ICUMSA News n°24 – 1995

Message from the President

With the appointment of Referees and their Associates and the publication of the Proceedings of the Havana session, we are now able to commence the work of the 22nd Session. To our new Referees and Associates, I extend my best wishes and my commitment to assist you in any way possible. We face challenging times because the financial and human resources to achieve our programme are probably more stretched than at any time in the Commission’s history. We all appreciate the reasons for the financial stringency being experienced in our industry, but we might question the wisdom of reducing expenditure on the development of sound analytical methods.

The case for maintaining or increasing resources for our work is compelling. Firstly, there are too many old classical methods which are expensive to carry out. These need to be replaced with cost effective, modern instrumental methods. These new methods need to be better developed than previously, because the requirements for method validation are more demanding. With the advent of Total Quality Management and ISO 9000 accreditation, the need for ICUMSA methods has not been greater. We now have a Methods book which will be kept up to date, as necessary, to remove obsolete methods and admit new ones. Our highest priority should be to complete the work necessary to make all important methods Official. I look forward to a productive and rewarding Session which will be culminated in the celebration of our Centenary in Berlin in 1998.

Near Infrared Polarimetry –

Report on the state of the work

A. Emmerich, Referee Subject 4: Polarimetry and quartz plates

Important contributions to Near Infrared Polarimetry have in recent years come from Altenburg and Chou (1991) and from Paton et al. (1993). In the first paper it was shown that the method was suited for routine analyses of raw sugar samples and that the deviation from the standard method with clarification on average is small, although the difference in certain cases was much higher than desired.

The reasons for the differences were investigated in the second paper and it was found that the differences were connected to the composition of the non-sugars in the samples. It was further shown, that the equations given by Bünnagel (1966) for dispersion of optical rotation of sucrose solutions and quartz in the visible part of the spectrum do not have the necessary accuracy when extrapolated to the NIR-region. This is important for the adjustment and control of polarimeters.

We therefore have the following tasks:

1. To make the necessary documentation for the definition of the sucrose scale and for the verification of quartz control plates in the near infrared. In order to do this, it is necessary to make similar precision measurements for the optical rotation of NIR-light with a wavelength of up to around 880 nm for sucrose solutions and quartz such as those made for visible light in 1974 and which were recognised as Official by ICUMSA in 1986.

2. In addition to this it is necessary to determine the difference in polarisation between clarified and unclarified solutions for a large number of samples. This is especially important for raw sugar as being the main commercial product.

In addition, it is however also important to determine the difference for intermediate products and syrups analysed during sugar production. The second task can be carried out by well-equipped commercial or industrial laboratories, whereas it is necessary that the first task is done by special laboratories. The base measuring values for visual light were made by two institutes in Braunschweig.

The Sugar Institute produced the ultrapure sucrose and the solutions having known sucrose contents and made also the necessary analytical determinations. The Physikalisch-Technische Bundesanstalt PTB made the precision measurements of the optical rotation with an exactly known wavelength of the light. Both Institutes are willing to use their resources for the necessary measurements to the near infrared, but there was a lack of funds for adapting the equipment required for precise polarimetric measurements in the near infrared as well as for a scientist. This scientist has to plan, make and evaluate the measurements.

By request from the Schmidt + Haensch company the Senate in Berlin has given a grant which allows us to make the measurements at 20 °C. Moreover, ICUMSA has at the meeting in Cuba in 1994 decided to give another grant thus making it possible to determine the temperature dependency.

When all these measurements are finished there will be the necessary documentation to make it possible to determine the polarimetric sugar scale and temperature compensation in the near infrared. The following preparatory work has been carried out:

PTB has made the equipment ready for the polarimeters and also made the necessary light sources ready. The light sources are 3 Lasers having wavelengths 731 nm, 780 nm and 852 nm. The wavelengths of these lasers have been used in connection with other measurements by PTB with the highest possible accuracy and the wavelength uncertainty is less than the ±0.003 nm needed for the light rotation measurements. There is no light source ready at or above 880 nm, which is the most frequently used wavelength for polarimetry, but so far it seems that extrapolation is possible within the required accuracy. The Braunschweiger Zuckerinstitut also has the necessary amount of ultrapure sucrose ready.

In the summer of 1994, the 50 mm quartz plate used for the measurements in visible light was measured at the three infrared wavelengths at 20 °C. These measurements showed that the values calculated according to the Bünnagel equation in the NIR-range were somewhat lower than those measured, whereas control measurements made within the visible range were exact. The difference was for example around 0.04% at 880 nm. A difference of this size was what could be expected according to the measurements by Altenburg and Chou and Paton et al. It is not yet possible to give exact figures and virial coefficients in the equations, because there is a deviation in the measurements at 731 nm. This deviation is small but still outside the allowed uncertainty. Control measurements are therefore needed. These measurements can hopefully be made in the spring of 1995.

It was originally scheduled, that the measurements on sucrose solutions at 20 °C should take place before the end of 1994. This has not been possible due to rebuilding work at the PTB where the polarimeter is located. It is now planned, that the measurements on sucrose solutions at 20 °C will finished in 1995. The measurements of the temperature dependency can probably not be made before 1996.

Although the holdup is disappointing it is necessary to recognise, that two things are destructive for precision measurements, disturbancies and haste. Everybody involved must have patience and we hope that those in ICUMSA waiting for the results will also have patience.


R.Bünnagel: Proc. 14th Session ICUMSA 1966, 17 and 33

W.Altenburg, C.C. Chou: Zuckerind. 116 (1991) 1041-1046

N.H. Paton, M.A. Player, R.M. Urquhart, M. Duong: 1993. Zuckerind. 118 (1993) 907–909

General Subject 3: Specialty sugars

M.L. Burge, Referee for General Subject 3

Following the Recommendations adopted at the 21st Session in Cuba 1994 a number of collaborative studies and investigations were identified within the terms of reference of General Subject 3. It was obvious from the list of Recommendations that most of the collaborative studies will require a high degree of involvement with other ICUMSA Subjects.

In Recommendation 1, the Boehringer Mannheim enzymatic method for the determination of sulphite, although Tentatively adopted following a number of investigations, needs to be collaboratively tested to gain Official acceptance at the next Session. This method is needed by Codex. The collaborative tests should include all the product categories defined in the Specialty Sugars classification according to Proc. 20th Session ICUMSA 1990.

It could be argued that since the product category d)- very pure syrups is understood to be solutions of white sugar, this method may be Officially adopted in keeping with the Recommendation for Official action for solid white sugar in General Subject 2.

In organising a collaborative study for determining sulphite in solutions, some thoughts must be applied to how samples are to be circulated since the presence of dissolved oxygen may affect the stability of free SO2 and therefore the reproducibility of the analytical results.

Recommendation 2 concerned the determination of anti-caking agents in powdered sugars but as stated in the Subject Report in Cuba, the Accepted Reichel method has a number of drawbacks if the method is to be applied to the wide range of possible additives, some of which may be partially soluble in water. This method relies upon a gravimetric determination in which a solution of the sugar in an alcohol/water mixture is filtered through a pre-weighed membrane which is then dried and reweighed. The weight of the residual matter obtained by difference is attributed to the insoluble anti-caking agent or additive.

In spite of the Recommendation to conduct further collaborative tests on the Reichel method, the Referee will seek the views of Associate Referees, arguing that a polarimetric procedure may be a better option to pursue. Since a procedure for determination of anticaking agents is required for Codex Alimentarius, some degree of priority should be attached to this collaboration.

The colour of brown sugars (Recommendation 3) is an important trading specification, yet the results of collaborative tests have been disappointing. In his Report to the 21st Session Riffer suggested a comprehensive list of concentrations which would help to improve reproducibility. Since the Cuba meeting, Riffer has circulated a paper to the Associate Referees for Subject 7 and to the appropriate General Subject Referees, citing a paper by J.B. Kuntz (Sugar y Azucar, Feb. 1993, 30–35) in which the effects of different filtering media on colour are discussed. This would appear to be a significant factor and will be an essential issue in defining further collaborative trials. The above investigations are believed to be the most pressing for General Subject 3 and will form the basis of work for the next Session.

General Subject 6: Beet

M. Kunz, Referee for General Subject 6.

The work will focus on the methods used in connection with the payment of beet and used for this purpose in the EU and other countries.

At present ICUMSA does not have any methods for sampling beet and preparing the brei. Given the obvious importance of these preparatory operations, effort will be made to standardise a method.

As routine methods to determine sucrose content two methods have Official status: the macerator method using lead acetate as the clarifying agent (GS6-1) and the macerator method using aluminium sulphate as the clarifying agent (GS6-3).

For the measurement of α-amino nitrogen there are also two Official methods: The ninhydrin method and the ‘blue number’ method. These together with a third non-ICUMSA method are used in connection with the payment of beet. The methods give different results as the different amino acids influence each method in a different way. There is urgent need for a comparison of the methods and an agreement to standardise one method. We will try to solve this problem.

There exist no Official methods for potassium and sodium. A collaborative study of the methods for the determination of potassium and sodium by flame-photometry is needed in order to make these methods Official.

There is also a need for the study of the influence of the clarification method using aluminium sulphate on the measurement of α-amino nitrogen.