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Dehydration

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Book cover Fundamentals of Food Process Engineering

Part of the book series: Food Science Text Series ((FSTS))

Abstract

Dehydration is a major process for food preservation. The reduced weight and bulk of dehydrated products and their dry shelf stability reduce product storage and distribution costs. As dehydration techniques that produce good-quality convenience foods are developed, more dehydrated products will be commercially produced. At present, instant beverage powders, dry soup mixes, spices, and ingredients used in further processing are the major food products dehydrated.

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Suggested Reading

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Author information

Authors and Affiliations

  1. Department of Food Science & Technology, University of Georgia, Athens, GA, USA

    Romeo T. Toledo, Rakesh K. Singh & Fanbin Kong

Authors
  1. Romeo T. Toledo
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  2. Rakesh K. Singh
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  3. Fanbin Kong
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Problems

Problems

  1. 12.1.

    Pork has aw of 1.00 at a moisture content of 50% (wet basis) and higher. If pork is infused with sucrose and NaCl and dehydrated such that at the end of the dehydration process, the moisture content is 60% (wet basis) and the concentration of sugar and NaCl are 10% and 3%, respectively, calculate the aw of the cured product.

  2. 12.2.

    What concentration of NaCl in water would give the same water activity as a 20% solution of sucrose?

  3. 12.3.

    The following data were obtained on the dehydration of a food product: initial moisture content = 89.7% (wet basis).

Drying time (min)

Net weight (kg)

0

24.0

10

17.4

20

12.9

30

9.7

40

7.8

50

6.2

60

5.2

70

4.5

80

3.9

90

3.5

Draw the drying curve for this material, and construct a curve for the drying rate as a function of the moisture content.

  1. (a)

    What is the critical moisture content for each of the falling rate zones?

  2. (b)

    What is the constant drying rate?

  3. (c)

    Determine the residual moisture content for each of the falling rate stages .

  4. (d)

    The dehydration was conducted at an air flow rate of 50 m/s at a dry bulb temperature of 82 °C and a wet bulb temperature of 43 °C. The wet material has a density of 947 kg/m3 and was dried in a layer 2.5 cm thick. If the same conditions were used but the initial moisture content was 91% (wet basis) and a thicker layer of material (3.5 cm) was used on the drying trays, how long will it take to dry this material to a final moisture content of 12% (wb)?

  1. 12.4.

    A continuous countercurrent drier is to be designed to dry 500 kg/h of food product from 60% (wet basis) moisture to 10% (wet basis) moisture. The equilibrium moisture content for the material is 5% (wet basis), and the critical moisture content is 30% (wet basis). The drying curve of the material in preliminary drying studies showed only one falling rate zone. Air at 66 °C dry bulb and 30 °C wet bulb will be used for drying. The exit air relative humidity is 40%. Assume adiabatic humidification of the air . The drying air is drawn from room temperature at 18 °C and 50% RH. The wet material has a density of 920 kg/m3. The drying tunnel should use trucks that hold a stack of 14 trays, each 122 cm wide, 76 cm deep along the length of the tunnel, and 5 cm thick. The distance between trays on the stack is 10 cm. The drying tunnel has a cross-sectional area of 2.93 m2. The material in the trays will be loaded at a depth of 12.7 mm. Calculate:

    1. (a)

      The number of trays of product through the tunnel/h.

    2. (b)

      The rate of travel by the trucks through the tunnel. Assume distance between trucks is 30 cm.

    3. (c)

      The constant drying rate and the total time for drying.

    4. (d)

      The length of the tunnel.

    5. (e)

      If air recycling is used, the fraction of the inlet air to the drier that must come from recycled air.

    6. (f)

      The capacity of the heater required for the operation with recycling.

  2. 12.5.

    A laboratory drier is operated with a wet bulb temperature of 115 °F and a dry bulb temperature of 160 °F. The air leaving the drier is at 145 °F dry bulb. Assume adiabatic operation. Part of the discharge air is recycled. Ambient air at 70 °F and 60% RH is heated and mixed with the recycled hot air. Calculate the proportion of fresh air and recycled hot air that must be mixed to achieve the desired inlet dry and wet bulb temperatures .

  3. 12.6.

    If it takes 8 h to dry a material in a freeze drier from 80% H2O to 10% H2O (wet basis) at an absolute pressure of 100 Φm and a temperature of 110 °F (43.3 °C), how long will it take to dry this material from 80% to 40% water if the dehydration is carried out at 500 Φm and 80 °F (26.7 °C)? The material is 25 mm thick and has a density of 950 kg/m3, and the thermal conductivity of the dried material is 0.35 W/m · K. Thermal conductivity and heat transfer coefficients are independent of plate temperature and vacuum.

  4. 12.7.

    Calculate the constant rate of drying in a countercurrent continuous belt dehydrator that processes 200 lb/h (90.8 kg/h) of wet material containing 80% water to 30% water. Air at 80° EF (26.7 °C) and 80% RH is heated to 180 °F (82.2 °C) in an electric heater, enters the drier, and leaves at 10% RH. The critical moisture content of the material is 28%. The drier is 4 ft (1.21 m) wide and the belt loaded to a depth of 2 in. (5.08 cm) of material, and the clearance from the top of the drier to the top of the material on the belt is 10 in. (25.4 cm). The density of the dry solids in the material is 12 lb/ft3 (193 kg/m 3).

  5. 12.8.

    In a spray-drying experiment, a sample containing 2.15% solids and 97.8% water was fed at the rate of 6.9 lb per hour (3.126 kg/h), and this sample was dried at 392 °F (200 °C) inlet air temperature. The exit air temperature was 200 °F (93.3 °C). The dried product was 94.5% solids, and the outside air was at 79 °F (26.1 °C) and 20% RH.

Calculate:

  1. (a)

    The weight water evaporated per hour.

  2. (b)

    The % RH of the exit air.

  3. (c)

    The mass flow rate of air through the drier in weight dry air/h.

  4. (d)

    In this same drier, if the inlet air temperature is changed to 440 °F (226.7 °C) and the % RH of the exit air was kept the same as in (b), weight of a sample containing 5% solids and 98% water can be dried to 2% water in 1 h. (Air flow rate is the same as before.) At what temperature would air exit the drier under the specified conditions of operation. Assume adiabatic drying .

  1. 12.9.

    A dehydrator when operated in the winter where the outside air was 10 °F(−12.2 °C) and 100% RH (H = 0.001) can dry 100 lb (45.5 kg) of fruit per hour from 90% water to 10% water. The inlet temperature of the air to the drier is 150 °F (65.6 °C) and leaves at 100 °F (37.8 °C). In the summer when the outside air is at 90 °F (32.2 °C) and 80% RH, determine the moisture content of the product leaving the drier if the operator maintains the same rate of 100 lb (45.4 kg) of wet fruit/h and the exit air from the drier has the same % RH as it was in the winter.

  2. 12.10.

    The desorption isotherm of water in carrots at 70° EC is reported to fit Iglesias and Chirife’s equation (Eq. 12.36) with the constants B1 = 3.2841 and B2 = 1.3923.

    1. (a)

      Determine the moisture contents where a shift in drying rate may be expected in the dehydration of carrots .

    2. (b)

      The following data represents the equilibrium water activity (aw) for carrots at various moisture contents in kg water/kg dry matter (X): (aw, X); (0.02, 0.0045), (0.04, 0.009), (0.06, 0.0125), (0.08, 0.016), (0.10, 0.019), (0.12, 0.0225), (0.14, 0.025), (0.016, 0.028), (0.18, 0.031), and (0.20, 0.034). Fit this data to the BET isotherm, and determine the moisture content for a unimolecular layer, Xm.

    3. (c)

      Fit the data to the GAB equation, and determine the constants.

  3. 12.11.

    The diffusivity of water in scalded potatoes at 69° EC and 80% moisture (wet basis) has been determined to be 0.22 × 10−5 m2/h. If 1 cm potato cubes are dried using air at 1.5 m/s velocity and 1% relative humidity, calculate the dry bulb temperature of the air that can be used such that the diffusion rate from the interior to the surface will be equal to the surface dehydration rate. Assume the air flows parallel to the cubes and that dehydration proceeds from all faces of each cube. The density of the potato cube is 1002 kg/m3 at 80% moisture .

  4. 12.12.

    Puffing can be induced during dehydration of diced carrots if the dehydration rate at the constant rate period is of the order 1 kg water/(min kg DM). In a fluidized bed drier where the air contacts individual particles at a velocity of 12 m/s, calculate the minimum dry bulb temperature of the drying air that would induce this rate of drying at the constant rate period. Assume drying air has a humidity of 0.001 kg water/kg dry air and surface temperature under these conditions is 5° EC higher than the wet bulb temperature.

Calculate the mass transfer rate under these conditions. Is dehydration rate heat or mass transfer controlled?

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Toledo, R.T., Singh, R.K., Kong, F. (2018). Dehydration. In: Fundamentals of Food Process Engineering. Food Science Text Series. Springer, Cham. https://doi.org/10.1007/978-3-319-90098-8_12

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