Chpter 6 FREEZING AND FROZEN-FOOD STORAGE

1. Chpter 6 FREEZING ANDFROZEN-FOOD STORAGE Principles of freezing DESCRIPTION OF FOOD FREEZING SYSTEMS INDRECT-CONTACT FREEZING SYSTEMS DIRECT-CONTACT FREEZING SYSTEMS INDIVIDUAL QUICK FREEZING (IQF) ESTIMATION OF FREEZING TIME FOOD FREEZING AND PRODUCT QUALITY SUMMARY

2. Problem • Explain the cause of freezing temperature depression. • Why should we use rapid freezing process? • How many factors influence the freezing time? What are they? • What changes occur to the product during freezing? • Why the storage temp. should be kept in constant?

3. vocabulary • freeze, ice crystals , irreversible, negative changes, latent heat of fusion , depression of the freezing temperature , supercooling region , freezing plateau , solutes , depression of the freezing temperature, freezing--point depression , freezing--temperature depression, eutectic temperature ,

4. vocabulary • barrier,air blat system, spiral conveyor, fluidized bed,immersion, cryogenic, tunnel, spray of liquid refrigerant, intimate contact , fundamental, estimation, critical, individual, freezing medium, Plank's equation, temperature gradient, thermal conductivity, cylinder, cylindrical, sphere, spherical, infinite plate, convective heat transfer coefficients

5. Freezing Process • The freezing process is the removal of thermal energy from the food product to the extent required to reduce the temperature below the freezing temperature of water. The thermal energy removed as a part of freezing is primarily latent heat of fusion required to convert water to ice within the product

6. DEFINITION OF FREEZING AND FROZEN-FOOD STORAGE • Food freezing is the preservation process that depends on the reduction of product temperature to levels well below the temperature at which ice crystals begin to form within the food. By reducing the temperature of the product to -10 to -20℃, the normal reactions that cause deterioration of foods are reduced to negligible or minimal rates.

8. The two curves compare the characteristic temperature/time relationship during freezing of water with that of a food product. The first characteristic of the food--freezing curve is the depression of the freezing temperature. All food products will exhibit an initial ice crystal formation temperature below that of pure water. The second feature of the food-freezing curve is the gradual decline in temperature during removal of the latent heat of fusion.

10. Freezing-temperature Depression • The formation of ice crystals within the water phase results in a concentration of solutes within the water phase. The ice crystals are pure water in the solid state. The result of the increased concentration of the solute in the liquid phase causes depression of the freezing temp. , a lower temp. at which ice crystals will form. Since these changes occur on a continuous basis, the gradual temp. decline with time is a direct result of freezing-temperature depression.

11. Liquid Concentrated in Foods • In very pure solutions, the solute concentration in unfrozen liquid may become sufficiently high to cause crystallization of the solute. This change occurs at the eutectic temp. In frozen foods, the temp. used for food freezing are typically well above the eutectic temp. for solutes found in food products. Most often, eutectic temp. are in -55 to -75℃ range. It should be recognized that all frozen foods, even at temp. as low as -40℃, will contain small fractions of unfrozen water.

12. Definition of Freezing Time • time required to reduce the product temperature from the initial freezing temperature to a temperature of 5 degrees below the initial freezing temperature

13. Effect of Freezing Rate on Product Quality • The formation of large ice crystals causes greater damage to the product structure and negative impact on the quality attributes of that product. As might be anticipated, more rapid freezing and formation of small ice crystals usually result in a more desirable product after freezing and frozen--food storage

14. INDIRECT CONTACT FREEZING SYSTEMS • Plate freezing system • Indirect-contact freezing system • Cabinet freezingsystem • Scraped surface, continuous system

16. cabinet freezingsystem • The product is placed in a package prior to freezing, and the packages are p1aced on trays before they are moved into the freezing system. These types of freezing systems operate as batch systems, with ti1e freezing time established by the length of time that the product remains in the cabinet. The environment in the room is maintained at a low temperature, and air movement is established by fans within the cabinet.

18. Plate freezing system • In these types of freezing systems, the product is held firmly between two plates throughout the period of time required for product temperature reduction. The plates are the primary barrier between the cold refrigerant and the product. These types of freezing systems have a definite advantage when the product configuration allows for direct and close contact between the plate surface and the product surface.

20. Indirect-contact freezing system • indirect-contact freezing system using high-velocity air. In these types of systems the product package is the barrier between the cold air as a freezing medium and the product within the package. As illustrated, the systems are designed to be continuous, with continuous product movement in a direction concurrent to air flow

22. Scraped surface, continuous system • These types of freezing systems utilize a scraped surface heat exchanger as a primary component of the continuous system used to convert liquid product into a frozen slurry. In these systems, the outer wall of the heat exchanger barrel represents the barrier between the product and the low-temperature refrigerant used for product freezing.

24. DIRECT-CONTACT FREEZING SYSTEMS • Air blat system • Continuous spiral conveyor system • Continuous fluidized bed system • Continuous immersion freezing system • Continuous cryogenic freezing systems

26. Direct Contact Freezing System direct contact between the product and medium used for reduction of product temperature is appropriate and effective. In these types of systems, there is no barrier between product surface and the medium utilized for product freezing.

28. Air Blat System • One type of air blat system is used for direct contact freezing. The product is carried through the entire freezing system on a conveyor, and low temperature air is directed over the product throughout the period of time the product is held in the freezing tunnel.

30. Continuous Spiral Conveyor System • In this type of system the product is carried on a spiral conveyor from the time it enters the low-temperature environment until it leaves the system. For these types of systems, the freezing time is established by the rate of movement of the conveyor systems through the low-temperature environment

32. Continuous Fluidized Bed System • In these types of freezing systems, the product moves on a conveyor into the cold environment in a manner similar to air blast systems. In a fluidized bed system, the cold air used as a freezing medium is directed upward through the mesh conveyor at velocities sufficient to cause vibration and movement of product on the conveying system. The vibration or movement of product while being conveyed, increases the contact between cold air and the product and reduces the time required for freezing.

34. Continuous Immersion Freezing System • For products where rapid freezing is appropriate, direct contact between a liquid refrigerant such as nitrogen or carbon dioxide may be used. The product is carried on a conveyor through a bath of liquid refrigerant to establish direct and intimate contact with the liquid refrigerant.

36. Continuous Cryogenic Freezing Systems • The product on a conveyor moves through a tunnel where it is exposed to a spray of liquid refrigerant as it changes phase to vapor state. The length of time for freezing is established by the rate of conveyor movement through the tunnel where the product is exposed to the cryogenic refrigerant.

37. Individual Quick Freezing (IQF) • The concept of IQF involves exposure of individual pieces of the product to a low temp. medium for a relatively short period of time. In some situations, the rate of freezing is increased by promoting more intimate contact between the product pieces and the cold refrigerant. In other situations, IQF is achieved by using very low temperature cryogenic refrigerants as a freezing medium.

38. Estimation of Freezing Time • The primary dimension of the product is the thickness (L) in the horizontal dimension. The freezing medium temp. is (Tm) and temp. of unfrozen product in the middle of the diagram is the initial freezing temp. of the product (TF).The vertical sections of product in contact with the freezing medium are referred to as frozen zones. As the freezing process continues, the thickness of the frozen zones increases. When the two zones meet the freezing process is complete.

40. Factors Influencing Freezing Time • 1.Thermal conductivity of frozen food (k) • 2.Convective heat transfer coefficient (h) • 3.Area for heat transfer (geometry) • 4.Temperature of the freezing medium (TF) • 5.Size of the product defined as the thickness (L)

44. Food Freezing & Product Quality • An unfrozen product could have 70% water and 30% total solids. Within a temperature range of 5 degrees below the initial freezing point,a product might have 30% unfrozen water, 40% ice or frozen water, and the same 30% total solids. The changes occur gradually, As the temperature continues to decrease, the percentage of water in the frozen state increases. At a temperature low enough, a small fraction of the water will remain in the liquid state, The changes will have impact on the product quality attributes.

45. Influence of Freezing Rate • In slow rate freezing, ice crystals will become much larger. • The movement of water from one cell to another leads to dehydration of the cell and irreversible changes . • Large ice crystals within the product will lead to a frozen product with a rough texture. • But extremely high freezing rates lead to stress cracking .

46. STORAGE OF FROZEN FOODS • The quality of a frozen-food is influenced by storage conditions. The changes in quality decrease as temperature is decreased, maintaining low storage temperatures increases the cost of frozen-food storage. Higher temperatures in frozen-food storage must be avoided due to the sensitivity of the frozen-food to temperature. Experience has established that a frozen-food storage temperature of -18℃ is accepted as a safe storage temperature for extended shelf life of a frozen food.

50. Changes in quality • Microbial activity (negligible at –18C) • Biochemical reaction • Enzymatic reaction (blanching for vegetables, sulfur dioxide/ reduction of oxygen/ addition of acid for fruits)