Glucose syrup – Refractive index

The Refractive Index (RI) of specific glucose syrup, as determined using a refractometer, is directly related to its dry solids content and is used as such for this purpose industrially to control evaporative processes. It is also used as a routine quality control acceptance technique for dry solids in syrup as it is infinitely quicker to carry out a RI determination than to carry out the oven drying of a sample. It is important to note that RI is dependent on certain parameters of the syrup:

  • RI increases as the solids content of the syrup increases.
  • RI decreases as the DE of the syrup increases at the same solids content.
  • RI decreases as the temperature of the syrup increases at the same solids content.

It is thus important to measure RI at a specific temperature (normally 20°C or 40°C) and by reference to appropriate tables, which take account of the effects of DE, obtain a value for the solids content of the syrup under test. This value will usually be slightly different from the true solids of the syrup owing to a combination of errors introduced from the above relationship. The difference will not usually be of commercial significance however and providing the purchaser and the manufacturer of the syrup use the same technique there will be few problems. The refractometer used for the test must of course be calibrated at the temperature of measurement.

The method by which the glucose syrup is produced, e.g. acid or acid! enzyme or enzyme/enzyme also influences the RI reading, all else being equal. Mineral content (ash) will also affect the RI of syrup and corrections to the RI value may be necessary to take account of this as described in the literature.

DEType of conversionDry solids
(% w/w)
Table 5.8 Typical refractive index values of glucose syrups

Examples of the effects of temperature, solids content, DE, and method of hydrolysis of the syrup on RI are shown in Table 5.8 and detailed information on the effects may be found in the literature (Wartman et al., 1976). It is interesting to note in the literature (Okada and Nakakuki, 1992) that when the Refractive Index of different pure glucose oligomers from DP1 to DP10 was measured there was no difference between different products at the same concentration. This would tend to contradict to a certain extent the above findings.

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