Ultrasound in Food

ULTRASOUND PROCESSING IN FOOD
SANTHOSH. R
M. Tech (FPE)

Introduction
• Ultrasound is defined as sonic waves with frequencies more than 20kHz.
• Ultrasound is developed to minimize processing, maximize quality and
ensuring the safety of the food.
• Power ultrasound has been reported to be sufficient to meet the FDA’s
mandatory 5 log reduction of food born pathogens in fruit juices.(Baumann
et al, 2005)

Types
• High frequency low intensity ultrasound
- low energy [less than 1 W/cm²], high frequency [1-10MHz]
- non destructive imaging, diagnostic purposes
• Low frequency high intensity ultrasound
- high energy [more than 10 W/cm²], low frequency [20-100kHz]
- generates strong shear and mechanical forces
- destructing microbes, protein denaturation, stimulating seed
germination, enhancing crystallization

Principle - Cavitation
• When a sonic wave propagates in a liquid media as a longitudinal wave, it
creates alternating compression and expansion cycles.
• When negative pressure in the liquid, created by expansion cycles, is low
enough to overcome intermolecular forces, small bubbles are formed.
• During subsequent expansion/compression cycles, bubbles expand and
contract.

Stable cavitation
• Cavitation that originates from low power ultrasound in small bubbles, the
sizes of which oscillate slightly during thousands of cycles.
• Ultrasound makes bubble vibrate, causing microstreaming that act as
shock waves.

Transient cavitation
• When high power ultrasound hits liquid media, the size of the bubble
oscillates strongly.
• The surface area of the bubble increased during expansion, which also
increases gas diffusion.
• These bubbles, distributed throughout the liquid, grow over a period of few
cycles to a critical size until they become unstable and violently collapse.
• The phenomenon of growth and collapse of microbubbles under ultrasonic
field is known as ‘Acoustic cavitation’

During cavitation
• When cavitation bubbles oscillate and collapse, several physical effects are
generated, namely, shock waves, microjets, turbulence, shear forces, etc.
• The collapse of the acoustic cavitation bubbles is also near adiabatic and
generates temperatures (5500°C) and pressure peaks (10⁵ kPa) within the
bubbles for a short period of time. Under this extreme temperature conditions,
highly reactive radicals are generated.
• These radicals have been used to achieve chemical reactions that include the
synthesis of nanomaterials, polymers, degradation of organic pollutants, etc.

Equipment
• Consists of
Electrical power generator
Transducer
Emitter
• Others systems include liquid whistle [without electrical generator] and
airborne systems [which do not require an emitter].

• Generator provides the required electrical energy to transducer.
• Transducer as a central element, that converts electrical energy into
sound energy by vibrating mechanically at ultrasonic frequencies. Three
types – liquid driven, magnetostrictive and Piezoelectric transducer. Later
is the most common and effective type. Ex - pzt [lead zirconate titanate]
• Emitter/reactor/ultrasonic cell radiates ultrasonic wave from transducer to
medium.
• Two main form of emitters are bath and horns [i.e. probes]

Probe type
• The probe-type sonoreactors feature a high sound intensity (W/cm²) at the
probe surface and a high acoustic power density (APD) (W/cm³) in the
reactor.
• The probe is often in direct contact with the food. Processes that require a
high energy input, such as cell rupturing, extraction, enzyme inactivation,
etc., are often performed with a probe or a radial horn.
• Titanium and silica are used, due of their mechanical strength.

Bath/tank type
• Tank-type ultrasound treatment devices have a lower sound intensity and
APD, due to the larger volume of the liquid in the chamber, as well as the
large surface area that emits the ultrasound.
• Here, Power is often low in order to avoid the cavitation damage to the
tank walls.
• Ultrasonic baths often find application in surface cleaning,
sonocrystallization, freezing and other applications that need a relatively
low APD.

Calorimetry method
• The ultrasound power level or energy that is delivered to food is expressed
as ultrasound power(W), ultrasound intensity(W/𝑐𝑚2
), acoustic energy
density(W/𝑐𝑚3 or W/mL).
𝑃 = 𝑚𝑐 𝑝
𝑑𝑇
𝑑𝑡
𝑈𝐼 =
4𝑃
𝜋𝐷2, D – diameter of probe(cm)
𝑨𝑬𝑫 =
𝑷
𝒗
, v – volume of medium(𝑐𝑚3)

Product variables
• Volume
• Temperature
• Viscosity
• pH
• Soluble solids
• Gas concentration
Process variables – frequency, amplitude cycle, exposure time & Acoustic
energy density.

Effect on microorganisms
• Appropriate ultrasound can promote the growth of microbial cells.
• Low intensity ultrasound provides steady cavitation and causes repairable
damages. It changes the living state of microbial cells leading to accelerate of
their proliferation and more products of metabolism.
• High intensity ultrasound can not provide accelerating proliferation effect due to
its unrepairable damages.
• Resistance to ultrasound in the order of Gram negative bacteria<Gram positive
bacteria<Yeast<spores , depend upon their complex structures.

Inactivation mechanisms
• Bacteria cell wall damage, due to mechanical effects induced by pressure
gradients generated during the collapse of cavitation bubbles within or
near the bacteria.
• Shear forces induced by micro-streaming which occurs in the bacterial cell
itself.
• Chemical attack due to formation of free radicals during cavitation which
attack the cell wall structure leading to disintegration. In addition there will
be the formation of a small amount of hydrogen peroxide via sonication,
which itself is a bactericide.

• Sonication has been less effective to gram positive bacteria such as
Staphylococci aureus and Enterococci due to their tough cell wall
structure.
• Ultrasound is relatively less effective against spores due to their resistance
to damage.
• Yeast cells may be resistant to physical effects of ultrasound because they
are relatively rigid structure and they are not disrupted by microstreaming.

Effect on enzymes
• Acoustic cavitation induced by ultrasound waves, both physically and
chemically affect enzymes.
• Denaturation of protein is highly responsible for enzyme inactivation either
by free radicals in sonolysis of water molecules or shear forces resulting
from the formation or collapse of cavitation bubbles.
• Enzymes such as catalase, yeast invertase & pepsin are resistant to
ultrasound.
• Enzyme inactivation mechanism is complex and depends upon several
factors such as fruit juice composition, pH, enzyme type and processing
parameters.

• Enzyme inactivation increases with increase in solid content and
decreases with increase in enzyme concentration.
• Decrease in inactivation is observed at high fat food molecules.
• Generally, higher enzyme inactivation is reported for probe type systems
than ultrasound baths.

Fruit juice enzymes
• Pectinmethylesterase (PME) – hydrolyses pectin results in reduced
viscosity. (D-value reduced from 45 min to 0.85 min in tomato juice. Lopez,
1998)
• Polyphenoloxidase (PPO) – copper containing enzyme causes enzymatic
browning.
• Peroxidases (POD) – heme containing enzyme causing off-flavors and
browning pigments. Thermally stable.
• Lipoxygenase (LOX) – related to oxidation of fatty acids and pigments.

Dairy enzymes
• Sonication of milk results in a diversity of physicochemical changes in
macromolecules including enzyme inactivation, homogenization, reduction
in fermentation time during yoghurt preparation and improvement of
yoghurt rheological properties.
• Heat resistant lipase and protease were destructed (can withstand UHT).
• Differences observed in the inactivation of the native milk enzymes such
as alkaline phosphatase, γ-glutamyltranspeptidase, lactoperoxidase, whey
proteins (α -lactalbumin and β-lactoglobulin) in whole and skim milk due to
its composition.
• Very less effect of sonication on milk enzymes without high temperature.

Critical Factors
Governing microorganism and/or enzyme inactivation are,
• Nature of ultrasonic waves or amplitude
• Exposure time
• Microorganism or enzyme type
• Volume of food to be processed
• Composition of food
• Temperature

Application
• Application of ultrasound to food processing divided into two categories.
- To replace traditional processing techniques.
- Ex : food cutting, emulsification/homogenization,
pasteurization/sterilization, meat tenderization and degassing, etc.
- To assist the traditional techniques.
- Ex : extraction, freezing, thawing, oxidation, brining, filtration and drying,
etc.

In diary (PU)
-Due to shear forces – viscosity decreases.
-In encaps. Solution containing polymer
(protein) is denatured and absorbed on surface
Of liq. Droplets & radicals cross links proteins

Disadvantages
• The lack of standardization in ultrasound operating frequencies and power
levels makes comparison between different effects were difficult.
• Direct contact between probe and food medium.
• Effect on color & antioxidants has been reported in some sonicated
samples due to degradation of pigments (anthocyanin, ascorbic acid,
lycopene & carotenoid) at high energy levels.
• However, during emulsification and processing of vegetable oils, a metallic
and rancid odor has been detected only for insonated oil and foods.

Ultrasound in Food

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