Mechanism of Development of Pigment Value in Zinc Sulfide I. Theory of Development of Pigment Value and Methods of Test LINCOLKT. WORKAKD I. HERBERTODELL,Columbia University, New York, N. Y. depend largely on the nature of The experimental data to be presented later the pigment surface and on the vestigation is to study show that the deeelopment of pigment properlies amount of such surface exposed. t h o s e f a c t o r s in the in the calcination of zinc suljide is closely related Therefore, the hiding power may manufacture of zinc sulfide which to changes in particle size. Such changes can be be expected to increase as the pigcontrol t h e ultimate p i g m e n t evaluated by tests for common pigment properties, ment is subdivided until the div a l u e to be obtained. In the mensions of t h e particles apmanufacture of this pigment, as including obscuring power, tinting strength, and proach the range of t h e w a v e in the case of some other white oil absorption. Further information on particle length of l i g h t , w h e r e u p o n pigments, the process consists in size is obtained through microscopic obsereation further d i m i n u t i o n of size reprecipitation followed by caland x-ray &fraction patterns. Other changes duces the reflection and refraccination. Lithopone, z i n c sulin chemical and physical properties are evaluated tion effects, so that the hiding fide, and t i t a n i u m oxide are poQ-er falls off. Thus there is among the pigments so treated. by means of refractive index determinations and an optimum size of p i g m e n t In such a process the precipiby chemical analyses. giving the maximum of h i d i n g tated material usually has little A new method has been deceloped and tested for p o v e r . I n g e n e r a l , hiding of that p r o p e r t y which lends evaluating changes in speci$c surface and particle power is d e t e r m i n e d b y t h e opacity or hiding power to the size. This is the equilibrium sorption of water nature of the substance and its paint, and it is t h a t q u a l i t y physical f o r m , with refractive which is increased by calcination. vapor under constant conditions of temperature index and Darticle size as domiThis improvement in the propera& relative humidity. nant factors, and some of these ties of the pigment brought about factors must account for the deby calcination has been variously attributed to a decomposition of hydrated materials of low re- velopment of pigment values in calcination. As for the chemical composition, there is likely to be some fractive index, to a change from an amorphous to a crystalline structure, and to an increase in the size of the ultimate par- action of the atmosphere of the calcining chamber on the zinc ticles of the pigment. The part played by precipitation con- sulfide. Although the atmospheres are chosen for their ditions in the character of the final product is also an open ques- inertness, many of the gases used in practice do react to a tion. This research was therefore undertaken in order to evalu- slight extent with zinc sulfide. For example, steam reacts ate the nature and extent of the factors involved in the acquisi- a t high temperatures to form zinc oxide and hydrogen sulfide; tion of pigment values through calcination, and, as a corol- carbon dioxide, often present in the form of flue gas, reacts to lary, the effects of the conditions of precipitation on these give zinc oxide, sulfur dioxide, and carbon monoxide. The possible effects of these reactions on pigment properties must changes. The choice of a specific material was influenced by several be considered in any thorough study of the problem. Another type of change sometimes classed as chemical is a conditions. Zinc sulfide was selected because it is a pure compound used to some extent as a pigment, and also because loss of water. The raw precipitates contain a certain amount it is the chief pigmenting ingredient of lithopone. Further, of water united so closely with the sulfide that it cannot be its crystal characteristics and tendencies toward compound removed by drying a t temperatures in the neighborhood of 100" C. The nature of this union, whether physical or formation are reasonably well known. chemical, has not as yet been definitely established. It has DEVELOPMENT O F PIGMENT VALUE been stated by Coffignier (3) that the precipitate obtained When normally precipitated zinc sulfide is calcined a t a from alkaline solutions is a hydrate, ZnS H20, or Zn\/OH temperature of 500" to 900" C., the material acquires certain SH ' characteristics which make it valuable as a pigment. Prob- and also, by de Stuckle (17), that a series of hydrates of varying ably the most significant of these is hiding power, or the composition is found a t different temperatures up to red heat. quality imparted by the pigment to the paint whereby the On the other hand, a standard reference book (15) makes latter is enabled t o hide or obscure the background over the statement that freshly precipitated zinc sulfide contains which it is spread. The usual raw precipitates are more or adsorbed water which is gradually removed on drying. To less deficient in this respect, and, although the hiding power illustrate this idea of adsorption as opposed t o hydrate forniamay be altered to some extent by varying the conditions of tion, there are the experiments of Glixelli (5) in which the precipitation, it reaches its full development in present prac- amount of water remaining with zinc sulfide under different tice only by means of heating to an elevated temperature conditions of drying up t o 100" C. showed no definite ratios, In the hiding or obscuring of the background, the particles but varied with the conditions of precipitation. (In this inof pigment in the paint film must intercept the incident light vestigation no effort is made t o differentiate betm-een absorprays. This result can be accomplished in three ways: by tion and adsorption, the general term "sorption" being used absorption, by reflection from the surfaces of the particles, in any case, with the one exception of the oil absorption test and by bending of the rays due to a difference in refractive which has a special meaning to the paint technologists.) index between pigment and vehicle. The actions obviously The absence of definite hydrates in the precipitates is also 411
T
HE purpose of this in-
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‘Tlie coialitiuns of precii’itatiori alfcet the clmacter of tlre raw preciiitate, and this inaterial uiidergocs further cliatiges in calcinntion. These clinnges relate to loss of water, to growtli of crystals, :ind possi1:ly also t o variations ill chemical composit,ioii and refractive index. ILi.i\.e\.er, the exact rlatiire of tlieso alterations has not becii previoiisly evaliiatrd. :\\-AI I.AB1.E
~~ETHoDs OF T
A qrtniitit:itivc stiidy of precipitation and calcirration varial,lc:s has lieen made by precipitnt.ing zinc SUIfide unrler a few conditiolis and calciriirig these precipitates at different t e m p e r a t u r e s for difiercnt lengths of time, aiid by tlien observing tire dcveliqrment of pigment value in cacli case. To evaluate tire results of these tests a niiiiilxr (if rrretliods is a v a i l a b l e . The rciiioval OF m t c r i t 1 the calcination of zinc 8111fide (wi iic studied liy determinations ol thc water wntent, of the sampler;. The refraetivc index bas a I~eariiigon t,lie cliemicd coriilxrsition n r r d ervstalline form. 1”or the iitudy of changes there is a considerable variety of inetliods, the fundainental one being a direct microscopic m e a s u r e m e n t . Otlrera ivliicli are relative include the DebyeSclierrer x-ray pattern and the nieasiiremcnt of turbidity or opacity. Another groupof relative methods is found in t h e c o m m o n or standard tests triade oii wliite pigments, inclodiiig Iiidiug power, tinting st,rcngth, and oil absorption. While direct and complete measiirerneiit of the size characteristics of 11 pigmotit or a raw precipitate is not always feasible, rcasonably definite infomiation can be ubtained by combitiing tlir dat,:i OII several properties which are functions of particle size. The folhiwing is a discussiiin of tlie availablc methods for
“cryptu-eryrtaliiiie~~ cxiriditiiin, wliicli woiilii Ije likely to increase t,lx rcflcctiiig timl refrarting elfeot. of the gartides. lloucever, t.lrc work of J e v i and l ~ o n t n t m( 1 4 ) :mil of Ilaiicr (7) slirnved a definite x-riiy pattcrn uf sphalerite i n t.tic r a s e of ziitc sulfide jirecijritated brit not calciiied. I t appears tlierefor? tlint tlie increase in tlie hiding p w o r of zinc solfkle on c:iIaiiiatiiiii is riot priiriarily doe t,n tile devciopnrriit~ (ifa cva1ir:iting tlre derelopment. of pigment properties, and a deeryst,allins stmrture from an arriorlhoits ~~recipitate. s~rilitiotiof tlie procedures chosen. It sc.ims triore p n ~ l ~ a l ~ that l c tlie nttainirmit of ~iig~iicirt W.crm< CO. vwliie is eaosed by cliange iri iiimticlc size. The knowledge tlrtrt the raw precipitates are very fine, sootetirrres semi-colThe water content mas determitied as tlie loss in weight of liiidal, loads to the concept that t,lie increased liirliiig power the sample on heating iii an atmosphere of nitrogen a t is atttiiried by gmrvtli of these extrernelg niinute particles 000” C., tlie nitrogen being dried by calcium chloride aud d ~ l o w r d or beyond tlit: size giving niaxitriiriri hidiiig powcr. oxygenated by passing over a Iiot copper gauze. The percent\Vhile this view has not l’iiiiiid miicli expression in published age was cliecked 0.y alisorbiiig tire evolred water in calciiiiii litcrat,iire, it is rather videly believed in the industr.v that cliloride and rioting the gain in might. gnnvt,li of particles takes place ill the caleitiation of prccipittrted pigrrient,s. llowerer, qitantitative rncasiirctiieiits are c:MEMIcAT, Bsar.rars lacking to est,alilisb tlie fiiets of crystal gruwth and tlieir TIie changes iii clieiiiical coiriposition resulting from tire rrlatioii to the conditims of ciilcinutioo. aatioii US tlic steam on t i l e zinc sulfide mere followed by deterIn considering this qiiestion of particlc initiations of total zinc and total sulfur in enough s a m i h ctilrinwtioii, :itteritiini slionhl be givan tu t to follow the trend of tlie reaction. The zinc was detrr\-:rriat,ions iii the jsart,iclc size of the prccipit :is rail materia1 fur tile calcitiatiiiri. Such variations might be inined by dissolving tiit: sample in hydroeliloric acid and titrating d l r ferrocyanide, rising uranyl acetate i ~ san out,expected to oeoiir nlren the rnmiicr of precipitation is iiltered, side indicator. Td,al sulfur was deteriiiincd by the I,ung? since it is comimiiily kiioi~int,liat the furiri and ap1x:nraiicr of method of oxidieiiig to su1f:ite with firiiiirig nitric acid arid i,tmy lirecipitatcs can be viiriid 111, cliniigiiig tlin eonditiuii3 ium clilorate, a i d precipitating with barioin chloride. of t,lrcir ,precipitation. The laws gworiiirig thcic rclntioiis have i m n forritrtlated, imiii tlrenrctinsl i,~j,i,~i(ler~iti(iiis aiid ~ Q U i J . l l l I i l U M hfiiiSl‘UltE ~ O l i P T I o X imictical experiment,,-, iiy voii Wcitrterii ( x i ) . He coiicliidcs As oiie irretlrod of evaliiating clianges in particle Size atid that the fineness of precipitatd p;irt.iclcs is Irriqiortioiial d tlie degrer of siipcr~~~tiiratiin, and irivcrsely to tire i t l r f a r e chiiractcristies, the samples were tested for a property of the jrrt:eipit,ated siitistancc iri tlie mediutn from wliicli has not, bccn cmiiriiinly considered as a method for w h i c h precipitatioii occurs. This Ixing tlir I the Iinrtiole size of Iiigmenta~~ia.mely, the equilibriiim s o r p of a oiric siilfide precipitate is pro1,aIily aflectcil by a tioir of water vapiir under definite condit.iiins of terriperat.itrr~ tirtiriber r l f factors, siicli as temperature, coriceirt.ratior1, t i r i d relative Iiiioiidity. Since this sorption is an action of :xidit,y, arid degree UF agitation, and t,licse factors rniiy like- tire surface, it is apparent that the arriouiit of gas or vapor t,he clrnracter of tlic final cnlcinnl product. wise t;aken nl? by a given rnass of the same powder under any This completes the somnmry of present knowledge regard- givcii external conditiritis will depend, among other things, ing the deyelopiiii~iit of iriniirnt prolicrt,ie.; i n airic! s o l f i d c , oii t,lie arnniiiit of iiirfacc exiinsd. ?ici:ording to the Freund-
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E N G 1 N 13 E R I N G C H h: M I S T 1%Y
Vol. 25, No 4
Fzoune 3. Mrcaosco~icP H ~ J E C ~APPARATUS ION (Elutriation freotion of groand rook is shown.)
are likely to be complicated. The same turbidity might result from two different types of size characteristics, as has been shown by Stutz and Pfund (18). However, it is likely that such measurements can be used to obsewe the direction and approximate extent of the changes in particle size which occur in the calcination of zinc sulfide, provided that it is known on which side of the optimum size the materiale lie. This assumes, of course, that the effect of other physical or cliemical change is t,aken into consideration. The methods devised for the determination of turbidity are varied. The simplest instruments, such as the Jackson t.urbidimeter (11), use very dilute aqnwus suspensions and measure the depth of a column of suspension required to obscure a light image at the bottom of a tube into which the suspension is poured. Similar instruments have been devised by others. A somewhat different class of instruments measures the intensity of light transmitted by the suspension. Stutz and Pfund, for example, make this measurement by matching the transniitted beam against the intensity of a variable lamp. In this study, turbidity was measured with an improved form of the Jackson turbidimeter. I n this instrument the tube is surrounded and covered with a dark box, and the image of a cross-shaped opening in a metal plate below the tube is observed in an inclined mirror at the top of the box. Constant intensity of light is obtained by means of an electric lamp, and the apparatus is arranged so as to exclude practically all external light from the eye of the observer. With care the readings so obtained can be relied upon within a variation of *2 per cent. The applicability of this method to pigments was tested by turbidity measurements on a series of microscopically sized samples of zinc oxide. The results showed a smooth curve of turbidity us. average particle size, similar to that obtained by tltutz and Pfund, with a maximum of turbidity a t approximately the same particle size (0.23 micron). I n preparing a susponsion, a 0.25-gram sample was ground for 8 minutes in a glass mortar Tvith 0.05 gram of saponin and 0.1 gram of y m arabic, in 1 cc. of a 0.05 N solution of potassium ferrocyanide. This is the procedure used by Stutz and I’und. The resulting thick suspension was diluted to 100 ce. and allowed to stand for an hour, a t which time the bubbles
had disappeared. Small portions of this susperision were then measured to give the desired concentrations of solid. Readings of depth of column for extinction of the cross were made a t concentrations of 0.05, 0.025, 0.0125, and sometimes also a t 0.00625 gram of solid per 1W cc. The plot of depth for extinction 1:s. dilution-i. e., reciprocal of concentration-was a straight line. Prom this was read the dilution, in hundreds of cc. per gram, required to give extinction with a column of IO-cm. depth in the tube. This method gives a relative turbidity for comparison of samples. HrnrNo POWER.This property has been defined as an obscuring of the background under a paint. Its dependence on particle size follows laws similar to those applying in the case of turbidity, but the laws are probably not quantitatively the same, and have been less exactly worked out. It is known, however, that there is an optimum size for hiding power in the same ranye a8 that for maximum turbidity. Therefore, if the material is growing in size toward the optimum value, this growth can be followed, at least qualitatively, by a study of the increase in hiding power. A variety of instruments has been developed for the measurement of hiding power. The choice here was dictated by considerations of the necessary precision combined with adaptability to the samples available. I n preference to the instruments which quantitatively measure the changing contrasts io brightness or changes in absolute brightness with inweaving film thickness, it was decided to use the type which measures the thickness of a wet fdm of paint required to render a black-white contrast invisible to the naked eye. The instrument used was that form of the Pfund cryptometer (18) which measures the film thickness required to obscure the contrast between the white paint itself and a black glass plate. Measurements in this study were made on paints containing 5 grams of pigment in 4 cc. of pale linseed oil. Ten readings were taken for disappearance and ten for reappearance of the line of demarcation, and from the average of these twenty readings, using the table furnished with the instrument, the hiding power was obtained in square feet per gallon. The samples were all tested by the same observer, and the average deviation of the twenty readings on any one paint was usually 2 or 3 per cent.
April? 1933
INDUSTRIAL AND ENGIKEERING CHEMISTRY
TINTIKG STRENGTH Another important property of a pigment is tinting strength, which may be defined as the power of maintaining its own color or brightness when diluted with one of another color. In the case of white pigments the strength is usually measured by some comparison with a standard white pigment, as to the quantity of carbon black or ultramarine blue required to produce a certain gray or blue tint n-hen mixed with the white pigment. The matching of tints is ordinarily done by eye, with pastes of pigment mixed with linseed oil, although more refined methods have been developed. Since tinting strength is usually a t least approximately proportional to hiding power, and is sometimes used as a measure of hiding power, it is reasonable to conclude that tinting strength also is related to particle size. There is an optimum particle size, analogous to that in the case of turbidity and hiding power, below which reflection and absorption fall off and the tinting strength decreases. The exact relations between particle size and tinting strength, however, are not so well understood. The relative particle size of the white pigment and the dark diluent is likely t o have some effect, since fine particles of the latter may group themselves around larger particles of the white material. However, tinting strength measurements do furnish information as to particle size changes, and are desirable as a supplement to the obscuring power tests. The tinting strength tests were made by dilution with a black pigment consisting of carbon black plus precipitated chalk. The procedure was similar to one used by Hallett (8). -4 standard was made by tinting a sample of commercial white pigment with yellow to match the slightly yellow tint of the samples. The tests were carried out by mixing 0.05 gram of the black pigment with 1 gram of the sample being tested, rubbing out in pale linseed oil on a glass plate with a glass muller, and determining by visual observation how much black must be mixed with 1 gram of the standard to match the gray tint of the darkened sample. The ratio of amount of black used with sample to amount of black used with standard gives a figure representing relative tinting strength, conveniently expressed in percentage of standard strength. The quality of oil was varied so as to give equal consistency in all cases. The precision may be considered 1 5 per cent. MICROSCOPIC OBSERVATION The basic method for the determination of particle size naturally is a direct measurement of the dimensions of the particles. A number of procedures have been developed for such measurement, and several of them have been recommended as standard methods to the American Society for Testing Materials ( 1 ) . I n these procedures the material, dispersed in a suitable medium, is placed under a highpowered microscope and observed, either directly by projection on a screen or by means of a photomicrograph. A count is made of the number of particles in the various size intervals so as to give a reliable average size, and the results are also reported in the form of curves showing frequency or weight percentages in the different intervals. The accuracy of such measurements is limited by the possibility of errors in the elaborate apparatus and technic required, by the difficulty of obtaining representative samples in the small quantities used, and by the fact that only two dimensions of a particle can be seen. Moreover the applicability of the method is limited by the resolving power of the microscope. Particles less than about 0.25 micron in diameter cannot be measured with dependable accuracy. This difficulty excludes any complete measurements on precipitated or partially calcined zinc sulfide which contains considerable submicroscopic material. By means of photomicro-
415
graphs made with ultra-violet light according to the method of Haslam and Hall (9), it is possible to extend the range to somewhat smaller sizes. Owing to the lack of ultra-violet equipment, and also to difficulties with dispersion in some cases, completely accurate measurement was impossible on most of the samples in this study. However, in order to have visual evidence of the changes in particle size evaluated by other methods, photomicrographs of several of the products were obtained xith as good dispersion and resolution as was feasible. The Green turpentine-damar method of dispersion as reported to Committee E-1 (1) was used and the pictures were taken with blue light. Approximate microscopic counts were also made on a few of the samples using turpentine-damar dispersion and observation by projection. The size intervals were slightly larger than those of the A. S. T. hl. method. The count was made on the basis of short diameter in the plane of the microscope. These counts furnish general indications of the size characteristics. X-RAY PATTERNS Another method which appears to show some promise for the measurement of very fine particles is the DebyeScherrer x-ray pattern ( 2 ) . A fine beam of monochromatic x-rays is sent through the powdered material, and the resulting diffraction pattern is photographed. This pattern in its entirety is a series of concentric circles caused by cones of rays striking the plate, but a segment only is usually photographed, resulting in a series of bands. The possibility of determining relative particle size lies in the fact that, as the particles become extremely fine, the bands become broader and more diffuse. This x-ray method is a relative rather than an absolute method of measuring particle size, and gives little or no indication of the distribution of sizes. However, it can be used in a qualitative way to follow the growth of zinc sulfide particles, a t least in the early stages of calcination. X-ray patterns by this method were photographed by Paul F. Kerr of the Department of Mineralogy, Columbia University, on several of the samples prepared in this study. REFRACTIVE INDEX As additional evidence bearing on physical and chemical changes in calcination, refractive indices were determined on a number of the samples. The tests were made by the immersion method of Larsen (IS) in which small lumps of pigment are imbedded in solidified melts of materials of known refractive index on glass slides and observed in a microscope with central illumination. When the objective is moved up from the position of perfect focus, the bright bands around the lumps of pigment move inward if the pigment has the higher index, and vice versa. The index is thus measured usually within *0.01. The known materials were furnished by the Department of Geology and Mineralogy of Columbia University. SUMMARY The above methods form a series of tests for tracing the development of pigment value in the calcination of zinc sulfide. The data on the various properties related t o particle size, taken as a whole, will indicate the extent of crystal growth under different conditions. With the addition of the other tests, it will be possible to draw definite conclusions as to the nature and extent of all essential changes in the calcination. LITERATURE CITED (1) A. S. T. M. Committee E-1, Proc. Am. Soc. Testing MateriaZs, 30, Pt. I, 923-7 (1930).
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(2) Bragg, TT'. H., and Bragg, 11'. L., "X-Rays and Crystal Structure," 4th ed., pp. 132-4, G. Bell & Sons, London, 1924. (3) Coffignier, C.. Ret. chim.ind., 18, 5-8 (1906). (4) Gardner, H. A,, "Physical and Chemical Examination of Paints, Varnishes and Colors," Chap. 5, Paint Mfrs.' hssoc. U. S., 1922. (5) Glixelli, S., 2. anorg. allgem. Chem., 55, 297-320 (1907). (6) Grohn, H., Farben-Ztg., 33, 1 6 6 0 4 (1928). (7) Haber, F., Ber., 55B,1717-33 (1922). (8) Hallett, R. L., Proc. Am. Soc. Testing M a f e r i a l s , 22,Pt. 11, 52331 (1922). (9) Haslam, G. S., and Hall, C. H., J . Franklin Inst., 209, 777-89 (1930). (10) Heaton, iY., "Outlines of Paint Technology," pp. 90, 96, Charles Griffen & Co.. London. 1928. (11) Jackson, D. D., Tech. Quart., 13, 278 (1900); bluer, H. F., J. ISD. ESG. CHEW.,3,553-7 (1911). (12) Klumpp, E., Kolloid-Z., 55, 3.18-51 (1931).
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(13) Larsen, E. S.,U. S. Geol. Survey, Bull. 679 (1921). (14) Levi, G. R., and Fontana, C. G., Atti accad. Lincei, [GI 7, 502-8 (1928). (15) Mellor, J. W.,"Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. Is', p. 607, Longmans, 1923. (16) Pfund, A. H., J . Franklin Inst., 188:675-81 (1919): 196,69-7b (1923). (17) Stuckle, H. W.de, U. S. Patent 884,874 (1906). (18) Stutn, G. F., and Pfund, A. H., ISD. EKQ. CHEY., 19, 51-3 (1927). (19) Wagner, H., and Pfanner, H., Farhen-Ztg., 34,2513-14 (1929). (20) Weimarn, P. P. von, "Die Allgemeinheit des Kolloidaustandes," 2nd ed., Theodor Steinkopff, Dresden, 1925; Alexander, .J., "Colloid Chemistry," Chap. 2, Chemical Catalog, 1926. R E C E I V E D.iugust 29, 1932. Presented before t h e Division of Paint a n d Yarnish Chemistry a t t h e 84th X e e t i n g of t h e American Chemical Society, Denver, Colo., .Lugus1 2 2 t o 2 6 , 1932.
Inversion of' Sucrose by Invertase a t Low Temperatures Preliminary Report &I.A. JOSLYNAKD \I. SHERRILL, University of California, Berkeley, California URING the course of investigations on the chemical and physical changes occurring in fruits during freezing and subsequent thawing, a marked inversion of sucrose was observed in certain crushed fruits packed with sugar and stored for a period of 8 months or longer a t temperatures of -16" to - 12" C. . From 10 to 50 per cent of the sucrose added to crushed Cuthbert raspberries was inverted during storage for a period of 2 years. The storage temperatures during this period mere subject to some fluctuations; temperatures as high as -4" C. were reached and held for several days. Cnder similar conditions 25 to 80 per cent of
-12" C. for a year. Practically no inversion of sucrose wa5 found in California cling peaches. Since no studies of invertase activity a t these temperatures had been reported in the literature, a preliminary study of the rate of inversion of sucrose by invertase during freezing storage was made. I n these studies, "red label" dry yeast invertase scales, free from melibiase, procured through the Xulonioline Company, were used. Cane sugar solutions of varying concentrations were prepared, cooled to 0 " C., adjusted to a pH of about 4.5, the reported optimum' pH for yeast invertase (0, and mixed in the cold with qufficient amounts of invertase suspension to yield concentrations of 0.001, 0.01, and 0.05 mg. per cc. of resulting solution, respectively. T ~ B LI.E DEGREE OF INVERSIOX OF SUCROSE IS R ~ ~ P B E R R I E S ASD STR~WBERRIES FRUIT
SUCROSE IVVERTED T h a u e d at room temp for 98 hours
RITIO OF FRUIT T h a u e d in TO S U G A R boiling u a t e r
70
I 30
DAYS
70
I 75
IW
FIGURE 1. RATEOF
DECRE.4SE OF POLARIZ.4TION IX S U G 4 R SOLUTIOSS OF v-ARYIVG CONCENTRdTIONS COSTAISING 0.01 MG. ISVERTASE PER cc. OF SOLUTIOS (Curve based on black dots represents decrease of reading for solution whose initial reading i s 54.4)
the sucrose added to crushed California Banner strawberries was inverted. The degree of inversion increased with decrease in the concentration of added sugar and increased with increase in time of thawing as shown in Table I. There is some indication that complete inversion of added sucrose occurred in frozen Hachiya persimmons stored a t -16" to
Although care was taken to mix the samples thoroughly, small variations in the initial concentration of the control samples and in the pH values in samples stored for 55 day; indicated that these samples were not entirely homogeneous. The variations were particularly noticeable in the samples of higher density which were fairly viscous when mixed. HOWever, considerable care was used to obtain the proper concenA t t h e present nothing I 1 This 18 t h e o p t i m u m a t room temperature. k n o u n a b o u t t h e possible changes in p H of t h e frozen solution a n d of t h e possible changes in t h e pH optimums of enzymes in t h e frozen s t a t e Thls pH value, however, approximates t h a t of t h e fruits i n which inversion of sucro3e was ob-erved.
