1. Introduction
The hydrolysis of oils and fats to produce free fatty acids has noteworthy industrial importance as it is required in the pharmaceutical, energy, food, chemical, and cosmetic industries, including coatings, adhesives, biofuels, surfactants, shampoos, and other personal care products (Avelar et al., 2013; Noor et al., 2003). The usual industrial process for the hydrolysis to take place, the Colgate-Emery process, entails high temperature (at least 250°C) and pressure (4.82 MPa) (Barnebey and Brown, 1948), implying in a high energy cost in addition to undesirable reactions and by-products formation (Rooney and Weatherley, 2001). Oil hydrolysis can also be performed by employing chemical catalysts (Da Silva et al., 2016; Devaraj et al., 2018; Ong et al., 2016), enzymatic catalysts (lipases) (Cavalcanti-Oliveira et al., 2010; Corradini et al., 2019; de Sousa et al., 2010; El-Hefnawy and Sakran, 2014; Goñi et al., 2018; Santos et al., 2013), using subcritical water (Almeida et al., 2017; dos Santos et al., 2019; Ilham and Saka, 2010; Xiao et al., 2017) or other less conventional methods as microwave irradiation (Nguyen et al., 2020) and the use of ionic liquids (Han et al., 2019).
The enzymatic method presents the advantage of being performed under mild conditions of pressure and temperature, which allows the reduction of costs associates with energy and equipment; nonetheless, waste treatments are also reduced (Posorske, 1984). A common drawback from the use of enzymes, however, is the high price of isolation and purification of microbial and animal enzymes, which restrict their industrial application on a large scale (Du et al., 2008). In this sense, the use of vegetable lipases can be an attractive alternative considering that it can be obtained from renewable sources, produced on a large scale, and does not require a high level of purification (Campillo-Alvarado and Tovar-Miranda, 2013; Villeneuve, 2003).
During the germination of oilseeds, hydrolysis of acylglycerols takes place, and lipases are activated to catalyze the process (Abigor et al., 2002; Hills and Murphy, 1988; Muto and Beevers, 1974). Nevertheless, it has been reported that in castor bean seeds, lipases were activated not only during germination, as most of the known seeds, but also in dormant seeds (Eastmond, 2004; Fuchs et al., 1996; Muto and Beevers, 1974). The castor seed oil is widely used for pharmaceutical and cosmetic industries, the castor seed cake, however, is poisonous to humans and animals, since the toxic protein ricin is present (Martínez et al., 2018; Meneguelli de Souza et al., 2018). The use of castor seeds, only in the oil-free form, as lipase source has been reported in the literature before [1,10,11,13,31–34], and reaction times range between 2 to 10 hours. It is not from the author’s knowledge of any study involving ultrasound to the enzymatic hydrolysis reaction using vegetal seeds as a catalyst.
In the oil hydrolysis reaction catalyzed by lipases, the reaction takes place on the aqueous-organic interface (Al-Zuhair et al., 2003; Ferreira-Dias and da Fonseca, 1995); however, the viscosity and immiscibility of the substrates are high, and the phenomenon of mass transfer controls the reaction rate (Romero et al., 2007; Talukder et al., 2006). Therefore, increasing the interface by generating an emulsion can improve the rate (Feiten et al., 2014; Lerin et al., 2014; Talukder et al., 2006) and facilitate the use of biocatalysts on the industrial scale (De Freitas et al., 2019). Ultrasound creates emulsion by generating the phenomenon of cavitation, which consists of the growth and collapse of bubbles and, consequently, the generation of shockwaves (Ho et al., 2016). Reduced and dispersed liquid droplets can be formed through this system (Leong et al., 2009; Talukder et al., 2006). Compared to typical mixing methods, enzymatic reactions under ultrasonic treatment achieve higher equilibrium values as well as higher selectivity and reduction (Chiplunkar et al., 2018; Lerin et al., 2014; Polachini et al., 2019). However, due to thermal and mechanical (high shear stress and pressure, causing dissociation of enzyme) effects (Froment et al., 1998; Vercet et al., 2001), some studies reported a loss of enzymatic activity during the process [48,49]. Therefore, its impact on the activity of enzymes is intricate (Barton et al., 1996). The information on the kinetics, along with the assessment of the variables process conditions, can provide practical information about the performance of ultrasound-assisted hydrolysis of oil with vegetal lipase to contribute to the scale-up of the process (Polachini et al., 2019).
The non-edible oilseed Crambe Abyssinian (Brassicaceae family) is a low-cost raw material with high oil content and a high percentage of monounsaturated fatty acids (mostly erucic acid, C22:1, 50–60% on average) which grants high stability to the oil (Lazzeri et al., 1994; Wazilewski et al., 2013), representing an attractive option of sustainable raw material. Furthermore, the advantages of using a low-cost raw material, castor bean seeds, as a lipase source for hydrolysis reactions are clear. However, studies are still necessary for the process to become industrial. Understanding of the reaction mechanism and optimization of reaction conditions are essential tools for scaling-up. In this work, we studied the optimation of conditions for the enzymatic hydrolysis reaction performed under ultrasound treatment, using in natura and oil-free castor seeds. Under this treatment, the reaction rate is fast as well as the lipase deactivation; therefore, the modeling of the kinetic experiments can be relevant for optimized use of this enzymatic catalyst under harsh conditions as it is the use of ultrasound power.