Removal of Colorants and Polysaccharides and the Quality of White Sugar

Transcription

1 Association AVH 6 e Symposium Reims, mars 1999 Removal of Colorants and Polysaccharides and the Quality of White Sugar Mary An GODSHALL, Sugar Processing Research Institute, Inc. New Orleans, Louisiana, USA ABSTRACT The entire process of producing refined white sugar, from either sugarcane or sugarbeet, is directed toward the removal of extraneous components that adversely affect the final quality of white sugar. Many of these components, while considered extraneous because it is desirable that they be removed, actually are quite normal constituents of the cane or beet plant, examples being soluble cell wall polysaccharides, starch, and other smaller metabolites. Additionally, reactions occur during processing as a result of ph changes, thermal effects and autocatalytic effects, which lead to the formation of polymeric colorant. In recent years, innovations in membrane technology and pre-treatment (such as centrifugation) have taken place, and some emphasis will be placed on how these processes affect polysaccharide and colorant concentrations. Ideally, the final refined white sugar product would contain nothing more than pure sucrose, which, in reality, of course, is not possible. Very small quantities (in the mg/kg range) of polysaccharides, colorant, ash, volatiles and organic molecules such as lactic acid, HMF, pyrrolidone carboxylic acid and oleanolic acid may remain. Nevertheless, white refined sugar is one of the purest foodstuffs available. This presentation will first provide an overview of the general effects of polysaccharides and colorants on the quality of white sugar. This will be followed by an update on recent research at SPRI (Sugar Processing Research Institute, Inc.) on cane polysaccharides and cane and beet colorants along with recent findings on the role of oleanolic acid in floc formation in beet sugars. INTRODUCTION Many factors can affect the quality of a refined, white cane or beet sugar. These factors generally have to do with the non-sugars left in the product after refining or processing and they include moisture, invert, ash, colorant molecules, polysaccharides, metal ions (especially iron), insoluble sediment or particles, and various small organic molecules, including volatiles and organic acids. Each of these items can contribute one or more quality defects. Their detrimental effects may appear only with time, such as, during storage, either with or without environmental abuses, such as, too high humidity or temperature. In the case of deterioration during storage, the damage is often caused by further interaction of the constituents, especially in the syrup layer. Among the most troublesome of constituents that may remain in white sugar are the colorants and the polysaccharides. Both types of molecules are of high molecular weight, are polymeric, and have molecular elements that enhance their tendency to occlude within the crystal. In this presentation, we will summarize the types of colorants and polysaccharides found in beet and cane processing and the removal of same by various unit processes. 28

2 AVH Association 6 th Symposium Reims, March 1999 COLORANTS CANE AND BEET Types of colorants. Beet and cane colorants fall into roughly the same types of categories, although the individual constituents responsible for a colorant category will be different. The general categories of colorants are listed in Table 1. Unfortunately, except for phenolic colorants, no good tests exist to distinguish the various colorants from one another. Removal of colorant types. Since no specific tests exist to determine the different types of colorants, tests for phenolics, ph sensitivity, molecular weight ranges and distribution inside the crystal or in the syrup layer are the most effective means of determining the fate of colorants in processing. Gel permeation chromatography is an excellent technique for examining the high molecular weight constituents in sugar processing, and diode array detectors and evaporative light scattering detectors are yielding a great deal of information about individual high molecular weight colorants (1). In a laboratory scale study conducted at SPRI (2), several color variables were studied in raw cane sugars to determine the removal of color and polysaccharide by adsorbents, which included new carbon, regenerated carbon, IRA-958 (strong base acrylic resin) and IRA-900 (strong base polystyrene resin). The results are summarized in Table 2. What becomes immediately obvious is, that while total color is quite well removed by the four adsorbents tested, high molecular weight colorant is less efficiently removed, except by IRA-958; and polysaccharides are very little removed. Phenolic colorant is better removed by carbon than by resins. These experiments were conducted on raw sugars that had been affined (syrup layer removed by washing) and clarified by a tight filter aid filtration. Affination, the process of washing off the syrup layer around the sugar crystal, is often the first step in refining of raw cane sugar. Affination is effective at removing a significant proportion of the color but less so the polysaccharide. In recent years, with the advent of higher quality raw sugars, the affination step is often omitted, as it has essentially been done by the raw sugar producer. A study at SPRI (3) showed that only 30% of polysaccharide was removed from raw sugar by affination, meaning that the majority, 70%, was occluded within the crystal. A similar study by Chou (4) showed that 80% of dextran in raw sugar is inside the crystal. By contrast, 69% of total color was removed by affination, meaning only 31% was occluded in the crystal. However, this same study showed that only 34% of color of greater than 20,000 Da molecular weight was removed by affination, showing that 66% of the high molecular weight colorant was preferentially found inside the crystal. As for beet white sugar, the distribution of colorant constituents inside the crystal and on the syrup layer can be quite variable. Table 3 summarizes some data based on a limited study at SPRI (5). In general, it may be said that a slim majority of the constituents listed here are in the outer syrup coating rather than occluded within the crystal. It is of interest to note that in this study, two white sugars considered to be of very high qua- Table 1: Types of cane and beet sugar colorants. Colorant type Phenolic Caramel Alkaline degradation products of fructose Melanoidin Melanin Characteristics Plant pigments; low molecular weight; may be attached to polysaccharides; ph sensitive; darker at high ph; pale yellow to orange color; react with iron to produce very dark color; may dimerize or oxidize to form darker color. Cane; a lesser extent in beet. Process colorant; thermal degradation of sugar; high molecular weight. Low net charge. Beet and cane. Process colorants; reddish to dark brown in color; low molecular weight up to polymeric depending on degree of degradation. Beet; a lesser extent in cane. Browning reaction products of sugars and amino acids; high molecular weight. Not ph sensitive. Beet; a lesser extent in cane. Reaction products of amino acids with phenolics; very dark; medium molecular weight, not much charge. Also, black enzymatic oxidation products of phenolics. Beet. 29

3 Association AVH 6 e Symposium Reims, mars 1999 Table 2: Summary of laboratory scale treatment by various adsorbents on removal of colorant and polysaccharide components from seven raw cane sugars. Component % of component removed by adsorbent New carbon Regen. carbon IRA-958 IRA-900 Total color Color >20,000 Da Phenolic colorant Nondialyzable fxn.* Total polysacch Starch Dextran *Nondialyzable fraction is all material above 12,000 Da in the raw sugar and includes both colorant and polysaccharide. It is usually composed of about 50-70% polysaccharide. Table 3: Percentage of color and color precursors inside crystals of white beet sugar. The remaining constituents were in the syrup layer surrounding the crystals. Constituent Range Mean Value Color % 46.0% Phenolics % 56.5% Amino Nitrogen % 36.4% lity (low color, no color development on storage, no turbidity) had no color above 20,000 Da molecular weight, based on membrane separation, whereas two sugars considered to be of poor quality contained 48.3% and 78.0% respectively, of color above 20,000 Da molecular weight. These findings are suggestive and further studies should done. Another sugar in this study, which had developed a great deal of color on room temperature storage, was shown to contain most of its color (73%) in the syrup film. This sugar was shown to be quite sensitive to heat, with color increasing dramatically. However, when the sugar was washed with methanol, it did not form color on heating. It is quite common when washing beet sugars with methanol or ethanol to obtain, upon evaporation of the solvent, a bright yellow extract. Recent studies at SPRI (6, 7) have shown that lactic acid, pyrrolidone carboxylic acid, and many other possibly reactive components, are found in the syrup layer around white beet sugars. These findings would suggest that careful washing of white beet sugar crystals will help to decrease problems caused by color build-up on storage, as most of the colorant precursors are in the syrup layer. Differences between beet and cane colorants In both beet and cane refining, in excess of 99% of the starting color is usually removed. However, in cane sugar refining, this degree of removal is achieved by a series of unit processes, each of which removes an incremental amount of color. In beet sugar processing, fewer steps are required to remove the same degree of color, and an adsorption/decolorization step is not practiced. (Sometimes, a polish filtration with powdered carbon might be necessary.) This suggests a fundamental difference in the nature of cane and beet sugar colorants. The main differences, based on studies at SPRI and elsewhere, appears to be that (1) beet colorants are generally of a lower average molecular weight than cane colorants and (2) there is far less polysaccharide in beet processing than in cane processing. Crystallization is, of course, among the most effective ways to remove color. In cane refining, up to 95% of the color in first liquor will be removed upon crystallization. Conversely, only 40% of polysaccharide is likely to be removed (8). Differences between beet standard liquor color and chromatographic extract color It has been observed that a relatively lower color 30

4 AVH Association 6 th Symposium Reims, March 1999 white sugar can be obtained from the chromatographic extract obtained from beet molasses desugarization compared to standard liquor of the same color. We have begun a study at SPRI to examine these differences, and indications are that standard liquor has more polysaccharide and slightly more high molecular weight fraction than the extract, as shown in Table 4. Of course, the colorant remaining in the molasses represents colorant that was excluded and so, a priori, is of a different nature than the colorant remaining in standard liquor. Table 4: Comparison of several color factors in beet standard liquor and chromatographic extract. Constituent Standard Extract liquor Color, I.U Turbidity Total polysaccharide, ppm Tenate*, ppm % polysaccharide in tenate Indicator value** *Tenate = fraction >12,000 Da obtained by dialysis. **Indicator value = ratio of color at ph 9/color at ph4. An indication of phenolic colorant. POLYSACCHARIDES CANE AND BEET Types of polysaccharides Probably more is known about the polysaccharides in cane processing, especially since they seem to be more prevalent in refined cane sugar than in beet sugar. The total polysaccharide concentration in beet white sugars is almost always very low, normally about 0-20 ppm, while in cane sugar, it may be, on occasions, higher than 100 ppm, and is usually not less than 50 ppm. We have, however, found quite high levels of polysaccharides (>100 ppm) in some beet sugars with strong off-odors (9). The predominant polysaccharides in cane sugar refining are starch, dextran and ISP (indigenous sugarcane polysaccharide, a soluble cell wall polysaccharide). Rarely, there may be fructan and other polysaccharides caused by various sugarcane diseases, but in general, the three listed above are the important polysaccharides that impact white sugar quality. We have found both dextran and starch to be present in refined cane sugar, as well as ISP. The main quality problems associated with polysaccharides in white cane sugar are a propensity to cause floc, turbidity, or haze. If the dextran concentration is very high, the sugar will have a dull appearance. High dextran, along with oligosaccharides associated with dextran, are also implicated in abnormal crystal shapes, particularly elongated crystals, which changes the packing density of sugar so that, for example, 1 kg of sugar will not fit in a 1 kg bag. Figure 1 shows the progressive removal of polysaccharides, as monitored by GPC, during cane sugar refining. In the white sugar, the predominant polysaccharides remaining are a very high molecular weight (VHMW) species of about 1,000,000 Da and a glucan of about 30,000 Da molecular weight. The VHMW polysaccharide is present in all cane sugars and upon isolation and concentration, has a pale yellow, turbid appearance. It has a clear tendency to occlude within the crystal, and is responsible for most of the color of white cane sugar. The polysaccharides in white beet sugar have been less well characterized, although extensive work has been done by many workers on the polysaccharides in the sugarbeet itself. Polysaccharides arising from the beet plant consist of araban and galactan. Pectin, while the predominant polysaccharide, is mostly removed in process. Certainly dextran can be present, and in many cases, the values for total polysaccharide and dextran in beet sugar are almost the same, indicating that the predominant polysaccharide present in white beet sugar often is dextran. We have not found starch to be present in white beet sugar. Removal of polysaccharides The removal of polysaccharides was included in the section on colorants. In general, polyaccharides are difficult to remove and have a tendency to occlude within the sugar crystal. They are partially removed (20% or less) by most traditional unit processes in cane sugar refining, but there is no one process that stands out as having a superior ability to remove polysaccharides. It does, however, appear that the resins used in desugarization of beet molasses do produce sugar-enriched fractions that are low in polysaccharides. 31

5 Association AVH 6 e Symposium Reims, mars 1999 Figure 1: Changes in the polysaccharide composition throughout cane sugar refining. A beet polysaccharide responsible for turbidity and color Several years ago, a polysaccharide was identified in several white beet sugars, and was also found to be enriched in a set of molasses desugarization process samples (10). This compound, isolated by gel permeation chromatography, was dubbed BCC (beet crystal colorant) because it was found to be occluded in white beet sugar crystals and imparted a light yellow color to the sugar if present in sufficient quantities, as well as turbidity. It is not found in all beet sugar, being noted in 6 out of 11 sugars surveyed from both North America and Europe. The isolated compound was granular and free-flowing, with a light yellow to light brown color. The molecular weight was in the range of 20,000 Da. Hydrolysis showed that it was composed of arabinose and glucose. Precipitation reactions with barium chloride and oxalic acid indicated the presence of calcium sulfate. This compound was felt to be a colorant-polysaccharide-caso 4 complex. The reasons for its appearance in some sugars and not in others has not been elucidated. Figure 2 shows the BCC peak in a white beet sugar. Floc in beet sugars One of the last research projects Margaret Clarke worked on before her death was the role of olea- 32

6 AVH Association 6 th Symposium Reims, March 1999 Figure 2: GPC profile of high molecular weight colorant-polysaccharide complex (BCC) in white beet sugar. nolic acid in floc formation in solutions of beet sugar (11). While beet floc is not specifically caused by a polysaccharide or colorant, it is nevertheless a quality issue for white beet sugar. Saponin has traditionally been considered the sole cause of acid floc in beet sugar, but Clarke showed that in acid solutions, oleanolic acid can hydrolyze off the saponin, and, in combination with a small amount of protein, will form a floc. The presence of polysaccharides in the sugar enhances the turbid appearance of the floc. Interactions between colorants and polysaccharides In cane processing, it has long been felt that interactions occur between plant pigments (phenolics), process colorants and polysaccharides, particularly the ISP (indigenous sugarcane polysaccharide). Vercellotti, et al., (12) showed that a buildup of colorant polymers occurs during processing, and that the molecular weight also increased, at the expense of the lower molecular weight species. Later, the same workers showed that sugarcane colorants agglomerate during membrane processing to form a fouling layer (13). Nmr studies (13, 14) showed the presence of lipids along with phenolic and polysaccharide functionalities, and it was thought that lipids and bifunctional organic acids were involved in the agglomeration. Work by Godshall (15) showed the presence of lipids and bifunctional organic acids esterified onto the nondialyzable fraction of cane juice, which remained throughout the process into raw sugar. Although refinery studies were not reported, a trace of this complex does carry through into some white sugars. NEW PROCESSES The sugar industry is always undergoing changes, experimenting with innovative processes in hopes of gaining a competitive edge and producing an economical advantage through energy savings or other improvements. In recent years, we have seen increased usage of mechanical harvesting of cane, widespread use of growth regulators in cane fields, use of membrane separation in cane and beet, and even some experimentation with centrifugation (13, 16). Inevitably, these changes will bring about changes in the composition of the process samples. Each of these should prove to be areas of interesting and worthwhile research. Mechanical harvesting of cane: Effects of leaves and tops in cane sugar processing With the advent of the mechanical chopper-harvester in the cane industry, there is a tendency for more leaves and tops to be harvested with the cane, especially if the cane has been lodged by wind or its own weight, if the topping height is set too high, or if the cane is unusually short. Juice milled from leaves and tops is considerably higher in color and polysaccharides, as shown in Table 5. Furthermore, the color from leaves is of a different character, having a tendency to quickly oxidize/polymerize into a greenish-black color and having a lower indicator value, which suggests that the phenolics have been oxidized and/or polymerized. These data imply that a higher burden of color and polysaccharide may enter the mill, and processes will need to be per- 33

7 Association AVH 6 e Symposium Reims, mars 1999 fected that can either avoid this, or more effectively remove the additional color and polysaccharide. Table 5: Some differences between juice from leaves and tops compared to juice from clean cane stalks. Component Clean stalk Juice juice from tops ph Color 6,280 77,660 Indicator value Total polysaccharide, ppm 1,352 20,044 Dextran, ppm 372 3,498 Starch, ppm 710 4,037 Filtration rate, min SUMMARY To briefly summarize, colorants and polysaccharides are among the constituents that most impact the final quality of refined cane or beet sugar. Most processes, except possibly membrane filtration, are not very efficient at removing polysaccharides, and, because of their carbohydrate nature, they have an affinity for the sugar crystal, preferentially occluding within the crystal. Colorants are of many types, and are different between cane and beet, with beet having lower molecular weight colorant which is easier to remove by crystallization. Perhaps because of the lower molecular weight of beet colorant, it is often found in the syrup layer surrounding the crystal, rather than occluded within the crystal. In cane sugar, higher molecular weight colorants tend, also, to occlude within the crystal, possibly because the colorant is associated with polysaccharide. New processes may cause changes to occur in the composition of the process samples. REFERENCES (1) Bento, L.S.M., and Sá, S. (1999) Study of HMW compounds in sugar liquors from carbonatation and ion exchange resins using gel permeation chromatography with an evaporative light scattering detector. Proc Sugar Proc. Research Conf., pp (2) Godshall, M.A., Clarke, M.A., Miranda, X.M., and Blanco, R.S. (1993) Comparison of refinery decolorization systems. Proc Sugar Proc. Res. Conf., pp (3) Godshall, M.A., Clarke, M.A., Dooley, C.D., and Roberts, E.J. (1987) Large colorant and polysaccharide molecules in raw cane sugars. Proc. Sugar Industry Technolog., Vol. 46, pp (4) Chou, C.C., and Wnukowski, M. (1981) Dextran problems in sugar refining: A critical laboratory evaluation. Proc Technical Session Cane Sugar Refining Research, pp (5) Godshall, M.A., Clarke, M.A., Dooley, C.D., and Blanco, R.S. (1991) Progress in beet sugar colorant research. J. Sugar Beet Research, Vol. 28, pp (6) Godshall, M.A., Buchler, I.P., and Clarke, M.A. (1999) Identification and measurement of compounds in the surface film of white beet sugars. Proc Sugar Proc. Research Conf., pp (7) Godshall, M.A., Buchler, I.P., Boyle, H.G., and Clarke, M.A. (1999) Changes in white sugars under accelerated storage conditions. Proc Sugar Proc. Research Conf., pp (8) Godshall, M.A., Clarke, M.A., Dooley, C.D., and Roberts, E.J. (1989) High molecular weight color in refineries. Proc Sugar Processing Research Conf., pp (9) Clarke, M.A., Godshall, M.A., Blanco, R.S., and Miranda, X.M. (1997) Color and odor in beet sugar manufacture and storage. L Industria Saccarifera Italiana, Vol. 90, pp (10) Godshall, M.A. (1993) Isolation of a high molecular weight colorant from white beet sugar. Proc Sugar Processing Research Conf., pp (11) Clarke, M.A., Roberts, E.J., and Godshall, M.A. (1999) Acid beverage floc from Beet Sugars. Proc Sugar Processing Research Conf., pp (12) Vercellotti, J.R., Clarke, M.A., and Edye, L.A. (1996) Components of molasses: I. Sugarcane molasses: Factory and seasonal variables Sugar Processing Research Conf., pp (13) Vercellotti, J.R., Clarke, M.A., Godshall, M.A., Blanco, R.S., Patout, III, W.S., and Florence, R.A. (1999) Chemistry of membrane separation processes in sugar industry applications. Zuckerindustrie, Vol. 48, pp

8 AVH Association 6 th Symposium Reims, March 1999 (14) Vercellotti, J.R., Clarke, M.A., and Godshall, M.A. (1999) Sugarcane components that affect efficiency of membrane filtration: Identification and removal. Twenty-third Congress of the International Society of Sugarcane Technologists, in press. (15) Godshall, M.A., Buchler, I.P., and Vercellotti, J.R. (1999) Identification of esterified phenolic and organic acids in the high molecular weight colorant/poysaccharide fraction in cane sugar processing. Proc Sugar Proc. Research Conf., pp (16) Clarke, M.A., Vercellotti, J.R., Blanco, R.S., and Godshall, M.A. (1999) Report on treatment of sugarcane juice with disc-stack centrifugation Sugar Processing Research Conf., pp