Fluorescence microscopy to study galactomannan structure in frozen sucroseand 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 p owder after freezing and during temperature cycling between - 18 and - 10 d egrees C was visualized with fluorescence and brightfield microscopy. LBG w as observed to produce a structured gel-like network around the ice crystal s, which became more distinct with repeated temperature cycling. Such a net work was not evident with guar gum. Image analysis of the ice crystal size distributions showed that LBG had provided much greater resistance to ice r ecrystallization under these conditions than guar. The same trends were evi dent in sucrose plus skim milk powder solutions, but the overall increase i n 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. Ca rrageenan successfully reduced the extent of protein/galactomannan phase se paration, 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 netw ork and structural heterogeneity in the unfrozen phase. (C) 1999 Elsevier S cience Ltd. All rights reserved.