Color Formation in Sugar Solutions under Simulated Cane Sugar Mill Conditions M. L. WOLFROM, W-. W. BINKLEY, AND J. N. SCHUYIACHER Department of Chemistry, The Ohio State University, Columbus 10, Ohio
C
ANE juice contains invertase, and under harvesting and
mill conditions the sucrose hydrolytic components, Dglucose and D-fructose, are present in the liquors. The principal nonnitrogenous plant acid of the cane is aconitic acid and considerable quantities are present under the harvesting conditions characteristic of the Louisiana region. The principal amino acid present is asparagine. I n the cane mill defecation process the slightly diluted juice is brought to p H 8 i 0.5 with calcium hydroxide and heated near 100" C. for several hours. This process clarifies the liquors through the precipitation of suspended material, proteins, waxes, and fats. Bt the same time, the hot alkaline treatment effects undesirable changes in the nonsucrose components, producing colored substances and by-products which inhibit the crystallization of sucrose and accumulate in the mother liquors or molasses. It was the objective of the present work to devise model experiments that might interpret, in part, some of the chemical trends in this complex defecation reaction. In the simplified model experiments, the principal functional types of compounds present, in major concentrations, in the cane
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juice were utilized in aqueous solution in approximately the concentration in which they are known (1, 4 ) to be present in Louisiana cane crusher juice. The total free amino acids present were collectively represented by their equivalent of asparagine. In turn, the organic anions were represented by aconitate, as the other organic anions present in the cane juice occur in a low concentration relative to that of the aconitate ion ( 2 ) . The rate of color formation was employed as a rough index of reactivity, and measurements of the rate were facilitated by extending the time of observation to 24 hours (see Figure 1 for details of these measurements). Calcium ion was replaced by potassium ion in order to maintain homogeneity in the solutions. While it is recognized that the heterogeneous character of the actual mill defecation process, particularly with respect to calcium precipitates, may have some bearing on the reactions considered in this paper, it is believed that the main trends of the colorforming reactions will not be grossly changed by this replacement of calcium ion by potassium ion. Adjustment to an initial p H 7.8 f 0.1 brought the tribasic aconitic acid to slightly beyond the neutral tripotassium salt stage. The results of the color formation measurements are plotted in a series of curves (Figures 1 to 4). The curves tend to be sigmoid in shape and exhibit evidence of an initial induction period. The solutions showed a downward drift in p H with time and the initial and final pH values are given in Table I. Figure 1 contains a collection of curves illustrating the data obtained in simple two-component systems. These fall into three sharply defined groups: The reducing sugars gave (1) a
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IO TIME, hours
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Figure 1. Rate of color formation at 100" C. and initial pH 8 i n two-component systems containing combinations of D-glucose (G, 0.7q0, 0.39 mM), D-fructose (F, 0.3%, 0.17 mM), sucrose lo%, 0.29 m M ) , tripotassium trans-aconitate (IGAcon, 0.4q0, 0.23 mM), and asparagine (Asp, 0.370, 0.23 mM)
(s,
TIME, hours Figure 2. Rate of color formation a t 100' C. and initial pH 8 in aqueous solutions of D-glucose ( G ) , D-fructose (F), and sucrose (S) containing tripotassium trans-aconitate (KaAcon) and asparagine (Asp)
Lumetron photoelectric colorimeter (Model 400). Measurements were made, immediately on withdrawal from the bath, on warm (90-96' C.) solutions contained in 1.4 (diame?,er) X 12 om. glass tubes, employing filter calibrated for transmittance at 490 mp. Warm (90-95' C . ) water in same size tube employed under identical operation, as standard for 100% transmittance
Under conditions cited in Figure I
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INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1955
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IO TIME,
20 hours
Figure 3. Rate of color formation at 100’ C. and initial p H 8 in aqueous solutions containing both D-glucose (G) and D-fructose (F) in tripotassium trans-aconitate (ICaAcon) and asparagine (Asp) Under conditions cited in Figuie I
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Table I.
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Changes in pH on Heating at 100’ C.
[Aqueous solutions containing various combinations of D-glucose (GI, Dructose (F), and sucroae (S) with tripotassium trans-aconitate (KaAcon) and asparagine (Asp) ] Figure Where Plotted I 1 1 1
pHa Initial At 24 hr. 5.80 7.85 F ASP 7.88 6.10 G ASP F KaAcon 7.85 6.10 G KaAcon 7.85 6.05 1 S KaAcon 7.85 6.45 AS” 1 7.90 7.00 ~. 2 7.72 6.15 K;Acon -f- Asp 2 7.70 6.18 KaAcon Asp 2 7.70 6.60 KaAcon Asp 3 F KaAcon Asa 7.72 5.88 3 7.77 5.38 F ASP 3 F KsAcon 7.86 5.68 4 5.62 F 9 KaAcon 7.75 4 7.78 5.88 F S Asa F S KsAcon 4 5.10 7.78 a Beckman pH meter (Model G ) standardized a t pH 7.00 against phosphate buffer.
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high rate of final color formation with asparagine and (2) a higher initial but a lower final rate with tripotassium trans-aconitate; and (3) the nonreducing disaccharide sucrose exhibited a low order of coloration with asparagine and aconitate alone. The rate of color enhancement with each of the sugars, including sucrose, was increased when each was heated with asparagine in the presence of tripotassium aconitate (Figure 2). The significant coloration with sucrose probably indicates some hydrolysis under these conditions. The concentration of sucrose employed was much greater than those used for the reducing sugars. The rate of coloration of D-fructose was always markedly higher than that of D-glucose, even though its concentration was only one half that of D-glucose (Figures 1 and 2). When the two reducing sugars were present together (Figure 3), a still higher rate of coloration was noted in the presence of both asparagine and tripotassium trans-aconitate, followed b y a lower rate with asparagine and aconitate, individually, and in that order. Finally, in the top‘ curve of Figure 4, we find the
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TIME,
hours
Figure 4. Rate of color formation at 100’ C. and initial pH 8 in aqueous solutions each containing Dglucose (G), D-fructose (F), and sucrose (S) in tripotassium trans-aconitate (IGAcon) and asparagine
(ASP) Under conditions cited in Figure 1
highest rate of coloration of all, when the combined reactants were all present. This was again followed, as in Figure 3, by lower rates with asparagine and aconitate alone, and in that order. CONCLUSION
These model experiments suggest that. at pH 8 one of the main color-producing systems in cane crusher juice is that of D-fructose and D-glucose with asparagine, followed by that of D-fructose and D-glucose with the aconitate ion. The former is a Maillard t y e of reaction involving interaction of the reducing sugar with tEe amino group of the amino acid (5,5 ) . The second reaction is undoubtedly an alkaline interaction of the reducing sugars, in which the tripotassium aconitate contributes to the general acidbase environment but does not enter into actual chemical combination with the sugars. Both of these reaction types are under further investigation in this laboratory. ACKNOWLEDGMENT
This work was carried out under contract A-1s-32692 between The Ohio State University Research Foundation (Project 427) and the United States Department of Agriculture as authorized under the Research and Marketing Act and su ervised by the Southern Regional Research Laboratory of the Sureau of Agricultural and Industrial Chemistry. Acknowledgment is made of the counsel of L. F. Martin, Southern Regional Research Laboratory. LITERATURE CITED
(1) Binkley, W. W., and Wolfrom, M. L., Advances i n Carbohydrate Chem., 8 , 291 (1953). (2) Roberts, E. J., and Martin, L. F., A n a l . Chem., 26, 815 (1954). (3) Sattler, L., and Zerban, F. W., IND.ENG.CHEM., 37, 1133 (1945). (4) Spencer, G. L., and Meade, G. P., “Cane Sugar Handbook,” 8th
ed., Wiley, New York, 1945.
(5) Wolfrom, M. L., Schlicht, R. C.. Langer, A. W., Jr., and Roomy, C. S., J. Am. Chem. SOC.,7 5 , 1013 (1953). RECEIVED for review July 20, 1954.
ACCEPTEDFebruary 12, 1955.
