Food Preservation Methods_Drying_Chilling_Freezing PDF

Drying  Learning Outcomes  At the end of this section you should be able to explain  What drying is  The reasons for drying  The different types of drying  The common equipment used in drying  The parameters that control a drying process ...

Drying  Learning Outcomes  At the end of this section you should be able to explain  What drying is  The reasons for drying  The different types of drying  The common equipment used in drying  The parameters that control a drying process  Drying  A drying process involves the removal of liquid from a solid product using heat.  The first step is to heat the solid, the liquid to be removed from the solid increases in temperature to form a vapor.  The next step is the transfer of the vapor from the surface of the solid material into the air.  Drying and evaporation are common terms and have similar meanings.  The term drying is applied when the amount of liquid to be removed is small.  The term evaporation is commonly used when large amounts of liquid are being removed.  Drying is a very important and common process in food processing 3  Use of Heat  Heat influences food processing in a number of ways:  it is the most convenient way of extending the shelf life of foods by destroying enzymatic and microbiological activity, or by removing water to inhibit deterioration  it changes the nutritional and sensory qualities of foods  generation of heat is a major processing cost  Dehydration  Dehydration (or drying) is defined as ‘the application of heat under controlled conditions to remove the majority of the water normally present in a food by evaporation’ (or in the case of freeze drying by sublimation).  The main purpose of dehydration is to extend the shelf life of foods by a reduction in water activity This inhibits microbial growth and enzyme activity, but the processing temperature is usually insufficient to cause their inactivation.  Dehydration  Drying causes deterioration of both the eating quality and the nutritional value of the food.  Examples of commercially important dried foods  coffee, milk, raisins, and other fruits, pasta, flours (including bakery mixes), beans, nuts, breakfast cereals, tea and spices. There are a large number of factors that control the rate at which foods dry, which can be grouped into the following categories  those related to the processing conditions  those related to the nature of the food  those related to the drier design.  Drying using heated air There are three inter-related factors that control the capacity of air to remove moisture from a food: 1. the amount of water vapor already carried by the air 2. the air temperature 3. the amount of air that passes over the food.  The amount of water vapour in air is expressed as either absolute humidity or relative humidity (RH) (in per cent).  Psychrometry is the study of inter-related properties of air–water vapour systems. Mechanism of drying  Mechanism of drying Drying curves. The temperature and humidity of the drying air are constant and all heat is supplied to the food surface by convection.  Factors affecting drying  The composition and structure of the food has an influence on the mechanism of moisture removal.  For example, the orientation of fibres in vegetables (e.g. celery) and protein strands in meat allow more rapid moisture movement along their length than across the structure.  The amount of food placed into a drier in relation to its capacity (in a given drier, faster drying is achieved with smaller quantities of food).  Process Control Parameters  The drying process is controlled by a series of critical parameters.  These include:  Load size  Temperature  Relative humidity  Airflow rate  Drying time 12 Types of Drying  TYPES OF DRYING - SUN DRYING A. SUN DRYING This method is one of the Slow process most traditional methods of Problems: no control drying. It is slow and only Microorganisms and practical in hot, dry pests can attack climates. However, it is still Rain used today e.g. sun dried chillies, raisins or tomatoes. High nutrient loss Inexpensive products: grains, acid fruits, spices The food, such as fish, is also vulnerable to contamination through pollution and vermin, e.g. rodents and flies. 14  TYPES OF DRYING (cont.) B. HOT AIR DRYING  More efficient/control  Lower nutrient loss  More expensive  Products: dried vegetables, pasta, some fruits C. DRUM DRYING  More efficient than hot air  Lower nutrient loss  Products: potato pastes & slurries  TYPES OF DRYING (cont.) D. SPRAY DRYING  Low nutrient loss  More expensive than drum or air drying  Good control/efficiency  Use only for liquids  Products: milk, instant tea and coffee E. PUFF DRYING: PRESSURE DROP  Using heating systems;  The process includes removing the free water from fruits and vegetables which have been washed, then put the fruits and vegetables into a reaction vessel, and vacuumizing the reaction vessel , injecting carbon dioxide maintaining 30 seconds to 60 minutes, depressurizing to atmospheric pressure over 0.5-4 minutes so as to puff- dry the materials.  TYPES OF DRYING (cont.) F. FREEZE DRYING  Best nutrient quality  Best product quality (shape; rehydration)  Most expensive  Good control  Products: coffee, camping foods, military, NASA Types of Driers  Types of Driers Hot-air driers  Bin driers Bin driers are large, cylindrical or rectangular containers fitted with a mesh base. Hot air passes up through a bed of food at relatively low velocities  Cabinet driers (tray driers) These consist of an insulated cabinet fitted with shallow mesh or perforated trays, each of which contains a thin (2–6 cm deep) layer of food. Hot air is blown at 0.5–5 m s-1 through a system of ducts and baffles to promote uniform air distribution over and/or through each tray.  Tray Drier  Tray drier.  Air flows in direction of the arrows over each shelf in turn.  The wet material is spread on shallow trays resting on the shelves.  Electrical elements or steam-heated pipes are positioned as shown, so that the air is periodically reheated after it has cooled by passage over the wet material on one shelf before it passes on the next.  Tunnel driers Hot air is blown over the product, such as vegetables. The concurrent system dries the food rapidly with little shrinkage, but leaves a relatively high moisture content. The counter-current system is slower, but produces a product with a low moisture level. A disadvantage of this process is that the product tends to shrink and is less easy to rehydrate.  Conveyor driers (belt driers) (a) Conveyor  Continuous conveyor drier driers are up to 20m long and 3m wide.  Food is dried on a mesh belt in beds 5–15 cm deep.  The air flow is initially directed upwards through the bed of food and then downwards in later stages to prevent dried food from blowing out of the bed. (b) three-stage conveyor drier. 22  Fluidised bed drying Warm air is blown upwards directly underneath the food, causing it to flow and remain separated. This procedure is suitable for small items such as peas and coffee.  Drum Dryer (Film Drying)  It consists of a drum of about 0, 75-1.5 m in diameter and 2-4 m in length, heated internally, usually by steam, and rotated on its longitudinal axis.  Operation: The liquid is applied to the surface and spread to a film, this may be done in various ways, but the simplest method is that shown in the diagram, where the drum dips into a feed pan. Drying rate is controlled by using a suitable speed of rotation and the drum temperature. The product is scraped from the surface of the drum by means of a knife. Fig. Drum dryer  25  Advantages of the drum dryer 1- The method gives rapid drying, the thin film spread over a large area resulting in rapid heat and mass transfer. 2- The equipment is compact, occupying much less space than other dryers. 3- Heating time is short, being only a few seconds. 4- The drum can be enclosed in a vacuum jacket, enabling the temperature of drying to be reduced. 5- The product is obtained in flake form, which is convenient for many purposes.  The only disadvantage : is that operating conditions are critical and it is necessary to introduce careful control on feed rate, film thickness, speed of drum rotation and drum temperature.  Uses: It can handle a variety of materials, either as solutions or as suspensions e.g. starch products, ferrous salts  Spray drying  This method is suitable for producing products such as dried milk and coffee powder.  A fine spray of liquid is injected into a blast of hot air in a chamber. Water evaporates within seconds, leaving the solid part of the product behind in powdered form.  Usually this powder is too fine to disperse in water, so a little moisture is added to make it ‘clump’ together into larger particles.  This improves the wettability of the product and helps it dissolve more fully when added to water. Fluidise bed drying is used to granulate these powders.  Freeze-drying  Is a commercial process that combines freezing and drying to preserve foods.  First food is frozen; then its treated to remove the solvent from dispersed or dissolved solids.  In most food, this means removing the water.  Technical name: lyophilisation.  Freeze-drying  During freeze-drying, water in the form of ice is removed through sublimation – occurs on the surface and continues inward.  Wet clothes on the line during winter example.  By the time the ice at the very center has sublimed, up to 99% of the food’s moisture has been removed.  The result is dried food.  Flash frozen or frozen very quickly – a special, low-pressure chamber. It is then held at or below freezing temperature.  The low temperature keeps the water frozen, and the low pressure speeds the rate at which ice crystals in the food escapes as water vapor. 29  Advantages of freeze drying 1- Drying takes place at very low temperatures, so the chemical decomposition, particularly hydrolysis is minimized. 2- The solution is frozen occupying the same volume as the original solution, thus, the product is light and porous. 3- There is no concentration of solution prior to drying. Hence, salts do not concentrate and denature proteins, as occurs with other drying methods. 4- As the process takes place under high vacuum there is little contact with air, and oxidation is minimized.  Disadvantages & Uses of freeze drying There are two main disadvantages:  1-Unless products are dried in their final container and sealed in situ, packing require special conditions.  2-The process is very slow and uses complicated plant, which is very expensive.It is not a general method of drying but limited to certain types of valuable products.  Using Freeze Dried Foods  Foods could be stored for months even years  Must be protected in airtight packaging  Examples are fruit in cereal and chicken in instant soup  Makes foods extremely light – should be reconstituted. Effect of Drying on Food characteristics  Drying Effect on foods  All products undergo changes during drying and storage that reduce their quality compared to the fresh material and the aim of improved drying technologies is to minimize these changes while maximizing process efficiency.  The main changes to dried foods are to the texture and loss of flavor or aroma, but changes in color and nutritional value are also significant in some foods.  Flavor and aroma  Heat not only vaporises water during drying but also causes loss of volatile components from the food and as a result most dried foods have less flavour than the original material.  The extent of volatile loss depends on the temperature and moisture content of the food and on the vapour pressure of the volatiles and their solubility in water vapour.  Volatiles which have a high relative volatility and diffusivity are lost at an early stage in drying.  Foods that have a high economic value due to their characteristic flavours (for example herbs and spices) are dried at low temperatures  Flavour and Aroma Flavour changes, due to oxidative or hydrolytic enzymes are prevented in fruits by the use of sulphur dioxide, ascorbic acid or citric acid, by pasteurisation of milk or fruit juices and by blanching of vegetables. Other methods which are used to retain flavours in dried foods include:  recovery of volatiles and their return to the product during drying  mixing recovered volatiles with flavour fixing compounds, which are then granulated and added back to the dried product (for example dried meat powders)  addition of enzymes, or activation of naturally occurring enzymes, to produce flavours from flavour precursors in the food (for example onion and garlic are dried under conditions that protect the enzymes that release characteristic flavours).  Colour  There are a number of causes of colour loss or change in dried foods; drying changes the surface characteristics of a food and hence alters its reflectivity and colour.  In fruits and vegetables, chemical changes to carotenoid and chlorophyll pigments are caused by heat and oxidation during drying and residual polyphenoloxidase enzyme activity causes browning during storage.  This is prevented by blanching or treatment of fruits with ascorbic acid or sulphur dioxide.  Nutritional value  Large differences in reported data on the nutritional value of dried foods are due to wide variations in the preparation procedures, the drying temperature and time, and the storage conditions. In fruits and vegetables, losses during preparation usually exceed those caused by the drying operation  For example Escher and Neukom (1970) showed that losses of vitamin C during preparation of apple flakes were 8% during slicing, 62% from blanching, 10% from pureeing and 5% from drum drying  Rehydration  Water that is removed from a food during dehydration cannot be replaced in the same way when the food is rehydrated (that is, rehydration is not the reverse of drying);  Learning Outcomes  At the end of this section you should be able to explain  What drying is  The reasons for drying  The different types of drying  The common equipment used in drying  The parameters that control a drying process Preservation Operations - Chilling  Learning Outcomes  At the end of this section, you should have an understanding of  the unit operation of chilling  The equipment used  Safety features required  Effect on food as a processing method  Chilling  Chilling is the unit operation in which the temperature of a food is reduced to between 1ºC and 8ºC.  It is used to reduce the rate of biochemical and microbiological changes, and hence to extend the shelf life of fresh and processed foods.  It causes minimal changes to sensory characteristics and nutritional properties of foods and, as a result, chilled foods are perceived by consumers as being convenient, easy to prepare, high quality and ‘healthy’, ‘natural’ and ‘fresh’.  Chilling is often used in combination with other unit operations (for example fermentation or pasteurization) to extend the shelf life of mildly processed foods. There is a greater preservative effect when chilling is combined with control of the composition of the storage atmosphere than that found using either unit operation alone. Chilled foods are grouped into three categories according to their storage temperature range as follows: 1. -1ºC to +1ºC (fresh fish, meats, sausages and ground meats, smoked meats and breaded fish). 2. 0ºC to +5ºC (pasteurised canned meat, milk, cream, yoghurt, prepared salads, sandwiches, baked goods, fresh pasta, fresh soups and sauces, pizzas, pastries and unbaked dough). 3. 0ºC to +8ºC (fully cooked meats and fish pies, cooked or uncooked cured meats, butter, margarine, hard cheese, cooked rice, fruit juices and soft fruits).  Theory on chilling Fresh foods  The rate of biochemical changes caused by either micro-organisms or naturally occurring enzymes increases logarithmically with temperature.  Chilling therefore reduces the rate of enzymic and microbiological change and retards respiration of fresh foods  The factors that control the shelf life of fresh crops in chill storage include:  the type of food and variety or cultivar  the part of the crop selected (the fastest growing parts have the highest metabolic rates and the shortest storage lives )  the condition of the food at harvest (for example the presence of mechanical damage or microbial contamination, and the degree of maturity)  the temperature of harvest, storage, distribution and retail display  the relative humidity of the storage atmosphere, which influences dehydration losses. Botanical function related to respiration rate and storage life for selected products Optimum storage conditions for some fruits and vegetables  Processed foods  There are four broad categories of micro-organism, based on the temperature range for growth 1. thermophilic (minimum: 30–40ºC, optimum: 55–65ºC) 2. mesophilic (minimum: 5–10ºC, optimum: 30–40ºC) 3. psychrotrophic (minimum: 0–5ºC, optimum: 20– 30ºC) 4. psychrophilic (minimum: 0–5ºC, optimum: 12–18ºC).  Chilling  Chilling prevents the growth of thermophilic and many mesophilic micro-organisms.  The main microbiological concerns with chilled foods are a number of pathogens that can grow during extended refrigerated storage below 5ºC, or as a result of any increase in temperature (temperature abuse) and thus cause food poisoning  Examples of these pathogens that survive chilling conditions are Aeromonas hydrophilia, Listeria spp, Yersinia enterocolitica, some strains of Bacillus cereus, and enteropathogenic Escherichia coli Pathogenic or spoilage bacteria in high–risk chilled foods  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.  The range of chilled foods can be characterised by the class of microbial risk that they pose to consumers as follows:  Class 1 foods containing raw or uncooked ingredients, such as salad or cheese as ready-to-eat (RTE) foods (also includes chill-stable raw foods, such as meat, fish, etc.)  Class 2 products made from a mixture of cooked and low risk raw ingredients  Class 3 cooked products that are then packaged  Class 4 products that are cooked after packaging, including ready-to-eat-products for- extended- durability (REPFEDs) having a shelf life of 40+ days (the acronym is also used to mean refrigerated- pasteurised-foods-for-extended durability).  Chilled Foods  After preparation, cooked–chilled foods are portioned and chilled within 30 min of cooking. Chilling to 3ºC should be completed within 90 min and the food should be stored at 0–3ºC.  In the cook–pasteurise–chill system, hot food is filled into a flexible container, a partial vacuum is formed to remove oxygen and the pack is heat sealed. It is then pasteurised to a minimum temperature of 80ºC for 10 min at the thermal centre, followed by immediate cooling to 3ºC. These foods have a shelf life of 2–3 weeks Chilling Equipment  Mechanical refrigerators  Mechanical refrigerators have four basic elements: an evaporator, a compressor, a condenser and an expansion valve  In the evaporator: the liquid refrigerant evaporates under reduced pressure, and in doing so absorbs latent heat of vaporization and cools the freezing medium. This is the most important part of the refrigerator; the remaining equipment is used to recycle the refrigerant. Refrigerant vapour passes from the evaporator to the compressor where the pressure is increased. The vapour then passes to the condenser where the high pressure is maintained and the vapour is condensed. The liquid passes through the expansion valve where the pressure is reduced to restart the refrigeration cycle.  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 non-flammable low miscibility with oil in the compressor low cost.  Refrigerants  Main refrigerants that are now used are Freon-22 and ammonia, with the possibility of future use of propane.  However, the latter two in particular are more expensive and could cause localised hazards, thus requiring additional safety precautions and training for equipment users  Cryogenic chilling  A cryogen is a refrigerant that changes phase by absorbing latent heat to cool the food.  Cryogenic chillers use solid carbon dioxide, liquid carbon dioxide or liquid nitrogen.  The main limitation of carbon dioxide, and to a lesser extent nitrogen, is its ability to cause asphyxia.  There is therefore a maximum safe limit for operators of 0.5% CO2 by volume and excess carbon dioxide is removed from the processing area by an exhaust system to ensure operator safety, which incurs additional setup costs.  Other hazards associated with liquefied gases include cold burns, frostbite and hypothermia after exposure to intense cold  Chill storage  Once a product has been chilled, the temperature must be maintained by refrigerated storage.  Chill stores are normally cooled by circulation of cold air produced by mechanical refrigeration units, and foods may be stored on pallets, racks, or in the case of carcass meats, hung from hooks.  Control of storage conditions  In all stores it is important to maintain an adequate circulation of air using fans, to control the temperature, relative humidity or atmospheric composition.  Foods are therefore stacked in ways that enable air to circulate freely around all sides.  This is particularly important for respiring foods, to remove heat generated by respiration or for foods, such as cheese, in which flavour development takes place during storage.  Adequate air circulation is also important when high storage humidities are used for fresh fruits and vegetables  Temperature monitoring  Temperature monitoring is an integral part of quality management and product safety management throughout the production and distribution chain.  Improvements to microelectronics over the last ten years has enabled the development of monitoring devices that can both store large amounts of data and integrate this into computerized management systems  Effect on foods  The most significant effect of chilling on the sensory characteristics of processed foods is hardening due to solidification of fats and oils.  enzymic browning, lipolysis, colour and flavour deterioration in some products and retrogradation of starch to cause staling of baked products  Lipid oxidation is one of the main causes of quality loss in cook–chilled products, and cooked meats in particular rapidly develop an oxidised flavour termed ‘warmed-over flavour’ (WOF),  Learning Outcomes  At the end of this section, you should have an understanding of  the unit operation of chilling  The equipment used  Safety features required  Effect on food as a processing method Preservation Operations - Freezing  Learning Outcomes  At the end of this section, you should have an understanding of  The unit operation of freezing  The equipment used  Safety features required  Effect on food as a processing method  Freezing  Freezing is the unit operation in which the temperature of a food is reduced below its freezing point and a proportion of the water undergoes a change in state to form ice crystals. The immobilisation of water to ice and the resulting concentration of dissolved solutes in unfrozen water lower the water activity (aw) of the food  Preservation is achieved by a combination of low temperatures, reduced water activity and, in some foods, pre-treatment by blanching.  The major groups of commercially frozen foods are as follows: fruits (strawberries, oranges, raspberries) either whole or pureed, or as juice concentrates vegetables (peas, green beans, sweet corn, spinach, and potatoes) fish fillets and sea foods (cod, plaice, shrimps and crab meat) including fish fingers, fish cakes or prepared dishes with an accompanying sauce 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).  Theory  During freezing, sensible heat is first removed to lower the temperature of a food to the freezing point.  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.  Theory  A substantial amount of energy is therefore needed to remove latent heat, form ice crystals and hence to freeze foods.  The latent heat of other components of the food (for example fats) must also be removed before they can solidify but in most foods these other components are present in smaller amounts and removal of a relatively small amount of heat is needed for crystallisation to take place Equipment  Equipment Freezers are broadly categorized into: mechanical refrigerators, which 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, which use solid or liquid carbon dioxide, liquid nitrogen (or until recently, liquid Freon) directly in contact with the food.  Cooled-air freezers  Chest freezers food is frozen in stationary (natural-circulation) air at between -20ºC and - 30ºC. Chest freezers are not used for commercial freezing owing to low freezing rates (3–72 h).  A major problem with cold stores is ice formation on floors, walls and evaporator coils, caused by moisture from the air or from unpackaged products in the store.  Blast freezers  air is recirculated over food at between -30ºC and - 40ºC at a velocity of 1.5–6.0 m s-1. 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.  The trolleys should be fully loaded to prevent air from bypassing the food through spaces between the trays  Blast freezing Batches of food are subjected to a constant, steady stream of cold air (-40ºC or lower) in a tunnel or large cabinet. This process can freeze irregular shaped foods, including those which have already been packaged, e.g. battered fish pieces.  Fluidised-bed freezers  Fluidised-bed freezers are modified blast freezers in which air at between -25ºC and -35ºC is passed at a high velocity (2–6 m s-1) through a 2–13 cm bed of food, contained on a perforated tray or conveyor belt.  In some designs there are two stages; after initial rapid freezing in a shallow bed to produce an ice glaze on the surface of the food, freezing is completed on a second belt in beds 10–15 cm deep.  Fluidised bed freezing Vertical jets of refrigerated air are blown up through the product, causing it to float and remain separated. This is a continuous process which takes up to 10 minutes. The product, e.g. peas, beans, chopped vegetables or prawns, move along a conveyor belt.  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  Traditionally foods were immersed in solutions of salt and ice for several hours, e.g. brine, freezing of fish at sea.  However, modern methods of freezing have meant that this process is rarely used. Refrigerants are now sprayed directly onto the food  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.  Plate freezing The food is prepared as normal, then packed between flat, hollow, refrigerated metal plates. These are adjusted to press tightly on the food and reduce any air gaps. The plates may be horizontal or vertical, the latter being used for many bulky products, such as blocks of fish for fish fingers. This system is ideal for freezing large blocks of product, but cannot easily freeze irregular shaped items.  Scraped heat exchangers Products such as ice cream are frozen using this method in order to stir and freeze simultaneously. It reduces large ice crystal formation, producing a smooth end product.  Cryogenic freezers  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.  Cryogenic freezing Liquid nitrogen or carbon dioxide is sprayed directly onto small food items such as soft fruit and prawns. Due to the liquids’ extremely low temperatures (-196ºC) and - 78ºC respectively) freezing is almost instant. The nitrogen gas is removed by fans. Carbon dioxide is used for larger products. The carbon dioxide system is more economical and the gas can be recycled into the system.  Changes in foods Effect of freezing  The main effect of freezing on food quality is damage caused to cells by ice crystal growth.  Freezing causes negligible changes to pigments, flavours or nutritionally important components, although these may be lost in preparation procedures or deteriorate later during frozen storage. Effect of freezing on plant tissues: (a) slow freezing; (b) fast freezing.  The main changes to frozen foods during storage are as follows: Degradation of pigments. Chloroplasts and chromoplasts are broken down and chlorophyll is slowly degraded to brown pheophytin even in blanched vegetables.  In fruits, changes in pH due to precipitation of salts in concentrated solutions change the colour of anthocyanins.  Loss of vitamins. Water-soluble vitamins (for example vitamin C and pantothenic acid) are lost at sub-freezing temperatures.  Vitamin C losses are highly temperature dependent; a 10ºC increase in temperature causes a sixfold to twentyfold increase in the rate of vitamin C degradation in vegetables and a thirtyfold to seventyfold increase in fruits.  Learning Outcomes  At the end of this section, you should have an understanding of  the unit operation of freezing  The equipment used  Safety features required  Effect on food as a processing method