GLASSY TRANSITION STATE IN FROZEN FOODS
- 1. GLASSY STATE IN FROZEN FOODS
- 2. CONTENTS 1. Introduction 2. Understanding glass transition temperature 3. Impact on food quality 4. Controlling Tg in frozen foods 5. Analytical techniques 6. Measuring and interpreting the glass transition in frozen foods and model systems 7. Implications of non-equilibrium states and glass transition in frozen and dried fish and meat products 8. Formation of the glassy phase in frozen foods 9. Solute crystallization 10. Ice recrystallization
- 3. INTRODUCTION The glass transition occurs as an endothermic steps in heat flow, and the onset temperature of the maximally freeze-concentrated unfrozen phase is taken as Tg'. A single solute may concentrate in unfrozen phase forms a continuous matrix around ice crystals .
- 4. Understanding Glass Transition Temperature (Tg): • Glass transition temperature (Tg) marks the shift from a hard, glassy state to a rubbery or viscous state. • In frozen foods, Tg is crucial as it indicates the change from a rigid, glass-like structure to a more flexible state. • Understanding Tg helps predict how frozen foods will behave during storage and thawing. • Tg is a key factor in maintaining the quality and texture of frozen foods
- 5. Effects of Temperature on Food Structure: • Below Tg: Restricted molecular motion, ice crystal formation maintains frozen state. • Near and above Tg: Increased molecular mobility. • Tg crossing leads to structural and textural changes in the food.
- 6. Impact on Food Quality: 1 Texture Change: • Near Tg, frozen foods soften, become more elastic, or collapse. 2 Shelf Life Impact: • Above Tg, increased molecular mobility accelerates spoilage. 3 Recrystallization Effect: • Close to Tg, ice crystals recrystallize, altering texture. • Significant in frozen desserts like ice cream.
- 7. Controlling Tg in Frozen Foods: • Freezing rate determines ice crystal size. • Slower freezing = larger crystals, impacts texture. • Ingredient selection alters Tg for desired properties. • Sugars, stabilizers modify Tg, prevent crystallization. • Formulation adjustments change Tg, affect texture. • Modify water, cryoprotectant levels for Tg control. • Adjust fat, protein levels for texture and stability.
- 8. ANALYTICAL TECHNIQUES • Analytical Techniques: • DSC and DMA determine Tg in frozen foods. • Provide insights into thermal and mechanical properties. • Importance of Tg: • Influences texture, stability, and quality. • Crucial for manufacturers to control. • Texture and Stability: • Tg affects texture and accelerates spoilage. • Quality Control: • Understanding Tg ensures desirable attributes and longer shelf life.
- 9. Measuring and interpreting the glass transition in frozen foods and model systems • Frozen foods have an unfrozen phase due to water removal during freezing. • Reactions like enzymatic degradation occur in this phase, limiting shelf life. • Recent research emphasizes viscosity and diffusion rates for stability. • High viscosities lead to glass formation, slowing diffusion. • Proper temperatures are crucial for stability. • Adjusting formulations can enhance shelf life.
- 10. Implications of Non-Equilibrium States and Glass Transitions in Frozen and Dried Fish and Meat Products • Frozen and dried foods resist microbial growth but degrade chemically and physically during storage. • Issues include enzymatic changes, oxidation, and solute crystallization. • Desiccated foods benefit from storage in an amorphous glassy state for prolonged stability. • Exceeding the glass-transition temperature leads to structural and chemical alterations. • Such changes degrade the quality and stability of the food.
- 11. Formation of the Glassy Phase in Frozen Foods • Ice Formation: • Water crystallizes, concentrating solutes. • Equilibrium Freezing: • Solute concentration determines freezing point. • Glass Transition: • Below freezing point, solute crystallization slows. • Viscosity increases, transitioning to a glassy state. • State Diagram: • Integrates freezing and glass transition. • Freeze Concentration: • Cooling initiates crystallization. • Water removal depresses freezing point. • Viscosity rises rapidly. • UFP becomes a solid glass.
- 12. SOLUTE CRYSTALLIZATION • Solute crystallization in frozen foods occurs due to thermodynamic driving forces toward equilibrium. • Common in poorly soluble components like lactose, especially below -18°C. • Lactose in ice cream can remain in a glassy state, preventing gritty texture. • Crystallization can happen above Tg' due to supersaturation. • Lactose's poor solubility at low temperatures increases the likelihood of crystallization. • Crystallization rate peaks at a temperature balancing supersaturation and viscosity. • Inhibition methods include blending sugars and using hydrocolloids to increase viscosity.
- 13. ICE RECRYSTALLIZATION • Ice Formation and Freezing: • Rapid freezing crucial for small, numerous ice crystals. • Initial crystal size determined by nucleation during rapid cooling. • Ice Formation and Temperature: • Ice formation ceases at Tm′ due to water-solute chemical potential. • Rapid cooling below unfrozen Tg leads to vitrification; reheating causes devitrification. • Recrystallization: • Modifies ice crystal size and shape post-crystal growth. • Types: isomass, migratory, accretive. • Rates depend on temperature, diffusion, and dilution above Tm′. • Effects of Recrystallization: • Larger crystals lower food quality, damage cellular structures. • Alters food texture during thawing. • Impact of Temperature Fluctuations: • Accelerates quality decline during storage. • Normal variations cause significant quality loss.
- 14. CONCLUSION • GLASS TRANSITION HAVE HIGH IMPACTS ON STABILITY AND OTHER FOOD QUALITY PARAMETERS. • GLASS TRANSITIONS ARE HIGHLY DEPENDENT ON CONSTITUENTS AND FREEZING CONDITIONS.