Ultrasound assisted freezing

ULTRASOUND ASSISTED FREEZING(UAF)

BYSIVA SHANKAR.V

2015 804 106

• INTRODUCTION

• MECHANISM

• EQUIPMENT

• CASE STUDY

• CONCLUSION

• REFERENCES

Contents

FREEZING• Freezing is the unit operation in which the temperature of a food is

reduced below its freezing point and a proportion of the water undergoes a change in liquid state to form ice crystals.

• Freezing consists of three stages: 1. Precooling or chilling stage.

2. Phase transition stage.

3. Tempering stage.

NUCLEATION

• Nucleation, the initial process that occurs in the formation of a crystal from a solution, in which a small number of ions, atoms, or molecules become arranged in a pattern characteristic of a crystalline solid.

• In the freezing of tissue foods, formation of large ice crystals, which are mostly extracellular, results in significant damages to the tissue.

• Formation of fine crystals that are evenly distributed both inside and outside the cells leads to the quality of the product being better preserved since less damage is caused to the tissue. (Kiani, 2011).

ULTRASOUND• Ultrasound is a form of energy generated by sound waves of frequencies that are too

high to be detected by human ear.

• It is usually used as a non-destructive analytical technique for quality assurance and process control.•Ultrasound has been proved to be useful in promoting the nucleation of ice in water.

• The uses of ultrasound are broadly classified into two groups.• Low energy - Higher than 100 kHz • High energy - Frequencies between 18 and 100 kHz.•Ultrasound wave with frequencies in the range 20 to 100 kHz, and high sound power

or sound intensity, can improve or alter the freezing process via different mechanisms. (Li and sun, 2002).

ACOUSTIC MECHANISMS ON THE FREEZING PROCESS

• Hickling (1965, 1994) studied that the vigorous collapses of cavitation bubbles produced by ultrasound can create local zones of high pressure (5 gpa or more) for very short periods of time (nanoseconds), resulting in high degrees of supercooling.

• This supercooling can act as a driving force for instantaneous nucleation.

• Cavitation can be divided into two categories:

1.Stable

2.Transient cavitations (depending on whether the bubbles break or not)

STABLE AND TRANSIENT CAVITATION

• 1) freezer cavity,

• 2) sample holder,

• 3) samples,

• 4) coolant reservoir,

• 5) thermocouple,

• 6) compressor,

• 7) condenser,

• 8) reservoir for refrigerant,

• 9) valve,

• 10) ultrasound transducer,

• 11) immersion fluid circulation pump,

• 12) control panel,

• 13) digital screen,

• 14) computer,

• 15) digital thermometer.

Major functions of ultrasound in food freezing

• Initiation of ice nucleation.

• Acceleration of the freezing process.

• Control of crystal size distribution in the frozen product.

• Preventing encrustation on cold surfaces.

ULTRASOUND GENERATING EQUIPMENT• Power generator

• Ultrasound transducers

• Ultrasonic bath

• Ultrasonic probe system

POWER GENERATOR

• To convert a standard electrical frequency (typically 5 to 60 Hz) into the high alternating frequency (over 20 kHz) required for ultrasonic transmission.

• The sweep frequency technology developed and managed to solve the frequency output of the generator.

• Each transducer can be operated at its resonant frequency and thus maximum efficiency is achieved.

ULTRASOUND TRANSDUCERS

ULTRASOUND TRANSDUCERS

1. MAGNETOSTRICTIVE TRANSDUCERS

2. PIEZOELECTRIC

Magnetostrictive transducers

Piezoelectric transducer

ULTRASONIC BATH• Process medium with

transducers bonded to its base.• Stainless steel tank with

refrigerant flowing inside the jacket.

ULTRASONIC PROBE

• The power ultrasonic system can also be designed as a probe, in which one or several shaped metal horns are attached to the transducer so that high power intensity can be obtained.

• The horns must be designed to resonate at the same frequency as the transducer that drives them.

• It is important to obtain the correct amplitude of movement of the horn tips, which is dependent on the horn shape and dimensions.

• The intermediate horns are usually called boosters.

Ultrasonic Probe

FACTORS AFFECTING POWERULTRASOUND EFFICIENCY

• PRODUCT FACTORS• Temperature

• Viscosity

• Density

• SOUND FACTORS• Frequency

• Acoustic power level

• Acoustic duration

CASE STUDY 1

• Title : Effect of power ultrasound on freezing rate during immersion freezing of potatoes

•Author : Bing Li, Da-Wen Sun

• Journal : Journal of food engineering (2010).

• Sample : Potato samples

• Sample dimensions : 76 mm length, 17 mm width, 17 mm height.

• Frequency : 25 kHz

To characterize freezing efficiency, freezing rate is defined as the characteristic freezing times to transverse the temperature range from -1 to 7 ◦C in which maximum ice crystals are formed.

The actual ultrasonic power corresponding to the power settings

Power setting Actual power (W)1 02 4.663 7.344 15.855 25.896 66.777 90.008 140.209 170.00

Cryogenic scanning electron microscopic image of frozen potato

CONCLUSION• Power ultrasound showed enhancing effect on freezing rate during immersion

freezing of potato samples.

• Violent agitation produced by ultrasonic cavitation would improve the rate of heat removal from the samples.

• Factors such as ultrasonic power, exposure time and the freezing phase to which ultrasound was applied would influence the enhancing effects.

• 15.85 W ultrasonic power was applied for 2 min in the phase change period during freezing process and the freezing process was improved greatly.

CASE STUDY 2• Title : The effect of power ultrasound-assisted immersion freezing on mushroom

• Author : Md. Nahidul Islama, Min Zhanga, Benu Adhikarib, Cheng Xinfenga, Bao-guo Xua

• Journal : International journal of refrigeration (2014).

• Sample : Agaricus bisporus (Button mushroom)

• Sample dimensions : 17 mm sides of cubic pieces

• Frequency : 20 kHz

• Ultrasound Intensity : AU0 - Control, AU1 - 0.13W/cm2, AU2 - 0.27 W/cm2, AU3 - 0.39 W/cm2

Initial freezing

temp,(oC)

Nucleation temp,

(oC)

Nucleation time, (sec)

Transition phase

time, (sec)

Freezing time, (sec)

AU0 -1.57 ± 0.108 -1.96 ± 0.56 194.91 ± 3.17 155.67 ± 6.06

350.58 ± 2.94

AU1 -1.49 ± 0.105 -2.07 ± 0.50 137.58 ± 2.39 203.15 ± 7.52

340.73 ± 5.17

AU2 -1.61 ± 0.16 -2.04 ± 0.37 147.06 ± 6.62 123.10 ± 3.84

270.17 ± 5.26

AU3 -2.17 ± 0.032 -2.64 ± 0.09 91.41 ± 4.63 109.20 ± 0.83

200.62 ± 3.96

CONCLUSION• Power ultrasound reduced the nucleation time by 53% were observed.

• The shock wave and micro-jet from the localized high pressure during the collapse of the cavitating bubles can accelerate the agitation, which helps in accelerating the nucleation as well as increasing the freezing rate.

• Cavitation phenomenon results into stronger agitation, which is responsible for the formation of smaller ice crystals having better size uniformity.

CONCLUSION• Resulting from these acoustic effects, power ultrasound has proved itself an

effective method in assisting food freezing, and its benefits are wide-ranging.

• It can be utilized to induce nucleation and to control crystal size distribution in frozen products during the solidification of fluid food.

• It is not only increase the freezing rate but also improve the quality of the frozen products.

• Application of power ultrasound can also help prevent incrustation on the freezing surface.

REFERENCES•Chemat, F., Khan, M.K., 2011. Applications of ultrasound in food technology: processing, preservation and extraction. Ultrasonics sonochemistry 18 (4), 813-835.•Chow, R., Blindt, R., Chivers, R., Povey, M., 2003. The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation. Ultrasonics 41 (8), 595-604.•Delgado, A., Zheng, L., Sun, D.W., 2009. Influence of ultrasound on freezing rate of immersion-frozen apples. Food and bioprocess technology 2 (3), 263-270.•Hickling, R., 1965. Nucleation of freezing by cavitation in sub-cooled bismuth and gallium. Nature 207 (4998), 742.•Kiani, H., Sun, D.W., 2011. Water crystallization and its importance to freezing of foods: a review. Trends in food science & technology 22 (8), 407-426.•Kiani, H., Zhang, Z., Delgado, A., Sun, D.W., 2011b. Ultrasound assisted nucleation of some liquid and solid model foods during freezing. Food research international 44 (9), 2915-2921.•Li, B., Sun, D.W., 2010. Effect of power ultrasound on freezing rate during immersion freezing of potatoes. Journal of food engineering 55 (3), 277-282.

•Mason, t.J., Paniwnyk, L., Lorimer, J.P., 1996. The uses of ultrasound in food technology. Ultrasonics sonochemistry 3 (3), S253-S260.•Sun, d.W., Li, B., 2003. Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing. Journal of food engineering 57 (4), 337-345.•Zhang, x., Inada, T., Tezuka, A., 2003. Ultrasonic-induced nucleation of ice in water containing air bubbles. Ultrasonics sonochemistry 10 (2), 71-76.

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