Glucose syrup – Osmotic pressure and water activity

Glucose syrups are essentially self-preserving and no preservatives need to be added to them to prevent microbial growth. This is due to the high dissolved solids content in the syrups effectively binding all the free water and thus generating a high osmotic pressure and low water activity. With no free water, microbial growth does not take place. It has been reported that some glucose oligomers possess antimicrobial activity which would undoubtedly contribute to their stability (Kondo et ai., 1975). Osmotic pressure is directly proportional to molecular concentration or average molecular weight and the lower the molecular weight the higher the osmotic pressure at the same concentration. Thus high DE syrups produce a correspondingly higher osmotic pressure than low DE syrups at the same concentration (higher molecular concentration) and are thus more effective against microbial spoilage. For glucose syrups below about 30 DE the solids content required to prevent microbial growth is so high as to prevent easy handling of the syrup and thus such products are commonly only available as spray dried powders.

Glucose syrups are not sterile and although no microbial development may occur in the syrup owing to the high osmotic pressure and low water activity the microorganisms in the syrup are not necessarily killed during storage although some reduction in numbers may occur. Any dilution of the syrup will reduce the osmotic pressure and increase the water activity and microbial growth may occur. Correct storage of glucose syrups is therefore essential to prevent condensation on the interior of the tank above the syrup as this could run back onto the surface of the syrup causing localized dilution. Trace heating of the headspace or airflow through the headspace is normally carried out to prevent condensation occurring.

Water activity is related both to the water content and the temperature of the particular syrup as well as its concentration and composition. The table below shows typical water activity values of glucose syrups. Note that bacteria do not develop when the water activity is below 0.9, moulds when below 0.8 and osmophilic yeasts when below 0.6. Thus all glucose syrups can be made safe from bacterial and mould spoilage but care should be taken to avoid osmophilic yeast contamination.

SyrupSolids content
(% w/v)
Water activity
42 DE Acid
glucose syrup
60 DE Acid-enzyme
glucose syrup
90-94 DE
glucose syrup
High fructose
glucose syrup
Liquid sugar670.85
Table. Water activity of glucose syrups at 20°C

There is a linear relationship between DE and osmotic pressure at the same solids content and also between osmotic pressure and concentration at the same DE (Kearsley and Birch, 1978) and a method of DE determination has been developed based upon this principle (Kearsley, 1978b; Fitton, 1979). However, this method is only suitable for syrups which have been demineralised since small amounts of minerals can lead to errors in the determination owing to their effect on the osmotic pressure.

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