Chapter 8 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

2. Vocabulary • Drying, dehydrate, rehydrate, equilibrium relative humidity, water activity, isotherms sorption desorption hysteresis behavior hypothesis capillary semiempirical empirical, critical moisture content

3. DEHYDRATION • Drying 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 liquid food, whereas drying typically produces product with water content sufficiently low to give solid food.

4. Reasons for drying foods • Historically, there was a need to preserve foods for longer times so that food was available 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 to preserve foods as they are harvested, so that we can enjoy them at later times.

5. Reasons for drying foods • One of the easiest ways to preserve foods is to remove water, since microorganisms need water to survive and grow, 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 that we can enjoy at any time.

6. 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 also provide convenience. Dried convenience foods may be used for special expedition--type (military) foods where weight is a major concern.

7. There are many methods and technologies by which we can dehydrate foods. We must 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 way that the product regains its initial form when rehydrated is not so easy.

10. 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 still leave a high-quality product. Food technologists often use the thermodynamic measure of water activity to describe how water interacts in food products.

11. Water activity • Water activity (aw)is defined as the ratio of the vapor pressure of water measured at the food surface (Pw)to the saturation vapor pressure of pure water at the same temperature (Pwº)

12. Water activity • For a cup of water, the vapor pressure over the surface is measured 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 some value less than 1. The reduction in water activity depends on the type of solutes present and their levels.

13. Water Activity • For food products, the water activity is generally less than 1. aw is related to the moisture content of the food, the types and concentrations of different solutes, and the structure or physical characteristics of the food.

16. Relationship between RH & aw • The water activity of a food can be related to an equilibrium 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 relative humidity is the "equilibrium relative humidity" or ERH.

17. Free" water & "bound" water • In the past people simplified the state of water in foods by denoting two types: "free" water 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.

18. 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 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.

19. physical changes • 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 through several regions of properties, where viscosities are intermediate between a pumpable liquid and a stationary solid.

20. The state diagram • For a simple system of solute and solvent The glass transition curve represents a metastable 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

21. For example, powdered milk products 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 and be less stable. In this case, powdered milk would be likely to get sticky, and the powder would cake together.

23. EFFECTS OF DRYING ON PRODUCT QUALITY • After rehydrating the food cannot reach the original quality. There 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 main problems are loss and change of flavors, and change in physical qualities of dried products.

24. Effect on flavor • One problem with dried foods is that the flavor of the rehydrated product is not the same as that of the original. 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 volatile compounds (alcohols, aldehydes, ketones, etc.) to be removed.

25. burnt flavor • Dried products have less of these volatile flavoring compounds than the original starting material. In addition, the rates of chemical reactions 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 flavor. (reconstituted milk from a dried powder)

26. 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 drying, which causes increasd viscosity, Thermal degradation of vitamins and proteins may also influence the nutritional status of dried products.

27. The extent of these changes depends on the nature of the drying process. Some 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 flavor and aroma compounds are not lost during drying, and freeze-dried products have higher quality

28. MOISTURE SORPTION AND DESORPTION • During drying, both moisture content and water activity change. At any given relative 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 in a food product that can be reached for any condition of drying air.

29. Isotherms • By holding a food product in air at different relative humidities and measuring the equilibrium water content, the curve of water content and 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 water content and water activity. Isotherms

30. MOISTURE SORPTION AND DESORPTION • Moisture sorption (picking up water) curves typically are slightly lower in water contents than moisture desorption (drying) curves. Several mechanisms have been proposed for this hysteresis behavior. • capillary forces • volume expansion

32. RAT E OF DEHYDRATION • In drying, water molecules must make their way through the food to the surface (internal resistance to 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 vapor pressure in the air reaches the same value as the vapor pressure of water at the surface of the food, drying ceases.

33. The rate of drying may be limited by either the rate of internal migration of water molecules to the 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 drying process as the product dries out.

35. RAT E OF DEHYDRATION • Drying Curves • Constant Rate Period • Falling Rate Period

36. Drying Curves • A curve of loss of moisture during drying of a food product are typically generated by 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).

37. The kg of dry matter (initial product 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.

39. The shape of the drying curve is similar for many food products. 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 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.

40. constant rate period • drying rate is plotted against 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 nearly 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 "constant rate period," or CRP.

41. 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 moisture 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 the dryer.

43. 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 lost 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.

46. Energy equation • For the simplest case, where 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 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 surface (liquid to vapor), an amount of energy equivalent to the latent heat of vaporization is required.

48. The constant rate drying period lasts as long as the rate of moisture migration from the interior of the product 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 and the time for constant rate drying, tCRP can be found as: