ERT 426 Food Engineering Semester 1 Academic Session 2017/18 Food Freezing ERT 426 Food Engineering Semester 1 Academic Session 2017/18

Subtopics Freezing. Freezing time Thawing Thawing time Freezing equipment Effect of freezing on food quality ERT 426 Food Engineering

1. Freezing Freezing is a unit operation that is intended to preserve foods without causing significant changes to their sensory qualities or nutritional value. It involves a reduction in the temperature of a food to below its freezing point, which causes a proportion of the water in the food to undergo a change in state to form ice crystals. The immobilisation of water as ice and the resulting concentration of dissolved solutes in unfrozen water lower the water activity (aw) of the food ERT 426 Food Engineering

Freezing Preservation is achieved by a combination of low temperatures that reduce biochemical changes, enzymatic and microbial activity, reduced water activity and, in some foods, pretreatment by blanching. There are only small changes to nutritional or sensory qualities of foods when correct freezing, storage and thawing procedures are followed. ERT 426 Food Engineering

Freezing There are different stages involved in lowering the temperature of a food below its freezing point. First, sensible heat is removed and in fresh foods, heat produced by respiration is also removed. This is termed the `heat load‘ and is important in determining the correct size of freezing equipment for a particular production rate. ERT 426 Food Engineering

Freezing Latent heat is removed when water freezes to ice. Most foods contain a large proportion of water which has a high specific heat (4182 J kg-1K-1) and a high latent heat of crystallisation (335 kJ kg-1). A substantial amount of energy is therefore needed to remove sensible and latent heat to form ice crystals. ERT 426 Food Engineering

Freezing The latent heat of other components of the food (e.g. fats) must also be removed before they can solidify but in most foods they are present in smaller amounts and require removal of a relatively small amount of heat for crystallisation. Energy for freezing is supplied as electrical energy, which is used to compress refrigerants in mechanical freezing equipment or to compress and cool cryogens. ERT 426 Food Engineering

Freezing In foods that contain a large proportion of water the formation of ice has a dramatic effect on their thermo-physical properties: The density falls as the proportion of ice increases. The thermal conductivity increases (the thermal conductivity of ice is approximately four times greater than that of water. The enthalpy decreases. ERT 426 Food Engineering

Freezing The specific heat rises substantially as ice is formed and then falls back to approximately the same value as water when the temperature of the food is reduced to ≈20 0C. The thermal diffusivity of the food increases after initial ice formation as the temperature is further reduced. ERT 426 Food Engineering

Freezing Figure 1: Freezing diagram of water and a food material. ERT 426 Food Engineering

2. Freezing time Knowledge of the freezing time for a particular food is important for both ensuring its quality and in determining the throughput of a freezing plant. During freezing a moving front inside the food separates the frozen layer from unfrozen food. Heat is generated at the moving front as latent heat of fusion is released. ERT 426 Food Engineering

Freezing time This heat is transferred by conduction through the frozen layer to the surface, and then by convective heat transfer through a boundary film to the freezing medium. The factors that influence the rate of heat transfer are: thermal conductivity of the food; area of food available for heat transfer; distance that the heat must travel through the food (size and shape of the pieces); ERT 426 Food Engineering

Freezing time temperature difference between the food and the freezing medium; insulating effect of the boundary film of air surrounding the food; packaging, if present, is an additional barrier to heat flow. ERT 426 Food Engineering

2.1 Plank’s Equation A number of methods have been developed to calculate freezing time, the earliest: Plank’s equation: Constant (i) P’ = 1/6 (Sphere) (ii) P’ = 1/2 (Slab) (iii) P’ = 1/4 (Cylinder) Constant (i) R’ = 1/24 (Sphere) (ii) R’ = 1/8 (Slab) (iii) R’ = 1/16 (Cylinder) Latent heat of fusion of the food (kJ kg-1) Density of the food (kg m-3) Thickness/ diameter of the material (m) Freezing time (s) Freezing temperature (0C) Thermal conductivity of the frozen food (Wm-2K-1) Convective heat transfer coefficient at the surface of the food (W m-2K-1) Temperature of the freezing medium (0C) ERT 426 Food Engineering

Freezing Plank’s equation shows that the freezing time: increases with higher food density and increased size of the food, and decreases with higher temperature differences between the food and the freezing medium, decreases with higher thermal conductivity of the frozen food and higher surface heat transfer coefficient. ERT 426 Food Engineering

Freezing Assumptions: Freezing starts with all water in the food unfrozen but at its freezing point, and loss of sensible heat is ignored. Heat transfer takes place in one direction and is sufficiently slow for steady state conditions to operate. The freezing front maintains a similar shape to that of the food (e.g. in a rectangular block the freezing front remains rectangular). There is a single freezing point. The thermal conductivity and specific heat of the food are constant when unfrozen and then change to a different constant value when the food is frozen. ERT 426 Food Engineering

Freezing Limitations to Plank’s equation are related primarily to assignment of quantitative values to the components of the equation. Density values for frozen foods are difficult to locate or measure. Although the initial freezing temperature is tabulated for many foods, the initial and final product temperatures are not accounted for in the equation for computation of freezing time. The thermal conductivity k should be for the frozen product, and accurate values are not readily available for most foods. ERT 426 Food Engineering

2.2 Pham’s Method Pham (1986) developed a simplified equation that included the time taken to lose sensible heat (to overcome some of the limitations of the Plank’s equation). Enthalpy change for phase change and cooling of frozen food Enthalpy change for precooling unfrozen food A characteristic dimension (radius or shortest distance to the centre) (m) Biot number Shape factor 1 - slab, 2 - cylinder , 3 - sphere, Convective heat transfer coefficient (W/[m 2 K]) Temperature gradients ERT 426 Food Engineering

Pham’s Method Assumption: The environmental conditions are constant. The initial temperature, θi, is constant. The value for the final temperature, θc, is fixed. The convective heat transfer at the surface of an object is described by Newton’s law of cooling. ERT 426 Food Engineering

Pham’s Method A “ mean freezing temperature, ” θfm, is used to separate the diagram into two parts: the precooling period with some phase change component, the phase change (largely) and the postcooling periods. Figure 2: Freezing diagram of food, divided into sections for Pham’s method. ERT 426 Food Engineering

Pham’s Method Mean freezing temperature (0C) Final temperature at the centre of the food (0C) Temperature of the freezing medium(0C) Density of unfrozen material (kg m-3) Specific heat of unfrozen material (J kg-1K-1) Initial freezing temperatures of the material (0C) Density of frozen material (kg m-3) Latent heat of fusion of the food (kJ/kg) Specific heat of frozen material (J kg-1 K-1) ERT 426 Food Engineering

3. Thawing When frozen food is thawed using air or water, surface ice melts to form a layer of water. Water has a lower thermal conductivity and a lower thermal diffusivity than ice and the surface layer of water therefore reduces the rate at which heat is conducted to the frozen interior. This insulating effect increases as the layer of thawed food grows thicker in contrast, during freezing, the increase in thickness of ice causes heat transfer to accelerate because of the higher thermal conductivity of the ice. Thawing is therefore a substantially longer process than freezing when temperature differences and other conditions are similar. ERT 426 Food Engineering

Figure 3: Temperature changes during thawing ERT 426 Food Engineering

Thawing The initial rapid rise in temperature (AB) is due to the absence of a significant layer of water around the food. There is then a long period when the temperature of the food is near to that of melting ice (BC). During this period any cellular damage caused by slow freezing or recrystallisation results in the release of cell constituents to form drip losses. ERT 426 Food Engineering

4. Thawing time Cleland’s model: A characteristic dimension (radius or shortest distance to the centre) (m) Enthalpy of the product from 0 to -100C Biot number Thawing time (s) Freezing Temperature (0C) Shape factor 1 - slab, 2 - cylinder , 3 - sphere, Temperature of the freezing medium (0C) Convective heat transfer coefficient (W/[m 2 K]) Planck number Stephen number ERT 426 Food Engineering

5. Freezing equipment The selection of freezing equipment should take the following factors into consideration: the rate of freezing required; the size, shape and packaging requirements of the food; batch or continuous operation, the scale of production; range of products to be processed; the capital and operating costs. ERT 426 Food Engineering

Freezing equipment Freezers are categorised into: mechanical refrigerators evaporate and compress a refrigerant in a continuous cycle and use cooled air, cooled liquid or cooled surfaces to remove heat from foods cryogenic freezers use solid or liquid carbon dioxide, or liquid nitrogen directly in contact with the food. ERT 426 Food Engineering

Freezing equipment In general, mechanical freezers operate at -40 0C and have higher capital costs than cryogenic freezers. Cryogenic freezers operate at -50 to -70 0C and have higher operating costs because refrigerant is not recirculated and is lost to the atmosphere. ERT 426 Food Engineering

Freezing equipment Freezers can also be grouped according to the rate of movement of the ice front: slow freezers and sharp freezers (0.2 cm h-1) still-air freezers and cold stores quick freezers (0.5-3 cm h-1) air-blast and plate freezers rapid freezers (5-10 cm h-1) fluidised-bed freezers ultrarapid freezers (10-100 cm h-1) cryogenic freezers ERT 426 Food Engineering

Freezing equipment All types of freezers are constructed from stainless steel and are insulated with expanded polystyrene, polyurethane or other materials that have low thermal conductivity. Most freezing equipment has microprocessor control, using programmable logic controllers (PLCs) to monitor process parameters and equipment status, display trends, identify faults and automatically control processing conditions for different products. ERT 426 Food Engineering

Freezing equipment Table 1: A comparison of freezing equipment ERT 426 Food Engineering

Freezing equipment Belt freezers: (a) Spiral freezer, and (b) self-staking belt ERT 426 Food Engineering

A fluidized-bed freezing system. Horizontal plate freezer Freezing equipment A fluidized-bed freezing system. Horizontal plate freezer ERT 426 Food Engineering

Freezing equipment Schematic illustration of an immersion freezing system. Individual quick freezing (IQF) using liquid refrigerant. ERT 426 Food Engineering

Freezing equipment Advantages of cryogenic freezers as compared to mechanical freezers: Simple continuous operation with relatively low capital costs (30% of the capital cost and 5% of the power requirement of mechanical systems because there is no requirement for a compressor or evaporator). Smaller units for the same production rates because heat exchanger coils are not used. Smaller product weight losses due to dehydration (0.5% compared with up to 8.0% in mechanical air-blast systems). ERT 426 Food Engineering

Freezing equipment Rapid freezing produces small ice crystals that cause smaller changes to the sensory and nutritional characteristics of the product. Exclusion of oxygen during freezing which reduces oxidative changes to products. Rapid start-up and no defrosting time. ERT 426 Food Engineering

because cryogens are not recirculated and are lost to the atmosphere. Freezing equipment The main disadvantage of cryogenic freezers is the relatively high cost of the cryogens, which results in operating costs that are 6-8 times higher than those of mechanical refrigeration systems. because cryogens are not recirculated and are lost to the atmosphere. ERT 426 Food Engineering

6. Effect of freezing on food quality The main effect of freezing on food quality is damage caused to cells by dehydration; the extent of damage depends on the size of the crystals and hence on the rate of freezing. Volume changes: The volume of ice is 9% greater than that of pure water, and an expansion of foods after freezing would therefore be expected. Recrystallisation : Physical changes to ice crystals (e.g. changes in shape, size or orientation of ice crystals). ERT 426 Food Engineering

Effect of freezing on food quality Effect on food microorganism: Freezing prolongs the shelf-life of products by slowing microbial growth. In general, the lower the temperature of frozen storage, the lower is the rate of microbiological and biochemical changes. ERT 426 Food Engineering

Effect of freezing on food quality The main changes to frozen foods during storage: Degradation of pigments Loss of vitamins Enzyme activity (browning, lipoxygenase activity which produces off-flavours and off-odours from lipids and causes degradation of carotene). Oxidation of lipids ERT 426 Food Engineering