This document discusses superheated steam drying (SSD) of foods. SSD uses steam instead of hot air for drying, allowing for higher drying rates and better product quality. It describes the basic principles of SSD, including that steam has better heat transfer properties than air. SSD can reduce energy consumption compared to hot air drying if the latent heat of exhaust steam is recovered. The document presents a case study on SSD of cashew kernels, finding that higher steam temperatures and velocities reduced drying time and improved product quality with low final moisture contents and color changes.

1. SUPERHEATED STEAM DRYING OF
FOODS
By: Rohit Varma
18AG63R19
AGRICULTURAL AND FOOD ENGINEERING DEPARTMENT
INDIAN INSTITUTE OF TECHNOLOGY
KHARAGPUR

2. CONTENT
Introduction
Basic principles of SSD
Set up for SSD
SSD and Energy
Advantages and Disadvantages
Types of SSD
Case study
Material
Experiment
Drying characterization
Result and discussion
Conclusion
References

3. SUPERHEATED STEAM DRYING (SSD)
• Proposed over 100 years ago; received serious attention only
during the past 20 years
• Uses steam in place of hot air or combustion/flue gases
in a direct dryer
• More complex than hot-air drying system
• Lower net energy consumption
• Better product quality
• Ability to inactivate microorganisms

4. SUPERHEATED STEAM DRYING (SSD)
• If steam pressure is kept constant and more energy is added,
its temperature increases and saturated steam becomes
superheated steam (SHS)
• If extra heat can be transferred to an available heat sink, SHS
returns to saturated conditions
• Low or high pressure operation possible

5. BASIC PRINCIPLES OF SSD
• Drying rate in CRP depends only on heat transfer rate
• If sensible heat effects, heat losses and other modes of heat
transfer are neglected, CRP drying rate is :
N 
q

h(Tmedium  Tsurface )
 

6. BASIC PRINCIPLES OF SSD
• SHS has superior thermal properties to air at the same
temperature: h is higher!
• In hot-air drying ΔT is higher at low drying temperatures;
reverse is true at higher drying temperatures
• The counter-acting effects lead to phenomenon of inversion;
beyond inversion temperature SSD is faster than hot-air
drying

7. CRP drying rate
Temp.Inversion temp.
SSD
An inversion phenomenon
hot-air drying
N 
q

h(Tmedium  Tsurface )
 
Air drying: Tsurface =Twet-bulb
SSD: Tsurface =Tsaturation

8. BASIC PRINCIPLES OF SSD
• FRP drying rate of SSD is higher than that of hot air drying
• FRP drying rate of SSD is higher than air drying rate since
product temperature is higher. Casehardening is unlikely to
form and product is likely to be more porous as well

9. Closed
steam drying
system
Fan/blower
Direct use of
steam
Energy recovery via
heat exchanger
Removal of
condensate
Heater
purged steam
steam from boiler
Recycled steam
Typical SSD set-up

10. SUPERHEATED STEAM DRYER
Saturated Steam Feed
Assume 100°C, 1 bar; H = 2,675 kJ/kg
Steam
Superheater
Superheated Steam
Assume 140°C, 1 bar; H = 2,756 kJ/kg
Drying chamber
Saturated Steam Exhaust
Back to 100°C, 1 bar; H = 2,675 kJ/kg

11. SSD & ENERGY
• If exhausted steam can be used elsewhere or can be recycled,
latent heat is not charged
• Net energy consumption is 1000-1500 kJ/kg water removed
• Reduced net energy consumption is a clear advantage of SSD

12. Change in Energy Use
Conventional
AnnualEnergyInput(GJ)
0
80,000
SSD
Thermal
Electricity

13. ADVANTAGES OF SSD
• Energy saving: All latent heat of exhaust steam can be
reused by thermo-compression
• Dryer exhaust is steam so it is possible to recover all
latent heat supplied to SSD
• No oxidative reactions possible due to lack of O2; color and
some nutrients are better preserved
• Higher drying rates possible in both CRP and FRP depending
on steam temperature (above the so-called inversion
temperature in SSD is faster than air drying)

14. ADVANTAGES OF SSD
• Toxic or organic liquids can be recovered easily
• SSD yields higher product porosity due to evolution of steam
within the product - bulk density is thus lower while
rehydration behavior is better
• Sterilization, deodorization or other heat treatments (e.g.
blanching, boiling, cooking) can be performed simultaneously
with drying

15. DISADVANTAGES OF SSD
• SSD system is more complex
• Initial condensation is inevitable
• Products that may melt, undergo glass transition or be
damaged at saturation temperature of steam cannot be dried
in SSD
• Products that may require oxidation reactions to develop
desired quality parameters cannot be dried in superheated
steam dryer
• Cost of the ancillaries (e.g., feeding systems,product-collection
systems, exhaust steam recovery systems, etc.) is much more

16. Fluid bed
POSSIBLE TYPES OF SSDs
•Flash dryers
•Fluidized bed dryer
•Spray dryers
•Impinging jet dryers
•Conveyor dryers
•Rotary dryers

17. CASE STUDY
• Thermal processing of cashew kernels was determined by
removal of moisture, and therefore, an indication of the
product quality.
• The main objectives was determining the drying
characteristics and the effect of superheated steam on product
quality.
• The range of drying steam temperature at atmospheric
pressure was 120- 140ºC with velocity varied from 1-3 m/s.
• The total colour difference was used to examine the change in
colour of dried kernels.

18. MATERIAL
• Shelled cashew kernel with testa layer.
• The average moisture content of cashew kernel was
determined and found to be in range between 10 to 11% dry-
basis.
• The weight of 100 numbers of cashew kernel with testa layer
was 200 ± 5 g.
• The cashew kernels were stored for about one week for
equilibration of moisture and then 50 g of cashew kernels (~25
kernels) was used for each experimental run.

19. SCHEMATIC DIAGRAM OF SSD

20. EXPERIMENT
• The experiments were conducted for a preliminary study of
drying cashew kernels with testa layer at the steam
temperature of 120, 130, and 140ºC for different steam velocity
of 1, 2, and 3 m/s.
• The samples were drawn after 30 min and cooled in a
desiccator.
• After that, the samples were weighed and dried in a vacuum
oven at 103°C for 72 h.
• The moisture content of cashew kernel was expressed in terms
of percentage of water per kg of dry solids. Changes in colour
of the peeled kernels were evaluated visually and
instrumentally.

21. DRYING CHARACTERIZATION
• Moisture ratio:
Ratio of weight of water in the product to the weight of
dried sample
MR = Mt - Meq
Mi - Meq
Where,
MR : moisture ratio of the sample
Mt : average moisture content at time t, % dry-basis
Mi : initial moisture content at time t=0, % dry-basis
Meq : equilibrium moisture content at time t=∞, % dry-basis

22. • Colour measurement:
The colour value of the dried samples after removing the
testa was used to describe the quality of product in terms of
Total colour difference (ΔE).
Where,
ΔE indicates the degree of overall colour change of a sample
L0 = initial brightness
a00 = initial redness
b0 = initial yellowness
Lf, af, and bf values of final brightness, redness and
yellowness of the sample

23. RESULTS AND DISCUSSION
(1) Variation of moisture ratio for steam velocity V = 2m/s

24. (2) Variation of moisture ratio for steam temp. T = 140ºC

25. COLOUR CHARACTERIZATION

26. • Colour deterioration of fruits during thermal processing is due
to pigment degradation and browning reaction.
• Many fruits are rich in sugar, so that possible browning
reaction
• Total colour difference (ΔE) found to be ranged between 4.5 to
24
• ΔE increases as the steam temp. and velocity increases
COLOUR CHARACTERIZATION

27. Moisture content:
• The moisture content of cashew kernel was found to
be 0.06±0.02% after superheated steam drying.

28. CONCLUSION
• Superheated steam dryer was more effective in shortening
drying time and improving product quality.
• SSD was found to be more energy saving as compared to
other conventional drying method.
• Short drying time and low final moisture content can be
obtained when high temperature and/or high velocity of
steam were applied to the cashew kernels.

29. REFERENCES
• Deventer,H.C., & Heijmans,R.M.H. (2001). DRYING WITH
SUPERHEATED STEAM. Drying Technology, 19(8): 2033-
2045
• Eanga,R., & Tippayawonga N. (2017). Superheated Steam
Drying of Cashew Kernels with Testa. Energy Procedia, 138:
674–679.
• Karimi,F. (2010) Applications of superheated steam for the
drying of food products. INTERNATIONAL Agrophysics, 24:
195-204.

30. Thank You