Elsevier

Food Research International

Volume 74, August 2015, Pages 151-159
Food Research International

Peroxidase (POD) and polyphenol oxidase (PPO) photo-inactivation in a coconut water model solution using ultraviolet (UV)

https://doi.org/10.1016/j.foodres.2015.04.046Get rights and content

Highlights

  • The POD and PPO photo-inactivation was studied in a coconut water solution model.

  • Three different models for describing the phenomenon were evaluated.

  • A possible mechanism for the observed photo-inactivation was proposed.

  • The absorbed radiation power was modelled.

  • The UV processing has been shown to be efficient for the inactivation of the enzymes.

Abstract

The interest in coconut water as a beverage is increasing due, not only to its sensory properties, but also to its nutritional characteristics. Even so, several challenges limit its processing, the inactivation of the polyphenol oxidase (PPO) and peroxidase (POD) enzymes being the most important. Although the inactivation of these enzymes has been extensively studied in coconut water, both by conventional and emerging technologies, the technologies evaluated so far are either not effective in the inactivation of these enzymes and/or result in undesirable changes. This work evaluated the photo-inactivation of POD and PPO in a coconut water model solution using ultraviolet radiation (UV). Both enzymes showed continuous inactivation behaviour in relation to the processing time, this being described by a two-portion inactivation kinetics. A possible mechanism for the observed photo-inactivation was proposed, involving steps of molecular unfolding and aggregation. The POD activity after 15 min of processing was ~ 5% of its original value, and reduced to ~ 1% after 30 min of UV processing. After 15 min of processing, PPO activity was ~ 8% of its original value, falling to ~ 2% after 30 min of UV processing. The results obtained highlight the potential use of the ultraviolet radiation to inactivate both enzymes in coconut water.

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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