE OF SWELLINQ ( X 900) FIGWEE 32. THIN-BOILING RYE STAHLCH, PASTINO STAGE AT ABOUT '35' e., ST.tINED ( X 400) I'LGUEF.
31.
to the paste, as compared to a paste made from another starch in which the relative voluine of the gelatinized granules is about the same. As will be seen, rye starch shows the saine behavior. A knowledge of this peculiarity has a practical interest; it has, for instance, enabled the writer to identify wheat starch (rye starch being practically excluded) iii tecbnical products in several cases where identification in any other way would have heen impossible. Thin-boiling wheat starch is shown in Figure 27 at a tcrnperature near the boiling point. There is a ewtain amorlilt, of swelling (no determination was made of the relative fluidity of the sample), and the mode of disintegration is different from that of the starches already described. Coricent,ric layers are split off in a manner siniilar to the disintegration of dextrin. However, i t is not surprising that we should find a difference in this respect when we consider that the hahitiis of the wheat starch is different from that of the others. The latter are developed in all three directions, the former has length and width but is lacking in thickness.
Rye Starch
Wheat Starch The wheat starch granules (Figure 24) are, as a rule, either large or small, although the large ones rather dominate the field; there are few granules of intermediate size. The largest granule shonn has a diameter of 3-5 microns. The granules are thin and lenticnlar in form, and do not show any distinct striation. Figures 25 and 26 show the pasted forms of wheat starch. At lower temperatures (65" to 70" C.) the granules swell into a baglike shape (Figure 2 5 ) , but near the boiling point they assnine a peculiar and characteristic curved shape (Figure 26). The granules are stained for contrast. Probably this shape of the graniilrs gives some specific jihysieal character
Rye starch resembles wheat starch, but the granulos are usually larger and thicker (Figure 28). They often show fine striations, radial as well as concentric. Faint indications of striae can be seen on some of the granules, but the diffraction seams around their outlines should not be confused with striations. The pasted forms of the granules arc similar to those of wheat starch. The curling of the granules is more pronounced in the rye (Figure 29), so superficial inspection seems to indicate that the granule has cracked radially down to the hilum. Upon continued heating this difference soon disappears (Figure 30), and the completely gelatinized grannles of the tnvo starches ~.ppearto be practically identical. There in an interesting phenomenon in connection with the earliest stage of the swelling which may be observed in several
INDUSTRIAL AND ENGINEENING CHEMISTRY
70
starches hut which is more marked in wheat and rye and is especially conspicuous in the latter. As the granules begin to swell, the original faint structural indications become prominent and give a fair evidence of an orderly arrangement of micellar units in the granules, even if we make allowance for the limitations due to optical and other imperfections of the art. Attention has already been called to indications of micellar structure under similar conditions in connection with tapioca (Figure 17) and sweet potato (Figure 20). In Figure 31 a rye starch granule is shown in the earliest stage of swelling (at about 55" C.), photographed under immersion lens a t high magnification in order to bring out details. In the central part of this granule the structure is not exklent. Granules can be found in which only the central part shows a strueturd arrangement, but the writer has so far not seen any granule with complete visual development of the struc-
FIQUIIE 33. SAGOSTARCK GRANULES ( X 400) FIGURE 34. SAGO STARCH. PASTING STAQE AT 95°C. ( X 400)
TmN-BOILIN0 SAOOSTARCH, PASTEDAT 70° TO 75" c. ( X 400) F I G W E 35.
VOL. 28, NO. 1
ture. The granule in Figure 31, however, illustrates fairly well the condition described. As the swelling increases, these structural markings become very diffuse rapidly, and by the time the curling begins they are usually obliterated (Figure 29). Gelatinized thin-boiling rye starch is shown in Figure 32. The relationship to wheat starch is evident, but in rye starch the indications of micellar structure are more easily demonstrated. The granules have been stained lightly with methyl violet in order to hring out the structural units which otherwise would have been practically invisible for photographic purposes. The disadvantage with staining is that i t is apt to give the structure a rather coarse and blurred appearance because of the irregular adsorption of the dye. At any rate, it 8erves to indicate that a micellar structure actually does exist.
JANUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
71
72
INDUSTRIAL AND ENGINEERING CHEMISTKY
nization is the transformation of the granule into a ring. The granule opens up partially along the median line, the parts on both sides of the opening elongate in the course of gelatinization, and the final result is a ring of more or less irregular shape. Some of these rings are smooth, as shown in (Figure 38). In others (Figure 39) the bydration develops peculiar units of rounded form, arranged in a more or less irregular manner and often shorr.ing individual concentric
VOL. 28, NO. 1
of the granules which muses them to split np into innumerable small remnants, when the paste is agitltted, so that the result is actually a complete final disiukgration of the cornpoiients of t.he paste. Figure 42 shows a potato dextrin disintegrating in contact with cold water iiiixed with glycerol in order to retard the solution. Here again we note the characteristic dextrin feat,ure-the splitting up inio concentric layers. The grenule is stained for contrast with a small amount of iodine contained in the 1%-ater and has been photographed at high magnification
JANUARY, 1936
ISDUSTRIAL AXD ENGlSEERlNG CHIS.CIlSTRY
in order to bring out the details. The staining is effective; the many exceedingly thin layers whieh make up the bulkier tissues can be seen distinctly. There seems to be a greater difference between the mode of disintegration of the thin-boiling starch and that of the dextrin with potato starch than with the cereal starches. In other words, the disintegration of potato dextrin tells us something about the structure which does not appear to agree with the informationwe derive from the study of the phenomenon in thin-boiling starch. How to explain and reconcile these differences is a complicated problem which will be left to future investigations.
Canna Starch The canna starch granules (Figure 43) are among the largest granules known; the average size is considerably greater than that of potato granules. Tliey are elliptical in form, mostly oyster-shapd, and show orna,mental striations. In the early stage of pasting (Figure44) the outer thinlayer cracks readily and gathers into numerous skin folds. As the temperature increases, the latter are obliterated and the granules show a tendency to curl, which is evident even in the irregular mass formed a t the stage of complete gelatinization (Figure 45). A thin-boiling canna granule, gelatinized at 95' C., is shown in Figure 46. It is similar to thin-boiling potato starch in so far as i t shows the same mode of swelling in partial units of the granule. The chain of units is curled but there is no pronounced ring formation. The granule has a curious shrimplike appearance, due to the unequal manner in which the process of hydration and disintegration has taken place in the different parts of the granule. Here and there are indications of micellar structure. However, the diffraction seams on the outer side of the curvature of the granule should not he confused with striations.
13
Figure 17 shows canna dextrin disintegrating and dissolving in a water-glycerol mixture. The mode of disintegration is tlie same as that of the dextrins from tlie other starches. Figure 47 is unusual; the photograph irappened to be taken just at the right moment to show the procedure in a rather spectacular manner with the flimsy l a y e r s f l o a t i n g away gossamer-like, to be rapidly dissolved. I n trying t o form an idea of the structure from observations o n this s t a r c h we a r e faced w i t h t h e s a m e difficulty 9s in the case of potato s t a r c h . The dextrin picture (Figure 47) shows a beautiful example of uniform s t r u c tural a r r a n g e ment throughout thegranule. On the other hand, the thin-boiling modification (Figure 46) indicates great
74
INDUSTRIAL AND ENGINEERING CHEMISTRY
irregularities in this respect. The puzzle may perhaps be solved in the future through detailed and systematic investigations.
Ground and Crushed Starch Granules During the last years several workers have carried out investigations on starch structure and related problems (3) and have advanced the theory that the structure of the starch granule is a “growth structure” which resists gelatinizing under ordinary conditions and is affected by contact with water from the outside only a t higher temperatures. If, however, the granule is mechanically injured, for instance by grinding, the structure is damaged or broken down and the granule gelatinizes in contact with cold water. On the other hand, if the granule is broken or split along the natural cleavage planes, the swelling should be insignificant, if any, and the broken granules or parts should retain their original shape. Even if the arguments and assumptions in favor of this theory appear rather vague and the investigations so far have not furnished sufficient material for a satisfactory proof, the theory nevertheless represents an important effort to unravel some of the mysteries in the behavior of starch. Some facts from the writer’s experience, illustrated by the photomicrographs to be described, seem to confirm the theory in principle. Figure 48 shows the appearance of corn starch (suspended in oil) after it has been subjected to long continued grinding in a ball mill. Corn starch has a dense, tough, compact structure compared to the majority of starches; and while all the granules are more or less deformed, broken parts are rather scarce. I n many of the granules a bubble is seen in the hilum (sometimes as a black spot, depending upon the focusing) which has probably been formed from moisture vaporized by the heat generated during the grinding. Figure 49 shows the gelatinization of the ground starch in contact with cold water. It is well marked, although the swelling is not so great as that of the original starch with hot water. There is a time element in the reaction; the maximum of swelling is reached rather slowly. This, then, is an example of abnormal hydration due to a deformation of the structural arrangement in the bruised granules. As an illustration of the reverse condition-that of granules broken along natural cleavage planes-Figures 50 and 51 have been selected from a number of reproductions of crushed granules. Figure 50 shows an ordinary and 51 a thin-boiling potato starch. The crushing was done by pressing a cover glass down upon a few drops of a water suspension of starch spread on a slide. The cleavage of the granules in Figure 50 is rather irregular, but in a general way i t has taken place along natural planes; although the direction of the planes is not distinct, they converge towards the excentrically located hilum. The structure is largely preserved and the cracks are mainly along natural boundary planes. There is consequently very little swelling, in accordance with the theory. The effect of the crushing as shown in Figure 51 demonstrates the relative brittleness which is characteristic of thin boiling starches in general. The granules are broken up intc numerous wedge-shaped pieces, approximately along natural planes of cleavage in a radial direction. The fragments show no perceptible signs of swelling. In potato starch the striations run, as a rule, in a concentric direction, Radial striae are rarely seen. One would therefore expect to find concentric cracks in the crushed granules, but there is no indication of cleavage in that direction. Other starches with concentric striation-for example, rye starch-show, when crushed, occasional cracks roughly parallel to the outline of the granule, although these cracks cannot be coordinated with the striae. The typical direction of
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cleavage, however, is radial, the cracks extending more ok less to the center of the granule, depending upon the pressure exerted. Another example of splitting along natural cleavage planes is shown in Figure 4. The large granule in the picture is damaged and a sector is missing; but the separation has taken place along structural partitions and there is, consequently, scarcely any swelling and no gelatinization, such as is visible on the damaged granules in Figure 49.
Flaked Products (Cold-Water Pasting Starch) For a number of years there have been on the market starch preparations of varying nature and origin which form a paste with cold water and are used for a number of purposes. The principle of their manufacture is always the same: starch or the starchy part of grains, containing varying amounts of moisture, are forced between heated rolls and pressed out in the form of a thin sheet which is broken up and ground. I n this operation the starch is gelatinized and the granules lose their identity. The ground particles appear under the microscope as more or less glassy pieces of irregular shape. As in many operations which are simple in principle, there are difficulties and complications in the production of these modifications on a commercial scale. Great care and close control of the details of operation are required in order to obtain products of special properties, suitable for various definite technical needs. However, these products have in common that they gelatinize instantly in cold water, the extent of hydration varying with the condition of the product and the origin of the starch. They are of interest as examples of reversible gelatinization and serve also to prove that, after the starch structure has been destroyed, there is nothing in the way for rapid hydration, provided that the shape and size of the individual particles is such that the water has easy access to the starch substance. Two photomicrographs illustrate the behavior of a coldwater pasting starch modification. Figure 52 shows a t a magnification of 90 times a flake of Amijel, a commercial corn starch preparation, mounted dry on the slide. The dark parts on the flake are due to the optical effect of enclosed air bubbles. Figure 53 shows the same flake after addition of water. There is instant gelatinization and consequent increase in volume. The refractiveness is largely lost and with that the contrast between object and field, but there is sufficient distinction left to show the change in the appearance of the flake.
Literature Cited (1) Hanson, E. A., and Katz, J. R., 2. physik. Chem., (A) 168, 341 (1934). (2) Katz, J. R., in Abderhalden’s Handbuch der Biologischen Arbeitsmethoden,Abt. 11, Teil3, Heft 6 (1934). (3) Katz, J. R., and eo-workers, numerous articles in 2. physik. C h m . , especially during 1934; for a review of theories, see van der Hoeve and others, RolZo&%Beihefte,39,104 (1934). (4) Meyer, A., “Untersuohungenilber die Stilrkekfirner,”1896. (5) Nilgeli, “Die Stiirkekamer,” 1858. (6) Samec, M., “Kolloidchemieder Stirke,” 1927. RH~CE~IVHID May 23, 1936. Presented as part of the Symposium on Starch before the Division of Agricultural and Food Chemistry at the 89th Meeting of the American Chemioal Society, New York, N . Y . , April 22 to 26,1936.
