FE 483 Operation Laboratory TRAT DRYING EXPERIMENT DRYING

FE 483 Operation Laboratory TRAT DRYING EXPERIMENT

DRYING • Dehydration is a food unit operation that is used to extend the shelf-life of a foodstuff by reducing its water activity (aw). • Heat transferred to the surface of a foodstuff exposed to hot air, provides the latent heat of vaporization to evaporate moisture from the food surface. Hot air flow Increase the sample temperature Remove water from sample

TRAY DRYING • Drying with hot air using a tray or cabinet dryer is one of the methods of food dehydration. • Tray drying are operated as batch systems. • Tray dryers are useful for the small production rate. • These dryers are suitable for wide range of food items, especially for fruit and vegetables.

Drying Curves • A drying process can be represented graphically by recording changes in moisture content of a foodstuff. • Drying curves are typically represented as drying rate curves, showing different stages or periods of drying. • The drying process mainly occur in three stages; 1. initial 2. contant rate 3. falling rate periods

• The initial short period: allows for the food surface that is heated to reach similar conditions as the hot air (A to B). • The constant rate period: (B to C) follows the initial period, where the drying rate is constant. ***At this stage the food surface is covered completely by a continuous film of free water (water that is not bound to the food surface).

• The falling rate period: start when the surface begins to dry and, as the rate of drying decreases. At that certain point in the drying process, the continuous film of water at the surface cannot be maintained, which results in a significant reduction in water and It is at this point where the critical moisture content is reached *** A single falling rate period occurs with non-hydroscopic foods whereas with hydroscopic foods, two (CD and DE) or more periods may occur

Aim of the Experiment • Drying mint with using tray dryer • Learning basic information about drying, tray drying and also drying curves • Learn to the plot drying curves • Determine to the drying time, drying periods and critical moisture content from drying curves, obtained from experimental drying data.

Experimental Apparatus and Materials 1. Tray dryer • The experimental tray dryer is a small scale dryer unit designed. • It has a tunnel in which an axial flow fan is mounted at one end. Before the fan there is a set of electrically heated elements which heat the air.

• The chamber contains a rack of trays suspended from a scale mounted on top of the drier. • The total capacity of the scale is approximately 6 kg with 0. 1 g increments. • A transparent access door, permits viewing the product while drying. • A control panel mounted near the fan allows setting the air speed and air temperature by varying the power of the heater ( 2 KW).

2. Anemometer used to measure the air flow speed ( m/s) in the dryer

3. Digital Psychrometer used to measure dry bulb temperature and RH of drying air

4. Infrared dryer used to measure initial moisture content of fresh mint 5. Food material to be dried: Fresh mint

Procedure 1. Measure tray dimensions and Place one of the tray into middle shelf of the tray dryer

2. Turn on air flow. Adjust flow knob. Turn on electrical heater.

3. Record exit flow area where you measure velocity to be able to calculate volumetric flow rate

4. Measure the initial moisture content of mint using the infrared dryer

5. Make several measurements of exit drying air speed(m/s, use mean value in your calculations).

6. Place approximately 30 grams of mint on the tray and record the initial weight of the material to be dried. ( Don't forget setting tare to zero before placing mint)

7. Register the weight of the mints every 5 minute for the first 20 minutes of drying. After this initial period, record these measurements every 10 min. Use this data to calculate and follow moisture content of mint during drying process. 8. Continue process until a constant weight is reached for the mints.

9. Measure the temperature and RH with the digital psychrometer before the drying chamber before starting the drying process. Measure the temperature and RH of the air after the drying trays. before trays after trays

DATA SHEET Initial moisture content of mint: 86. 09% Dimensions of tray: 18. 3 cm*27. 4 cm Exit flow area dimensions ( where air velocity was measured ): Air flow rates(m/s): 22. 5*22. 5 Inlet air to trays Outlet air from drying trays Mint 1 - 0. 5 3 - 0. 82 5 - 0. 55 2 - 0. 72 4 - 0. 75 Temp: 75◦C RH: 5. 0% Temp: 71◦C RH: 5. 4% t (min) Weight (g) 0 5 10 15 20 30 40 50 60 70 80 31 26 21 16 14 10 8 7, 5 7 6, 65

CALCULATION AND DATA ANALYSIS 1. Calculate the free moisture content (X) for all times in data sheet with using excell program • X: Free moisture content X= W-Ws W: weight Ws Ws: dry solid Example: Finding X value for initial time of drying W 0 = 31 g Ws 0 =31*(1 -0, 8609) = 4, 31 g X 0 = (31 -4, 31)/ 4, 31 = 6, 19

2. Calculate Drying Rate ( R ) data for all times in data sheet Ls = Ws dry solid A = area of drying sample (area of tray) !!! Ws for althroughout drying not change because drying not effect the dry solid (drying remove free water not change the solid part just effect the water amount)

Please put your excell datas as colums in your reports as example; t (min) Weight (g) X Free Moisture 0 31 5 26 10 21 15 16 20 14 30 10 40 8 50 7, 5 60 7 70 6, 7 80 6, 65 Rate !!! Fill the table and attached to your calculation part.

3. Plot moisture content ( kg H 2 O / kg dry solid) vs time ( min), free moisture content ( kg H 2 O / kg dry solid) vs time ( min), drying rate ( kg H 2 O/h. m 2) vs time, drying rate ( kg H 2 O/h. m 2) vs free moisture content. ( for drying rate calculations to find slopes use linear regression and curve fitting techniques) m. c. R X time

4. On each graph identify the different periods of drying Explain the periods such as; • The initial period : can be detect or not detect why? • Constant rate period: observed in which time interval and in which moisture content interval • Falling rate period: one period or more period? Please indicate and explain

5. Find equilibrium and critical moisture content. 6. Calculate (i) the drying time during the constant rate period and (ii) the drying time during the falling rate period using all possible procedures with available data. 7. Calculate Reynolds Number. NRE = D * v *� �=density of air find from table with µ µ = viscosity of air D = diameter of exit flow area using air temperature !!! Find equilibrium diameter because exit is not circle V = velocity of air

Good Luck Don’t forget write your group names and ‘Tray Drying Report’ in your mail title You will send your tray drying experiment report in to my e-mail adress ‘ebasarici@gantep. edu. tr’