The two most common ingredients in the kitchen that affect the freezing point are salt and sugar. Sugar lowers the freezing point of water, making frozen desserts subject to changes in the freezing point. Most desserts freeze between 29.5 to 26.6 degrees F (-1.4 to -3.0 C), depending on the sugar concentration. Infrared videothermography was used to observe ice nucleation temperatures, ice formation patterns and freezing rates in leaves that were not acclimated and acclimated to the cold of a rapeseed (Brassica napus) spring (cv Quest) and winter (cv Express).
Clearly different freezing patterns were observed and the effect of water content, sugars and soluble proteins on the freezing process was characterized. When freezing began at a warm temperature below zero, ice growth spread rapidly through unacclimated leaves. On the contrary, acclimated leaves started freezing following a horseshoe pattern that began at the upper edge, followed by a slow progression of ice formation along the leaf. However, when leaves that were previously acclimated by slow freezing (2°C h−) or by direct immersion in liquid nitrogen were refrozen, their freezing pattern was similar to that of non-acclimated leaves.
A novel technique was developed using strips of filter paper to determine the effects of sugars and proteins on the freezing rate of cellular extracts. Cell sap from non-acclimated leaves froze 3 times faster than extracts from acclimated leaves. The rate of freezing of the leaves was largely dependent on the osmotic potential of the leaves. Simple sugars had a much greater effect on freezing speed than proteins.
Unacclimated leaves containing high water content were not supercooled as much as acclimated leaves. In addition, the wet leaves were not supercooled as much as the non-wet leaves. As expected, cellular solutes reduced the nucleation temperature of the leaves. The use of infrared thermography has revealed that the freezing process in plants is a complex process, which reminds us that many aspects of freeze tolerance occur at the level of the whole plant, related to aspects of the plant's structure and metabolites, and not just to the expression of specific genes.
The addition of sugar disrupts the liquid state because sugar molecules move aimlessly, making liquid water molecules less organized. Sugar molecules aren't packaged with water molecules, so when water molecules start to freeze, the sugar molecules remain in liquid water. Sugar analysis revealed that acclimated CV Express leaf extracts had a total sugar content of 92.9 mg mL−1, compared to 74.9 mg mL−1 of cell sap extracted from acclimated cv Quest leaves (Table II).
