**4. General principles of ultrasound and sonocrystallization**

Sonochemistry and sonoprocessing have a wide range of applications in food technology, medicine, nanotechnology, chemical synthesis, materials extraction, polymerization, phase separation, surface and water cleaning, catalysis, enhancing the enzyme activity, and so on. Sonication of a liquid generates acoustic cavitation depending upon the experimental conditions used. Strong physical effects and highly reactive radicals are generated during acoustic cavitation [33]. An overview of the general principles of ultrasound is discussed in this section.

#### **4.1. Ultrasound**

Ultrasound refers to sound waves of a frequency that cannot be detected by human ear. The ultrasonic frequency ranges from 20 kHz to >10 MHz within which they are further divided into low frequency (20–100 kHz), intermediate frequency (100 kHz–1 MHz), and high frequency (1–10 MHz) regions. The interaction of ultrasound gas bubbles in liquids can lead to the generation of chemical reactions and physical forces. The driving force behind such forces is acoustic cavitation [34].

#### **4.2. Acoustic cavitation**

Acoustic cavitation is the phenomenon of formation, growth, and violent collapse of microbubbles in a liquid medium under the influence of acoustic field (**Figure 2**). Bubbles which are inherently present as small nuclei will grow to a critical size under the applied ultrasonic energy. The growth of the acoustic bubbles is due to the phenomenon called "rectified

**Figure 2.** Schematic representation of acoustic cavitation.

and purity. For this lactose refining, the crystals are re-dissolved, treated with charcoal to remove impurities (salts and proteins), and recrystallized as previously described [4, 31].

The process of lactose crystallization is very slow (up to 72 h), the quality of lactose is usually poor, and the yields of crystallization are very low. One of the oldest methods used to improve the process of crystallization is the seeding of lactose. This approach consists in the addition of small lactose crystals into whey concentrate (seeding of nuclei) just before the second cooling step. The addition of lactose crystals may induce a secondary nucleation that accelerates the crystallization process and reduces the CSD [5]. However, this method has low reproducibility because its success depends on the addition of crystals in the appropriate timing [13]. More recently, alternative methods such as the use of antisolvent or sonocrystallization have been explored to assist the crystallization of lactose. The addition of nonsolvent compounds into whey concentrate (antisolvent crystallization) decreases the solubility of lactose, narrows the metastable zone, and reduces the induction times of nucleation. In general, the antisolvent crystallization improves the yield of crystallization and reduces the size of lactose crystals [20, 32]. The main drawbacks of antisolvent crystallization are the large amounts of solvent used, and the expensive separation and purification steps required to remove the antisolvent from the product [5, 9]. The crystallization of lactose assisted with low-frequency

power ultrasound (sonocrystallization) is discussed later in the chapter.

56 Technological Approaches for Novel Applications in Dairy Processing

**4. General principles of ultrasound and sonocrystallization**

**4.1. Ultrasound**

is acoustic cavitation [34].

**4.2. Acoustic cavitation**

Sonochemistry and sonoprocessing have a wide range of applications in food technology, medicine, nanotechnology, chemical synthesis, materials extraction, polymerization, phase separation, surface and water cleaning, catalysis, enhancing the enzyme activity, and so on. Sonication of a liquid generates acoustic cavitation depending upon the experimental conditions used. Strong physical effects and highly reactive radicals are generated during acoustic cavitation [33]. An overview of the general principles of ultrasound is discussed in this section.

Ultrasound refers to sound waves of a frequency that cannot be detected by human ear. The ultrasonic frequency ranges from 20 kHz to >10 MHz within which they are further divided into low frequency (20–100 kHz), intermediate frequency (100 kHz–1 MHz), and high frequency (1–10 MHz) regions. The interaction of ultrasound gas bubbles in liquids can lead to the generation of chemical reactions and physical forces. The driving force behind such forces

Acoustic cavitation is the phenomenon of formation, growth, and violent collapse of microbubbles in a liquid medium under the influence of acoustic field (**Figure 2**). Bubbles which are inherently present as small nuclei will grow to a critical size under the applied ultrasonic energy. The growth of the acoustic bubbles is due to the phenomenon called "rectified diffusion" which is defined as slow growth of the acoustic bubble as a function of time due to unequal mass transfer across the air/water interface [35]. There are two types of cavitation bubbles that exist depending upon the ultrasonic intensity, i.e., transient cavitation bubbles and stable cavitation bubbles. When the ultrasonic intensity is very high, transient cavitation bubbles last for a few acoustic cycles. On the other hand, stable cavitation bubbles can oscillate for many acoustic cycles. The size of stable cavitation bubbles grows over time due to coalescence and also by rectified diffusion until the size is reached, where the coupling of bubble's resonance frequency and driving frequency of the ultrasound occurs. In a multibubble cavitation field, bubbles with a range of size are generated and grown toward the critical size. Miinaert's equation (Eq. 1) provides a relationship between linear resonance radius (critical size) of the bubble with frequency [36].

$$R\_{ss} = \frac{3}{f} \tag{1}$$

where Rres is the linear resonance radius (m) and f is the ultrasonic frequency (Hz).

Transient cavitation bubbles dominate at the lower frequency where they can grow rapidly above a threshold size during the rarefaction cycle. The nature of cavitation bubble is controlled by numerous parameters, such as acoustic pressure, frequency, type of reactor, and bubble size.

#### **4.3. Chemical and physical effects of ultrasound**

The collapse of cavitation bubble leads to the generation of a very high temperature of >5000 K and pressure (>1000 atm) within the bubble (**Figure 2**) [34, 36, 39]. The collapse of the bubble takes place in a very short period of time, and thermodynamically, the work done leads to a near adiabatic heating of the bubble contents, which lead to extreme conditions [36–38]. The maximum temperature and pressure generated within the cavitation bubble can be theoretically calculated using Eqs. (2) and (3), respectively, to a near adiabatic heating of the bubble contents which lead to extreme conditions [36–38].

$$\mathbf{T}\_{\text{max}} = \,^\*T\_0 \left\{ \frac{P\_n(\gamma - 1)}{P\_v} \right\} \tag{2}$$

where T0 is the temperature of the solution, Pm is the pressure inside the liquid, γ is the ratio of specific heat of gas-vapor mixture, and P<sup>v</sup> is the pressure of the bubble when it has maximum size.

$$P\_{\text{max}} = P\_v \left\{ \frac{P\_v(\gamma - 1)}{P\_v} \right\}^{\frac{\gamma}{(\gamma - 1)}} \tag{3}$$

physical as well as chemical properties that can affect the bioavailability, solubility, stability, and other characteristics of the drug. Reduction in particle size can significantly enhance the bioavailability and solubility in most of the pharmaceutical drugs. Therefore, production of smaller size particles with uniform size distribution and desired properties is very important in the development of pharmaceutical drugs. Applying ultrasound during crystallization also results in a number of other benefits, such as nucleation at lower level of supersaturation, narrowing of metastable zone width, highly repeatable and predictable crystallization, reduction

Sonocrystallization of Lactose from Whey http://dx.doi.org/10.5772/intechopen.74759 59

Ultrasound has also shown the tendency to significantly influence the agglomeration of the particles. There are different physical effects generated by ultrasound that can contribute to reduce the agglomeration. This includes shock waves generated due to acoustic cavitation, which can decrease the time of contact between particles that can hinder the interaction of particles together. Also, sometimes agglomeration occurs at the stage of nucleation. Nuclei usually have high surface area to volume ratio, which can lead to high surface tension and nuclei tend to lower the surface tension by interacting to one another. Then, the surface tension tends to drop during the crystal growth when the particles become more stable, which can prohibit agglomeration [48]. Lastly, the uniform mixing of the sonication mixture due to physical forces of the ultrasound can help to reduce agglomeration by locally controlling the nucleus population [48].

Hunt and Jackson [49] demonstrated that nucleation occurs during the collapse of a cavitation bubble rather than when it expands. They have demonstrated this by slowing down the formation and collapse of cavitation bubble in pure liquid sealed in U-tube during isolated cavitation events. The pressure variations and very high pressures generated when the cavity collapses tend to lower the crystallization temperature of the liquid, which results in nucleation [50]. Another reason may be the rapid cooling that occurs after the bubble collapse around the collapsing bubble, thus creating a region of high supersaturation. According to another report [36], nucleation occurs due to the negative pressures generated during the collapse of cavitation bubbles. A possible nucleation mechanism, proposed during ice crystallization, is that the concentration and agglomeration of ice clusters can occur near the bubble surface because of the diffusion of species from low- to high-pressure zones. Louisnard et al. [52] have also suggested that high-pressure gradients are required, for pressure diffusion to be effective and that can only be attained during the collapse of the bubbles, and thus, stable cavitation can also act as a potential nucleation initiator. It has been provided different physical mechanisms that can possibly influence crystallization process with sonication. It was suggested that high pressures due to cavitation, agitation intensive mixing of the liquid medium by ultrasound, supercooling at the bubble surface, and enhanced heterogeneous nucleation are the possible factors responsible for the observed benefits of the sonocrystallization [52]. Author [53] has suggested that high pressure generated is strong enough to initiate the nucleation, as it increases the melting point of the liquid; therefore, cavitation bubble is important to initiate the crystallization. The enhancement of heterogeneous nucleation occurred as the ultrasound can lead to production of different nuclei from the single seed [53]. On the other hand, Virone et al. [54] determined the physical mechanism of ultrasound-induced crystallization based on the bubble dynamics for the first time. The authors correlated the nucleation

in the induction time, improved morphology, and polymorphs selectivity [45–47].

The extreme conditions generated on bubble collapse results in: (1) light emission—sonoluminescence, (2) radical generation, and (3) shock waves, microjetting, microstreaming, shear forces, and microturbulence [40]. The violent collapse of cavitation bubbles can sometimes lead to the emission of light called sonoluminescence [41]. The intensity of light emitted and the sonochemical yield depends on different factors, such as amount and type of dissolved gases in a liquid, ultrasonic frequency and power, hydrostatic pressure, and addition of some solutes. When ultrasound is passed through a liquid medium, the formation of standing waves takes place. Mostly, active bubble formation occurs at the pressure antinodes; therefore, the number of bubbles increases with an increase in the antinodes formed in a liquid. The increase in ultrasonic frequency leads to (i) an increase in number of antinodes and active cavitation bubbles and (ii) a decrease in size and collapse intensity of cavitation bubbles [40].

In air-saturated water, different radicals and molecular products such as H<sup>2</sup> O2 , HO<sup>2</sup> , H, and OH radicals are generated. These radical are formed through the following reactions: (i) *H2 O* ➝ *H˙ + OH˙*, (ii) *OH˙ + OH˙* ➝ *H2 O2* , (iii) *H˙ + O<sup>2</sup>* ➝ *HO2 ˙*. Primary radicals can be used to initiate a number of chemical reactions such as polymerization, synthesis, and degradation. The detailed information on the fundamentals and applications of the ultrasound could be found in literature [42–44]. The physical and chemical effects of ultrasound can be utilized and applied in a number of fields such as material synthesis, water treatment, and crystallization. In the following section, various reported investigations dealing with crystallization processes under ultrasound and possible mechanisms involved are summarized.

#### **4.4. Sonocrystallization (ultrasound-assisted crystallization)**

Ultrasound is found to influence crystallization process, which is referred to as sonocrystallization. Ultrasound has been used from a long time to initiate nucleation and control growth during cooling and antisolvent crystallization process, but the mechanism behind sonocrystallization is still debatable and unclear. It is generally accepted that the physical effects of acoustic cavitation are responsible for the effects observed during sonocrystallization [13]. Sonocrystallization is known to have a number of specific features. For most materials, such features include enhancing the primary nucleation, due to uniform mixing throughout the liquid medium; relatively easier nucleation in some systems which are otherwise hard to nucleate under conventional procedures; ultrasound also has the tendency to initiate secondary nucleation and formation crystals with uniform and small size with high purity. Supersaturation is the driving force behind crystallization process, which is accompanied by nucleation and growth of the crystals, and ultrasound can affect both processes. Active pharmaceutical ingredients are found in a variety of crystalline solid forms, which include polymorphs, hydrates, salts, co-crystals, and amorphous solids. Such different solid forms exhibit different and unique physical as well as chemical properties that can affect the bioavailability, solubility, stability, and other characteristics of the drug. Reduction in particle size can significantly enhance the bioavailability and solubility in most of the pharmaceutical drugs. Therefore, production of smaller size particles with uniform size distribution and desired properties is very important in the development of pharmaceutical drugs. Applying ultrasound during crystallization also results in a number of other benefits, such as nucleation at lower level of supersaturation, narrowing of metastable zone width, highly repeatable and predictable crystallization, reduction in the induction time, improved morphology, and polymorphs selectivity [45–47].

where T0

specific heat of gas-vapor mixture, and P<sup>v</sup>

➝ *H˙ + OH˙*, (ii) *OH˙ + OH˙* ➝ *H2*

*Pmax* = *Pv* {

58 Technological Approaches for Novel Applications in Dairy Processing

is the temperature of the solution, Pm is the pressure inside the liquid, γ is the ratio of

*γ* \_\_\_\_\_ (*γ*−1)

*Pm*(*<sup>γ</sup>* <sup>−</sup> <sup>1</sup>) \_\_\_\_\_\_\_ *Pv* }

The extreme conditions generated on bubble collapse results in: (1) light emission—sonoluminescence, (2) radical generation, and (3) shock waves, microjetting, microstreaming, shear forces, and microturbulence [40]. The violent collapse of cavitation bubbles can sometimes lead to the emission of light called sonoluminescence [41]. The intensity of light emitted and the sonochemical yield depends on different factors, such as amount and type of dissolved gases in a liquid, ultrasonic frequency and power, hydrostatic pressure, and addition of some solutes. When ultrasound is passed through a liquid medium, the formation of standing waves takes place. Mostly, active bubble formation occurs at the pressure antinodes; therefore, the number of bubbles increases with an increase in the antinodes formed in a liquid. The increase in ultrasonic frequency leads to (i) an increase in number of antinodes and active cavitation bubbles and (ii) a decrease in size and collapse intensity of cavitation bubbles [40].

In air-saturated water, different radicals and molecular products such as H<sup>2</sup>

processes under ultrasound and possible mechanisms involved are summarized.

*O2*

**4.4. Sonocrystallization (ultrasound-assisted crystallization)**

OH radicals are generated. These radical are formed through the following reactions: (i) *H2*

, (iii) *H˙ + O<sup>2</sup>* ➝ *HO2*

initiate a number of chemical reactions such as polymerization, synthesis, and degradation. The detailed information on the fundamentals and applications of the ultrasound could be found in literature [42–44]. The physical and chemical effects of ultrasound can be utilized and applied in a number of fields such as material synthesis, water treatment, and crystallization. In the following section, various reported investigations dealing with crystallization

Ultrasound is found to influence crystallization process, which is referred to as sonocrystallization. Ultrasound has been used from a long time to initiate nucleation and control growth during cooling and antisolvent crystallization process, but the mechanism behind sonocrystallization is still debatable and unclear. It is generally accepted that the physical effects of acoustic cavitation are responsible for the effects observed during sonocrystallization [13]. Sonocrystallization is known to have a number of specific features. For most materials, such features include enhancing the primary nucleation, due to uniform mixing throughout the liquid medium; relatively easier nucleation in some systems which are otherwise hard to nucleate under conventional procedures; ultrasound also has the tendency to initiate secondary nucleation and formation crystals with uniform and small size with high purity. Supersaturation is the driving force behind crystallization process, which is accompanied by nucleation and growth of the crystals, and ultrasound can affect both processes. Active pharmaceutical ingredients are found in a variety of crystalline solid forms, which include polymorphs, hydrates, salts, co-crystals, and amorphous solids. Such different solid forms exhibit different and unique

is the pressure of the bubble when it has maximum size.

O2 , HO<sup>2</sup>

*˙*. Primary radicals can be used to

(3)

, H, and

*O*

Ultrasound has also shown the tendency to significantly influence the agglomeration of the particles. There are different physical effects generated by ultrasound that can contribute to reduce the agglomeration. This includes shock waves generated due to acoustic cavitation, which can decrease the time of contact between particles that can hinder the interaction of particles together. Also, sometimes agglomeration occurs at the stage of nucleation. Nuclei usually have high surface area to volume ratio, which can lead to high surface tension and nuclei tend to lower the surface tension by interacting to one another. Then, the surface tension tends to drop during the crystal growth when the particles become more stable, which can prohibit agglomeration [48]. Lastly, the uniform mixing of the sonication mixture due to physical forces of the ultrasound can help to reduce agglomeration by locally controlling the nucleus population [48].

Hunt and Jackson [49] demonstrated that nucleation occurs during the collapse of a cavitation bubble rather than when it expands. They have demonstrated this by slowing down the formation and collapse of cavitation bubble in pure liquid sealed in U-tube during isolated cavitation events. The pressure variations and very high pressures generated when the cavity collapses tend to lower the crystallization temperature of the liquid, which results in nucleation [50]. Another reason may be the rapid cooling that occurs after the bubble collapse around the collapsing bubble, thus creating a region of high supersaturation. According to another report [36], nucleation occurs due to the negative pressures generated during the collapse of cavitation bubbles. A possible nucleation mechanism, proposed during ice crystallization, is that the concentration and agglomeration of ice clusters can occur near the bubble surface because of the diffusion of species from low- to high-pressure zones. Louisnard et al. [52] have also suggested that high-pressure gradients are required, for pressure diffusion to be effective and that can only be attained during the collapse of the bubbles, and thus, stable cavitation can also act as a potential nucleation initiator. It has been provided different physical mechanisms that can possibly influence crystallization process with sonication. It was suggested that high pressures due to cavitation, agitation intensive mixing of the liquid medium by ultrasound, supercooling at the bubble surface, and enhanced heterogeneous nucleation are the possible factors responsible for the observed benefits of the sonocrystallization [52]. Author [53] has suggested that high pressure generated is strong enough to initiate the nucleation, as it increases the melting point of the liquid; therefore, cavitation bubble is important to initiate the crystallization. The enhancement of heterogeneous nucleation occurred as the ultrasound can lead to production of different nuclei from the single seed [53]. On the other hand, Virone et al. [54] determined the physical mechanism of ultrasound-induced crystallization based on the bubble dynamics for the first time. The authors correlated the nucleation rate to the maximum pressure reached inside the cavitation bubble. To correlate such factors, they used numerical simulations on bubble dynamics.

**Effect on Antisolvent Experimental** 

85% n-propanol

85% n-propanol

Model solution system

Concentrated cheese whey

Size and morphology

Crystal growth rate

Size and morphology **setup**

80% Acetone 12–16% lactose, 120 W

Ethanol 20–30% lactose,

85% Ethanol 11.5–17.5%

85% Ethanol 22–33 kHz,

Yield 85% Ethanol 11.5–17.5%lactose,

Purity Ethanol 20–30% lactose,

65–85% Acetone

Yield 85% Ethanol 22–33 kHz,

Purity 85% Ethanol 22–33 kHz,

12–18% lactose, 20 kHz, 120 W

20 kHz, 10–30 W

12–18% lactose, 20 kHz, 120 W

lactose, 22 kHz, 12.3 W

22 kHz, 12.3 W

20 kHz, 10–30 W

5–15% of lactose content, 120 W

40–120 W

40–120 W

40–120 W

**Table 1.** Reported effects of ultrasound in combination with antisolvents on lactose crystallization.

**Key outcomes Ref.**

Sonocrystallization of Lactose from Whey http://dx.doi.org/10.5772/intechopen.74759

Growth rates from 0.007 to 0.027 μm s−1 [32]

[20]

61

[19]

[32]

[28]

[28]

[58]

[6]

[6]

[6]

The crystal diameter decreased from 4 to 2.48 μm. Appearance of rod shaped

There was no correlation between ultrasound and particle size. Appearance of rods, needles and

Mean diameter from 12 to 15 μm. Appearance of elongated and rod/ needle-shaped lactose crystals in sonicated samples and tomahawk shape for stirring samples.

Smaller crystals with more uniform shape than those obtained in the absence of sonication (stirring). Tomahawk and needle-shaped crystals

Lactose recovery after sonication treatment (91.48%) was much higher that nonsonicated samples (14.63%)

Narrow crystal size distribution (2.5–6.5 μm) at pH 6.5, 15% of lactose concentration and 75% acetone concentration. Appearance of needle

An increase in power from 40 to 120 W and frequency from 22 to 33 kHz had a marked reduction in particle size.

The yield without sonication was 74.7% and increased to 97.6% with 40 W of ultrasonic power. In a range between 60 and 120 W the yield decreased from 98.6 to 75.6%. An increase in frequency from 22 to 33 kHz did not change the lactose recovery (94 to 92%)

An increase in dissipation power (40 to 120 W) decreases the lactose purity from 96.9 to 88.7%. An increase in frequency from 22 to 33 kHz increased the purity from 79.5 to 91.5%.

Decreases β-lactose incorporation. [19]

crystals

tomahawks shape.

were observed.

shaped crystals.

Since sonocrystallization has many benefits over the conventional crystallization, numerous studies have been reported on the impact of various ultrasonic parameters on crystallization process for a variety of solutes such as acetylsalicylic acid, sodium acetate, sucrose, glycine, lactose, adipic acid, carbamazepine, NaCl, KCl, benzoic acid, and paracetamol. Various parameters investigated include sonication time, frequency and power, horn diameter, and supersaturation ratio. Besides affecting the MZW, crystal size distribution (CSD), and yield, ultrasound also provides control over polymorph forms of some solutes. It was shown that sonication can influence the primary nucleation and crystal growth of roxithromycin during antisolvent crystallization. With intensive amount of shear generated, ultrasound helped to reduce agglomeration and change the roxithromycin crystal morphology from a hexagonal to rhombus shape [55]. Further study by Hatkar et al. [51] on salicylic acid clearly established that ultrasound can be effectively used to control the antisolvent crystallization process in terms of the mean size of obtained crystals and size distribution. During sonocrystallization experiments, ultrasoundrelated variables like irradiation time and power of ultrasound were found to affect the crystal size distribution, whereas frequency did not have much effect over the range of frequencies investigated. It was found that irradiation time and power of ultrasound decreased the average particle size, as well as a reduction in the agglomeration was observed [36].
