Protected application of high intensity ultrasound for food drying enhancement

The purpose of this chapter is to summarize potential applications of the highpower ultrasound technology (5 Wcm2; 20–100 kHz) in the food industry. Those applications are mainly related to the improvement in mass and energy transfer in different processes when ultrasound is applied in water or through air, e.g., reduction in dehydration; thawing and freezing times and energy costs of plant, meat, or fishbased products; increase the extraction yields of intracellular compounds with biological activity; reduction of chemical health risks such as cadmium or acrylamide; etc. The influence of some physical parameters like temperature and pressure in cavitation intensity and the potential of this technology to even inactivate microorganisms in food products and surfaces in contact with food will be discussed. Several examples of these applications will be presented, with reference to some of the industrial or pilot plant systems available in the market to be implemented in the food industry.

Application of High Intensity Ultrasound for Food Drying Enhancement: A review

Abstract: Foods are usually divided into two major categories: natural foods which structure occursnaturally; structured foods which structure forms as a result of processing Drying alters food structure

or even creates new food structures Food drying is an ancient practice and is one of the most popular

preservation methods for foods around the world due to many advantages it offers High intensityultrasound-assisted drying represents as a means for food dehydration without affecting the maincharacteristics and quality of the product Drying in high intensity ultrasound assistance with modern

drying systems has great potential to accelerate drying, improve product quality, reduce investment

and production costs This review paper provides up-to-date researches of application of high intensityultrasound as pre-treatments before drying and during drying process as well as its effects on thequality of different driedproducts

Keywords: Drying, high intensity ultrasound, pretreatment, quality.

INTRODUCTION

In the 2009 World Summit on Food Security, it was recognized that by 2050 food production must

increase by about 60-70%, which is 34% higher than it is today, to feed the anticipated 9 billion people

Therefore, agricultural production must make progress significantly from today’s levels and foodmanufacturing systems must become more efficient, use less energy, generate less waste, and produce

food with extended shelf life Drying is hugely important technique for the food industry and offers feasibility for ingredient development and novel products Dried foods offer multiple benefits including: extended product shelf-life, reduced packaging, storage, handling and transportation costs,

and extends the possibility of out-of-season availability and provides a wider range of products forconsumers

Ultrasound technology as an essential field of research and development in the food industry is derivefrom mechanical waves at a frequency above the threshold of human hearing (>16 kHz), and can becategorized into two ranges: low and high energy Low-energy (low power, low intensity) ultrasoundhas frequencies higher than 100 kHz at intensities below 1 Wcm-2 while high-energy (high power, high-intensity) ultrasound uses intensities higher than 1 Wcm-2 at frequencies between 20 and 500 kHz

Normally, low intensity ultrasounds are used to characterize the properties of food materials and forprocess control Whereas , high intensity ultrasound is used to alter physical and chemicalproperties of

food or to facilitate the progress of a process Other typical applications of high-intensity ultrasonic waves are welding, machining of brittle materials, sorting and particle motion in fluids, surface cleaning, atomization of liquids, medical therapy, sonochemistry, formation and processing of

nanomaterials, etc

The use of ultrasound in drying of food has been carried out in two ways: using ultrasound treatments prior to drying and using ultrasonic drying Application of ultrasonic waves is associated

pre-with a cavitation phenomenon as well as alternative cycles of compressions and rarefactions of the

material The forces involved by the sponge effect caused by ultrasonic waves can create microscopic channels that may ease moisture removal These microscopic channels are utilized by water molecules

as a special pathway to diffuse toward the surface of materials, increasing its effective water diffusivity Water removal is influenced by both internal and external resistances The drying process has been

intensified highly when applying ultrasound pre-treatment, followed by high intensity assisted (HIU) drying

ultrasound-The present work centers on reviewing current scientific literatures on application of high intensityultrasound (HIUS) to enhance food drying and the effects of HIUS support on physicochemicalproperties of dried food Thus, this way could function as an always ready element to meetsimultaneously the needs of academic, food processing industry and consumers at large

APPLICATIONS OF HIGH INTENSITY ULTRASOUND IN FOOD DRYING PROCESSES

There are many high-intensity applications remarkably in food processing such as viscosity alteration, emulsion generation, cell disruption, aggregate dispersion, polymerization, degassing of liquid food, extraction of enzymes and proteins, microorganism inactivation, cutting, improvement of freezing and

defrosting, crystallization, filtration, pasteurization and sterilization, etc

New drying methods such as microwave, infrared, vacuum, freeze, or combined hybrid drying are usedincreasingly more often in the industry The positive effects of these methods are aimed at a reduction

of both the drying time and energy consumption, in particular when using non-conventional orcombined techniques, such as ultrasound, microwave and ultrasound ,, convective-microwave drying and convective–infrared drying, acoustic and solar energy Hybrid drying is a very attractive andpromising solution from the product quality point of view and for process economy There is both atime and energy profit due to the diversity of mechanisms providing the energy needed for moistureremoval, the synergistic effects, the reversion of adversethermo-diffusion effects.

The application of HIUS in food drying is a relatively new and emerging technology in food industry Itsmain purpose is to accelerate the drying process, improve product quality and ensure duration of thefood products Hot air with HIUS support resulting in an acceleration of heat and mass transfer andreduction of the drying time without a significant increase in product temperature Considering thegreat importance of HIUS in food drying, many researches have applied HIUS as pretreatment (Table 1)and during drying (Table 2) The results obtained is successful and remarkable

Ultrasonic Pretreatment Before Drying Process

To improve dried product quality and accelerate the drying process with minimal cost, pretreatment are processes often employed before drying Pretreatment of the material prior to drying is a well- explored area, and many methods have been developed New pretreatment techniques have drawn the attention of researchers in improving their efficiency in food drying technology The principle of ultrasound application in liquid systems is based on the effects of mechanical waves which are mainly

related to the cavitation phenomenon Cavitation is considered to be responsible for the creation of microscopic channels in the material, which facilitates mass transport Bubbles are produced during cavitation they collapse and form tiny jets directly interacting with the surface of objects in liquid This

micro jet raises the mass and heat transfers between liquid and solid by breaking the respectivediffusion boundary layers Acoustic energy during the interaction with the medium is converted intothermal energy

Propagation Mode

Ultrasound technology has been directly or indirectly used as a pretreatment in many drying and or

dehydration applications which differ intheir effectiveness, performance and capability There are twotypes of devices which are used as direct mode and indirect mode The directly ultrasonic probe which

has a vibrating titanium tip in different diameters and can be immersed in the liquid near the sample The liquid is irradiated with an ultrasonic wave directly from the horn tip so that a high power intensity could be obtained The indirectly ultrasonic bath, an ultrasonic transducer is attached to the outer surface of the liquid container and the liquid irradiates with an ultrasonic wave from the surface of the liquid container Ultrasonic baths are available commercially in different tank capacities, ultrasonicpowers or frequencies The typical acoustic amplitude in a standing wave type sonochemical reactor is

much smaller than that in a horn-type sonochemical reactor Several researches have used ultrasonicbath in the pretreatment of fruits and vegetables such as apple, pineapple, guava, carrot, Malayapple,melon,mushrooms, cherries and strawberry, Brussels sprout Besides, some investigatorstended to ultrasonic probe in pretreatment of apple, carrot, cauliflower, onion , mulberry leaves, sea-cucumber, guava

Kek et al studied differences between direct and indirect application of HIUS to ultrasonic pretreatment using an ultrasonic probe and ultrasonic bath, respectively Indirect sonication in osmotic solutions contributed to high water loss and solid gain with acceptable total color change than direct

sonication Applying ultrasound pre-osmotic treatment in 700Brix prior to hot-air drying reduced thedrying time by 33%, increased the effective diffusivity by 35% Another work conducted directly andindirectly on button mushroom, cauliflower, Brussels sprouts which pH of the surrounding water of

vegetables decreased after the ultrasound treatments with the probe and the bath (for Brussels sprout), the highest decrease occurring with 20 kHz probe for 10 min in all samples because the probe

is more powerful that the 40 kHz bath Staining of cauliflower, button mushroom and Brussels sproutstissue surfaces performed for the damage determination showed that cavitation damage (blue spots)

was present after the ultrasonic treatment with 20 kHz probe for 3 min, followed by 20 kHz probe for

10 min, while very little cavitational damage occurred on cauliflower and button mushroom but no cavitation on Brussels sprouts after sonication with 40 kHz bath for 3 and 10 min Most cases provedthat theultrasonic assistance increased the efficiency of the total osmotic dehydration process Fromreports of Rodríguez et al and Cárcel et al, the higher the ultrasonic power applied, the higherthe solidgain and the water loss Using of different ultrasound amplitudes causes reduction of the drying time

and allows elimination of more water from the apple slices

The selection of liquid medium that is used in the ultrasonicallyassisted pretreatment depends on thedesirable purpose Hence,the pH or solute concentration can induce a great influence withrespect tothe effect of acoustic energy on the food product and interaction between the solid and thesurroundingmedium The opposite mass transport occurs when an osmotic solution is used instead of distilled water These mass transfer processes are important factors in pretreatment because it influences the natural change in the taste in dried product and are used to evaluate performance of drying after pretreatments. The common solutions for osmotic pretreatments of fruits and vegetablesare distilled water and sugar dissolved in distilled water The self-juice of the fruit has been recentlyused in a few studies as an osmotic solution with the purpose of evaluating the effect of HIUapplication when the liquid medium and the food product areof the same nature.

If the liquid medium is distilled, water-soluble solids migrate from the fruit to the medium and water moves into the fruit as a result of the mass transfer exchange promoted by the solute gradientbetween the food product and the surrounding media It is worth declaring that the increase in theinitial moisture content of treated samples is indicative of the water intake during the treatment,although this water might have a free character inside the fruit tissue, leading to a weak interactionwith the solutes in the sample and easier removal during the drying process Fernandes et al

conducted ultrasonic pre-treatment with distilled water for banana, the amount of sugars lost during the process was 21.3% of reducing sugars of the fruit after 30 min while water gain was 11.1% Moreover, the overall drying time was reduced by 11%, which represents an economy of energy since

air-drying is energy cost intensive Thus, the ultrasonic pre-treatment can be an interesting process to

produce dried fruits with low sugar content However, the same ultrasound intensity in pre-treatment pointed out varying effects on different materials which some fruits gain water during exposure to

ultrasound, whereas others show loss of water

Concentration has been carried out using sugar solutions ranging from 25 to 700Brix The workexamined the influence of ultrasonic pre-treatment prior to air drying of Malayapple Distilledwater asthe liquid medium, caused loss of soluble solidsfrom the fruit to the liquid medium 33.4% after 60 min.The results concluded that the increase in effective water diffusivity was estimated in 28.1% (5.84×10-

10m2s-1) after 30 min of ultrasound for the process carried out using an osmotic solution of 25°Brixleading to faster air drying of the fruit The exception occurred with the pre-treatment using an

osmotic solution of 50°Bx, which reduced the water effective diffusivity (De) to 3.98×10-10m2s-1 in 10 minand 3.74×10-10m2s-1 in 60 min The reason could be due to the saturation of the surface of the fruit withsucrose creating an extraresistance for mass transfer Similar results was reported by Garcia-Noguera

et al where neither higher osmotic concentrations nor extended periods in ultrasonic applications had resulted higher effective diffusivities in pre-treated strawberries Kek et al stated the effective diffusivity of ultrasound pre-osmotic treated dried guava increased 18% for the 35°Brix and 35% for the 70°Brix when compared with the De of hot-air dried guava When comparing with the osmo-

dehydrated dried guava, the De of ultrasound pre-osmotic treated dried guava was 12% higher for the

35 ◦Brix and there was no significant difference for the 70°Brix

Table 1 Application of high intensity ultrasound as pretreatment

Raw material Acoustically assisted pretreatment Drying Refe

renc e

Ultrasonic device

conditions

Method/

Device

Drying conditio n

85°C

Apple

(Malus

domestica L

var Royal Gala)

10 min

assisted convective drier

ultrasound-45-60°C 1-3 m/s

2.1, 12.9 W /cm2 25°C;

25-45 kHz

65 W 30°C

30 min

hot air drying oven

60°C 0.3 m/s

0.05% sodium nitrite

hot air dryer 40–60

50°C 0.3 m/s

30–60 min

Convective drying oven

40– 60°C;

Convective tray dryer

46°C; 4.9 m/s

120 min

chamber dryer

70°C 1.1 m/ s

air-drying oven

60°C

1 m /s

Cauliflower Bath/ Immersing Distilled water 0.5–43 W /cm 2 hot air dryer 60°C

0–2.5 kW

25 kHz 30°C 20–60 min

convection oven

70°C 0.06 m/ s Guava

400 W

20 kHz 6–20 min

convective oven

70°C 0.06 m/ s

20, 35,80 kHz

25°C

10 min

microwave vacuum dryer

Low temperature oven

Convective air-drying oven

60°C 2.5 m/s

Hot air drier 50°C

hot-air drier

freeze-drier

60°C 0.3 m /s

8 h 0.04mb

20 min

convective dryer

microwave-20-40°

C 0.7 m /s

air-drying oven

60°C 0.5 m/ s

Distilled water Sucrose 35°Bx

25KW/m 3

40kHz 25°C 30min

air blast drying oven.

Microwave vacuum dryer.

65°C 1.5 m/s 22W/g, 90kPa

Ultrasonic Treatment during the Drying Process

Currently, different systems have been developed to efficiently transmit the acoustic energy from theemitter for the acoustically assisted convective drying There are two main ways for ultrasound devices assisting food drying processes: the ultrasound transducers are directly contacted into the samples during drying (direct contact systems); the ultrasound transducers work without direct contact between the vibrating element and the samples (contactless systems) Some specific ultrasonic

transducers undertake higher intensity capacities have partially solved problems observed in theapplication of contact systems One of the most popular contactless system is the aluminum vibratingcylinder driven by a piezoelectric composite transducer (21.8 kHz) capable of converting the electricenergy in vibration movement generating a high-intensity ultrasonic field inside the cylinder (75 W,154.3 dB)with a relative low energy consumption The HIU convective drying process permits the use

of lower temperatures and may be useful for drying heat-sensitive materials such as fruits and

Gallego-Juárez and co-workers carried out two experimental procedures by airborne ultrasound and ultrasonic vibration in direct contact with the vegetable Forced-air dehydration assisted by airborne ultrasound for acoustic intensity applied 155 dB and 163dB Comparing results between these

intensities showed that the effect of increasing the acoustic pressure in about 8 dB allows the

temperature to be diminished by about 10°C with little increase in the airflow velocity. Ultrasonic

dehydration by direct coupling of the vibration, at two different powers applied by contact ultrasound,

25 W and 50 W Influence of the ultrasonic power applied is clearly observed, the moisture is rapidly

released by 50 W treatment De increased significantly from 1.33×10-9 m2/s by applying a power of 45

W and was even greater when the applied power reached 90 W (1.72×10-9 m2/s) on HIU drying system

(154.3 dB) for orange peel In order to examine the influence of ultrasound on the drying process of olive leaves, air drying experiments were carried out (40°C and 1 m/s) without and with ultrasound

application (8, 16, 25 and 33 kW/m3) Samples dried without ultrasound spent 7.4 h to achieve a

moisture content of 0.12 kg water / kg dry matter (kg W/kg dm), while samples dried with application

of ultrasound (33 kW/m3) only 3.6 h as well as the mass transfer coefficient was increased Based onresults the authors stated that the power of the applied ultrasound had a meaningful influence on the

kinetics of the process, the higher the applied ultrasonic intensity, the greater De, the faster the drying

Regarding the effects of the use of HIUS on De in the drying of apples, the increase of De was of 1.6– 1.8

times at 18.5 kW/m3 and 2.4–2.2 times at 30.8 kW/m3 for temperatures between 30°C and 50°C, and

smaller when the drying temperature was of 70°C (1.3 times at 18.5 kW/m3 and 1.4 times at 30.8 kW/

m3) Similarly, an experiment carried out at 40, 50, 60, 70°C with ultrasound application (21,7 kHz, 30.8kW/m3) reduced the drying time by 60%, 49%, 38%, 28% respectively on drying passion fruit peel.Other studies that HIU application increased the drying rate, however, as the drying temperature

increases, the influence of the US application on the drying rate decreases, ,

An alternative means to hot air drying for retaining sensory, nutritional and functional properties of

foods is low-temperature air drying (LTD) or atmospheric freeze drying (AFD) This technique could

combine the advantages of both freeze drying (high product quality) and convective drying (low cost

and continuous process) However, reducing the air temperature to below the product’s freezing point

entails low drying rates, which largely places restriction on industrial application In this case, processintensification can be also addressed by coupling new technologies such as microwave, radio-

frequency, and infrared radiation, should be carefully controlled due to the risk of overheating and product thawing, with the subsequent reduction in product quality Therefore, to avoid this risk

without a costly strict control of the process, the HIU drying meets this requirement This system hasbeen applied onthe drying of fruits and vegetables, cereal and protein matrices such as apple, carrot,

eggplant, peas, fish During the AFD of apple, Santacatalina et al observed that the effect of ultrasound

on the drying time was more dominated than other factors, such as air temperature or velocity The ultrasonic performance variable found in the literature for LTD applications could be linked to the

efficiency of the transducer used, and so to the different ultrasonic intensity applied The higher theultrasonic power level, the shorter the drying time From another report of Santacatalina and co-workers , the maximum shortening of the drying time achieved was 80.3% (at -10°C and 75 W) The

faster drying in ultrasonic-assisted AFD is assumed because of a higher mass transfer rate at the gas interface, caused by a reduced boundary layer due to a higher turbulent interface

solid-The effectiveness of ultrasound greatly depends on the process and product variables, such astemperature, air velocity, acoustic intensity applied, and product porosity Influence of the acousticenergy was affected by porosity and the role of external resistance The effective moisture diffusivity

was valid over the whole range tested (4–37 kW/m3) for lemon peel, whereas it was necessary to

exceed a minimum acoustic intensity threshold (8–12 kW/m3) for carrot To clarify to theunderstanding of how the properties of the material being dried affect air-borne ultrasonic application.Some researchers examined application of HIUS at 31 kW/m3 during the drying of eggplant, potato,carrot and lemon peel, cassava and apple and the drying time reduction was 67, 39, 34, 49, 32, 56%,respectively

Therefore, from results reported that soft and open-porous product structures exhibited a better

transmission whereas materials with a hard and closed-compact structure were less affected by

acoustic energy due to the fact that the significant impedance differences between the product andthe air cause high energy losses on the interface Another study showed that both low porosityproducts of carrot and persimmon, power ultrasound only improved mass transfer process at low airvelocities, since acoustic energy decreases as air velocity gets higher due to the disruption of the

acoustic field by air flow In a highly porous product, like lemon peel, the influence of ultrasound

intensity was observed in the whole air velocity range, even at high air velocity levels when the

acoustic energy was low HIUS has a greater influence at low air velocities (1 m/ s), being negligible at the highest velocity tested (3 m/ s) on drying thyme leaves Herein, the influence of ultrasound power

density applied on the internal resistance to the mass transfer was significantly lower than its influence

of the external resistance Therefore, process intensification is mainly linked to external resistance

Table 2 Application of high intensity ultrasound during drying Raw material Drying

CL

Lemon peel (Fino) CV 154.3 dB

75 W 21.8 kHz

1.2 ±0.2 m/s 15.0 ± 3.0%

1 m/s 40°C

1 m/s 40°C

CL

30.8 kW/m 21.7 kHz

40, 50, 60, 70°C

20 kHz

3.1, 3.2, 3.4 m/s -6, -3, 0, 10, 20°C

100 W and 200 W

55°C 0.4 m/s

AFD- Atmospheric Freeze Drying, LTD- Low Temperature Drying, CV- Convective Drying, FD-Freeze Drying, VD- Vacuum

Drying, IFVD-infrared-vacuum , CL-contactless, CO-contact

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