dehydration

1、Chapter 8 DEHYDRATION,STATE OF WAER IN FOODS EFFECTS OF DRYING ON PRODUCT QUALITY MOISTURE SORPTION AND DESORPTION RATE OF DEHYDRATION FACTORS THAT INFLUENCE DRYING DRYING METHODS SPRAY DRYING FREEZE DRYING,Vocabu

2、lary,Drying, dehydrate, rehydrate, equilibrium relative humidity, water activity, isotherms sorption desorption hysteresis behavior hypothesis capillary semiempirical empirical, critical moisture content,DEHYDRATION,Dryi

3、ng of foods is an important food processing operation used to preserve foods. The distinguishing features between drying and concentration are the final level of water and nature of the product. Concentration leaves a li

4、quid food, whereas drying typically produces product with water content sufficiently low to give solid food.,Reasons for drying foods,Historically, there was a need to preserve foods for longer times so that food was ava

5、ilable during times of limited food production or availability. Hunters needed a technique to preserve meat for more than a few days to ensure a continuous food supply In the same manner, we have techniques that allow us

6、 to preserve foods as they are harvested, so that we can enjoy them at later times.,Reasons for drying foods,One of the easiest ways to preserve foods is to remove water, since microorganisms need water to survive and gr

7、ow, and many chemical reactions require water to proceed. Early hunters dried their meat to help maintain a more continuous food supply. Nowadays, we dry foods for the same reason: to provide a continuous supply of foods

8、 that we can enjoy at any time.,Other reasons for drying foods,Removal of water leaves a product reduced in weight and often in bulk. This reduces shipping costs and makes the food supply more economical. Dried foods als

9、o provide convenience. Dried convenience foods may be used for special expedition--type (military) foods where weight is a major concern.,,There are many methods and technologies by which we can dehydrate foods. We must

10、first understand the nature of water in food products to appreciate the difficulties in producing high-quality dried products. Removal of water from foods is not a difficult task. However, removing the water in such a w

11、ay that the product regains its initial form when rehydrated is not so easy.,,STATE OF WAER IN FOODS,In dehydration, it is important to understand the behavior of water so that it can be removed most effectively and stil

12、l leave a high-quality product. Food technologists often use the thermodynamic measure of water activity to describe how water interacts in food products.,Water activity,Water activity (aw)is defined as the ratio of the

13、vapor pressure of water measured at the food surface (Pw)to the saturation vapor pressure of pure water at the same temperature (Pwº),Water activity,For a cup of water, the vapor pressure over the surface is measure

14、d as the saturation vapor pressure, and aw is 1. When there are solutes in the water such as sugars, salts, etc. the vapor pressure over the water surface is lower than the saturation vapor pressure, and aw is reduced to

15、 some value less than 1. The reduction in water activity depends on the type of solutes present and their levels.,Water Activity,For food products, the water activity is generally less than 1. aw is related to the moistu

16、re content of the food, the types and concentrations of different solutes, and the structure or physical characteristics of the food.,,,Relationship between RH & aw,The water activity of a food can be related to an e

17、quilibrium relative humidity in the air around the product. That is, at only one relative humidity will the air be in moisture equilibrium with the food product where the food neither gives up or adsorbs water. This rela

18、tive humidity is the "equilibrium relative humidity" or ERH.,Free" water & "bound" water,In the past people simplified the state of water in foods by denoting two types: "free" wate

19、r or "bound" water. "free" water or "bound" water. The working definition for these terms is: Free water is that which gives water activity of 1, bound water gives water activity less than 1

20、.,,Free water is relatively easy to remove from a food product while bound water takes more energy to release from the food. Thus, the latent heat required to remove a molecule of water from a food increases as the water

21、 activity decreases. This is important to those who design drying operations, since the energy requirement to provide sufficient driving force for drying is related to the latent energy of vaporization.,physical changes,

22、As a food product dries out and the water molecules become less mobile, physical changes also occur in the food. As water is removed, the remaining product generally becomes increasingly viscous. The product may go throu

23、gh several regions of properties, where viscosities are intermediate between a pumpable liquid and a stationary solid.,The state diagram,For a simple system of solute and solvent The glass transition curve represents a m

24、etastable transition where viscosity is so high that the product does not "flow". Below this curve, the food is stable to diffusion-limited processes for extremely long times,,For example, powdered milk product

25、s remain dry and stable when maintained below the glass transition temperature. However, if the powder picks up moisture from the air or experiences elevated storage temperature, it may exceed the glass transition curve

26、and be less stable. In this case, powdered milk would be likely to get sticky, and the powder would cake together.,,EFFECTS OF DRYING ON PRODUCT QUALITY,After rehydrating the food cannot reach the original quality. There

27、 is always some change that gives a loss of quality in the product. The goal is to minimize these changes, while optimizing process efficiency and minimizing costs. Several types of changes can occur during drying. Two m

28、ain problems are loss and change of flavors, and change in physical qualities of dried products.,Effect on flavor,One problem with dried foods is that the flavor of the rehydrated product is not the same as that of the o

29、riginal. During drying, flavor compounds that are typically more volatile than water are removed in the drying process. The physical forces that cause water molecules to be removed from the food during drying also cause

30、volatile compounds (alcohols, aldehydes, ketones, etc.) to be removed.,burnt flavor,Dried products have less of these volatile flavoring compounds than the original starting material. In addition, the rates of chemical r

31、eactions are enhanced at the elevated temp., and many of these reactions generate undesired flavor compounds. For example, the browning reaction (between reducing sugars and proteins) is enhanced and generates a burnt fl

32、avor. (reconstituted milk from a dried powder),Browning,Other chemical reactions may also take place during drying. Browning occurs in many foods which results in color changes. Protein denaturation can occur during dryi

33、ng, which causes increasd viscosity, Thermal degradation of vitamins and proteins may also influence the nutritional status of dried products.,,The extent of these changes depends on the nature of the drying process. So

34、me types of dryers produce products having superior properties on reconstitution. The instant coffee spray-dried and freeze-dried is different. Since freeze-drying does not involve a vapor-liquid interface, the volatile

35、 flavor and aroma compounds are not lost during drying, and freeze-dried products have higher quality,MOISTURE SORPTION AND DESORPTION,During drying, both moisture content and water activity change. At any given relative

36、 humidity of air used for drying, there is an equilibrium water content with the product, At this point the activity of water in the air is the same as that in the product. This relationship specifies the water content i

37、n a food product that can be reached for any condition of drying air.,Isotherms,By holding a food product in air at different relative humidities and measuring the equilibrium water content, the curve of water content an

38、d water activity can be obtained. It’s nature depends on whether the food product is being dried or allowed to pick up moisture from the air. The direction of the experimental measurement affects the relationship between

39、 water content and water activity. Isotherms,MOISTURE SORPTION AND DESORPTION,Moisture sorption (picking up water) curves typically are slightly lower in water contents than moisture desorption (drying) curves. Several m

40、echanisms have been proposed for this hysteresis behavior. capillary forces volume expansion,,RAT E OF DEHYDRATION,In drying, water molecules must make their way through the food to the surface (internal resistance to

41、drying) in contact with drying air. Once at the surface, water molecules are transfered into the air (external resistance to drying) based on the difference in vapor pressure between the air and the surface. When the vap

42、or pressure in the air reaches the same value as the vapor pressure of water at the surface of the food, drying ceases.,,The rate of drying may be limited by either the rate of internal migration of water molecules to th

43、e surface or the rate of evaporation of water molecules from the surface into the air, depending on the conditions of drying. In fact most foods switch from an external drying process during initial stages to an internal

44、 drying process as the product dries out.,,RAT E OF DEHYDRATION,Drying Curves Constant Rate Period Falling Rate Period,Drying Curves,A curve of loss of moisture during drying of a food product are typically generated b

45、y weighing a sample of food undergoing drying and relating weight loss to moisture content. Moisture content is most often expressed as kg of water per kg of dry product (or matter).,,The kg of dry matter (initial produ

46、ct weight minus weight from water) are always constant during drying, so a constant reference point is used when referring to drying in kg water/kg dry matter.,,,The shape of the drying curve is similar for many food pro

47、ducts. After short initial equilibration period (for thermal equilibration), the moisture content decreases rapidly, and almost linearly, with time. This initial drying period is followed by a much slower rate of drying

48、as the moisture content of the product decreases. The rate of drying is the slope of the moisture content change with time, expressed in kg water/kg dry matter-minute.,constant rate period,drying rate is plotted against

49、the moisture content (instead of time). Since moisture content goes from high to low during drying, the initial drying condition is given by the point at the right of the graph. Initially, the rate of drying may be nearl

50、y constant until some critical moisture content Xc, is reached. Xc represents the moisture content where drying changes from constant rate to falling rate. This initial period of constant rate drying is called the "

51、constant rate period," or CRP.,falling rate period,After the product is dried below Xc, the rate of drying decreases. This is called the "falling rate period," or FRP. Here, drying rate depends on the mois

52、ture content remaining in the product. If the product is dried extensively, the product eventually equilibrates with the drying air. The equilibration point depends on temperature and relative humidity of the air used in

53、 the dryer.,,Constant Rate Period,The initial rate of drying-- The rate at which water molecules arrive at the surface by migration from the interior is greater than (or equal to) the rate at which water molecules are lo

54、st from the surface to the drying air. So there is sufficient water to be evaporated, the thermal energy to the food is used as latent heat, the temp. of food is not elevated.,,,Energy equation,For the simplest case, whe

55、re only convective heat transfer occurs, all of the heat energy goes into vaporizing moisture during the constant rate period. That is, the rate of heat transfer into the product is balanced by the rate of energy removal

56、 due to the vaporizing moisture. The rate of energy removal with vaporized water can be found as the product of the rate of drying and the latent heat of vaporization. That is, for each molecule of water vaporized at the

57、 surface (liquid to vapor), an amount of energy equivalent to the latent heat of vaporization is required.,,,The constant rate drying period lasts as long as the rate of moisture migration from the interior of the produc

58、t to the surface is sufficiently rapid that the moisture content at the surface is constant. At the point where moisture migration from the interior is slower than the surface vaporization, the constant rate period ends

59、and the time for constant rate drying, tCRP can be found as:,,,Falling Rate Period,After the critical moisture point, the rate at which moisture migrates to the surface limits drying. That is, the rate of moisture loss f

60、rom the surface to the drying air is faster than the rate at which that moisture is replenished at the surface.,Mechanisms of internal mass transfer,1. Liquid diffusion. 2. Vapor diffusion. 3. Capillary flow. 4. Press

61、ure flow. 5. Thermal flow.,FACTORS THAT INFLUENCE DRYING,1.Process Conditions Temperature/Air Velocity/Relative Humidity/Pressure2.Food PropertiesSurface Area/Constituent /Orientation/Cellular Structure/ Type and Con

62、centration of Solutes.,,DRYING METHODS,1.Direct Contact Dryers Sun Dryer/Bin Dryer/Kiln Dryer/ Tray or Cabinet Dryer/Tunnel Dryer/Belt or Conveyor Dryer/ Fluidized Bed Dryer/Rotary Air Dryer/Spray Dryer. 2.Indirect Con

63、tact Dryers Drum Dryer 3.Infrared or Dielectric Dryers Infrared Dryers/Microwave Dryers.,,,,Tray or Cabinet Dryer,The food product placed in a pan is placed inside a drying chamber with hot air blowing across the prod

64、uct until drying is complete. Some of the hot air used for drying may be recirculated through the dryer to conserve energy However, increased relative humidity of the recirculated air decreases dryer efficiency.,,Tunnel

65、Dryer,The food product is loaded onto trays that are placed into carts. The carts are input at one end of the tunnel dryer and move through to the outlet. Air blowing within the tunnel causes drying at a specified rate,

66、so that the food product reaches the exit on completion of drying. (1) cocurrent; (2) countercurrent; or (3) mixed flow,,,,Belt or Conveyor Dryer,Product may also be moved through a dryer by placing it on a belt or conve

67、yor. In order to extend the time within a conveyor dryer, a series of conveyors may be arranged one above the other. In this case, product drops from an upper conveyor to a lower conveyor. air flow can be through the con

68、veyor and through the bed of food product laying on the conveyor.,,Fluidized Bed Dryer,Air flow through a bed of product is sufficient to lift the product. Since there is intimate contact between air and product drying r