Comparison of ice nucleating particles from fruit juices and their properties

Abstract

Heterogeneous ice nucleation is a crucial process for ice cloud formation in the atmosphere. Recent findings indicate the importance of ice nucleating particles of biological origin in this process. Previous investigations in our group (Pummer et al., 2012) revealed that pollen from several plants native to the boreal forests contain ice nucleating macromolecules, which are easily detached from the grains. Investigating a broader scope of boreal plants, Bichler (2015) showed that several juices of berries contain ice nucleation active macromolecules. Based on this research we examined ice nucleation activity and general properties of five juices from perennial plants: acerola, black currant, chokeberry, elderberry and sea buckthorn. With the only exception of acerola, which occurs primarily in the tropics and subtropics, the other four plants can be distributed up to the boreal zone. All of the examined fruit juices exhibit ice nucleation activity, T50,het values (median heterogeneous freezing temperature) of the pure juices are ranging between -17.9 °C and -29.6 °C. We elucidated particle size dependency of the ice nucleation activity to differentiate between coarse and submicron ice nucleating particles. For all analysed fruit juices ice nucleating particles were detected in both fractions: the coarse (> 0.45 μm) and the submicron size range (< 0.45 μm). For all five juices no direct correlation between dry mass and ice nucleation activity was found. Therefore no conclusions can be drawn from the dry mass of a juice concerning the ice nucleating particle quantity. The results of the investigations with different solvents (methanol, methylene chloride, and cyclohexane) suggest that polar components play an important role in the ice nucleation activity of acerola, black currant, chokeberry, elderberry and sea buckthorn. By FTIR spectroscopy numerous strong bands were assigned to saccharides, lipids and phenolic compounds. Further, broad bands in the different amid regions were present. The two juices that show the highest ice nucleation activity (sea buckthorn and black currant) were investigated with chaotropic agents (guanidinium chloride and urea) and a protein-digesting enzyme (Subtilisin). This way, properties like the importance of hydrogen bonds and other structure forming polar interactions were controlled. Our results suggest that proteinaceous compounds play a key role in the ice nucleation activity of pure black currant and sea buckthorn juice.