Chapter 5 Low Temperature Processing Preservation

[Audio] LOW PROCESSING TEMPERATURE. LOW. PROCESSING TEMPERATURE.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION OVERVIEW Refrigeration is the process of removing heat from an enclosed space. Refrigerant - a special liquid that has capability to evaporates at extremely low temperature. Refrigeration start when refrigerant absorb heat from food and indirectly evaporation occur. Refrigerant that has been vaporized will be compressed back into liquid form. MW bpoC/ 760torr Latent Heat kj/mole Flammable Pressure at 0oC Pressure at 50oC NH3 17 -34 24 Yes 4atm 20atm CO2 44 -78 25 No 35atm >>60atm SO2 64 -10 25 No 2atm 9atm CF2Cl2 121 -30 22 No 3atm 12atm TYPE OF REFRIGERANT.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION PROCESSING BY THE REMOVAL OF HEAT CHILLING FREEZING FREEZE DRYING.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING Chilling is the unit operation in which the temperature of a food is reduced to between -1oC and 8oC. Critical warm zone (10oC to 50oC) where maximum growth of microorganism. Purpose of chilling:Is used to extend the shelf life of food by reduced the rate of biochemical and microbiological changes. "Retard" – due to at low temperature most of enzymes/m'os activities occur at slow rate. The shelf life of chilled processed foods is determined by: The type of food The degree of microbial destruction or enzyme inactivation achieved by the process Control of hygiene during processing and packaging The barrier properties of the package Temperatures during processing, distribution and storage. Mechanical refrigerator – example domestic refrigerator. Cryogenic systems- used liquid nitrogen to reduce temperature. TYPES OF EQUIPMENT.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | TYPES OF EQUIPMENT | MECHANICAL REFRIGERATOR A refrigerant circulates between the four elements of the refrigerator, changing state from liquid to gas, and back to liquid as follows: (1) In the evaporator the liquid refrigerant evaporates under reduced pressure, and in doing so absorbs latent heat of vaporisation and cools the freezing medium. This is the most important part of the refrigerator; the remaining equipment is used to recycle the refrigerant. (2) Refrigerant vapour passes from the evaporator to the compressor where the pressure is increased. (3) The vapour then passes to the condenser where the high pressure is maintained and the vapour is condensed. (4) The liquid passes through the expansion valve where the pressure is reduced to restart the refrigeration cycle. . (1) (2) (3) (4) The important properties of refrigerants are as follows: A low boiling point and high latent heat of vaporisation A dense vapour to reduce the size of the compressor Low toxicity and nonflammable Low miscibility with oil in the compressor Low cost..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | TYPES OF EQUIPMENT | CRYOGENIC SYSTEM A cryogen is a refrigerant that changes phase by absorbing latent heat to cool the food. Cryogenic temperature lies below -180 oC/93.15K Cryogenic chillers use solid carbon dioxide, liquid carbon dioxide or liquid nitrogen, helium, oxygen (Boiling point <180 oC). Solid carbon dioxide removes latent heat of sublimation (352 kJ kg1 at 78ºC), and liquid cryogens remove latent heat of vaporisation (358 kJ kg1 at 196ºC for liquid nitrogen; liquid carbon dioxide has a similar latent heat to the solid). The gas also absorbs sensible heat as it warms from 78ºC (CO2) or from 196ºC (liquid nitrogen) to give a total refrigerant effect of 565 kJ kg1 and 690 kJ kg1 respectively. PROCESS 1) Gases compressed and heated in compressor. 2) Heat of gases is removed and used in compressor. 3) Refrigerant in expansion valve for cooling below condensation temperature, 4) Now cools the objects to cryogenic temperature and reheating of gases occurs COMPRESSOR (2) (3) (4) (1).

[Audio] APPLICATION OF CRYOGENIC SYSTEMS IN FOOD PRODUCTS: The advantages of carbon dioxide include: A higher boiling and sublimation point than nitrogen, and therefore a less severe effect on the food Most of enthalpy (heat capacity) arises from the conversion of solid or liquid to gas. LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | TYPES OF EQUIPMENT | CRYOGENIC SYSTEM Cryogenic cooling include sausage manufacture, where carbon dioxide snow removes the heat generated during size reduction and mixing Cryogenic grinding where the cryogen reduces dust levels, prevents dust explosions and improves the throughput of mills. In spice milling, cryogens also prevent the loss of aromatic compounds. In the production of multi-layer chilled foods (for example trifles and other desserts) Cooling and case-hardening of hot bakery products and Chilling flour to obtain accurate and consistent flour temperatures for dough preparation..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | CHILLED FOODS BUT NOT ALL FOOD SUITABLE TO BE STORED UNDER CHILLING CONDITION…FOR INSTANCE, TROPICAL FRUIT…CHILLING INJURY MAY OCCUR IF BANANA STORED AT CHILLING TEMPERATURE. EXAMPLE OF CHILLED FOODS CHARACTERISTICS OF CHILLED FOODS Wholesomeness/freshness, healthy, natural…minimal sensory effects..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | CHILLED FOODS 0 1 2 3 4 5 6 7 8 9 -1 Chilled foods can be grouped into 3 categories: -1oC to +1oC (used to store fresh fish, meats, sausages, ground meats, smoked meat, breaded fish). 0oC to +5oC (pasteurized canned meat, milk, cream, yoghurt, prepared salads, sandwiches, baked goods, fresh pasta, fresh soups, sauces, pizzas, pastries and unbaked dough). 0oC to +8oC (fully cooked meats and fish pies, cooked and uncooked cured meats, butter, margarine, hard cheese, cooked rice, fruit juices and soft fruits). oC.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | CHILLED FOODS.

[Audio] The successful supply of chilled food to consumer is heavily dependent on temperature. This expensive distribution system will involve: Chill store, refrigerated transport, retail chill display cabinet, suitable cold box and domestic refrigerator. Temperature should be monitored and controlled along the distribution line to avoid food spoilage/poisoning.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION CHILLING | OBJECTIVE Retard Growth of microorganisms Thermophilic, mesophilic and psychrophilic mo's. Post harvest metabolic activities for animal & plant. Deteriorative biochemical reactions (enzyme – browning & oxidation of lipid); chemical changes like color degradation, autolysis of fish, loss of nutritive value of foods. Moisture loss Chilling also is used as unit operation in crystallization, aging of beef, wine & cheese, and to facilitate operations like cutting of meat and slicing of bread. For optimum shelf life, chilling may combined with others food processing: Controlled/modified atmosphere storage Vacuum Packaging Others (pasteurization, chemical preservatives, packaging, temperature control etc.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING Water freezes when the molecules have slowed down enough to develop bonds upon collision. The rate at which freezing occurs is governed by nucleation and growth. Nucleation is the formation of small solids in a liquid. This small solids are called the nuclei. Once the nuclei have form, they become the landing side for other molecules to attach onto. Growth rate – is the radius of nuclei growth. Solidification rate is determined by the combination of nucleation and growth rate..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING Freezing point of water at sea level is 0oC. If impurities is added (e.g. salt), freezing point will be lowered. This concept is applied in icecream industries. Brine solution (salt-water solution, i.e. potassium chloride act as refrigerant) used to reduce ice cream temperature up to 5oC, and brine temperature itself is about 26oC. Impurities avoid complete freezing of icecream thus allowing retains the characteristic soft texture..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | FREEZING CURVE A B S C D E F tf θf θa Temperature Time The food is cooled to below its freezing point θf which, with the exception of pure water, is always below 0oC. At point S the water remains liquid, although the temperature is below the freezing point, this phenomenon is known as supercoiling and may be as much as 10oC below the freezing point. The temperature rises rapidly to the freezing point as ice crystals begin to form and latent heat of crystallization is released. Heat is removed from the food at the same rate as before, but it is latent heat 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 therefore falls slightly. It is during this stage that the major part of the ice is formed. One of the solutes becomes supersaturated and crystallises out. The latent heat of crystallisations is released and the temperature rises to the eutectic temperature for that solute. Crystallisation of water and solutes continues, the total time tf taken (the freezing plateau) is determined by the rate at which heat is removed, 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. For example, at a storage temperature of -20oC, the percentage of water frozen is 88% in lamb, 91% in egg albumin. AS SB BC CD DE EF The thermal centre of a food (the point that cools most slowly) as heat is removed, a characteristic curve is obtained..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | FROZEN FOODS Fruits (strawberries, oranges, raspberries, blackcurrants) either whole or pureed, or as juice concentrates Vegetables (peas, green beans, sweetcorn, spinach, sprouts and potatoes) Fish fillets and seafoods (cod, plaice, shrimps and crab meat) including fish fingers, fish cakes or prepared dishes with an accompanying sauce The major groups of commercially frozen foods are as follows:.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | FROZEN FOODS Meats (beef, lamb, poultry) as carcasses, boxed joints or cubes, and meat products (sausages, beefburgers, reformed steaks) Baked goods (bread, cakes, fruit and meat pies) Prepared foods (pizzas, desserts, ice cream, complete meals and cook–freeze dishes)..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | FROZEN FOODS Not all food suitable to be stored under freezing condition. Similar with chilling where some fruits and vegetables show characteristics of chilling injury. Attention should be given when thawing frozen food. Use water for thawing. Avoid repeat freezing, thawing, freezing, since texture of food may damage. EXAMPLE Cooked egg whites Cream based soups and sauces Cucumber Desserts with meringue Fried foods (become soggy) Frostings/icing that include raw egg whites Fully cooked pasta (can be frozen in dishes if undercooked) Fully cooked rice Gelatin Hard-boiled eggs Lettuce Mayonnaise and mayo-based salads (tuna salad, egg salad) Sour cream.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | TYPES OF FREEZER | COOLED AIR FREEZERS CHEST FREEZERS – frozen in stationary (natural-circulation) air at between - 20⁰C to 30⁰C BLAST FREEZERS – air is recirculated over food at between -30⁰C to -40⁰C at a velocity of 1.5 – 6.0 ms⁻1. BELT FREEZERS (spiral freezers) - have a continuous flexible mesh belt which is formed into spiral tiers and carries food up through a refrigerated chamber. In some designs each tier rests on the vertical sides of the tier beneath and the belt is therefore 'selfstacking'. FLUIDISED-BED FREEZERS - 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.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | TYPES OF FREEZER | COOLED LIQUID FREEZERS 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..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | TYPES OF FREEZER | COOLED SURFACE FREEZERS PLATE FREEZER - 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. This improves the contact between surfaces of the food and the plates and thereby increases the rate of heat transfer.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | TYPES OF FREEZER | COOLED SURFACE FREEZERS SCRAPED-SURFACE FREEZERS - used for liquid or semi-solid foods (for example ice cream). They are similar in design to equipment used for evaporation and heat sterilisation but are refrigerated with ammonia, brine, or other refrigerants..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | TYPES OF FREEZER | CRYOGENIC FREEZER In liquid-nitrogen freezers, packaged or unpackaged food travels on a perforated belt through a tunnel, where it is frozen by liquid-nitrogen sprays and by gaseous nitrogen. The temperature is either allowed to equilibrate at the required storage temperature (between -18ºC and -30ºC) before the food is removed from the freezer, or alternatively food is passed to a mechanical freezer to complete the freezing process. The use of gaseous nitrogen reduces the thermal shock to the food, and recirculation fans increase the rates of heat transfer. A newer design of tunnel, with fans located beneath the conveyor to produce gas vortices is described by Summers (1998). This design is said to double the output (faster) of conventional freezers of the same length, reduce nitrogen consumption by 20% and reduce already low levels of dehydration by 60%. E.g. A beef burger will be frozen in 1 minute at these extreme temperatures. This produces small crystals, and little moisture loss. This method is used when freezing prawns. The prawns are first dipped in liquid nitrogen to freeze the outside layer. This prevents the prawns sticking together and from sticking to the freezer belts..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | TYPES OF FREEZER | CRYOGENIC FREEZER ADVANTAGES Simple continuous operation with relatively low capital costs (approximately 30% of the capital cost of mechanical systems) Smaller weight losses from dehydration of the product (0.5% compared with 1.0–8.0% in mechanical air-blast systems) Rapid freezing which results in smaller changes to the sensory and nutritional characteristics of the product The exclusion of oxygen during freezing Rapid startup and no defrost time Low power consumption (leeson, 1987). DISADVANTAGES The main disadvantage is the relatively high cost of refrigerant (nitrogen and carbon dioxide consumption.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | EFFECTS, ADVANTAGES & DISADVANTAGES OF FREEZING EFFECTS Low temperatures do not significantly affect the nutritional value of food, but thiamin and vitamin C may be destroyed when vegetables are blanched (briefly immersed in boiling water) before freezing. If fish is frozen too slow, some of its cells may rupture and release nutrients into the liquid that drips from the fish when it thaws. Some flavours become weaker and some become stronger when food is frozen. ADVANTAGES Many foods can be frozen. Natural color, flavor, and nutritive value retained. Texture usually better than other methods of food preservation. Foods can be frozen in less time than they can be dried or canned. Simple procedures. Adds convenience to food preparation. Proportions can be adapted to needs unlike other home preservation methods. Kitchen remains cool and comfortable. DISADVANTAGES Texture of some foods is undesirable because of freezing process. Initial investment and cost of maintaining freezer is high. Storage space limited by capacity of freezer..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | HOW FREEZING AFFECTS FOOD CHEMICAL CHANGES Enzymes in vegetables Enzymes in fruit Rancidity TEXTURE CHANGES Expansion of food Ice crystals.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZING | ISSUES WITH FROZEN FOOD CHEMICAL REACTIONS CAN OCCUR IN UNFROZEN WATER. Some foods blanched or sulfited before freezing. Vacuum packaging to keep out oxygen UNDESIRABLE PHYSICAL CHANGES Fruits and vegetables lose crispness Drip loss in meats and colloidal type foods (starch, emulsions) Freeze product faster Control temperature fluctuations in storage. Modify starch, egg systems, etc Freezer burn Package properly Control temperature fluctuations in storage. Oxidation Off-flavors Vitamin loss Browning Recrystallization DRIP LOSS When ice crystals grow they may puncture the cell walls so the juice bleeds out. This causes 'drip loss' when the product is thawing. The larger the average crystal size is, the greater the number of punctured cells and the greater the drip loss. The loss of juice can result in a loss of firmness and flavor..

[Audio] If a water pressure of a food is held below 4.58 Torr (610.5 Pa) and frozen, when heat is introduced, solid ice sublimes directly to vapour without entering liquid phase. LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) Freeze drying involves the removal of water or other solvent from a frozen product by a process called sublimation. Sublimation occurs when a frozen liquid goes directly to the gaseous state without passing through the liquid phase. Advantage, freeze dried product do not need refrigeration and can be stored at ambient temperature, thus save money..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | PROCESS STEPS VACUUM: After freezing, the product is placed under vacuum. This enables the frozen solvent in the product to vaporize without passing through the liquid phase, a process known as sublimation. FREEZING: The product is frozen. This provides a necessary condition for low temperature drying. HEAT: Heat is applied to the frozen product to accelerate sublimation. CONDENSATION: Lowtemperature condenser plates remove the vaporized solvent from the vacuum chamber by converting it back to a liquid. This completes the separation process..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | HEAT & MASS TRANSFER IN FREEZE DRYING RATE OF HEAT TRANSFER 3 methods of transferring heat to the sublimation front. 1. Heat transfer through the frozen layer 2. Heat transfer through the dried layer 3. Heating by microwaves RATE OF MASS TRANSFER When heat reaches the sublimation front, it raises the temperature and the water vapour pressure of the ice. Vapour then moves through the dried food to a region of low vapour pressure in the drying chamber. The factors that control the water vapour pressure gradient are: 1. The pressure in the drying chamber 2. The temperature of the vapour condenser, both of which should be as low as economically possible 3. The temperature of ice at the sublimation front, which should be as high as possible, without melting.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | COMPONENTS OF FREEZE DRYER The DRYING CHAMBER, in which the sample is placed and heating/cooling takes place, must be vacuum tight and with temperature controlled shelves. The CONDENSER must have sufficient condensing surface and cooling capacity to collect water vapour released by the product. As vapours contact the condensing surface, they give up their heat energy and turn into ice crystals that will be removed from the system. A condenser temperature of -65 oC is typical for most commercial freeze dryers. The VACUUM PUMP removes non-condensable gases to achieve high vacuum levels (below 4 mmHg) in the chamber and condenser..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | APPLICATION OF FREEZE DRYER Biggest market for freeze-drying is the food industry. Freeze-dried food is used by hikers, hunters, astronauts, the military, as well as being used in the food industry for dehydrated soups and meals for consumers in the supermarket. The largest application is freeze –dried coffee. Freeze-dried microorganisms – frequently used for fermentation reactions, used in bioconversion reactions and stored for research. In the pharmaceutical industry – For saving many medicines lives and for life saving substance that is blood..

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | HOW IT WORKS.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | WHY CHOOSE FREEZE-DRIED FOODS SHELF LIFE – can be stored for 25+ years without losing taste or nutritional value. All of the nutrients and flavours of food stay intact after being freeze dried. 25+ YEARS EASY TO PREPARE – just add water to reconstitute. LIGHT WEIGHT – easier to transport and great for hiking and camping. VARIETY – Meats, dairies, fruits and vegetables can all be preserved by freezedrying. Other food preparation methods deplete the flavours and nutrients during the preservation process When food is freeze dried, only the water is removed. DID YOU KNOW?.

[Audio] LOW TEMPERATURE PROCESSING/PRESERVATION FREEZE DRYING (LYOPHILISATION) | CONVENTIONAL DRYING VS FREEZE DRYING CONVENTIONAL DRYING FREEZE DRYING Successful for easily dried foods (vegetables & grains) Successful for most food but limited to those that are difficult to dry by other methods Meat general unsatisfactory Successful with cooked and raw meats Temperature range 37-39oC Temperature below freezing point Atmospheric pressure Reduced pressures (27-133 Pa) Evaporation of water from surface of food Sublimation of water from ice front Movement of solutes and sometimes case hardening Minimal solute movement Stresses in solid foods cause structural damage and shrinkage Minimal structural changes or shrinkage Slow, incomplete rehydration Rapid complete rehydration Solid or porous dried particles often having a higher density than the original food Porous dried particles having a lower density than original food Odour and flavour frequently abnormal Odour and flavour usually normal Colour frequently darker Colour usually normal Reduced nutritional value Nutrients largely retained Costs generally low Costs generally high, up to four times those of conventional drying.