Ice crystallization in a scraped surface freezer

Abstract

Ice crystal size plays an important role in determining the quality of ice cream, with small ice crystal sizes desired to enhance consumer acceptance. During freezing, nuclei formation must be promoted and ice crystal growth and recrystallization minimized to create many small ice crystals. The effects of sweetener type, draw temperature, dasher speed, and throughput rate on ice crystal size distributions during freezing of ice cream were investigated. These operating variables affect the mechanisms by which ice crystals form and ripen into the disc-shaped crystals observed exiting the freezer. Increasing the throughput rate, reducing the residence time, in the freezer allows less time at warmer temperatures where recrystallization occurs rapidly, which leads to smaller ice crystals at the freezer exit. Smaller ice crystals were also found at lower draw temperatures, since coolant temperatures were reduced, increasing the driving force for nucleation. Increased ice crystal size at draw occurred when faster dasher speeds were utilized due to the addition of frictional heat into the system causing melting of small crystals. Residence time was found to have the most pronounced effect on mean ice crystal size followed by draw temperature and dasher speed, whereas type of sweetener had no significant effect.