Peroxidase (POD) and polyphenol oxidase (PPO) photo-inactivation in a coconut water model solution using ultraviolet (UV)☆
Introduction
Coconut water is a widely consumed beverage in Brazil, and interest in it is growing worldwide not only due to its sensory properties, but also due to its nutritional characteristics.
Its low acidity combined with the balanced sugar content and the isotonic mineral composition, make coconut water a drink with great potential for rehydration (Prades, Dornier, Diop, & Pain, 2012a) and health (Yong, Ge, Ng, & Tan, 2009). However, there is a challenge for developing processes to ensure that the product is available with safety and high nutritional and sensorial quality.
The most important problem related to the stability of coconut water during its shelflife is related to the activity of the polyphenol oxidase (PPO) and peroxidase (POD) enzymes (Prades, Dornier, Diop, & Pain, 2012b). Such enzymes have relatively high thermal resistance and their activity leads to yellow, brown or even pink colouring during storage (Prades et al., 2012b), even under refrigeration (Awua, Doe, & Agyare, 2011).
In fact, the thermal inactivation of PPO and POD in coconut water has been widely studied (Abreu and Faria, 2007, Campos et al., 1996, Matsui et al., 2007, Matsui et al., 2008, Murasaki-Aliberti et al., 2009, Prades et al., 2012b). However, although the thermal process has been shown to be effective for inactivating these enzymes, its negative impact makes it necessary to study alternatives, such as the non-conventional processes.
Such non-conventional technologies as membranes (Jayanti, Rai, Dasgupta, & De, 2010), high-pressure homogenization (Dosualdo, 2007), CO2 in dense phase (Damar, Balaban, & Sims, 2009), γ-irradiation (Awua et al., 2011) and microwave heating (Matsui et al., 2008) have all been studied for processing coconut water. The use of the high hydrostatic pressure technology has already been commercially adopted for this purpose (Harmless Harvest, 2015, Unoco, 2015).
However, although these technologies are effective for microbiological inactivation, they result in undesirable changes and/or are ineffective at inactivating the PPO and POD enzymes. As an example, there is a commercial coconut water processed through high pressure technology whose pink colour is due to the enzymatic action (Unoco, 2015). Furthermore, as described by Prades et al. (2012b), the combination of ultra-filtration and micro-filtration can ensure the product's sterility, but without stopping the enzyme activity, or it can ensure both sterility and removal of enzymes, but also removing desirable components, such as minerals. Therefore, the study of other technologies that can ensure the inactivation of the PPO and POD enzymes in coconut water is required.
The ultraviolet (UV) radiation technology has been shown to be effective for inactivating micro-organisms and enzymes (Bintsis et al., 2000, Falguera, Pagán, Garza, Garvín and Ibarz, 2011, Koutchma, 2009), including PPO and POD in different food matrices, such as apple juice (Başlar and Ertugay, 2013, Falguera, Pagán and Ibarz, 2011, Manzocco et al., 2009), must and grape juice (Falguera, Forns and Ibarz, 2012, Falguera, Garza, Pagán, Garvín and Ibarz, 2013), orange juice (Hirsch, Förch, Neidhart, Wolf, & Carle, 2008) and pear juice (Falguera, Garvín, Garza, Pagán, & Ibarz, 2014). However, it has not been studied for coconut water, even though the product's properties (especially its transparency, absence of suspended particles and low-fat content) make it suitable for the use of this technology.
The objective this work was to evaluate the photo-inactivation of peroxidase (POD) and polyphenol oxidase (PPO) in a coconut water model solution using ultraviolet radiation (UV).
Section snippets
Materials and methods
The use of model foods for process studies enables simple, cost-effective and continuous experiments to be carried out without significantly changing the products (Berto, Gratão, Vitali, & Silveira, 2003). Moreover, the main benefit of using model food systems in scientific studies is the experimental reproducibility, which minimizes the effects of inherent variations in food characteristics (Augusto, Tribst, & Cristianini, 2011). Consequently, this work was conducted using a coconut water
Peroxidase (POD) inactivation
Fig. 2 shows the relative activity (A/A0) of peroxidase (POD) in the coconut water model solution as a function of the photo-irradiation processing time (tUV). The enzyme activity decreased continuously, at a decreasing rate, in relation to the processing time, giving a curve with an upward concave shape. It can be seen that the POD activity after 15 min of processing was ~ 5% of its original value, falling to ~ 1% after 30 min and ~ 0.3% after 60 min of UV processing. This result highlights the
Conclusions
This work evaluated and modelled the polyphenol oxidase (PPO) and peroxidase (POD) photo-inactivation in a coconut water model solution using ultraviolet radiation (UV). Both enzymes showed continuous inactivation behaviour, at a decreasing rate, in relation to the processing time. Three models (simple first order kinetics, two-portions with first order kinetics and two-stages at first order kinetics) were evaluated, the two-portions inactivation kinetics being the one that best describes the
Acknowledgements
The authors are grateful to the São Paulo Research Foundation (FAPESP) for the PED Augusto post-doctoral fellowship at the University of Lleida (2014/14219-7).
Nomenclature
- α
- relative resistance of the sensitive enzyme portion in relation to the photo-inactivation using the two portion kinetics = AS(tUV = 0)/(AS(tUV = 0) + AR(tUV = 0)) (Eqs. (5), (6), (7)) [-]
- β
- angle defined by the emitting point in the lamp and the solution point receiving the radiation [-]
- λ
- wavelength [nm]
- Λ
- ratio between the activities of the native and intermediate enzyme states (Eqs. (5), (6), (7)) [–]
- μλ
- absorption coefficient at the λ wavelength [cm− 1]
- a,b
- parameters of model fit evaluation (Eq. (11)) [-]
- A
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2021, LWTCitation Excerpt :Also, transparency, low-fat content, and less particle content are the major factors affecting UV treatments (Rajashri, Rastogi et al., 2020). In addition, salts and sugars present in the system could expose the interaction sites during molecular unfolding, destabilizing the enzymes (Augusto et al., 2015). Furthermore, a study conducted with a model TCW solution suggests that even at constant UV lamp power, the absorbed power changed after 20 min creating two distinct phases of inactivation and a two-portion model kinetics study for POD (Augusto et al., 2015), which is similar to a biphasic model.
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By describing a potential application of light to food processing, the authors pay tribute to the United Nations International Year of Light and Light-based Technologies (IYL 2015).