Message from the President
Twenty first Session, 15-20 May 1994
It is now less than a year till our Session in Havana. I visited the Cuban National Committee last May during the week of their Diversification ’93 Conference. I reviewed the plans for our meeting, saw the hotels we will stay in, visited the places the accompanying persons will see, experienced the translation facilities at the conference centre and was impressed with their enthusiasm for the task ahead and the advanced state of their preparations. Others who have attended the ISSCT in 1983, or the recent ISSCT By-products workshop would know about Cuba’s first-class conference facilities and the friendly welcome extended to all visitors.
Collaborative Testing Programmes
I am aware of several programmes to collaboratively test methods in readiness for both the approval of the 21st Session and inclusion in our new Methods Book. Referees are urged to work up their test reports quickly and to circulate them among Associates so that matters are discussed in plenty of time for the Referees to write their reports for the 21 st Session.
Referees’ Reports
I expect to request these reports by 31 December 1993 so that there will be time for circulation and discussion before our Session in May. Anyone who wishes to make a submission to a Referee should do so before the end of October otherwise there is the risk that it may be too late. As Referee for Subject 1, Constitution and By-Laws, I have received a number of responses to the proposed revision of this document. All concerned relatively minor matters which do not require a further reworking of the changes. If any National Committee has substantial comments they still wish to make, I would be pleased to receive these before the end of October.
Methods Book
The publication of our new Methods Book is well in hand and it is proposed to offer it for sale by the end of the year in both a loose-leaf book form or by individual methods. Contributors who still have work to complete are urged to give it high priority so that we meet our deadlines with the printer. To all who have made a contribution, the Commission thanks you sincerely.
General Subject 3 – Specialty sugars
by R.W. Plews (Tate & Lyle Sugars, UK), Referee
Work on analytical methods allocated to General Subject 3 is progressing on several fronts but, in particular, in procedures for the determination of anti-caking agents in powdered sugars, solution colours of brown sugars and sulphite in products other than refined white sugar.
Anti-caking Agents
ICUMSA has been pressed (and has been pressing) for many years for a general method for the determination of levels of anti-caking agents in powdered sugars but the nearest the Commission has come to fulfilling this requirement has been to recommend the Reichel gravimetric procedure for further study. A specific method for the determination of calcium phosphate in powdered sugar was tentatively adopted some years ago and still pertains.
Following the Recommendation of the 20th Session of ICUMSA in Colorado Springs, Associate Referees of General Subject 3 were circulated on the current situation and the suitability of the Reichel procedure. I am grateful to those who were kind enough to express their views and for their helpful comments.
In general, it would seem that most organisations use specific methods designed to meet their own particular needs and an all-embracing method may have little application in typical Quality Assurance (QA) and Quality Control (QC) systems.
Nevertheless, ICUMSA has determined that a general method shall be defined. For a general procedure there would seem to be two lines of initiative. Either to attempt to determine sucrose and subtract the amount from 100%, to derive the additive level (assuming the original sugar to be pure enough to ignore invert, ash, water content and other organic matter or to make allowance for such constituents); or to determine the anti-caking agent directly as insoluble matter through a gravimetric procedure.
In either procedure there is a failure to address the role of original water in the additive unless a separate assessment of water content in the free additive is made independently. The latter is no easy task since, unless a general method can be prescribed, it is possible that each additive would require a specific method for the determination of original water content.
An assessment of possible procedures is being carried out with the help of a final year student at the South Bank Polytechnic in London and it is hoped that the results of this investigation will enable a suitable procedure to be defined for collaborative testing.
Sulphur dioxide
Various methods for the determination of sulphite in a number of products are being studied as a further South Bank student project. The applicability of the methods to soft sugars, brown sugars, syrups, etc., is of prime importance and it is hoped that, as with the exercise involving anti-caking agents, it will be possible to identify a procedure for collaborative study.
It is noted, also, that the French National Committee have proposed collaborative testing of an enzymatic method based on the Böhringer Mannheim procedure, although some modifications may be required to accommodate dark syrups and brown sugars.
Colour of brown sugars
The collaborative study of three brown sugars carried out for the last Session of ICUMSA produced such poor reproducibility that the derivation of a reliable method for such an important quality criterion was essential. Dr. R. Riffer, the recently appointed Referee for Subject 7 has been particularly helpful in designing a collaborative study which will address the problem. The result of this investigation will be of great interest and importance to General Subject 3.
HPIC determination of sucrose
A collaborative study is currently under way, organised by Mr. K. J. Schäffler, but at this stage the investigation is limited to analyses of molasses. It would be of interest to determine the applicability of the technique to sugars and syrups which, by their nature, contain high levels of sucrose.
Solution colour and turbidity’
by Dr. Richard Riffer (USA), Referee, Subject 7
ICUMSA collaborative studies over the years have shown the filtration technique to be an important component of poor precision in colour measurement. The Proceedings of the 1970 Session include reports on experiments using several filter aids in combination with various filter papers.
More recently, Joseph B. Kuntz has reported on variation in colour and turbidity measurement with the choice of 0.45 μm filtration membrane [l]. Highest colours were obtained with membranes of cellulosic composition and lower with those of nylon or polyvinylidene difluoride. In his article, Mr Kuntz discusses adsorptive capacity of the membrane, on which residual ionic charge might retain colorants electrostatically. In addition, he notes that pore geometry of the membranes could influence removal of high molecular weight colorants.
We know that most colorant molecules are much smaller than 0.45 μm, an important exception being some “high molecular weight colour”, which very likely consists of small colorant molecules tightly complexed or covalently bound to large polysaccharide units. Some clathrate structures could conceivably be present as well, although hollow helixes such as those of starch are internally hydrophobic, so that the guest molecule would have to be lipophilic and linear. A second exception is high molecular weight material resulting from thermal degradation, which ultimately forms graphite-like high polymers.
Most polysaccharides in cane sugar, such as cell-wall arabinogalactan and associated glucomannan, have molecular dimensions much smaller than 0.45 μm. The notable exceptions are starch and any high molecular weight extracellular dextran resulting from bacterial infection. At high degrees of polymerization, such molecules form highly entangled networks of semi-rigid impenetrable coils.
Surface coverage by particles of this sort, such as might occur on a membrane filter, has been studied by Monte Carlo computer simulation models. An investigation by E. Dickinson and S.R. Euston at the University of Leeds [2] suggests that such coverage is controlled by a “deformability parameter” dependent upon particle perimeter.
During filtration, hydrated polysaccharides can readily form a film that blocks membrane pores. One might expect greater interaction and spreading – and more severe blockage – with a carbohydrate membrane such as cellulose than with one of a material such as polyvinylidene difluoride. However, the latter type cannot be entirely hydrophobic, since its surface is easily wetted. An incompressible material such as kieselguhr acts as a depth pre-filter, that can retain gummy particles both on its surface and within its matrix.
In his article, Mr Kuntz noted similar but smaller variation in turbidity measurement. In this case, the situation was reversed, the membranes that had yielded the highest colours now yielded the lowest turbidities. At first glance, there is nothing untoward here; the membranes that best remove turbidity are apparently poorest at removing colour. But we have the makings of a paradox; if we are to assume that all substances separated by the membrane are somewhat larger than 0.45 μm and largely of the polysaccharide type, the membrane appears to be able to distinguish between coloured and uncoloured particles. How can this be?
M Kuntz determined turbidity by absorbance measurements at 720 nm before and after filtration. Note that this difference represents a turbidity lost, not a turbidity remaining in solution. He believes that colorant molecules are adsorbed onto suspended particles, thus reducing solution colour during filtration. Decolourization proceeding by such a mechanism would be largely independent of the membrane type but instead a function of the level of suspended solids. Colour removal would also take place when the membrane became blinded by polysaccharides.
Filtration procedures for colour measurement are based on the assumption that large particles cause turbidity only, and small particles only absorb light but do not cause scattering. We know, however, that neither of these propositions is rigorously true. Polysaccharide-associated “high molecular weight colour” can contribute a significant fraction to the total, and much white sugar turbidity results from particles considerably smaller than 0.45 μm. Furthermore, the choice of membrane cut-off pore size is dictated not by theoretical considerations but by a very practical need to filter the sample in a relatively short time.
There is no ICUMSA-recommended method for turbidity measurement at present. In former Method 1-A, the 420 nm reading is “corrected” for turbidity by subtracting twice the 720 nm reading. The rationale here is that 720 nm colour is very slight, but some measurable turbidity remains.
It should be emphasized, however, that there is no theoretical basis for such subtractive techniques and they tend to give erroneous results. As Referee Frank G. Carpenter stated in the 1982 Proceeding and elsewhere, the apparent turbidity contribution is often exaggerated, and the very purest sugars can give negative colours. The quantitative turbidity factor of 2 is also debatable. It should be noted that the subtractive method was abandoned by ICUMSA in 1970. The procedure remains standard, however, for the U.S. National Soft Drink Association.
Many industries quantify suspended solids by filtration and subsequent drying and weighing, but such methods overlook colloidal material. For colloidal systems of particle diameter considerably larger than 0.45 μm, sedimentation of course occurs at an appreciable rate. (The term ‘colloidal’ is used loosely here; large molecules form homogeneous systems, not highly dispersed aggregates. However, like true colloids, they have a very high surface area per unit mass.)
Turbidity can be properly measured only with some sort of light scattering instrument. Although turbidimeters that measure 90° scatter are relatively low-cost additions to the sugar laboratory, they are not widely used. Right-angle scatter is not without its limitations, however. Since sugar solutions scatter much more strongly at forward angles than at 90°, right-angle measurement is not the most sensitive to concentration. Furthermore, a scattering reading in Nephelometric Turbidity Units (NTUs) clearly cannot be subtracted from an attenuation index to provide a “corrected” colour.
One might suppose that ideally one would measure scatter at several angles, obtaining as a bonus, data on the molecular weights, dimensions, shapes, and concentrations of scattering species. But severe problems remain. Scattering from commercial sugar solutions is not of the Rayleigh type; the particles are neither spherical nor small compared to the wavelength. Moreover, the solutions are· coloured and the scattering species highly heterogeneous. And except for the special case of Rayleigh scattering, the Mie theory [3] that describes light scattering is so complex as to be of little practical value.
The issue whether a colour measurement is “correct” is a philosophical rather than a technical one, since there is no “true” colour in the same sense as there is a true calcium content. The matter has arisen again recently in connection with a Subject 7 collaborative test now in progress. The study in question is of TEA buffer for pH adjustment in measurement of colours of sugars other than white sugars. The corresponding ICUMSA method for white sugars has been shown to be statistically sound, but it will be recalled that in the test there was no parallel set on which pH adjustment was performed conventionally. Triethanolamine is of course a base having the formula N(C2H4OH)3. With a pK-value of about 9.50, at neutrality TEA is about 99.7 % in the salt form and thus is not a particularly good buffer at pH = 7. However, even with only a small fraction present as the free base, the concentration is nonetheless probably very high compared to that of colourant species. Since colorants are largely acidic, there is a potential for the formation of conjugates with TEA, with unknown consequences for the colour measurement.
There is however no doubt about the need to fix the pH-value at 7.0, as also from Mr Kuntz’s paper of a need to decide about the chemical cornposition of the membranes to be used in solution colour determinations.
Literature
1 Kuntz J. B. (1993): Sugary Azucar 88 (2) 30-35
2 Dickinson, E., and Euston, S. R. (1992): Food Hydrocolloids 6 (4) 345-357
3 Van de Hulst, H.C., ed. (1952): Light scattering by small particle. Chapman & Hall, London
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