Fluorescence microscopy to study galactomannan structure in frozen sucrose and milk protein solutions

Locust bean gum (LBG) and guar were labeled with rhodamine isothiocyanate, and their location within a solution of sucrose or sucrose plus skim milk powder after freezing and during temperature cycling between - 18 and - 10 degrees C was visualized with fluorescence and brightfield microscopy. LBG was observed to produce a structured gel-like network around the ice crystals, which became more distinct with repeated temperature cycling. Such a network was not evident with guar gum. Image analysis of the ice crystal size distributions showed that LBG had provided much greater resistance to ice recrystallization under these conditions than guar. The same trends were evident in sucrose plus skim milk powder solutions, but the overall increase in ice crystal size was less than it was without skim milk powder. Both the LBG and guar promoted the formation of a phase-separated protein region. Carrageenan successfully reduced the extent of protein/galactomannan phase separation, but solutions containing carrageenan recrystallized to a greater extent than those without carrageenan. It was suggested that a reduction in recrystallization resulted from the formation of a continuous polymer network and structural heterogeneity in the unfrozen phase. (C) 1999 Elsevier Science Ltd. All rights reserved.