Pulsed electric field (PEF) application on wheat malting process: Effect on hydration kinetics, germination and amylase expression

Wheat malt has been considered a potential ingredient to fully or partially replace barley malt in many food applications due to economic, availability, technological and sensory issues (Silva et al., 2021). Wheat can be more easily found in some regions than others with lower costs than barley (Poreda, Bijak, Zdaniewicz, Jakubowski, & Makarewicz, 2015). When malted, both can present comparable enzymatic profile while conferring different flavor to foods and beverages (Lentz, 2018). All of these properties are only possible when subjecting the raw grains to a malting process.

Malting (or partial sprouting) process is an ancient technique that is commonly applied to cereal grains in order to promote the development of several endogenous enzymes into the seed such as amylases, proteases, glucanases, xylanases, phytases and lipases (Guzmán-Ortiz et al., 2019). In this process, the grains are hydrated to breakdown their dormancy and to allow a better mobility of the enzymes towards the endosperm. As the enzymes start to be developed, the seed reserves are hydrolyzed to produce energy and structure modifications that make the sprout growth possible. When the desired germination degree is attained, a drying process takes place to stabilize the produced enzymes (Briggs, 1998; Carvalho, Monteiro, Laurindo, & Augusto, 2021; Dugulin, De Rouck, & Cook, 2021).

Although the malted grains may present the above-mentioned endogenous enzymes of different categories (Kalb, Seewald, Hofmann, & Granvogl, 2020; Rani & Bhardwaj, 2021), amylases, specifically, play an important role on the starch hydrolysis by releasing dextrins and reducing sugars. The cleavage of amylose and amylopectin chains is performed differently by the actuation of α-amylase and β-amylase. The first acts randomly in the α(1,4)-glycosidic linkages while the latter cleaves the non-reducing end of the amylose and amylopectin to release maltose units. Their technological role in the brewing sector is already well-known but recent applications in baked products have gained more and more attention due to their capacity to improve their volume, color, texture, bread-making and anti-staling properties (Marti, Cardone, Nicolodi, Quaglia, & Pagani, 2017; Olaoye, Ubbor, Okoro, & Lawrence, 2015; Yang, Guo, & Zhao, 2020). In this sense, the malted flour has been reported as a clean label ingredient – in replacement to synthetic enzymes – to improve the functional, nutritional and sensory properties of baked products.

Although several applications of malted cereals are reported in literature, the malting procedure is still considered a time- and energy-consuming process (Yaldagard, Mortazavi, & Tabatabaie, 2008). The hydration step may take long periods consisted of successive wet and resting cycles (Mosher & Trantham, 2021). After reached the targeted moisture content, the grains are still subjected to germination under controlled temperature and relative humidity. The germination process is interrupted when the desired enzymatic profile is obtained, which can take several days depending on the cereal and the application (Guzmán-Ortiz et al., 2019).

Many attempts aimed at intensifying the malting process and even the malt quality by improving the hydration kinetics and germination. Studies have proposed conventional methods focused on hydrating at higher temperatures than usual (Montanuci, Jorge, & Jorge, 2015), periodic operations (Mattioda, Jorge, & Jorge, 2019), different wet/resting cycles (Ezeogu & Okolo, 1995), incorporation of air and oxidizing agents in the aqueous medium (Ma et al., 2020), and so on. On the other hand, emerging techniques have been shown as potential and clean technologies to enhance the water uptake and/or the subsequent germination. It includes the microwave irradiation, ozonation, ultrasound and pulsed electric fields (PEF) application (Guimarães, Polachini, Augusto, & Telis-Romero, 2020; Rifna, Ratish Ramanan, & Mahendran, 2019).

PEF application consists of exposing the grains in aqueous medium to high-voltage short pulses generated by two electrodes. The very short treatment time (μs) is able to modify or disrupt the membrane of the cell by the electroporation theory. This important effect, which can be reversible or permanent, is responsible by increasing the trans-membrane potential as the micropores appears in the membrane – leading to enhanced mass transfer processes as the own water uptake process (Kumari, Tiwari, Hossain, Brunton, & Rai, 2018). These micropores are, in turn, formed when the electrical charges accumulated across the membrane exerts enough compression to overcome the elastic resistance of the membrane until causing its rupture (Soliva-Fortuny, Balasa, Knorr, & Martín-Belloso, 2009). However, the magnitude of the effects is dependent on the raw material as well as on the process conditions, mainly electrical strength and exposure time (Barba et al., 2015).

The waveform of the applied electric field can also induce different effects on biological tissues. Exponential decay and rectangular pulses, either in direct or alternating current, are the most widely used waveforms in PEF applications (Ngadi, Bazhal, & Raghavan, 2003). The generation of rectangular pulses is demonstrated to be more complex than of exponentials, but they are more effective to induce changes in vegetal tissues. Moreover, square pulses require wider pulses at high rise and drop in the electrical field (Barbosa-Canovas, Pierson, Zhang, & Schaffner, 2000). When the pulses are bipolar, the modulator needs a more complex and cost-effective construction – which is not clearly associated to superior performance compared to monopolar pulses (Arshad et al., 2020). Thus, as seen the singularity of the process design, it is important to study the effect of the different processing conditions on each raw material.

Some works have studied the effects of PEF in the amylase expression and germination of grains (Ahmed et al., 2020; L. Zhang et al., 2019) but, up to our knowledge, none has been focused on enhancing the hydration kinetics together with the germination parameters and amylolytic activity – especially when dealing with wheat. Therefore, the present paper aimed at investigating the effect of different PEF treatments on the hydration and rehydration (after the resting cyle) kinetics. After that, germination power and radicle length were determined over the germination period required for amylases development. The amylolytic activity (α and β-amylase) of the resulting malt was, then, evaluated as final quality parameter.

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