Analysis of Moisture in Sugar Products: Comparison of Two Rapid Moisture Analyzers

M. A. Godshall and X. M. Miranda Sugar Processing Research Institute, Inc.

1100 Robert E. Lee Blvd. New Orleans, LA 70124

ABSTRACT

The official ICUMSA (International Commission for Uniform Methods of Sugar Analysis) method for determining moisture in sugar products by loss-on-drying requires 3 hours of oven­drying followed by an extended cooling period to a temperature 2°C above ambient. Recently, instrumentation has become available that is capable of accurately measuring the small amounts of moisture in white sugar. This presentation reports on the evaluation of two of these. Both utilize the loss-on-drying method and both have a balance resolution of 0.1 mg. The Arizona Instruments Computrac MAX-2000 Moisture Analyzer uses a small convection air oven to effect drying. The Instrument Corporation Mark I High Performance Moisture Analyzer utilizes four quartz infrared heaters. Ranges and combinations of drying parameters have been studied fo r both the new instruments on sever al types of sugars. The choice of optimal drying parameters plays a critical role in successful moisture determination in white, raw, powdered and soft brown sugars. This report presents the results of the study. Both instruments give results comparable to the oven, with a great savings in time. Precision of measurement using the instrumental methods is at least doubled for white and powdered sugars and approximately the same for raw and soft sugar compared to the standard oven procedure.

Additional Key Words: Moisture analysis; white sugar; powdered sugar; raw sugar; soft sugar.

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A need exists in the sugar industry for a precise, reliable, safe and rapid method to

determine the moisture content of crystalline sugar products by loss-on-drying. The Officia~

ICUMSA (International Commission for Uniform Methods of Sugar Analysis) method for moisture analysis of both white and raw sugars (cane and beet) and specialty sugars is time-consuming, requiring a 3-hour drying time. While collaborative studies have shown the method to have acceptable performance characteristics for raw sugar (Vaccari, et aI., 1993), the method is only marginal for white sugar, and exists in the absence of a better method (ICUMSA, 1994). The oven­drying method is further complicated by the requirement that the weighing dishes be cooled to 2°C above ambient before weighing. Accurately determining the temperature of dishes inside a desiccator is difficult; dishes cool as they are being weighed so every dish is at a progressively slightly lower temperature; and ambient temperature can exhibit a wide range, even in the same location. These factors contribute to a great deal of variation in the moisture determination of low­moisture sugars, namely white sugar.

In recent years, drying ovens equipped with sensitive balances with enough resolution to be used to determine moisture in white sugar have become available. This report presents the results of a study comparing two of the new drying instruments with oven results on several crystall ine sugar products -- raw, white, powdered and soft sugars.

MATERIALS AND METHODS

Materials tested consisted of white beet and cane sugars, raw cane sugars, powdered sugars and soft brown sugars obtained either from a grocery store or from sponsors of Sugar Processing Research Institute.

The ICUMSA oven method (Method OS2/2/3-15, 1994), as described above, was used to determine the moisture in the sugars. The oven value was used as the standard for deciding on the parameters for programming the instruments.

Two rapid moisture analyzers were evaluated: The OmniMark Mark I High Performance Moisture Analyzer manufactured by Denver Instrument Company, Tempe, Arizona; and the Computrac MAX-2000 High Performance Moisture Analyzer manufactured by Arizona Instrument, Phoenix, Arizona. The Mark I utilizes four infrared quartz heaters while the MAX-2000 uses a small convection oven. Both operate on the principle of loss on drying. Both instruments have a similar weighing resolution of 0.1 mg and 0.001 % moisture.

RESUL TS AND DISCUSSION

A number of variables must be decided when programming these instruments to obtain optimum performance. Drying parameters include temperature, time, sample size, stand-by temperature, one or two-stage drying and choice of end of analysis mode. Although one oven method can accommodate the range of moisture in sugar products, from 0.01 % in white sugar up to 3.5+% in soft sugars, there is a choice of programmable drying options for instrumental methods, and the choice of parameters is dependent to some degree on the type of sample, its moisture content and particle size.

Instrumental moisture tests can be ended on time (not generally recommended) or on detennination of a constant rate of weight loss (slope or rate). End of analysis based on slope/rate

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is based on determining a percent of initial weight change over a chosen period of time. The test ends when slope conditions are met. The amount of slope/rate chosen depends on the amount of moisture in the sample. (Note: OmniMark refers to this mode as "slope" while Computrac refers to it as "rate".) The analyst has the choice of ending the test on the actual slope, when the slope conditions are met, or on a calculated (OmniMark) or predicted (Computrac) slope. In this mode, the test is ended before the actual end point (rate/slope) is reached, based on prediction algorithms. Advantages of this mode are a shortened analysis time, allows use of a higher starting temperature, and avoids sample decomposition. Prediction mode is best used for high moisture products.

White sugar. Because of the very low moisture content of white sugar, its determination is the most difficult for any method. Collaborative tests have shown that the within-laboratory repeatability relative standard deviation for white sugar moisture determined by the oven method is about 20%. Table 1 shows some comparative data for a white beet sugar, which indicates that the within laboratory deviation for the instrumental methods is about half that of the oven. The data in Table 1 were obtained at 130°C, using the rate mode, and 20 g of sample.

Table 1. Moisture in white sugar.

Oven OmniMark Max-2000

0.046 0.050 0.050 0.052 0.049 0.054 0.057 0.040 0.043 0.035 0.045 0.049

0.045

Oven OmniMark MAX-2000

0.048% ± 0.00940.046% ± 0.00390.049% ± 0.0045

7 (19.73%) 6 (8.61%) 5 (9.29%)

Powdered sugar. Moisture levels in powdered sugars are about ten times higher than those in white sugar because of the approximately 3% starch added as an anti-caking agent. Table 2 shows comparative data for several powdered sugars. OmniMark parameters were: 20 g; 120°C/3 min; slope 0.03%/1.5 min; stand-by lOO°C. Max-2000 parameters were: 109; 120°C; 95% of predicted slope; stand-by 95°C. The Max-2000 generally overestimated results, and it is possible that the parameters chosen will need to be further refined.

Table 2. Moisture in powdered sugar.

Sugar Oven Max-2000 OmniMark

1 0.411 0.463 0.366 2 0.448 0.599 0.464 3 0.482 0.626 0.471 4 0.373 0.437 0.354 5 0.475 0.372 0.463

Over-all 0.438 0.499 0.424

Time Max-2000: 2:55-4:23 min. Time OmniMark: 3 :24-5 :00 min.

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Raw sugar. Raw beet sugar is not very common, but millions of pounds of raw cane sugar are traded internationally, and moisture determination is one of the contractual quality determinants. Table 3 shows comparative data for raw sugars from different locations. Over-all the results for the three methods are not significantly different.

Table 3. Moisture in raw cane sugar.

Sugar Oven Max-2000 OmniMark

Florida 0.1 93 0.208 0.190 Phillippines 0.340 0.483 0.321 Peru 0.1 82 0.2 11 0.186 Mexico 0.180 0.1 90 0. 180

Over-all 0.224 0.273 0.219 Max-2000: OmniMark:

l30°C; 0.0140°C/4m

05% rate; 20 g. in; slope 0.01 %/2min; 20g.

Soft sugar. Soft or brown sugar is ten times again higher in moisture than raw or powdered sugars. Twenty-six paired analyses of soft sugars comparing the oven and the OmniMark showed:

Oven over-all mean = 2.77% OmniMark over-all mean = 2.59%

The OmniMark slightly underestimated moisture, but the error was not significant. Test conditions were: 140°C/5 min; slope 0.04%/min; 10 g; end on slope; stand-by temp 120 D C, and time of analysis ranged from 5:54 to 13:36 min with a mean analysis time of7:50 min.

Thirty-two paired analyses of soft sugars comparing the oven and the MAX-2000 showed no significant differences in the results:

Oven Mean = 1.93% MAX-2000 Mean = 2.01 %

Test conditions were: 140°C; rate 0.005%; 109; end on rate, stand-by temp 103 DC. Time of analysis ranged from 6:50 to 15 :57 min, with a mean analysis time of 11 :30 min.

CONCLUSIONS

Table 4 summarizes the repeatability data for the three methods of moisture analysis for four different sugar products, ranging over two orders of magnitude in moisture content. This brief study has shown that the two instrumental moisture analyzers are more precise and much faster than the

traditional oven method. The parameters of analysis can influence the results, and should be carefully chosen so as to give results comparable to the oven. Figure 1 gives a graphical representation of the moisture levels in various sugar products and the comparative results for each of the three methods examined.

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Table 4. Repeatability of moisture determination by various methods.

Sugar Moisture range Oven OmniMark MAX-2000

White 0.010-0.040 20 10 10 Raw 0.10-0.60 4.5 5.2 4.4 Powdered 0.15-0.55 13.7 2.8 6.1 Soft 1.50-4.00 4.0 2.1 3.9

Summ a ry of Comparative Data Over-A ll Mea ns

~ ::::J

·0 -en

~

eft.

3

2.5

2

1 .5

1

0.5

D Soft

Powd ered

Raw

White

Oven Max-2 000 OmniMark Method

Figure 1. Summary of moisture data as over-all means for four types of sugar products analyzed by three methods.

LITERATURE CITED

ICUMSA, Proceedings of the 21 st Session, May 1994, Report on General Subject 2, in press.

Vaccari, G., M. A. Godshall, and M. Nemeth. 1993. ICUMSA collaborative tests: 1. Determination of moisture content of raw cane sugar having a moisture content of less than 0.5%. Zuckerindustrie, 118:938-944.

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