Although specific rotation is a fundamental characteristic of a glucose syrup it has little relevance industrially except perhaps in the determination of complex mixture of sugars in foods (Pearson, 1976). When plane polarised light is passed through a glucose syrup solution the plane of polarization is rotated as a result of the optical activity of the carbohydrates present in that syrup. Each component of the glucose syrup, dextrose, maltose, maltotriose etc., contributes to the overall optical rotation each having a different rotatory effect on the polarised light.
From the figure for optical rotation, specific rotation can be calculated and this figure is therefore related not only to the DE but also the composition of the particular syrup. Different glucose syrups of the same DE but produced by different means (acid, enzyme, acid-enzyme hydrolysis) will have a different specific rotation owing to their different carbohydrate composition. It is however approximately linearly related to DE (Birch, 1968).
| DE | Specific rotation (degrees) |
| 15 | +163 |
| 19 | +168 |
| 25 | +160 |
| 33 | +153 |
| 42 | +140 |
| 49 | +130 |
| 55 | +106 |
| 67 | +97 |
| 100 | +52.7 |
The mineral content of the glucose syrup will affect the optical and hence specific rotation of a syrup possibly by inducing some conformational change in individual glucose polymers (Angyal and Pickles, 1972) or by forming helical complexes with the higher molecular weight oligomers. The table gives typical values for the specific rotation of glucose syrups.