Preserves are a food commodity that requires and uses large quantities of sugar and glucose syrup. During the manufacturing, process fruit is boiled with the sugars to a solids content which will give stability to microbiological spoilage, and to the state where provided other factors such as pH are correct the sugar-acid-pectin system forms a stable gel. Glucose syrups can and are used to replace up to 50% of the carbohydrate content. The most suitable and commonly used is a 63 DE glucose syrup which has an average molecular weight of approximately 300 Da, which provides a higher osmotic pressure than the equivalent weight of sucrose, and thus will contribute an improved effect on the microbiological stability of the preserve.
The presence of glucose syrup in preserves will also inhibit the recrystallization of the lower molecular weight carbohydrates such as sucrose. Sucrose is saturated at 67% solids, so if it were used alone in the commercial preserves, crystallization would occur, thus detracting from the organoleptic acceptability of the finished product. Such crystallisations can be prevented by the addition of a second sugar. Traditionally naturally inverted sugar was used which readily occurred in the more old-fashioned type of open pan boiling. However, with more modern vacuum techniques being used this natural inversion is minimized. Even if natural inversion or added invert sugar is used the maximum solids which can be attained before crystallization takes place is 76%. This produces only a fairly limited safety margin in commercial preserves, and is not suitable where high solids are used, for example in bakery jams. The inclusion of glucose syrup reduces this risk of crystallization.
When more than 50% of the carbohydrate portions in a preserve is glucose syrup, it is claimed that the gel strength is weakened and an undesirable texture (mouthfeel) is produced. This is thought to be due to the higher molecular weight sugars present and appears to be the limiting factor in the level of glucose syrup which can be incorporated. However, the new higher dextrose and higher maltose-containing syrups which automatically have a reduced proportion of higher sugars, should theoretically be capable of being used at levels greater than 50%.
It has been claimed that the inclusion of glucose syrup increases the damage caused by syneresis, however, the evidence is not conclusive since studies have demonstrated that whilst glucose syrup affects sugar-acid-pectin systems, if alterations to the set and manufacturing conditions are made, then extra cut gel syneresis can be avoided.
Fundamental work done by D. Howling (Cerestar) in conjunction with G. Stainsby, University of Leeds, showed that the type and level of glucose affected the setting temperature of the sugar-acid-pectin system. This varied according to the pectin type and pH but could be as much as 15°C. The raising of the setting temperature was causing premature setting in the process before filling of the finished product containers and this shearing of the infant gel in fact led to the syneresis. By increasing filling temperatures to avoid premature set glucose levels up to 50% were obtained in commercial preserves and recently this has been moved higher.
The type of glucose syrup used traditionally was 63 DE acid-enzyme glucose syrup but as discussed the higher sugars do tend to give a sticky, viscous non-elastic gel. Higher 68 DE enzyme-enzyme glucose syrups are now used to give superior gel properties. pH values are important in the sugar-acid-pectin system and adequate buffering must be provided in the product.
The use of all sucrose and invert sugar in preserves produces a very sweet product. This can be tempered by the inclusion of glucose syrup which allows the flavor of the fruit present to be sensed more readily by the consumer’s palate.