Chapter 5 Drying PDF

CHAPTER 5 PRESERVATION BY DEHYDRATION CHAPTER OUTCOMES At the end of this chapter, students should be able to:- a) State the principle food preservation by dehydration. b) List the advantages of dehydra...

CHAPTER 5 PRESERVATION BY DEHYDRATION CHAPTER OUTCOMES At the end of this chapter, students should be able to:- a) State the principle food preservation by dehydration. b) List the advantages of dehydrated foods. c) Describe the chemical and biochemical changes due to dehydration. d) State the factors affecting the drying rate. e) List the drying methods and equipments used.  HMF MOISTURE IN FOOD IMF LMF High moisture foods (HMF) a) Range of aw = 0.99 to [email protected] b) Examples: fresh milk, meat, fish, vegetables, fruits c) Easily spoiled by bacteria or fast-growing moulds - unless refrigerated or heated (canned).  Intermediate moisture foods (IMF) a) Range of aw = 0.90 to 0.60 b) Examples: Aged cheese, hard salami, heavily salted fish, jams, marshmallows and dried fruits. c) IMF products are preserved by the addition of considerable amounts of salt and/or sugar and intense drying process. d) Spoilage of IMF products is caused primarily by: i. Zero-tolerant moulds or osmo-tolerant yeasts. ii. Halo-tolerant bacteria (less frequent). e) Fungistatic substances such as sorbic acid, sulphur dioxide or smoke are used to preserve IMF products.  Low moisture foods (LMF) a) Range of aw = 0.60 to 0.20 b) Examples: Chocolate, confectioneries, honey, dried eggs, cocoa, cake mixes, powdered milk, dried cereals and vegetables. c) LMF product are dried to suitable (low) aw value and unsuitable for any microbial growth.  DEHYDRATION Principle: all living organisms need water to survive. Drying of foods reduces the moisture content and water activity (aw) that can stop the activity of spoilage microbes, enzymes and undesirable biochemical reaction. The objectives of drying are: a) To reduce the water content in the product (water content < 25% or aw between 0.00 to 0.60). b) To extend shelf life. c) To reduce enzyme activity and undesirable chemical reactions. The advantages of dehydrated foods are: a) Smaller and lighter end products - reduce cost of handling, storage and transport b) Maintain much of the original nutritional value c) Long shelf-life and requires no preservatives  CHANGES DUE TO DEHYDRATION The dehydration process causes many changes such as: a) Size and shape changes – due to shrinkage. b) Colour changes - due to removal of water, exposure to high temperature & exposed to air. c) Texture changes - due to shrinkage d) Flavour changes - due to loss of volatile flavor compounds & development of an undesirable cooked flavour e) Nutritional quality changes - due to loss of water-soluble nutrients. 4 FACTORS AFFECTING THE DRYING RATE 1) Speed of drying Need to be conducted rapidly, but not too fast till the outside of the food becomes hard before the moisture inside has a chance to evaporate. It is called "case hardening” (the cells on the outside of the pieces of food give up moisture faster than the cells on the inside. 2) Temperature Need to control temperature at optimum level to evaporate moisture Too high - case hardening & burning taste on food Too low - encourage microbial growth 3) Humidity and Ventilation Need to have low humidity in air – high humidity can slow down evaporation in food Need adequate ventilation in dryer - trapped air in the dryer will take on as much moisture as it can hold (saturation) and then drying can no longer take place. 4) Uniformity Need to dry food evenly by spread thin layer, flipping pieces of food frequently and shifting the racks in the dryer It is essential because heat is not the same in all parts of the dryer.  DEHYDRATION METHODS AND DRYING EQUIPMENT Factors that affecting the selection of dryers are: a) Initial water content of the product - Pre-drying may be required to partially remove water prior main drying. b) Physical form of feed and product - If the feed is in the form of paste or slurry, the choice of dryer is limited to spray, drum or fluidized beds. c) Heat-sensitivity of product - it is suitable to use vacuum dryer or freeze dryer. d) Energy efficiency - conduction or direct dryers are more efficient than convection dryers. e) Production rate and capacity – based on small, medium or large industry. FLUIDIZED BED DRYER  1 SUN DRYING (SOLAR) Oldest method of food preservation Advantages of direct sun drying: a) Energy source is renewable and free b) Simple and uncomplicated method Disadvantage of direct sun drying: a) Only practical in hot, dry climates. b) Need long time c) Hard to control weather, pollution & animals d) Large area is required e) Hard to control drying rate - uneven drying lead to unwanted biochemical or microbiological reactions Advantage of Indirect solar dryers: a) Can collect the sun’s rays using heat exchanger (solar collectors) b) Even drying - heating is conducted by hot air flows through or over the product using forced convection. c) Free from pollution and insects  SUN DRYING 2 (INDIRECT SOLAR DRYER) Indirect solar dryers 3 CABINET DRYING Cabinet and bed type dryer (kiln, tray, tunnel and rotary flow conveyor) are in this type of dryer. Suitable for solid food materials such as: a) Grains b) Sliced fruits and vegetables c) Chunked products. Advantages of hot air drying are: a) Efficient b) Simple c) Suitable for industrial application - can handle larger production rates HOT AIR DRYING CABINET (TRAY) HOT AIR DRYING CABINET (KILN) (a) through-circulated band dryer; (b) vibrated-band dryer; (c) suction-drum dryer; (d) rotary cascading dryer; (e) multitubular rotary cascading dryer; (f)through-circulation rotary dryer. After Keey (1992). HOT AIR DRYING CABINET (ROTARY FLOW) HOT AIR DRYING CABINET (TUNNEL AND CONVEYER)  The drying process involves forced flow of hot air along or through the trays in which the food material is placed to remove water from the surface. Air is heated indirectly, generally by heating element or steam coils. Tunnel dryer is similar to cabinet dryer except that the trays are made to move along a tunnel in which hot air flows in co- current or counter flow to the product movement.  4 MICROWAVEDRYING MICROWAVE DRYING Microwaves are high frequency electromagnetic waves of radiant energy. Microwave flow of work on food:- a) Magnetron converts alternating electric current into electromagnetic energy radiation b) A stirrer deflects the radiated microwaves and distributes to various parts of dryer. c) Some microwave energy are reflected back and some are absorbed by the food. d) Heat is created in the food as a result of the intermolecular friction produced as the molecules within the food rotates and oscillates.  A combination of microwave and conventional heating can be used for drying pasta, saving both time and energy. Advantages of microwaves are: a) Have penetrating heat leads to uniform drying b) Selective absorption by liquid water leads to uniform moisture profile within the particles c) Easy to control due to rapid response of heating. The disadvantages of microwave drying are the use of complicated equipment and the cost is expensive. TUNNEL MICROWAVE GRAIN DRYING  4 FREEZE FREEZE DRYING DRYING Freeze dryer flow of work:- a) Food is first frozen b) Placed in a vacuum chamber under reduced pressure c) Ice in the frozen food changes directly to water vapour (sublimation) and is carried away by the circulating heated air. d) The moisture content of the food is reduced (1 to 4%) e) The time for freeze drying depends on the product and its thickness - usually 10 to 20 hours. Products most often freeze-dried include: instant coffee, dried soup mixes, mushrooms, herbs, spices, cheese starter cultures, shrimp, fruits for ready-to- eat breakfast cereals.  The main advantages of food product preserve by freeze dryer are: a) Low thermal damage b) Maintain its nutritional values and vitamins c) No development of cooked flavour. d) Maintain of volatile flavours and aroma due to entrapment of volatiles in micro-regions within the solids or volatile compounds which are strongly adsorbed by the solid e) No damage in structure and texture, f) Low product shrinkage leads to minimal changes in shape, colour and appearance g) Rapid product rehydration h) Long product shelf-life when packed properly  The disadvantages of freeze drying are: a) high capital and operation costs b) Initial freezing can cause damage to certain products c) Products can deteriorate fast unless are packed in special materials to avoid oxidation and moisture pick-up. 5 SPRAY DRYING SPRAY DRYING Suitable for foods in the form of liquids or slurries. Spray dryer flow of work: a) Liquid food is converted into a fine mist then sprayed and mixed with hot air in drying chamber. b) Rapid drying will takes place – 1 to 20 sec c) dry powder is formed d) The residence time of the droplets /particles in the chamber must be sufficient to reach suitable low moisture content. e) Once particles are dry, it must be removed quickly from the chamber to avoid unnecessary heat damage. Spray drying is used to produce foods such as non-fat dried milk and some types of instant coffee. SPRAY DRYER 6 DRUM DRYING 6) DRUM DRYING A continuous contact dryer widely used in food industry for drying products initially in slurries (liquid form) which particles are too large for spray drying. Suit for many heat sensitive products since exposure to high temperature is limited to few seconds (also having vacuum condition if required). The slurries are poured over the hot surface of the drum, where the food will be dried on the dryer surface then are scraped off, ground into flakes and packed. This drying method is used to produce potatoes, corn syrup, baby foods. It is not suitable for materials which do not adhere to the drum and thermoplastic materials. Thick film of feedstock is applied to the external surface of a heated drum dryers. The drum rotates slowly about horizontal axis. The layer of materials remains attached to the drum for about 80% of a revolution. This is where moisture evaporates and leaves behind a layer of solids. Typical rotation speed : 4-12 rpm. The drums (size : 1.2 m diameter, 3.5 m length) are hollow and heated internally by condensing steam or heating media. The layer of solids will be removed from the drum surface by a scrapper/doctor knife. The drum rotates slowly about horizontal axis. The layer of materials remains attached to the drum for about 80% of a revolution. This is Thick film of feedstock is applied to the where moisture evaporates and leaves external surface of a heated drum dryers. behind a layer of solids. 2 1 3 The layer of solids will be removed from the drum surface by a scrapper/doctor knife. DRUM DRYER 6a SINGLE-DRUM DRIER SINGLE DRUM DRYING Single-drum drier consist of one hollow metal cylinder rotating about a horizontal axis and is heated internally by steam. The feed to be dried must be in solution form or a suspension of solids/paste. Feed is applied to the drum as thin film on drum surface and will be dried as the drum rotates. Dried film is then removed or scraped off from the drum surface by a knife located ½ to ¾ of a revolution from the point of feed application. 6b DOUBLE DRUMDRIER DOUBLE-DRUM DRYING The double-drum drier is similar in its drying operation with the single-drum drier except for it consist of two drums situated close together and rotating towards each other. The feed is introduced into the trough formed between the drums. It is suitable for low viscosity liquids that are not very heat sensitive. DOUBLE DRUM DRYER 6c VACUUM-DRUM VACUUM DRUMDRIER DRYING Vacuum-drum dryer is suitable for heat-sensitive material. The equipment consists of a single or double-drum enclosed within a vacuum tight chamber. Pressure is reduced so that the required low temperature can be use. The equipment is more expensive but the advantage is that it can be operated under sterile conditions. VACUUM DRUM DRYER END OF CHAPTER 5 PRESERVATION BY DEHYDRATION