Glucose syrup – boiling point elevation and freezing point depression

Both these colligative properties are dependent on the concentration and the molecular weight of the carbohydrate in solution and glucose syrups of different DE, and thus different average molecular weight, can be used to control these parameters. High DE syrups (low average molecular weight) will cause a greater elevation of boiling point and greater depression of freezing point than lower DE (high average molecular weight) syrups.

			BP ( °C)
			DS (% w/w)
DE	20	40	60	70	80	85
30	100.22	100.66	101.97	103.50	106.17	108.39
42	100.31	100.89	102.67	104.61	107.94	110.78
55	100.42	101.16	103.32	105.61	109.39	112.39
65	100.51	101.39	103.81	106.28	110.28	113.22
80	100.95	101.76	104.53	107.17	111.28	114.06
90	101.77	102.01	104.97	107.72	111.72	114.50

		FPD ( °C)
		DS (% w/w)
DE	10	20	30	40	50
15	0.17	0.38	0.66	1.02	1.53
19	0.20	0.46	0.78	1.22	1.83
25	0.27	0.61	1.05	1.63	2.45
33	0.36	0.80	1.37	2.13	3.20
37	0.37	0.83	1.43	2.22	3.33
42	0.43	0.97	1.66	2.59	3.88
49	0.52	1.19	2.03	3.16	4.75
55	0.66	1.49	2.55	3.96	5.94
67	0.85	1.91	3.27	5.08	7.62
100	1.15	2.58	4.43	6.89	10.33
sucrose	0.60	1.36	2.33	3.63	5.44

Table 5.3 shows the effect on boiling point elevation (Dziedzic and Kearsley, 1984) and Table 5.4 shows the effect on freezing point depression. Freezing point depression is calculated from average molecular weight of each syrup using the relationship:

Freezing point depression = (1.86 X weight of solute X 100) : (average molecular weight X weight of solvent)

where 1.86 = K_f = molal depression constant for water (Glasstone and Lewis, 1965).

With respect to boiling point elevation, this effect is important in the production of hard-boiled sweets where the judicious selection of the glucose syrup to blend with sucrose can result in a lowering of the boiling temperature of the mix. This in turn can lead to reduced browning of the sweets due to overcooking and also, potentially, a decrease in energy requirements to produce the sweet.

Freezing point depression is important with respect to ice cream. For ‘soft-scoop’ products, which can be easily handled straight from the freezer, a large depression of freezing point is required and thus a high DE glucose syrup is used or even dextrose or fructose. Too Iowa freezing point may be undesirable however as this can cause handling and packing problems during manufacture. In such cases a lower DE syrup may be the better choice. In ice cream other factors such as sweetness and foam stabilizing effects of the syrup are also important considerations.