Page 44 - FoodFocusThailand No.203 February 2023

SMART PRODUCTION

causes three main mechanisms, including cavitation, microflow, thermal and decreasing enzyme activity. For
mechanical effect, which works shown below: example, using the ultrasonic at a
frequency of 37 kHz and a power of
Cavitation effect The cavitation effect is ultrasonic waves’ rapid formation 29 watts can inhibit the metabolism
1 and explosion of many tiny bubbles. The resulting air bubbles can reduce of microorganisms and extends the
the temperature of the food below the freezing point, also known as shelf life of pineapples during cold
subcooling temperatures, to accelerate the formation of ice crystals and contribute storage.
to the primary nucleation. At the same time, a cavitation bubble also destroys large
ice crystal formations into smaller ones, which is an important factor in becoming a Thawing
new ice core and forming new ice crystals called secondary nucleation. Both Solid frozen foods require a rapid
primary and secondary nucleation of ice crystals can increase the freezing rate thawing process because enzyme
and provide small ice crystal evenly distribution resulting in lower damage in the and microbial activity are gradually
microstructure of the food. For example, using ultrasonic for sea bass frozen at a restored during thawing. The
frequency of 45 kHz and power of 320 watts under the secondary refrigerant traditional thawing methods include
temperature of -20 C can reduce the diameter of ice crystals by 32% compared with air thawing and water thawing. Air
o
traditional freezing. thawing takes a long time, which is
easy to cause lipid oxidation of food.
Microflow The microflow effect is closely related to the cavitation effect. Like water thawing, it causes the loss
2 The resulting bubbles are of different sizes, including collapse immediately with of soluble nutrients. As a result, the
a short time (temporary bubbles) and bubbles formed for a time period and then quality of the food decreases.
bursts (stable bubbles). In the steady state, the internal air is continuously exchanged Therefore, ultrasonic technology is
with the loaded cooling medium under ultrasonic. This exchange can generate commonly used to reduce thawing
eddy currents around the bubbles, resulting in a microfluidic effect. While generated time, lipid oxidation, maintain
bubble in transient state bursts and generates energy when its volume increases nutrition and improve food texture
and reaches the critical size, resulting in turbulence flow and improving heat because ultrasonic is based on
transfer efficiency. cavitation, vibration, and thermal
effect. Energy is generated when the
Thermal and mechanical effect The vibration energy generated when ultrasonic volume of air bubbles increases to
3 propagates to the secondary refrigerant causes adsorption and is converted into a critical size and then bursts. At the
thermal energy. Meanwhile, mechanical impact from ultrasonic waves also enhances same time, the vibration in the
the transfer of energy. So, it can be concluded that ultrasonic gives a faster freezing medium also drives power
rate than traditional freezing techniques (air freezing and immersion freezing). However, generation. So, it improves food
local high temperatures resulting from cavitation and thermal effects are important thawing efficiency. Moreover, the
criteria to consider for selecting the appropriate ultrasonic level and frequency according conversion of ultrasonic energy to
to different frozen products. heat energy accelerates the change
of ice crystal to water status,
Cooling increasing heat transfer and the rate
Ultrasonic technology is pollution free and can be applied to preserve solid food at of food thawing.
4 C. The energy is released from ultrasonic through the cavitation, and thermal The synergistic mechanisms of
o
effect can cause the structure change of enzymes, rupture of cell membranes, and ultrasonic technology, such as
inactivation of microbial growth. Moreover, the crumble of cavitation bubbles increases cavitation, microflow, thermal
the temperature and pressure, promoting free water to generate free radicals, thus generation, and vibration are useful
for frozen food, cooling, and thawing
processes, which shorten operation
time, increase heat transfer efficiency
and maintain the quality of the food
products.

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