The document discusses drying as a food preservation method that involves removing water through evaporation or sublimation. It describes the drying process as involving heat and mass transfer to remove moisture from food. There are typically three periods in a drying rate curve: a constant rate period where the surface remains saturated, a falling rate period where the surface starts drying out, and a critical moisture content point where the drying rate begins to decrease. Key parameters that influence drying include moisture diffusion, water activity, and moisture content expressed on wet or dry bases.

1. DRYING
• Unit operation involving removal of water from a food
product by evaporation/sublimation
• Involves reduction of water activity
• To be considered dried, the final product must be solid
• Widely used method of food preservation
• Energy-intensive & time consuming process

2. DRYING PROCESS
• Takes place by heat & mass transfer
• When food is subjected to thermal drying, processes
taking place are:
Transfer of heat to the food & transfer of moisture(mass)
from the food
(HEAT MASS TRANSFER)

3. HEAT & MASS TRANSFER
• Occurs at molecular level
• Heat transfer attributed to temperature
gradient b/w hot air & product surface &
limited by thermal conductivity
• The gradient causing moisture diffusion is
vapor pressure of water in air, compared to
vapor pressure of air at product surface and
depends on molecular diffusion of water
vapor in air

4. Typical drying rate curve
time (hrs)
A
B
C
D
time (hrs)
Moisture content
(kg /kg dry solids)
D
Drying rate, dW/dt
(kg/h)
A
B C
tc
tc
1a 1b

5. AB = Settling down period where the solid surface
conditions come into equilibrium with the drying air.
BC = Constant rate period which the surface of the
solid remains saturated with liquid because the
movement of water vapour to the surface equals the
evaporation rate. Thus the drying rate depends on the
rate of heat transfer to the drying surface and
temperature remains constant.
C = Critical moisture content where the drying rate
starts falling and surface temperature rises.
CD = Falling rate period which surface is drying out
and the drying rate falls. This is influenced by the
movement of moisture within the solid and take time.

6. PARAMETERS IMPORTANT TO DRYING
MECHANISM

7. MOISTURE DIFFUSION
• Drying occurs by the virtue of vapor pressure gradient b/w
air & product surface
• The rate of moisture diffusion can be estimated by the
expressions for molecular diffusion
• Expression for Fick’s first law of diffusion:
J= diffusion flux
D= diffusion coefficient
= concentration
x= distance (for vapor movement in the product
structure)

8. WATER ACTIVITY
• Amount of water available for biological
reactions. E.g. microbes
• One of the important parameters in drying is
the equilibrium condition that establishes a
limit to the process
• It represents an important portion of the
moisture movement process
• By definition, water activity is ERH/100

9. MOISTURE CONTENT ON DRY & WET
BASIS
• Moisture content on wet basis is
the amount of moisture present
per 100 g of product
• Wet basis moisture is used to
describe the water content of
agricultural materials and food
products
• When the term "moisture
content" is used in the food
industry it almost always refers to
wet basis moisture content.
• Moisture content on dry basis is
the amount of moisture present
per 100 g of dry solids in the bone
dry product
• In drying calculations, M.C is
commonly expressed on dry basis
• This can range from 0 to very
large percentages(amount of
water may be greater than dry
solids)

10. RELATION B/W MOISTURE CONTENT
ON WET & DRY BASIS
• This relationship is useful to calculate Mw when Md is known, similarly, if
Mw is known, then Md may be calculated from the following equation:
• Eg: Convert 85% M.C on wet basis to M.C on dry basis
(567%)

11. NUMERICAL
A wet food product contains 70% water. After drying, it is
found that 80% of original water has been removed.
Determine
(a) mass of water removed per kilogram of wet food and
(b) composition of dried food
• Select basis: 1 kg wet food product
• Mass of water in inlet stream: 0.7 kg
• Water removed in drying: (0.7) (0.8) = 0.56 kg/kg wet
food
• Now, water in dried food : 0.7(1)-0.56= 0.14 kg
• Similarly for solids, 0.3(1)= solids in exit stream
= 0.3 kg