(518i) Parametric Study on Factors Affecting Bubble Dynamics during the Immersion Frying Process

Frying
can be described as a conjugate boiling problem, consisting of simultaneous heat and mass transfer between the oil and
food.¹ Frying involves submersing a food material in oil heated
to temperatures above the boiling point of the water within the food material.^1,2
Heat is convected from the hot oil to the crust, and conducted from crust to
the core, resulting in water evaporation.¹ Thus, the food material
acts as a vapor generating matrix¹, where water is lost by
evaporation in the form of bubbles formed in hot oil at the food’s surface.²

Boiling
is a phase change process in which vapor bubbles are formed either on a heated
surface or in a superheated liquid layer adjacent to the heated surface.^3,4
Heat transfer during boiling is due to natural and forced convection caused by
discrete bubbles that are formed and released randomly from active sites on the
heated surface.^3,5 Factors such as surface roughness, fluid
viscosity, surface tension, density of fluid, surface wettability, and thermal
properties of the solid affect bubble dynamics during boling.⁶ Bubble
growth and detachment is a balance between stabilizing forces (viscous, surface
tension, inertia) and destabilizing forces (buoyancy, pressure).⁷ Frying
is similar to boiling, and the mechanisms and factors which affect nucleate
boiling can be used to study heat transfer during frying.^1,6

Mass
transfer during frying is due to the movement of water vapor from food to the
oil, and movement of oil into the food material.⁸ Oil adheres to the
surface of pores created by moisture escape from the food in the form of steam
during frying.⁸ Thus, the amount of moisture lost during frying, can
be related to the amount of oil uptake.⁹ Experiments on dynamics of
the gas–liquid interface inside a single orifice on a transparent perforated
plate have shown bubble formation time and waiting time to be governed by the
meniscus motion inside the orifice.¹⁰ Thus, bubble dynamics can be
hypothesized to govern the mass transfer rate during frying.

Change
in process variables including oil temperature, and solid and liquid physicochemical
properties can affect bubble dynamics during frying. The effect of these
parameters on bubble diameter, frequency and number of nucleation sites can
facilitate deeper understanding of overall heat and mass transfer rates during
frying. Hence, the aim of the present study was to experimentally understand
the impact of oil quality, temperature, pore diameter and surface wettability
on bubble dynamics and develop hypotheses to qualitative describe the impact on
heat and mass transfer rates.

An
assembly (Fig. 1) was built to simulate a single pore immersion frying process.
Pressurized nitrogen gas was supplied through an in-line regulator to a flow
meter connected to an orifice submerged in oil. Flow rate was maintained at 5
ml/min, based on calculated flow rate of steam found during frying of potato
slices. Variables studied were orifice size (100µm-1mm), oil temperature (room
and 170°C), and oil quality (fresh- Total polar material (TPM) content 4%,
used- TPM 15). A high-speed video camera (Photron FastCam) at 2000 fps and 4x zoom, was used to record the
bubble behavior, in the formation and ascendance processes. A purpose-built
MATLAB code was written to determine relevant bubble characteristics; bubble
volume, coordinates of bubble interface, and instantaneous contact angle. Image
processing steps and sequence was similar to the
process described by Di Bari and Robinson.¹¹ Volume was estimated using
MATLAB combined with SolidWorks. Dynamic angle was calculated by reading the
video and processing it single frame at a time in MATLAB. Bubble frequency was
calculated from the plot of dynamic angle vs time based on the formation and
lag time between subsequent bubbles on the plot.

Bubble
volume decreased with increase in temperature, and decrease in orifice diameter,
similar to previous findings.^6,12 Decrease in orifice
diameter led to increase in bubble frequency, and shifted the bubble pattern
from single bubbling to a pairing regime. Oil quality did not have an impact on
the bubble formation time, and thus bubble volume. This is supported by the
hypothesis that the primary factor affecting bubble growth is the surface
tension between liquid-air. Oil quality does not impact oil-air surface tension,
since the surfactants formed in used oil do not have affinity to air.¹³ Bubbles formed with
used oil had a shorter lag time compared to ones with fresh oil. When the
bubble pinches off, liquid slides on the solid surface to rewet the surface
before a new bubble can form. Hence, solid-liquid contact angle controls lag
time between subsequent bubbles. The decrease in lag time between bubbles with
degraded frying oil, could be a result of faster rewetting of the surface, due to
increased solid-liquid wettability in used oil.¹⁴ Thus, degradation of
oil during frying increased bubble frequency, and may be the cause of enhanced heat
transfer rate with used oil.¹ Based on the meniscus formation theory
by Ruzicka et
al.¹⁰, increased bubble
frequency with used oil could also be due to change in the meniscus depth
between subsequent bubble formations, and may affect the amount of oil absorbed
during frying.

References

(1)
     Farkas, B. E.; Hubbard, L. J.
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(2)
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(11)
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(13)
   Sahasrabudhe, S. N.;
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(14)
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