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.