Banana - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 138

Cano (1994) and Cano et al. (1990, 1997) have reported
diverse heat treatments to blanching the banana pulp be-
fore frozen. These studies included not only the use of
microwave heating but also the importance of selecting the
banana fruit to be frozen at the optimal level of ripeness.
The ripe state selected was a green-yellow peel color, pro-
ducing frozen banana slices without the appearance of
brown spots.

with a computerized data acquisition system. The unit's
drying chamber was equipped with heating, ventilation,
and a humidifying system (the humidifying system was for
the control humidity levels during drying). The cut fruit
of 1-2 cm cylindrical shape was placed in a stainless steel
mesh tray that was suspended from an electronic balance.
The computer-based data acquisition system recorded the
mass change, temperature, and humidity of surrounding air
as a function of drying time. A system like this can be
successfully used to develop modeling kinetics for drying
banana pieces of different sizes and shapes.
The drying time using hot air can be reduced by us-
ing microwave energy, which is rapidly absorbed by the
product's water molecules, which produces rapid evapora-
tion and thus higher drying rates. The interior temperature
of dried microwave-heated food is higher than the surface
temperature and moisture is transferred to the surface more
dynamically than during convective drying (Ohlsson, 1990;
Torringa et al., 2001). Pereira et al. (2007) studied the effect
of microwave power, air temperature, and velocity during
the final drying of osmotically dehydrated bananas. Their
results showed that increasing microwave power during the
final drying of banana slices increased the drying rate and
decreased drying time. Product quality evaluated in terms
of color, porosity, and apparent volume was not strongly af-
fected by processing parameters such as microwave power,

Drying
Bananas can be dried successfully to increase their shelf
life as a healthy snack food. Because of their physical and
chemical composition and structure (Sankat et al., 1996),
starch-rich fruits, like bananas, dry much more slowly than
other fruits. But prolonged drying times increase shrinkage
and toughness, reduce the bulk density and rehydration
capacity of the dried product, and cause serious damage to
flavor, color, and nutrients (Maskan, 2000).
Air-drying is the most common method employed for
foodstuffs and can offer dehydrated products that can have
an extended shelf life, but the quality of a dried product is
often reduced as compared to that of the original foodstuff,
with an impact in quality in terms of color, rehydration
ratio, texture, and other characteristics (Ratti, 2001).
Nguyen and Price (2007) used a lab-scale dryer (illus-
trated in Fig. 8.3), which consisted of a dehydration unit

Water in

Temperature controller

Electronic balance

Water out

Transmitter box

Heater

Fan

Tr ay
Drying chamber

Temp/RH
probe

Insulated
wall

Control valve

Exhaust air

Computer

Adjustable dampe r

Heating coil

Inlet air

Temp/RH probe

Interface

Variable speed motor

Transmitter box

Water catchment

Drain

Figure 8.3. Schematic of a lab-scale drying system (source: Nguyen and Price, 2007).