PHOTOIIICROGRAPHIC STUDIES OF SUCROSE CRYSTALS BT SYBIL WOODRUFF ASD HELEN VAN GILDER
Introduction Photomicrographs of fondants make it possible to compare the magnitude of crystals forming in syrups of varying composition. The sucrose crystals present in a fondant type of candy of good quality are of such small size that they cannot, be felt by the tongue as distinct particles. h high-grade product is said to be “creamy” or smooth in texture when it is entirely free from the grit of detectable crystals. It is rather surprising that the tongue and roof of the mouth give a tactile response to particles so slightly greater in size as are those of perceptibly coarse fondant compared with the crystals of a finely-textured one. The measurements of crystals accompanying these photomicrographic records show that in two fondants which would be rated as fine and slightly gritty respectively, we are able to sense a difference in crystal dimension of but 6 or I O microns. The factors which determine the size of crystals developing in a syrup are then of considerable importance if a difference in size of about I O microns between two samples represents the difference between a candy of poor and one of good texture. Besides these crystals of maximum size in a fondant there are many others barely visible at a magnification of 360 times and furthermore there are assuredly present still others of colloidal dimensions. The dispersion of these minute crystals of sucrose in a continuous phase of concentrated sugar syrup produces a product which is plastic and one whose consistency depends very largely upon the ratio existing between the two phases, and, therefore, in turn upon the composition of the syrup-phase. Rnce the presence of other sugars increases the solubility of the sucrose, the relative amount of syrup will be greater and the consistency of the fondant softer and more pliable the greater the amount present of sugar other than sucrose. The size to which sucrose crystals will grow as the mass of candy “grains out” depends upon physical conditions as well as upon the composition of the syrup. Cooling the syrup to a highly supersaturated state before agitation is permitted, favors crystallization a t countless points simultaneously and thereby reduces the opportunity for crystal nuclei to build up to great size. A syrup may contain such a proportion of invert sugar to sucrose that crystals of small size would be expected t o form but, due to faulty manipulation, opportunity may be given for the particles to grow to a size exceeding that found in an acceptable product. Examples of manipulative errors which might so hinder the desired graining of sucrose are failure to continue agitating the syrup until crystallization is complete, the accidental seeding of the hot syrup, or the cooling of the syrup in a vessel with rough surfaces. Thus * Department of Household Science, University of California, Berkeley.
1356
SYBIL WOODRUFF AND HELEN V A T GILDER
syrups containing even I 5 percent reducing sugars can form coarse-grained candies under circumstances such as these: on the other hand, fondants of surprisingly fine texture and plasticity can be obtained when no reducing sugar a t all is present, providing the physical conditions for crystallizat,ion are cleverly controlled. The only other published photomicrographs of fondants are those by Hallidxy and Koblr,' which illustrate only the differences caused by manipulative procedure and not those of different syrup composition. The photomicrographs of our study illustrate thc effect upon the size of sucrose crystals of varying amounts of invert sugar produced by boiling sucrose in turn with hydrochloric, tartaric and citric acids, and with acid pot'assium tartrate; also the action of added glucose or fructose with reference to maximal crystal size. Measurements of the largest and smallest crystals in the micrographic fields have been made i n freshly made fondant and in that which had been stored for some time. Variations in the physical conditions attending Crystallization were avoided as perfectly as possible.
Experimental Methods Preparation offondant. One volume of sugar to one-half volume of water, or zoo grams of commercial sucrose to 118 grams of distilled water, were boiled together in a beaker. h sugar solution of this concentration had a boiling temperature of approximately 103'C and the rate of boiling was always so adjusted that the syrup reached the desired concentration and a boiling temperature of 115' in 2 0 minutes. Analysis showed about 30 grams of the original 118 grams of water to remain in the final fondant. Such an excess of water a t the start was necessary since if much less than this was used there appeared to be an incomplete dissolving of the sucrose, thus making it less possible to control its recrystallization. The syrup boiled to I 15' was therefore seersaturated as soon as it began to cool and if permitted to cool to 40' before any nuclei of granulation appeared, agitation produced crystals of small size. The hot syrup was poured immediately into a shallow glass bowl, allowed to cool undisturbed to 40@,then was beaten with a wooden spoon until it had changed t o a plastic mass. It was kneaded smooth and stored in air-tight jars. The procedure was the same when acid was boiled with the sugar and also when other sugars were substituted for a part of the sucrose. It had been found by a preliminary test that 0.2 grams of potassium acid tartrate to 2 0 0 grams of sugar which is an amount frequently called for in the home-made article, gave a product of satisfactory grain and contained 7 . 5 percent of invert sugar on the basis of the moist fondant. Therefore, in substituting glucose or fructose for part of the sucrose, such an amount of each was used as would give this quantity of reducing sugar in the fondant. Photomicrographic Technique. The chief difficulty in mounting the specimen for microscopic observation was in spreading the fondant on the Halliday and Noble: "The Hows and Whys of Cooking," p. 139 (1928;
PHOTOMICROGRAPHIC STUDIES OF SUCROSE CRYSTALS
7357
slide in a sufficiently thin smear. A very small amount of the fondant from the center of the jar was transferred t o a drop of Canada balsam on the slide, a cover slip was pressed down upon it and moved to and fro between the thumb and forefinger until a thin layer of the specimen was obtained. A photomicrograph was made using a Leitz “Makam” camera and a magnification of 360 times. It was found that ordinary light with a ground-glass shield over the source, gave a better negative than did polarized light; also that a six-minute exposure under these circumstances was satisfactory. When samples were to be photographed after they had been stored, fresh smears from the center of the jar were prepared. Determination o j hydrogen i o n concentration. This measurement was made with a Leeds and Sorthrup quinhydrone electrode a t 25’C upon solutions containing I O grams of fondant in j o cc. The freshly boiled, distilled water used in making the dilutions had a pH of j.83. Determination of reducing sugar. The method used was the iodine titration modification of the well-known Munson and Walker one. The accuracy of the method for the case in hand was determined by checking the reducing sugar content of known combinations of sucrose with glucose or fructose or mixtures of the two wherein the reducing sugar varied from 7 to 30 percent of the total sugar present. The maximum deviation from the expected result was but, j percent of the reducing sugar known to have been added. In a fondant containing, as it well may, I O percent of its moist weight in the form of invert sugar or glucose, an error of 5 percent in the analysis of invert sugar would raise or lower the expected percent of invert sugar in the fondant to but 10.5 or 9.5. The characteristics of fondants do not change rapidly with small variations in the percent of invert sugar present; hence, it is believed that the accuracy obtainable with this method is sufficient for the purpose.
Discussion of Results Invert S u g a r produced b y Different A c i d s . The inverting effect of acids or acid salt,s other than potassium acid tartrate (cream of tartar) has not been studied in connection with candy making. A paper by Carrick? gave 6.28 as the percent, of invert sugar formed by boiling for I j minutes a sugar solution consisting of 2 1 0 grams of sucrose and 0.3 grams of cream of tartar; this quantity of invert sugar was said to have given a desirable plasticity to the fondant. Daniels and Cook3 have reported the invert sugar in a series of fondants containing from none to 1.2 grams of cream of tartar used with I I O grams of sugar and alkaline tap water. Their report did not describe the textures of the resulting candies, however. Before the days of commercial glucose fondant was made by boiling the ayrup with a small amount of “cream of tartar” and many rules for making the candy still suggest t,he use of acid. A study of this means of controlling crystal size was undertaken in a series of three fondants containing in turn 3
Carrick: J. Phgs. Chem., 23, j89 ( r y r y j . Daniels and Cook: J. Home Econ., 11, 65 ( 1 9 1 9 ) .
SYBIL WOODRUFF AND HELES V A S GILDER
1353
0 . 2 gram (an amount frequently recommended), 0.4 and 0.8 grams of potassium acid tartrate to zoo grams of sugar which served as a means of showing what effect the correspondingly increasing amount of invert sugar would have upon the size attained by the sucrose crystals. I t had been expected that when citric, tartaric and hydrochloric acids were used in such amounts as to give the same hydrogen ion concentrat’ion as t’hat shown by the “cream of tartar” samples, the same amounts of invert sugar would likewise be produced during boiling and therefore similar effects upon the crystallization of the sucrose. The actual results as they are recorded in Table I do not show this to be the case, liovvever.
TABLE I Amounts of invert sugar and pH of fondants containing various acids (Percent based upon moist fondant) Acid Sample Number
~b 4 a
b C
S a b C
Kind
Sone K-H-tartrate
,, ,,
I,
>I
,>
1,
Citric acid
,, ,,
,, ,,
6 a
Tartaric acid
b 7 a b
Hydrochloric acid
,,
1,
If
,f
Amount Milligrams
pH of Fondant Average
IO
5.33 4 . IO 3.82 3.73 4.72
20
4.20
40
3.85 4.37
2 00
400
800
IO
Average Invert Sugar Percent 0.42
6.49 11.10 15.80 7.00
16.30’ 43.43* 13.0j 23.55: 6.35
20
4.02
46 cc. 69 cc.
4.86 4 . 3 7 above ~ O . O O *
S o t e : The period of boiling to a temperature of I I j’C. was 2 0 minutes in each fondant. All syrups were cooled to 40°C. before crsytallization v a s induced. The amounts of acid are for zoo grams of sucrose. The HC1 was 0.001S . * Syrup did not crystallize. j
T Syrup crystallized after 50 to 60 minutes of beating !\-hereas others grained in from to 15 minutes.
There was so little relation between the manner and completeness with which sucrose crystallized from the syrup and the p H of the solution that it seemed as feasible a way as any to record results given by I O , 2 0 , and 4 0 mg. of citric acid, by I O and zo mg. of tartaric, and by 46 and 69 cc. of 0.001 N hydrochloric acid, The citric acid and hydrochloric acid fondants contained about’ 7 percent of invert sugar each, for instance, when their p H was but 4 . 7 2 and 4.86 respectively. This was approximately the same amount of invert sugar as that contained in the product carrying zoo mg. I
