Influence of ultrasound frequency and power on lactose nucleation
Introduction
Lactose is widely used as a sweetener in foods, an excipient in tablets, and a carrier in dry powder inhalers. Typically, this sugar is recovered by crystallization from cheese whey. The whey contains ∼1% of fat and proteins, 5–6% of lactose, and 90–92% of water. Before the process of crystallization, the cheese serum is defatted, deproteinized and evaporated to 40–65% (w/w) of dry matter. At this point, the concentrated whey has traces of proteins (0.1–0.2% w/w), and the rest is mostly lactose. Even though lactose becomes supersaturated in the concentrated whey (>25% w/w), this sugar will not crystallize unless the whey is cooled enough to reach the metastable zone (MZ) limit (Bund and Pandit, 2007, de Castro and Priego-Capote, 2007, Erdemir et al., 2009).
The MZ is a region defined by the solubility and super solubility of the sugar. Inside the MZ, the primary nucleation cannot occur spontaneously since the energy available is insufficient to induce nuclei formation. Depending on the temperature and the concentration of lactose, a number of ordered subcritical clusters of lactose molecules are formed in the solution. These assemblies of molecules have a critical size (n*); below which are unstable, and above which can grow into nuclei for a new phase. The free energy change (ΔG*) required for the formation of clusters and hence nucleation is largely determined by the number of molecules in the nucleus (n*) which in turn depends on the supersaturation level. Therefore, the primary nucleation of lactose will only occur when the critical energy of nucleation (ΔG*) is overcome under certain conditions of temperature and lactose concentration (Das and Langrish, 2012, McLeod et al., 2011, Zamanipoor and Mancera, 2014).
The time elapsed between the formation of supersaturation and the appearance of the new phase is referred to as the induction time. For lactose, the induction times are excessively long and can last 2 h at 130 g 100 g−1 of absolute supersaturation (Cα-Cαs) or 16 h at 22 g 100 g−1 of Cα-Cαs. In consequence, the industrial crystallization of lactose from cheese whey is excessively long (up to 72 h). Besides, the quality of lactose is frequently inadequate, and the yield of crystallization is low. Nowadays, different methods are explored to improve the process of lactose crystallization like the use of anti-solvents, the crystallization assisted by ultrasound (US), and the thermosonication. These methods are centered around reducing the induction time, increasing the rate of nucleation, the rate of crystal growth or the yield of crystallization (Khaire and Gogate, 2018, McLeod et al., 2011, Patel and Murthy, 2011a, Raghavan et al., 2001, Sánchez-García et al., 2018a, Zisu et al., 2014).
The crystallization of lactose assisted by the US has been successfully used to reduce the size distribution of lactose crystals and increase the yield of crystallization (Zisu et al., 2014). It also has been established that the US favors the formation of supersaturation, and narrows the MZ of lactose (Dincer et al., 2014). However, the information about the effect the US has on lactose nucleation is limited. In general, it is well accepted that sonocrystallization reduces the induction time, induces the primary nucleation at lower supersaturation levels, and accelerates the rate of nucleation (de Castro and Priego-Capote, 2007, Gajendragadkar and Gogate, 2017, Zamanipoor and Mancera, 2014). The mechanism of how the US modifies nucleation is still unclear, but some theories have been proposed. For instance, it has been argued that micro gas bubbles that remain stable during sonication (stable cavitation) generate flow streams which enhance the solid-liquid mass transfer promoting nucleation. Additionally, the cavitation bubble surfaces can act as nucleation sites leading to an increase in the nucleation rate. Meanwhile, the collapse of thousands or millions of micro gas bubbles during sonication (transient cavitation) releases shockwaves and energy (Gajendragadkar and Gogate, 2017; Ruecroft et al., 2005; Sánchez-García et al., 2018a). The energy supplied by transient cavitation creates localized changes of temperature. These temperature changes generate microzones of supersaturation which decrease the driving force for nucleation. The shockwaves released during the collapse of cavitation bubbles also provide energy to overcome ΔG*. Other authors have argued that the US can also assist the secondary nucleation by loosening assemblies of molecules that have been already formed or by fragmenting crystals (Gogate and Pandit, 2011, Sánchez-García et al., 2018a, Zhang et al., 2015).
Most of the studies on sonocrystallization of lactose has been conducted at a fixed ultrasound frequency, commonly 20–22 kHz, and in the presence of anti-solvents like acetone, ethanol, or n-propanol. The inclusion of antisolvents makes it difficult to determine the actual effect US has on lactose nucleation since high supersaturation of lactose is generated when these solvents are added (Patel and Murthy, 2009, Sánchez-García et al., 2018a, Zamanipoor and Mancera, 2014). Sonocrystallization studies at frequencies higher than 20–22 kHz are limited. According to Gajendragadkar and Gogate (2016), an increase in the US frequency from 22 to 33 kHz did not change the lactose recovery but decreased the particle size. Silva et al. (2017) reported that sonocrystallization of soybean oil at 40 kHz generated larger crystals compared with those created at 20 kHz. They also observed larger bubbles with low shear and microstreaming when the oil was sonicated at 40 kHz. Kaur et al. (2016) investigated the sonocrystallization of paracetamol at different frequencies (22, 44, 98 and 139 kHz) and found a reduction in the mean particle size by increasing the US frequency. These authors described a close relationship between cavitation activity (measured by sonoluminescence) and the rate of nucleation. In this sense, it has been established that increasing the US frequency raises the number of cavitation bubbles but the intensity of the bubble collapse decreases (Flynn, 1982, Lee et al., 2014). The present study is the first approach to evaluate the effect that different US frequencies and US power have on lactose nucleation when the crystallization is carried out in an aqueous system with traces of proteins.
Section snippets
Preparation of lactose solutions
Anhydrous lactose (Chem Supply; 99.9% purity) was dissolved in deionized water (18.2 MΩ/cm) to obtain 25% (w/v) lactose solutions. Solutions were warmed at 60 ± 2 °C and stirred to favor dissolution of lactose, cooled at 30 °C, and vacuum filtered with 0.45 μm pore-size filters. Lactose solutions were kept at 30 °C in a water bath circulator (Thermoline Scientific, AU) until the sonication step. Whey protein at 0.15,0.3, and 0.45% (w/v) were added to some solutions to study the effect of these
Effect of ultrasound frequency variation on nucleation
The non-sonicated treatment (control) had a coefficient k of 8.7 × 10−4 s−1, which can be considered as a slow process of nucleation (Nanev, 2017). Sonication of lactose solutions increased the k-value depending on the US-frequency used (Table 1, Fig. 3, Fig. 4). The highest k-value was observed at 44 kHz (k = 0.1482 s−1). Ultrasonic frequencies above or below 44 kHz reduced the k-value, but still higher than the control (Fig. 3, Fig. 4 and Table 1). Dincer et al. (2014) reported that the US
Conclusions
Ultrasound-assisted crystallization of lactose is one of the most promising methods to improve the recovery of lactose from cheese whey. However, it is still not clear how the US affects the crystallization of lactose, particularly during the stage of nucleation. From the results of this work, it was evident that the US has a different impact on lactose nucleation depending on the frequency used. In general, the nucleation of lactose was increased at 44 kHz but not at higher (98 and 142 kHz) or
Acknowledgments
The Mexican Nacional Council of Science and Technology (CONACYT) supported this research through the grant conceded to Yanira I. Sánchez-García. The authors also wish to thanks to Sukhvir Kaur Bhangu for her support in this work, and to Aracely Piñón-García for her contribution in the artwork of Fig. 1.
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