Production of a Palatable Artichoke Sirup 11:. Hydrolysis of the Polysaccharide Material F. A. DYKINSWITH D. T. ESGLIS,University of Illinois, Urbana, Ill. With the object of producing a palatable sirup since alteration in fhe concentration of salts and f r o m Jerusalem artichokes withoui a process of other materials in the extract has but slight effect. crystallization, a study is made of the conditions firhen hydrochloric acid is employed as a source of favorable to fhe hydrolysis of the polysaccharides hydrogen ion, the optimum conditions selected for zchich would result in a minimum qf added re- hydrolysis are a p H of 4 . 2 f o r 20 minutes at 130" agents and the least detrimental effect upon the C. Cnder these conditions the added salt resulting f r o m neutralization of the acid is less than one per Jacor and character of the final sirup. I t has been shown that the buffer capacity of the cent and a n apparent small loss of .fructose as inextract is such that the equimlents of hydrochloric dicated by polarimetric and chemical tests is probacid necessary to produce a desired p H are in ably not significant. A brief consideration f r o m the standpoint of sirup nearly constant ratio to the total solids, and no benejt is gained by rarying the concenfration of production is given to the use of other acids, particuthe extract at which the hydrolysis is carried larly those whose anions m a y be removed by prccipitation, and to acidification by means of electrodiout. The celocity constants are determined for the hy- alysis. I n these methods the proper conditions for drolysis at 80", loo", 120", and 130" C. with hydrolysis, following acidification io a dejnile p H , cariations in p f I and total solids. T h e catalysis of m a y be selected by consideration of the data presented the hydrolysis is due primarily to hydrogen ions, for hydrochloric acid.
A
S STATED in Part I (j), it is hoped and beliekd that a palatable sirup, gradually improved, will eventually lead to the ultimate goal-the production of pure levulose a t a lorn cost. Practically all of the investigations associated with the utilization of inulin-bearing plants have been attempts t o prepare the crystalline levulose. From the standpoint of hydrolysis, these studies may be classified into two principal groups: In the first are those experiments such as have been reported by Jackson, Silsbee, and Proffitt ( I f ) , Hoche ( I O ) , and Kleiderer and Englis (IS), which were concerned n-ith the separated inulin. These findings will be of greater importance when procedures can be worked out whereby the proportion of carbohydrate material obtained as inulin can be increased. I n the second group are those experiments related to the hydrolysis of the entire water-soluble fraction which contains additional levulose-yielding material, not separated by the usual processes for the precipitation of inulin. There is present also in the extract considerable nonsugar material. Experiments of this type have been reported by Jackson and associates ( l a ) , Buchanan and co-workers (a),and Golovin and others ( 6 ) . They are similar, in that acidification is generally accomplished by addition of sulfuric acid followed by neutralization with lime. These hydrolyses, satisfactory for the purposes for which they were developed, are unsuited for the production of a palatable sirup. The factors that made it inadvisable t o make use of them will be discussed later.
the extent of the hydrolysis is not the sole governing factor. Attention must also be given to the character and amount of the ash resultant from the neutralization of the acidity of the hydrolysis, the amount of caramelizaticin, the development of color, and particularly to the effect of the mode of hydrolysis on the flavor and palatability of the finished sirups. From the economic point of view, the conditions selected must be adaptable to plant-scale production without excessive cost.
IXITIAL COSSIDERATIOXS
The fact that the rate of hydrolysis of inulin by acids is essentially a function of pH was indicated by Jackson and associates ( l a ) . It was further recognized that the nonsugar material of the raw extract had a marked influence upon the acidity of the solution. Buchanan and co-workers (3) have furnished additional information upon these points and have established an approximate relationship between the total solids of the extract and the equivalents of acid necessary t o produce acidities in a pH range of 1.0 t o 2.60. I n a study of the acidification of raw extracts by electrodialysis, the work of Hardy ( 7 ) shows that this buffering action of the extract material is due chiefly to the potassium salts of organic acids. In addition to pH, two other major factors affecting hydrolysis have been studied. These are the temperature and time required for the process. A complete hydrolysis n-ith a minimum destruction of levulose has been the chief matter of concern. The approved conditions selected by Buchanan SIRUPHYDROLYSIS REQUIREMESTS and associates (3) are: 60-minute time interval, a temThe production of a sirup requires a different consideration perature of 80' C., and a pH of about 1.75. of the hydrolytic procedure than when the product sought Although acidification may be accomplished by several is the crystalline sugar. In the latter case natural impurities methods, for the present only the acidification by means of and the reagents of the hydrolysis will not interfere since they added acids will be considered. The acids which may be remain in the mother liquor. However, when the product used are of two types: (1) acids whose anions can be resought is the sirup itself, special attention must be given moved after hydrolysis by the formation of an insoluble preto the character and amount of the reagents associated with cipitate when neutralized (sulfuric, orthophosphoric, carbonic, the hydrolysis. Further precautions must also be taken since and sulfurous) and (2) acids whose anions must remain in the 1165
1166
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydrolyzed extract usually as the sodium salts (hydrochloric and various organic acids). It is evident that, if the acid used is of the first group, then the quantity of acid added may be any convenient amount, since it will be removed later. However when acids of the second group are used, the quantity added is important since the neutralization results in the formation of soluble salts which remain in the finished sirups. Since the present work is concerned principally with
m
FIGURE1. BUFFERCAPACITY OF ARTICHOKE EXTRACTS
the use of hydrochloric acid, calculations are given in Table I11 showing how the amount of added sodium chloride varies with the quantities of added hydrochloric acid and the concentration of the extract, when the final sirup contained 82 per cent solids. For the hydrolytic conditions recommended by Buchanan, the normality of hydrochloric acid used varied from 0 058 to 0.400 in a range of solids from 4 t o 40 per cent, and the resulting sodium chloride would vary from 7 . 7 to 5 . 3 per cent. It is obvious that under these conditions an excessive amount of salt would remain in the finished sirup. The purpose of the experiments lyas to discover the conditions which would make possible a suitable conversion using lesser amounts of added acids. Saturally the probability of achieving this end seemed to lie in the reduction of the acidity needed by a corresponding increase in temperature and the adoption of a proper time interval. It also seemed probable that alteration in the solid content of the extract might make possible a more favorable acid-solids ratio.
EXPERIMEXTAL PROCEDURE The extract from the diffusion battery as operated with dried material had a concentration of about 35 per cent solids. To determine the equivalents of acid necessary to produce a definite pH, experiments were carried out both below and above this value. Previous workers have noted that the buffer capacity varies somewhat with differences in methods of preparation of the extract. I n order that such slight discrepancies due to different extracts might be avoided, all determinations were made on the same extract. An extract prepared by the diffusion process was concentrated t o 61 per cent solids content. Using this as a stock extract, definite quantities of acid and water were added t o give the concentrations and apparent acidities indicated in Table I. Determination of the pH values were made by use of the quinhydrone electrode.
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same additional increments of acid are required to give the same pH when the solids concentrations are increased stepmise. Table I11 and Figure 1 indicate that for a definite p H there would be a slight variance in the salt content of the finished sirup, the advantage being in favor of the dilute extracts. This difference is hardly significant. Furthermore, the selection of the solids content of the extract is also governed by other conditions incident t o the sirup process. It should be readily obtainable by the diffusion process, suitable for filtration and the other steps involved, and be as concentrated as possible t o minimize subsequent evaporation procedures. These conditions are satisfied when the usual diffusion battery extract of about 35 per cent solids is employed. RATES OF HYDROLYSIS OF ARTICHOKE EXTRACTSUSISG HYDROCHLORIC ACID Blthough the experiments upon the buffer action of the extract had indicated that over a considerable range of pH the ratio of equivalents of acid added to total solids present was practically constant, it was deemed advisable t o include the concentration factor in the work on hydrolysis. The establishment of the optimum conditions for the reaction, involving variation of reaction time, temperature, acidity, and concentration, requires many experiments on account of the number of variables involved. From the results of preliminary work, four sets of conditions, which were believed to be within the range of practical use, were selected for intensive study. 46
41
38 34
$ 26 Pd
FIGURE2.
VELOCITY
CONST4UTS
vs. PH
The samples used for all the experiments a t a specified temperature were made from a solution of the extract of 35 per cent solids which had been concentrated under diminished pressure to the maximum value indicated, then diluted to give the other values. It is possible that a small amount of inversion may have taken place which would affect all of the velocity constants slightly. Although different solutions were prepared for the different temperatures, they were made in the same may from the same material. To definite amounts of the extract, measured volumes of standard hydrochloric acid were added and sufficient water to give the solids concentration and the acidity indicated in the tables. I n the experiments in which conversion was carried out under pressure, all the samples were hydrolyzed in the same autoclave a t the same time. Analyses for total reducing sugars OF ARTICHOKE EXTRACTS were.made by the method of Lane and Eynon (14). The TABLEI. BUFFERCAPACITY Lane and Eynon factor for the computation of reducing sugars CONCN. OF EXTRACT IN HC1 ADDED PH VlLUES AT FOLLOWING TO NORMALITY GR4MS/100 CC.. was modified for the approximate composition of the extract OF: 10 20 30 40 50 60 as recommended by Jackson and hlathews (11). The extent 5.30 6.19 5.22 5.17 5.17 5.17 0.000 5.00 4.88 4.91 4.63 4.80 0.015 4.23 of hydrolysis was calculated as the ratio of the quantity of 4.37 4.54 4.64 4.78 3.35 4.06 0.045 4.54 4.23 4.44 4.01 2.63 3.57 0.075 solids hydrolyzed to the quantity of solids hydrolyzable under 4.34 3.98 4.18 3.67 1.90 3.13 0.105 the conditions of the analytical method as recommended by 3.93 4.15 3.74 2.68 3.37 0,135 1.68 Jackson (12). The velocity constant was calculated, assumThe data obtained, represented graphically (Figure l), ing that the hydrolysis follows the course of a unimolecular indicate that in the pH region of 4.2, approximately the reaction. The results are given in TabIe 11.
October. 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
OF ARTICHOKE EXTRACTS USING TABLE11. HYDROLYSIS HYDROCHLORIC ACID
HC1 ADDED TO OF: SOLIDS NORMALITY
CONCN.OF
PH
Grams/iOO cc. T I M E . 80 M I F U T E S : ~~
30 40 50
0,090 0.135 0.180 0.135 0.180 0.225 0.180 0.225 0.270
T I M E , 60 X I K U T E S ;
30 40 .50
T I M E , 30 M I N U T E S ;
30 40 50
40 50
0.00523 0.01916 IO. 05672 0.00707 0.02099 11,05232 10.01133 0.02266 0.04496 C.
STABILITY OF LEVULOSEUSDER HYDROLYSIS CONDITIONS
I). 00832
39.3 72.9 93.4 51.9 72.2 90.5 54.8 74.2 88.2
11,02176 0,04528 1).01220 0 , 0 2 113 0.03926 10.01323 0.02268 0.03562
Owing to t'he generally prerailing opinion regarding the susceptibility of lerulose to destruction a t elevated temperatures, it' 11-as necessary to demonstrate whether or not the hydrolytic procedure adopted might result in a n appreciable loss of the sugar. Accordingly the following experiment was made :
P R E S S U R E , 1 . 1 K G . ; T E M P E R I T E R E , 120.5° C.
0,045 0.060 0.076 0.060 0,075 0.090 0,075 0,090 0.105
T I M E , 20 M I N U T E S ;
30
3.68 3.28 :1.97 3.63 3.37 3,95 3.71 3.47
C.
27.4 68.4 96.7 34.6 71.7 95.7 49.4 74.4 93.3
T E M P E R I T U R E , 100'
8.05
0.060 0.090 0.120 0.090 0.120 0.150 0.120 0.150 0.180
tion in salt content has little effect upon the rate of reaction, and the catalytic effect of materials other than the hydrogen ion are of little significance. Figure 2 shows that a comparison of the velocity constants obtained in this work with those of Buchanan and associates (3) is possible only a t 80" C. and in a limited range of pH. I n view of possible minor differences in the character of the extracts and of the fact that the rates of reaction were established in the one case by chemical methods and in the other by changes in rotation, the results are in good agreement.
EXTENT OF VELOCITY HYDROLYSISCONSTANT %
T E M P E R A T U R E . 80'
3.47 2.73 1.98 3,23 2.75 2.15 3.15 2.72 2.05
4.13 .?,go 3.71 4.20 3.98 3.81 4.20 4.01 3.84
63.5 SO. 5 89.5 64.7 79.4 87.3 66.3 76.1 85.1
I), 03357 0,05448 O.Oi5Oi 11,03469 0,06263 I). 06874 0,03623 0.04768 0.0634%
P R E S S U R E , 1 . 8 K G . ; T E M P E R I T U R E , 130. 5'
4.56 4.23 4.01 4.49 4.29 4.06 4.42 4.27 4.12
0.030 0,045 0.060 0.045 0.060 0.075 0.060 0,075 0,090
51.3 72.2 87.5 60.2 74.6 85.9 65.0 77.1 85.1
C.
0,03597 0.06401 0 . 1039i 0,04606 0.06852 0.09795 0.05250 0.07370 0.09519
From the data in Tables I1 and 111, it is found that the hydrolysis a t 80 " C . will require hydrochloric acid equivalent to a n added salt content of 3 per cent, while a t 100" C . this value is reduced to 2 per cent. Such a salt content is too high for a palatable sirup, and in addition the 60-minute time period required has been considered too long for application to commercial work. Since a reduction of both time and acidity was required, experiments were made using pressure. At 15 pounds per square inch (1.1 kg. per sq. em.) pressure the added salt content was about 1 . 2 5 per cent and the time interval 30 minutes. At 25 pounds per square inch (1.8 kg. per sq. cm.) the added salt content wah about one per cent and the time interval 20 minutes. The solids concentration of the extract had little effect on the added salt content or the extent of the hydrolysis under the conditions examined. -4s mentioned previously (5) these latter condition of hydrolysis have been applied t o semiplant scale and have been used in the preparation of sirups. TABLE111. ADDED SODIUNCHLORIDE IN FINAL SIRUP AS RESULTOF NEUTRALIZATION OF HYDROCHLORIC ACID HCl ADDED NORMALITY
C O N C N . OF EXTRACT,
TO
OF:
0.030 0.045 0.060 0.075 0.090 0.105 0.120 0.135 0.150 0.180 0.225 0.270 0.400
GRAMS/~OO CC.:
A
50
30
40
%
%
%
0.60
0.40
0.33 0.49
0.80 1 00 1 20 1 40 1.60 1.80 2.00 2.40 3 00 3.60 5 33
0.82 0.99 1.16 1.33 1.49 1.65 1.9s 2.46 2.99 4.44
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 3.00 3.75 4.50 6.66
1167
0.66
When the time and temperature are maintained constant extracts of the same pH, but with variation in total solids, show about the same degree of hydrolysis. Evidently altera-
A quantity of artichoke extract was acidified with standard hydrochloric acid and water added so that)the concentration was exactly 20.0 grams of solids in 50 cc., and the apparent acidity to hydrochloric acid was 0.075 N . Ten such samples were hydrolyzed in an autoclave at 1 . 8 kg. pressure for 20 minutes. At the end of this time two samples were removed for analysis, and the remaining eight samples were given an additional 10minute hydrolysis. Again, two samples were removed and the remaining six heated for another 10-minute period. This procedure was repeated, the final samples being in the autoclave for 60 minutes. As a control, two samples were hydrolyzed by the regular analytical technic. Analyses for total sugars were made by the method of Lane and Eynon ( I d ) , the factor being corrected as previously stated, and dextrose was determined by the method of Dykins and Englis (4). The samples, diluted to 250 cc., were polarized in a 2-dm. tube, and the values recorded are in Ventzke degrees. The data obtained are presented in Table IV.
TABLEIV.
HYDROLYSIS AT 1 . 8 KILOGRAMS PRESSURE
(20 grams solids in 50 c c . ; 0.075 N t o hydrochloric acid) R.iTIo
TIME Minutes 20 30 40 50 60 Control
BiSED ON
DEXTROSE: HYDROLYZABLE RESOLIDS DEX- REDUCIXGDUCING Reducing TROSE SUGAR S E G ~ RDextroae sugar ROTATION Grams 5.79 6.56 6.75 6.99 7.04 7.04
Grams 15.46 16.42 16.78 16.91 16.91 17.63
0.374 0,399 0.402 0.414 0.416 0.400
70
%
32.9 37.2 38.2 39.7 40.0 40.0
87.7 93.2 95.2 96.0 96.0 100.0
Venlrke -13.37 -15.05 -16.00 -16.66 -16.94 -17.64
Prolonging the period of hydrolysis resulted in a steady increase in the total reducing sugars formed, but the total failed by 4 per cent to equal that attained a t the higher acidity and lower temperature of the analytical procedure. The optical ralues show the same general trend as is indicated by the chemical analyses. It is probable that some portion of the polysaccharide material is not hydrolyzable a t the low acidity of the pressure treatment. This is indicated from the appearance of the defecated extracts in which caramelization was much less evident in the pressure-hydrolyzed samples than in those of the control. A small steady increase in the dextrose-reducing sugar ratio may be accounted for either by a slight destruction of levulose or by the increased production of amino acids or other substances affected by the alkaline iodine solution which would result in a higher apparent dextrose value. Previous workers (3, 12) hare devoted attention to the decomposition of levulose as a result of acidity and heat. I n general the acidities h a r e been fairly high and not comparable to those used in these experiments. Results with pure levulose or inulin cannot be accepted necessarily as a n indication of what would happen with a complex mixture such as artichoke juice. m70rk upon the well-recognized
I N D U S T R I A L A N D E N GI N E E R I I\; G C H E h l I S T R Y
1168
reaction between amino acids and reducing sugars has been reviewed by Ambler ( I ) , and additional experiments have been carried out which would indicate that this type of phenomenon should be of considerable significance in the hydrolysis of the artichoke juice. The fact that the changes in optical value and total reducing power are as closely parallel as they were in this experiment is perhaps as satisfactory evidence of a minimum destruction of levulose as can be advanced. OTHER
HYDROLYTIC hfETHODS
Experiments were also made in which the acidity was furnished by the direct addition of sulfuric or orthophosphoric acids. Since these acids are.removed from the sirup by the formation of an insoluble precipitate when neutralized, the quantity added and hence the adjustment of the conditions for hydrolysis gave no difficulty. However, some trouble was encountered in the effect on the flavor and palatability of the finished sirups due to the neutralization of these acids. The process using sulfuric acid neutralized with calcium hydroxide appeared rather promising since the defecating effect of the calcium hydroxide, similar t o that used in the cane sugar industry, could be utilized. Sirups made in this manner had a peculiar bitter flavor, believed to be due to the calcium ion, which prohibited their use. They had a further objectionable characteristic in that an additional precipitate of calcium sulfate slowly formed and settled out after the sirups were stored for some time. The use of strontium in the place of calcium modified to some extent the bitter taste, but in these sirups also an additional precipitate of strontium sulfate formed. It is evident that these sirups will always contain a saturation value of the insoluble precipitate a t the concentration a t which the last filtration was made. The final concentration then builds u p a supersaturation of the insoluble salt in the finished sirup and this, together with incomplete and slow precipitation due to the viscosity of the liquid, is responsible for the later formation and deposition of more precipitate. The extreme insolubility of barium sulfate favored the use of barium hydroxide in this procedure. Sirups made in this manner did not exhibit the objectionable characteristics previously noted for the sirups in which calcium and strontium were used. These sirups were satisfactory in every respect. However, the soluble salts of barium are known to be poisonous, and these salts are said to be cumulative in the body. The extreme insolubility of pure barium sulfate renders it nontoxic, and it is ingested in large quantities in certain x-ray clinical work. The question will arise as to the ability to convert completely all of the soluble barium compound into an insoluble precipitate under the conditions of this neutralization, in that the barium salt is being added to a sugar solution acidified with sulfuric acid. Pending further investigation and clinical tests, work on this method mas deferred and other procedures investigated. The use of orthophosphoric in the place of sulfuric acid in the foregoing procedures gave the same general results. The better defecating action due t o the formation of tricalcium phosphate as compared to calcium phosphate would not justify the added cost due to the use of orthophosphoric acid. The addition of acid and hence the salt resulting from its neutralization may be greatly minimized by the use of volatile acid anhydrides such as carbon dioxide and sulfur dioxide under high pressure. In this laboratory Kleiderer and Endis ( I S ) , using the reagents mentioned, hare accomplished successful hydrolyses of inulin a t pressures of 1000 pounds per square inch (10.3 kg. per sq. cm.) using a temperature of 150" C. for 60 minutes. Application of this method to the
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hydrolysis of artichoke extracts has not proved so satisfactory. With carbon dioxide the extent of hydrolysis a t 100" C. for 2 hours was negligible. At 130" C. a 60 per cent hydrolysis was attained in 3 hours. At 160" C. a 73 per cent hydrolysis was attained in 60 minutes. Under the latter conditions caramelization was quite marked. The buffering action of the salts present in the artichoke extract evidently prevents the development of a sufficient hydrogen-ion concentration for a hydrolysis comparable to that accomplished when pure inulin is used. The bombs employed in this work were of small capacity, and no attempt has been made t o work a t high pressure on a large scale. Sulfur dioxide under pressure would probably be satisfactory but would have some of the same objections in regard to the removal of residual sulfite as sulfuric acid. Some experiments carried out by Heubaum ( 8 ) in this laboratory have indicated that the hydrolysis of inulin and extracts of inulin-bearing plants may be accomplished by the acidity furnished by the hydrolysis of salts of metals such as iron, aluminum, titanium, and manganese. Some favorable results have been obtained using aluminum sulfate, but an extensive study has not been made. The problem of the satisfactory removal of such reagents from saccharine liquors must first be solved before their use will be of value. Successful hydrolyses of artichoke extracts have been accomplished in this work by utilizing the acidity resulting from the electrodialysis of the extract. A preliminary report on this method has been made by Heubaum (Q), and more complete information has been reported by Hardy ('7). This electrolytic treatment has several advantages: It reduces the buffer capacity and develops an acidity without the addition of acid; it decreases the colloid content of the extract: and it brings about a reduction of the natural salt content of the extract and, on neutralization of the sirup, results in a partial substitution of sodium salts for the less desirable potassium compounds of the organic acids. The proper conditions for hydrolysis, following the electrodialytical acidification to a definite pH, may be selected by consideration of the velocity constants in Table 11. The composition of the sirup depends on the variety of the Jerusalem artichokes utilized, the time of harvest, and the conditions of storage prior t o slicing and drying. Levulose always constitutes the major portion of the reducing sugars present, the percentage value varying from 60 for poor samples to 80 for the best samples thus far observed. The taste and color of the sirup vary somewhat, depending on the composition of the original dried material, the temperature and acidity of the solution during evaporation, and the extent of the active char treatment. LITER.4TURE CITED Ambler, IKD.EKG.CHEY.,21,47 (1929). Buchanan and co-workers, I b i d . , 23, 1202 (1031). Ibid., 24, 41 (1932). Dykins and Englis, Ihid., Anal. E d . , 3, 21 (1931). Englis and co-workers, ISD. ENQ.CHEX, 25, 937 (1933). Golovin, B r y u k h a n o r a , and Fridman, J.Sugar Ind. (U. S . S.R . ) , 3,140 (19291. H a r d y , to be published. Heubaum, Facts About Sugar, 28, 284 (1933). Heubaum, Report on Hydrolysis by Aluminum Sulfate (unpublished). Hoche, 2. Ver. deut. Zuckerznd., 76, 821 (1926). Jackson a n d Mathem% Bur. Standards, Research Paper 426 (1932). Jackson, Silsbee, and Proffitt, Bur. Standards, Sci. P a p e r 519 (1 926). Kleiderer and Englis, IKD. ESG.CHEW,23,33% (1931) Lane and Eynon, J . SOC.Chem. I n d . , 42, 82 (1923). RECEIVEDApril 28, 1933. The experimental work reported in this paper was done by F. A. Dykins in partial fulfilment of the requirements for the degree of doctor of philosophy in chemistry a t the University of Illinois.
