**5. Sonocrystallization of lactose**

Sonocrystallization of lactose has aroused great interest in the last decade. Consequently, there are a number of studies that provide relevant information on the effect of ultrasound on lactose crystallization, although such effect is not completely understood yet. The current information concerning the sonocrystallization of lactose is condensed in **Tables 1** and **2**.

#### **5.1. Effects on lactose supersaturation and nucleation**

It has already been discussed that sonication favors the formation of supersaturation and modifies the metastable zone [13, 14, 30]. The effect of ultrasound on lactose supersaturation has been scarcely documented chiefly because almost all the studies on lactose sonocrystallization have been done in combination with nonsolvents that greatly modify the solubility of lactose (**Table 1**) [4, 9, 18, 20, 28, 32]. The antisolvents used for these studies include ethanol, propanol, glycerol, and acetone, all of which decrease the solubility of lactose sharply and speed up the attainment of supersaturation [5]. On the other hand, there are a couple of studies that have been conducted in the absence of nonsolvent compounds (**Table 2**) [15, 22, 55]. From these works, it is reported that ultrasound energy densities of up to 0.15 W g−1 (at 20 kHz) narrow the MZW of lactose [22].

On the other hand, the effect of ultrasound on nucleation is not entirely clear, but it is generally accepted that sonication increases the rate of nucleation [15]. The long time of crystallization


rate to the maximum pressure reached inside the cavitation bubble. To correlate such factors,

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].

Sonocrystallization of lactose has aroused great interest in the last decade. Consequently, there are a number of studies that provide relevant information on the effect of ultrasound on lactose crystallization, although such effect is not completely understood yet. The current information concerning the sonocrystallization of lactose is condensed in **Tables 1** and **2**.

It has already been discussed that sonication favors the formation of supersaturation and modifies the metastable zone [13, 14, 30]. The effect of ultrasound on lactose supersaturation has been scarcely documented chiefly because almost all the studies on lactose sonocrystallization have been done in combination with nonsolvents that greatly modify the solubility of lactose (**Table 1**) [4, 9, 18, 20, 28, 32]. The antisolvents used for these studies include ethanol, propanol, glycerol, and acetone, all of which decrease the solubility of lactose sharply and speed up the attainment of supersaturation [5]. On the other hand, there are a couple of studies that have been conducted in the absence of nonsolvent compounds (**Table 2**) [15, 22, 55]. From these works, it is reported that ultrasound energy densities of up to 0.15 W g−1 (at

On the other hand, the effect of ultrasound on nucleation is not entirely clear, but it is generally accepted that sonication increases the rate of nucleation [15]. The long time of crystallization

**5. Sonocrystallization of lactose**

20 kHz) narrow the MZW of lactose [22].

**5.1. Effects on lactose supersaturation and nucleation**

they used numerical simulations on bubble dynamics.

60 Technological Approaches for Novel Applications in Dairy Processing

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


transfers, as well as the aeration, so does the nucleation rate [5, 14]. The stable bubbles may act as nucleation centers, since the rapid growth of bubbles during the acoustic cycles drops the temperature locally and increases the supersaturation nearby the bubbles [5]. Besides, the pressure gradient around the cavitation bubbles induces a controlled diffusion of particles or embryos (segregation effect) that also favor the nucleation process [57].

Sonocrystallization of Lactose from Whey http://dx.doi.org/10.5772/intechopen.74759 63

• during the transient cavitation, the vigorous collapse of bubbles releases shockwaves and creates local zones of high pressure and temperature. These release of energy promotes mass transfer, molecular collisions, and supply the driving force for instantaneous nucleation (*ΔG\**) [5, 13, 30]. Moreover, cavitation bubbles tend to locate themselves near the boundaries of the earlier formed crystals. When these bubbles collapse, the crystals are

Only a few works have addressed the effect of ultrasound on lactose nucleation, like in that reported by Dincer et al. [22]. In such study, it was observed that sonication of lactose solutions (60%) with an ultrasonic power density of 0.46 W g−1 (at 20 kHz) enhances the nucle-

crystals mL−1 min−1). The ultrasound affected primarily the heterogeneous nucleation, and this effect was improved at low levels of lactose supersaturation (1.6–2.1). This study also reported that sonication decreased nearly ten-fold the induction time of nucleation. Induction time has been used to determine the nucleation rate, and this is defined as the time elapsed from the creation of supersaturated solution and the appearance of the first crystals [13]. The conventional crystallization of lactose exhibits a long induction time, which makes the process uneconomical [32]. Therefore, the reduction of induction time by lactose sonocrystal-

In contrast to nucleation, there is no consensus to the effect of ultrasound on crystal growth. Although, it is theorized that ultrasound might promote the growth of crystals through the mass, and heat transfers enhancement [14]. Patel and Murthy [32] reported crystal growth rates between 0.007 and 0.027 μm s−1 for a crystallization process of lactose assisted with ultrasound (120 W) and antisolvents (n-propanol 85%). Meanwhile, Dincer et al. (2014) reported a growth rate of 0.14 μm min−1 for a lactose solution (60%) sonicated at 0.46 W g−1. Nevertheless, these authors did not observe a difference between the growth rates of sonication and stirring. The size of lactose crystals commonly falls between 2 and 50 μm. The desired lactose crystal size varies depending on the specific use. For instance, when lactose is employed as an excipient in dry powder inhalers, its size must range between 2 and 6 μm for an optimum drug delivery to the lung. When lactose is destined to the food industry, the size of crystals is regularly bigger than 20 μm. No matter the intended use, a narrow crystal size distribution (CSD) is always preferable [5, 19]. The principal factors that affect the crystal size of lactose are (a) initial levels of saturation, (b) presence of salts and proteins, (c) rate of nucleation/ crystallization, and (d) extent of secondary nucleation [5, 24, 32, 58]. Similarly, the addition of nonsolvents, as well as the seeding of lactose, decreases the size of lactose crystals and nar-

crystals mL−1 min−1) as compared to simple stirring at 300 rpm (0.16 × 105

disrupted in many small fragments promoting the secondary nucleation [14].

ation rate (5.3 × 105

lization becomes relevant.

rows the CSD [13, 18].

**5.2. Effects on the crystal size distribution (CSD) and yield**

**Table 2.** Reported effects of ultrasound on lactose crystallization (without antisolvents).

is one of the major issues to deal with during the recovering of lactose. Therefore, ultrasound has been used in lactose crystallization chiefly to accelerate nucleation and consequently to reduce the crystallization time [5]. The theoretical effects of ultrasound on nucleation can be summarized as follows:

• during the stable cavitation, the bubbles remain without collapsing for a number of ultrasonic cycles. The movement of these bubbles or flow streams enhances the mass and heat transfers, as well as the aeration, so does the nucleation rate [5, 14]. The stable bubbles may act as nucleation centers, since the rapid growth of bubbles during the acoustic cycles drops the temperature locally and increases the supersaturation nearby the bubbles [5]. Besides, the pressure gradient around the cavitation bubbles induces a controlled diffusion of particles or embryos (segregation effect) that also favor the nucleation process [57].

• during the transient cavitation, the vigorous collapse of bubbles releases shockwaves and creates local zones of high pressure and temperature. These release of energy promotes mass transfer, molecular collisions, and supply the driving force for instantaneous nucleation (*ΔG\**) [5, 13, 30]. Moreover, cavitation bubbles tend to locate themselves near the boundaries of the earlier formed crystals. When these bubbles collapse, the crystals are disrupted in many small fragments promoting the secondary nucleation [14].

Only a few works have addressed the effect of ultrasound on lactose nucleation, like in that reported by Dincer et al. [22]. In such study, it was observed that sonication of lactose solutions (60%) with an ultrasonic power density of 0.46 W g−1 (at 20 kHz) enhances the nucleation rate (5.3 × 105 crystals mL−1 min−1) as compared to simple stirring at 300 rpm (0.16 × 105 crystals mL−1 min−1). The ultrasound affected primarily the heterogeneous nucleation, and this effect was improved at low levels of lactose supersaturation (1.6–2.1). This study also reported that sonication decreased nearly ten-fold the induction time of nucleation. Induction time has been used to determine the nucleation rate, and this is defined as the time elapsed from the creation of supersaturated solution and the appearance of the first crystals [13]. The conventional crystallization of lactose exhibits a long induction time, which makes the process uneconomical [32]. Therefore, the reduction of induction time by lactose sonocrystallization becomes relevant.

#### **5.2. Effects on the crystal size distribution (CSD) and yield**

is one of the major issues to deal with during the recovering of lactose. Therefore, ultrasound has been used in lactose crystallization chiefly to accelerate nucleation and consequently to reduce the crystallization time [5]. The theoretical effects of ultrasound on nucleation can be

**Effect on Experimental setup Ref.**

nucleation.

induction.

stirring, 5.3 × 105

high elongated ratio.

were observed.

(57 ± 17 μm).

× 105

Ultrasound affected the heterogeneous

Sonication showed a very rapid nuclei

Did not change growth rate between ultrasound and stirring (0.14 μm min−1)

Number of crystals mL−1 was 2.8 × 106

Batch produces bigger crystals than continuous treatment. An increase in power produces smaller crystals. Tomahawk crystals

Sonication power from 0.10 to 0.15 W g−1 increased the yield from 17.4 to 25.1% (2.5 h of crystallization; and 19.7 to 28.3% after 4 h).

Narrow distribution of crystal sizes (38 ± 10 μm) than stirred solutions

A faster rate of crystallization were obtained for whey sonicated at a flow rate of 11 L min−1 and applied energy density of 3.3 J mL−1

Yield obtained from 75 to 85% after 24 h. [15]

Particles from 15 to 30 μm. Production of rodshaped lactose crystals with high elongation ratio. Appearance of rod-shaped crystals with

for sonicated and nonsonicated samples.

Yield of 84% with 5 min of sonication [18]

Induction time was faster than stirring but this decrease with an increasing power from 0.15 to 1.15 W g−1. Sonication resulted in a significantly faster nucleation rates than

and 1.6 × 10 <sup>4</sup> mL−1 min−1

More prominent effect of ultrasound at low supersaturation between 1.6 and 2.1

[22]

[18]

[22]

[22]

[18]

[22]

[55]

[56]

[15]

[15]

and 4.6

• during the stable cavitation, the bubbles remain without collapsing for a number of ultrasonic cycles. The movement of these bubbles or flow streams enhances the mass and heat

summarized as follows:

Concentrated cheese whey

Model solution system

Supersaturation 60% lactose,

62 Technological Approaches for Novel Applications in Dairy Processing

Nucleation 30–50% lactose,

Induction time 60% lactose,

Crystal growth

Crystal growth

rate

Size and morphology

rate

Size and morphology 0.46 W g−1

20 kHz, 750 W

0.46 W g−1

60% lactose, 0.46 W g−1

60% lactose, 0.46 W g−1

20 kHz, 750 W

3 to 16 J mL−1, 20 kHz

3 to 16 J mL−1, 20 kHz

20 kHz

**Table 2.** Reported effects of ultrasound on lactose crystallization (without antisolvents).

33% lactose, 20 kHz, 10-70 W, (oscillatory)

Yield 30–50% lactose,

Yield 3 to 16 J mL−1,

33% lactose, 20 kHz, 10-70 W, (oscillatory)

30–50% lactose, 20 kHz, 750 W

> In contrast to nucleation, there is no consensus to the effect of ultrasound on crystal growth. Although, it is theorized that ultrasound might promote the growth of crystals through the mass, and heat transfers enhancement [14]. Patel and Murthy [32] reported crystal growth rates between 0.007 and 0.027 μm s−1 for a crystallization process of lactose assisted with ultrasound (120 W) and antisolvents (n-propanol 85%). Meanwhile, Dincer et al. (2014) reported a growth rate of 0.14 μm min−1 for a lactose solution (60%) sonicated at 0.46 W g−1. Nevertheless, these authors did not observe a difference between the growth rates of sonication and stirring.

> The size of lactose crystals commonly falls between 2 and 50 μm. The desired lactose crystal size varies depending on the specific use. For instance, when lactose is employed as an excipient in dry powder inhalers, its size must range between 2 and 6 μm for an optimum drug delivery to the lung. When lactose is destined to the food industry, the size of crystals is regularly bigger than 20 μm. No matter the intended use, a narrow crystal size distribution (CSD) is always preferable [5, 19]. The principal factors that affect the crystal size of lactose are (a) initial levels of saturation, (b) presence of salts and proteins, (c) rate of nucleation/ crystallization, and (d) extent of secondary nucleation [5, 24, 32, 58]. Similarly, the addition of nonsolvents, as well as the seeding of lactose, decreases the size of lactose crystals and narrows the CSD [13, 18].

The shape and size of lactose crystals are also modified by sonication. The tomahawk shape (characteristic of α-lactose monohydrate) is the most reported in sonocrystallized lactose [5], although elongated shapes are described in some reports [9]. According to Dhumal et al. [18], applying ultrasound causes some faces of the lactose crystals grow faster than other producing elongated rod-shaped crystals. Besides, ultrasound increases the surface roughness of lactose crystals by reducing the incorporation of β-lactose into the crystal lattice [9, 18].

Regarding the crystal size, all the works agree that ultrasound increases the number of lactose crystals and produces smaller crystals with more homogeneous sizes [6, 15, 18, 20, 22, 32, 56]. Only one study has reported that there is no correlation between ultrasound (10–30 W, 20 kHz) and the size of lactose crystals [9]. The effects on the number and size of lactose crystal are primarily attributed to the increase in the number of nuclei that promotes the ultrasound, whether during the primary or secondary nucleation [56]. The extent of size reduction that is attained through sonocrystallization depends upon the ultrasound energy density (or power), time of sonication, and frequency applied (**Tables 1** and **2**). For example, we have observed that ultrasound energy densities of 9 J mL−1 were enough to decrease the size of lactose crystals by half and to significantly narrow the CSD Nevertheless, a higher energy density (50 J mL−1) did not produce a further change in the size of crystals or the CSD (**Figure 3A**). On the other hand, the effect of different ultrasonic frequencies on the size of lactose crystals has been hardly reported, because nearly all the studies have used the similar frequencies (20–22 kHz). So far, only the work of Gajendragadkar and Gogate [6] has explored an ultrasonic frequency of 33 kHz. According to these authors, a frequency increase from 22 to 33 kHz reduced the crystal size and improved the lactose purity but decreased the yield of crystallization.

size of crystal size significantly (9 J mL−1) in an aqueous system without nonsolvents. Besides,

**Figure 4.** Effect of time on lactose recovery in sonicated and nonsonicated samples (reproduced from Bund and Pandit [28]).

Speaking of lactose recovering, there is a consensus that ultrasound improves the yield of lactose crystallization. **Tables 1** and **2** summarize the published data on crystallization yield obtained from lactose sonocrystallization with or without nonsolvents. Just to mention some examples, Bund and Pandit [28] showed that sonicated samples had higher lactose recoveries both in absence and presence of protein compared to the nonsonicated samples at different pH values with an antisolvent crystallization method. They recovered ~88% of lactose with 2 min of sonication as compared to 55–60% in 12 to 72 h with conventional lactose recovery (**Figure 4**). Similarly, the recovery of lactose was reported from the paneer whey with the use of ethanol as an antisolvent. The ultrasonic frequency and power utilized were 22 kHz and 120 W, respectively. Almost 90% of lactose was recovered in just 20 min with ultrasound.

, Sukhvir Kaur Bhangu<sup>2</sup>

1 Facultad de Ciencias Químicas, Universidad Autónoma de Chihuahua, Chihuahua,

[1] Walstra P, Wouters J, Geurts T. Dairy Science and Technology. 2nd Ed. CRC press; 2005,

2 School of Chemistry, University of Melbourne, Melbourne, Australia

, Muthupandian Ashokkumar<sup>2</sup>

Sonocrystallization of Lactose from Whey http://dx.doi.org/10.5772/intechopen.74759 65

and

the CDS was narrowed by the presence of whey proteins (**Figure 3B**).

**Author details**

Mexico

**References**

782 p

Yanira Ivonne Sánchez-García1

\* \*Address all correspondence to: ngutierrez@uach.mx

Néstor Gutiérrez-Méndez1

The process of lactose crystallization is conventionally carried out in presence of residual whey proteins (0.1–0.2%), which also decrease significantly the crystals size. The water-binding capacity of whey proteins creates supersaturation spots that favor nucleation [24, 25]. A few studies have addressed the effect of whey proteins on lactose sonocrystallization. Bund and Pandit [28] reported an increase in the crystal size of lactose sonocrystallized with ethanol (85%) in the presence of 0.4% of bovine serum albumin (BSA). Patel and Murthy [32] described that 0.2 to 0.8% of BSA widened the CSD of lactose sonocrystallized (120 W, 20 kHz) with n-propanol (85%). In contrast, we have noted that 0.64% of whey proteins decreased the

**Figure 3.** Effect of different ultrasound energy densities on the crystal size distribution (CSD) of lactose: (A) solutions saturated with 25% (w/v) of lactose; (B) solutions with 25% (w/v) of lactose and 0.64% (w/v) of whey proteins.

**Figure 4.** Effect of time on lactose recovery in sonicated and nonsonicated samples (reproduced from Bund and Pandit [28]).

size of crystal size significantly (9 J mL−1) in an aqueous system without nonsolvents. Besides, the CDS was narrowed by the presence of whey proteins (**Figure 3B**).

Speaking of lactose recovering, there is a consensus that ultrasound improves the yield of lactose crystallization. **Tables 1** and **2** summarize the published data on crystallization yield obtained from lactose sonocrystallization with or without nonsolvents. Just to mention some examples, Bund and Pandit [28] showed that sonicated samples had higher lactose recoveries both in absence and presence of protein compared to the nonsonicated samples at different pH values with an antisolvent crystallization method. They recovered ~88% of lactose with 2 min of sonication as compared to 55–60% in 12 to 72 h with conventional lactose recovery (**Figure 4**). Similarly, the recovery of lactose was reported from the paneer whey with the use of ethanol as an antisolvent. The ultrasonic frequency and power utilized were 22 kHz and 120 W, respectively. Almost 90% of lactose was recovered in just 20 min with ultrasound.
