Sustainable and Reusable Gelatin-Based Hydrogel “Jelly Ice Cubes” as Food Coolant. II: Ideal Freeze–Thaw Conditions

In an earlier study, jelly ice cubes (JICs) based on 10% gelatin hydrogels were demonstrated as a new type of food coolant. One of the practical challenges associated with the use of JICs was the structural instability against temperature variations during the phase changes of water in hydrogels, which was closely associated with freeze–thaw (FT) conditions. Here, a systematic study on various freezing and thawing rates applied to JICs was conducted to explore the ideal manufacturing and application conditions. Three freezing conditions, −20 °C (F1), −78.5 °C (F2), and −198 °C (F3), and three thawing rates, 0.05 °C/min (T1), 0.25 °C/min (T2), and 2.5 °C/min (T3), were applied with repeated freeze–thaw cycles (FT cycles) to observe the changes of JICs in the latent heat of fusion, water content, mechanical properties, cooling efficiencies, and hydrogel inner structures. The JICs treated with the selected conditions showed significantly improved stability in water content, structural uniformity, heat-absorbing abilities, and lifecycles. With specified FT conditions, the cooling efficiency of JICs remained stable after at least 10 FT cycles. It is anticipated that both the rapid freezing rate and the slow thawing rate assist the formation of uniform polymer networks under repeated freeze–thaw cycles.