Freezing curve, freezing system & freezing time

Freezing curve, freezing system & freezing time

1. Freezing Curve, Freezing System & Freezing time calculation Muneeb Central university Of Punjab

2. Freezing Freezing is an unit operation In which the temperature of the food is reduced below the freezing point. Proportion of water undergoes change in system to form icecrystalls . Immobilisation of the water to ice and resulting concentration of dissolved solutes in unfrozen water will lower the water activity. During freezing preservation is achieved by low temperature and reduced water activity. Small changes to nutritional qualities or sensory quality. During freezing sensible heat is first removed to lower the temperature to the freezing point. In fresh foods the heat produced by the respiration also removed.

3. The material to be frozen first cools down to the temperature at which nucleation starts. Before ice can form, a nucleus, is required upon which the crystal can grow; the process of producing this seed is defined as nucleation. Once the first crystal appears in the solution, a phase change occurs from liquid to solid with further crystal growth. FREEZING PROCESS

4. Freezing curve

5. AS The food is cooled to below the freezing point ᶱ f . At the point S the water remains liquid, although the temperature is below the freezing point. The phenomenon is known as supercooling and may be as much as 10 c below freezing point SB The temperature rises rapidly to the freezing point as ice crystals begin to form and latent heat of crystallisation is released . BC heat is removed as same rate as before, but it is latent heat is being removed as ice forms and the temperature therefore remains almost constant. The freezing point is gradually depressed by the increase in solute concentration in the unfrozen liquor and the temperature falls slightly. CD One of the solutes become supersaturated and crystallises out, the latent heat of crystallisation is released and the temperature rises to the eutectic temperature for that solute.

6. DE Crystallisation of water and solute continues. The total time tf taken is determined by the rate at which is heat is removed EF The temperature of the ice–water mixture falls to the temperature of the freezer. A proportion of the water remains unfrozen at the temperatures used in commercial freezing; the amount depends on the type and composition of the food and the temperature of storage.

7. Crystallization occurs at point ‘B’ of the freezing curve and consists of nucleation and crystal growth Nucleation: occurs by combining molecules into an ordered particle of a size sufficient to survive and serve as a rate for crystal growth Homogeneous nucleation: in pure systems Heterogeneous nucleation: nucleus formation around suspended particle or at a cell wall, in food systems, takes place during supercooling Crystal growth: enlargement of nucleus by the orderly addition of molecules

8. IMPORTANT POINTS TO KNOW Freezing point is defined as the temperature at which the first ice crystal appears and the liquid at that temperature is in equilibrium with the solid. Freezing time is defined as time required to lower product temperature from its initial temperature to a given temperature at its thermal center. Freezing rate is defined as the ratio of difference between initial and final temperature of product to freezing time.

9. Calculation of freezing time During freezing, heat is conducted from the interior of a food to the surface and is removed by the freezing medium. The factors which influence the rate of heat transfer are : • the thermal conductivity of the food • the area of food available for heat transfer • the distance that the heat must travel through the food (size of the pieces) • the temperature difference between the food and the freezing medium • the insulating effect of the boundary film of air surrounding the food • packaging, if present, is an additional barrier to heat flow.

10. It is difficult to define the freezing time precisely but two approaches are taken. The effective freezing time measures the time that food spends in a freezer and is used to calculate the throughput of a manufacturing process. nominal freezing time can be used as an indicator of product damage as it takes no account of the initial conditions or the different rates of cooling at different points on the surface of the food The calculation of freezing time is complicated for the following reasons differences in the initial temperature, size and shape of individual pieces of food differences in the freezing point and the rate of ice crystal formation within different regions of a piece of food changes in density, thermal conductivity, specific heat and thermal diffusivity with a reduction in temperature of the food.

11. HOW TO DETERMINE FREEZING TIME ?? ONE OF THE WAY TO ESIIMATE FREEZING TIME IS THROUGH PLANCK’S EQUATION. THIS APPROCCH WAS DEVELPOED BY PLANCKS IN 1913. Plank’s equation was derived based on energy balance principle . He developed the equation to find out the freezing point of water. Although this equation has many limitations but it enables to find out freezing point with those limitations with spreadsheet or Calculator.

12. This involves the following 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 sufficiently slowly for steady-state conditions to operate the freezing front maintains a similar shape to that of the food (for example in a rectangular block the freezing front remains rectangular) there is a single freezing point the density of the food does not change 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. ] ]  

13. where t f (s) freezing time L (m) length of the cube, h (Wm2K1) surface heat transfer coefficient, f (ºC) freezing point of the food, a (ºC) temperature of the freezing medium, (J kg1) latent heat of crystallisation , (kg m3) density of the food, x (m) thickness of the packaging , k 1 (Wm1K1) thermal conductivity of the packaging, k 2 (W m1K1) thermal conductivity of the frozen zone, 6 and 24 are factors which represent the shortest distance between the centre and the surface of the food. Other shapes require different factors; these are 2 and 8 for a slab, 4 and 16 for a cylinder and 6 and 24 for a sphere  

14. Pham’s Method to Predict Freezing Time Pham proposed a method for predicting food freezing and thawing times. His method can be used for finite-size objects of irregular shapes by approximating them to be similar to an ellipsoid. Another advantage of this method is that it is easy to use, yet it provides answers with reasonable accuracy. The following assumptions are used in developing this method: The environmental conditions are constant. The initial temperature, T i , is constant. The value for the fi nal temperature, T c, is fi xed . The convective heat transfer at the surface of an object is described by Newton’s law of cooling.

15. T fm 1.8 0.263 T c +0.105 T a where T c is final center temperature (°C), and T a is freezing medium temperature This is an empirically derived equation that is valid for most water-rich biological materials The time for freezing of any simple-shaped object is calculated from the following equation: where dc is a characteristic dimension, either shortest distance to the center, or radius (m), h is the convective heat transfer coefficient (W/[m 2 K ]), Ef is the shape factor, an equivalent heat transfer dimension . E f 1 for an infinite slab, E f 2 for an infinite cylinder, and E f 3 for a sphere . Δ H 1 is the change in volumetric enthalpy (J/m 3) for the precooling period , Δ H 2 is the change in volumetric enthalpy (J/m 3) for the phase change and post cooling period  

16. TYPES OF FREEZING FAST FREEZING Quick or fast freezing occurs at –25ºC or less. Ice crystals are small and do not damage food cells. SLOW FREEZING Slow freezing occurs at -24 ºC or above. Ice crystals are big and damage the food cells causing loss of texture, nutrients, colour & flavour on thawing.

17. TYPES OF FREEZERS

18. Freezing syste m Indirect contact systems Plate freezers Air blast freezers Freezers for liquid foods Direct - contact systems Air blast immersion

19. 1) Cooled air freezer: Chest freezer Air blast freezer Fluidized bed freezer belt freezer Tunnel freezer 2) Cooled Liquid Immersion freezers. 3) Cooled surface freezer Plate freezer Scraped surface freezer Contact freezer 4) Cryogenic Freezers Liquid nitrogen freezers Liquid carbon dioxide freezers

20. Cooled air freezers Chest freezer The food is frozen in naturally circulated stationery air at temperature between -20 o c to -30 o c. Chest freezer takes longer time (3-72 h) for freezing Loss of product quality Air is circulated by fans for uniform distribution of temperature The heat transfer coefficients are low. 

21. B last freezers , A ir is recirculated over food at between -30C and -40C at a velocity of 1.5–6.0 m/s. The high air velocity reduces the thickness of boundary films surrounding the food and thus increases the surface heat transfer coefficient In batch equipment, food is stacked on trays in rooms or cabinets. Continuous equipment consists of trolleys stacked with trays of food or on conveyor belts which carry the food through an insulated tunnel relatively economical and highly flexible in that foods of different shapes and sizes can be frozen The equipment is compact and has a relatively low capital cost and a high throughput

22. Fluidised bed freezer Fluidised -bed freezers are modified blast freezers in which air at between 25ºC and 35ºC is passed at a high velocity (2–6 m s1) through a 2–13 cm bed of food, contained on a perforated tray or conveyor belt . Food comes into greater contact with the air than in blast freezers, and all surfaces are frozen simultaneously and uniformly The equipment therefore needs less frequent defrosting . However , the method is restricted to particulate foods (for example peas, sweetcorn kernels, shrimps, strawberries or French fried potatoes ).

23. Belt freezers (spiral freezers) have a continuous flexible mesh belt which is formed into spiral tiers and carries food up through a refrigerated chamber Cold air or sprays of liquid nitrogen are directed down through the belt stack in a countercurrent flow, which reduces weight losses due to evaporation of moisture. Spiral freezers require relatively small floor-space and have high capacity Other advantages include automatic loading and unloading , low maintenance costs and flexibility to freeze a wide range of foods including pizzas, cakes, pies , ice cream, whole fish and chicken portions.

24. Cooled-liquid freezers In immersion freezers , packaged food is passed through a bath of refrigerated propylene glycol , brine, glycerol or calcium chloride solution on a submerged mesh conveyor. In contrast with cryogenic freezing, the liquid remains fluid throughout the freezing operation and a change of state does not occur. The method has high rates of heat transfer and capital costs are relatively low. It is used commercially for concentrated orange juice in laminated card–polyethylene cans, and to pre-freeze filmwrapped poultry before blast freezing.

25. Cooled-surface freezers Plate freezers consist of a vertical or horizontal stack of hollow plates, through which refrigerant is pumped at 40ºC They may be batch, semi-continuous or continuous in operation. Flat, relatively thin foods (for example filleted fish, fish fingers or beef burgers ) are placed in single layers between the plates and a slight pressure is applied by moving the plates together.

26. Advantages good economy and space utilisation , relatively low operating costs compared with other methods , little dehydration of the product and therefore minimum defrosting of condensers, high rates of heat transfer Disadvantages relatively high capital costs, and restrictions on the shape of foods to those that are flat and relatively thin.

27. Scraped surface freezing These are used for liquid as well as for semi-solid foods like ice cream. In ice-cream manufacture, the rotor scrapes frozen food from the wall of the freezer barrel and incorporates air. In scraped surface freezers, the freezing is very fast and up to 50% of the water is frozen within a few seconds. this results in very small crystals which are not detectable in the mouth and thus given a smooth creamy consistency to the product. The temperature is reduced between -4 o c and -7 o c. the frozen aerated mixture is then pumped into containers and finally the freezing is completed in hardening room. 

28. Cryogenic freezing Freezers of this type are characterised by a change of state in the refrigerant (or cryogen) as heat is absorbed from the freezing food . The cryogen is in intimate contact with the food and rapidly removes heat from all surfaces of the food to produce high heat transfer coefficients and rapid freezing. The two most common refrigerants are liquid nitrogen and solid or liquid carbon dioxide THE PRODUCT CAN BE EXPOSED TO A CRYOGENIC MEDIUM IN THREE WAYS: Directly sprayed on the product in a tunnel freezer. Vaporized and blown over the food in a spiral freezer or batch freezer. The product is immersed in the cryogenic liquid in an immersion freezer.

29. LIQUID NITROGEN FREEZER The refrigerant consumption is in the range of 1.2-kg refrigerant per kg of the product. Liquid nitrogen refrigerants are colorless and odorless. Product is cooled by gaseous nitrogen and frozen by liquid nitrogen spray. Typical food products used in this system are fish fillets, seafood and fruits like berries. The use of gaseous nitrogen reduces the thermal shock to the food and recirculation fans increases the rate of heat transfer.

30. References Introduction to food engineering r.paul singh , dennis r.heldman Food science Norman N. Potter , Joseph H. Hotchkiss Food processing technology, Principle and practices P J Fellows. https://en.wikipedia.org/wiki/frozen_food www. slideshare.net

31. Thank you.