UC Davis

Novel environment-friendly heat-storage materials with efficient cooling capacity are in high demand by the food, pharmaceutical, and related industries and supply chains. Better control and prevention of microbial cross-contamination caused by meltwater from traditional ice have become critical to ensure the safety and quality of food and other commodities. In this dissertation, a novel food coolant, “jelly ice cubes” (JICs), based on gelatin hydrogels subject to various consolidating methods, was designed and tested to diminish meltwater while inheriting the high cooling efficiency of traditional ice. The new sustainable JICs can prevent meltwater-caused cross-contamination with high affordability, recyclability, sustainability, and biodegradability. Specifically, Chapter 1 introduces the challenges and current solutions regarding the temperature abuses in the supply chain, worldwide sustainability issues, and the application of phase change thermal energy storage materials. Chapter 1 also discussed the recent studies on the effect of cryogenic treatment on the properties of hydrogel materials and proposed the initial design of JICs. In Chapter 2, the feasibility of JICs is proposed and experimentally proven. JICs based on 10% gelatin hydrogels achieved 265.35 J·g-1 latent heat of fusion and cooling efficiency comparable with traditional ice. JICs survived the normal pressure equivalent to a food load as tall as 1 m on top throughout the repeated freeze−thaw cycles. After each freeze−thaw cycle, JICs could be effectively rehydrated, cleaned, or sanitized with brief water or diluted bleach rinse. The application of JICs can potentially reduce water consumption in the food supply chain and food waste by controlling microbial contaminations. In Chapter 3, since the stabilities of JICs are closely associated with freeze−thaw (FT) conditions, 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-1 (T1), 0.25 °C·min-1 (T2), and 2.5 °C·min-1 (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. 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. Chapter 4 reports the further development of JICs highlighting the features of microbial resistance, higher stability, and compostability, based on gelatin–menadione sodium bisulfite (MSB) hydrogels (Gel/MSB-JICs). The MSB-induced photo-crosslinking reaction is first-time proposed and validated theoretically and experimentally. In this chapter, photo-tuning is used collectively with rapid-freeze-slow-thaw treatment to construct robust hydrogel frameworks against the functional and structural damages caused by the phase transition of water during repeated uses. Exceptionally stable cooling efficiency and robust mechanical strength were observed across ten application cycles, with stable antimicrobial functions against bacteria, fungi, and yeast. At the end of the lifecycle of JICs, they can be composted or used as a soil treatment to promote plant growth. Until this step, the novel JICs are competent as promising, safer, and more eco-friendly substitutes for traditional ice and ice packs. Considering the potential general applicability of the protein hydro-rich strengthening method revealed in Chapter 4, Chapter 5 further details the mechanisms, optimizes the treatment conditions, and proposes a general protein-based hydrogel tuning strategy. The tuning strategy includes the initial formation of polymeric crystalline (via RFST treatment) and the subsequent formation of covalent bonds (induced by photoreactions) at the edge of the protein polymer crystalline. This novel hydrogel tuning strategy offers information guiding the future fabrications and applications of hydrogel materials, especially to those whose application conditions value the stability of the materials under repetitive solid-liquid phase changes of water in the hydrogels. Finally, Chapter 6 discusses the concerns found in the current development of JICs and the future possible development directions.