Elsevier

LWT

Volume 99, January 2019, Pages 497-504
LWT

Effect of microwave power and blanching time in relation to different geometric shapes of vegetables

https://doi.org/10.1016/j.lwt.2018.10.029Get rights and content

Highlights

  • Microwave (MW) blanching (MWB) was conducted at various MW power and time process.

  • POD inactivation extended from the sample core with the increase of MWB intensity.

  • The highest POD inactivation was at 160 W for 75 s in cuboid-shaped samples.

  • In cylinder and large parallelepiped samples the optimal MWB was: 160 W for 120 s.

  • In cube and small parallelepiped the optimal MWB was: 160 W for 75 s and 350 W for 45 s.

Abstract

Microwave (MW) blanching (MWB) of vegetables has been applied in food industry as alternative to conventional treatment. Therefore, this study was aimed to evaluate the efficacy of MWB in terms of peroxidase (POD) inactivation, by applying various process conditions (MW power and process time) to different vegetables (potatoes, savoy and white cabbage), mimicking the most common geometric shapes (cylindrical, cubed, parallelepiped and slabbed). In all 3-D shapes, POD inactivation was initially achieved at the core of sample and gradually extended toward the periphery with the increase of process intensity. However, MWB appeared to be not suitable for reaching the temperature level required for the POD irreversible inactivation on the surface of vegetables, neither in 3-D nor in slabbed shape.

The optimal blanching conditions (POD residual activity < 10%), determined by response surface methodology analysis were: 160 W for 120 s for cylinder and large parallelepiped samples; 160 W for 75 s for cube sample; 350 W for 45 s for small parallelepiped sample. Contrariwise, in vegetables mimicking slab geometric shape the POD inactivation did not reach the optimal endpoint of blanching treatment, neither by increasing the MW power nor extending the process time.

Introduction

Blanching is a mild heat treatment widely applied in food industry prior to freezing, canning or drying fresh vegetables and fruits (Ranjan, Dasgupta, Walia, Chand, & Ramalingam, 2017) in order to extend the shelf life of the products, stabilizing their nutritional quality and texture. This unit operation is mainly aimed to inactivate both microorganisms and quality changing enzymes responsible for deterioration reactions that contribute to off-flavors, odors, undesirable color and texture, and breakdown of nutrients (Manpreet, Shivhare, & Ahmed, 2000; Xiao et al., 2017).

Among the naturally occurring enzymes, peroxidase (POD) is used in the food industry as an indicator of blanching adequacy, due to its high thermal resistance (Ramesh, Wolf, Tevini, & Bognár, 2002). Based on the type of technique applied, blanching can be categorized as: i) wet blanching, which is achieved by dipping vegetables in hot water, hot solutions (containing acids and/or salts) or steam (Kidmose & Martens, 1999); ii) dry blanching, which is carried out using microwaves (MW) (Ramesh et al., 2002), infra-red radiation (Kalathur, Girish, & Hebbar, 2013) or high pressure (Castro et al., 2008).

In the last decade, the increasing consumer demand of minimum treated and fresh-like products lead food industries to the application of novel technologies for inactivating enzymes with minimum deleterious effects on texture, flavor and nutrients (Aziz, Mahrous, & Youssef, 2002; Demirdöven & Baysal, 2008). MW blanching (MWB) is of great interest for both academic and industrial point of view, and it has been applied on different vegetables such as potatoes (Severini, Baiano, De Pilli, Romaniello, & Derossi, 2004), broccoli (Eheart, 1967), carrots (Soysal & Söylemez, 2005), and bell peppers (Ramesh et al., 2002). In comparison with conventional wet blanching, MWB allows: i) greater nutrients retention due to the reduction of leaching losses during treatment; ii) faster heating rate (heat is internally produced, whereas in conventional blanching heat is transferred by conduction from the product surface to the inner part); iii) greater penetration depth; iv) space savings because the equipment takes up a smaller area (Benlloch-Tinoco, Igual, Rodrigo, & Martínez-Navarrete, 2015).

As described in literature (Chavan & Chavan, 2010), the effectiveness of MWB depends on: i) process conditions [i.e. MW frequency and power, heating speed, mass and mobility of the product inside the oven (rotation/non-rotation)]; ii) vegetable characteristics [i.e. dielectric properties, penetration depth, moisture content, density, physical geometry (size, thickness, shape), surface to volume ratio and specific heat]. The non-uniform temperature distribution during MW heating has been extensively studied (Geedipalli, Rakesh, & Datta, 2007; Gunasekaran & Yang, 2007; Manickavasagan, Jayas, & White, 2006; Vadivambal & Jayas, 2010) and several researchers have developed mathematical models in order to predict temperature distribution in the MW heated food (Campanone & Zaritzky, 2005; Ni & Datta, 1999; Vilayannur, Puri, & Anantheswaran, 1998; Yang & Gunasekaran, 2004). Among these, Van Remmen, Ponne, Nijhuis, Bartels, and Kerkhof (1996) tweaked simple models in order to obtain a qualitative prediction of temperature distributions in three basic geometries (i.e. sphere, cylinder and slab) for MW energy penetration in food products.

Brewer and Begum (2004) proved that microwave power levels applied for different times affected POD inactivation in broccoli, green beans and asparagus. Despite these researches on MWB, no studies have yet been carried out changing both the process parameters and the geometry of vegetable matter. Therefore, the aim of this study was to evaluate the efficacy of MWB, in terms of POD inactivation, applying various process conditions (i.e. MW power and process time) to different vegetables namely potato and leafy vegetables (savoy cabbage and white cabbage) mimicking the most common geometric shapes and assess the effect on selected quality attributes.

Section snippets

Sample preparation

Potatoes (Solanum tuberosum L. var. agria), savoy cabbage (Brassica oleracea L. var. sabauda) and white cabbage (Brassica oleracea L. var. capitata) were purchased from a local supermarket and stored in a refrigerator at 4 °C. Before the experiments, the samples were taken out and equilibrated to room temperature (25 °C) overnight. Potatoes were washed, hand-peeled, and cut into cylindrical- and cuboid-shaped. The cylinder samples (Cy) had a diameter of 24 mm and a height of 9 mm. For

Peroxidase inactivation

POD inactivation achieved by MWB, carried out applying various process conditions, is inferred by the colorless areas in potatoes and leafy vegetables, as shown in Fig. 1. For all MW powers applied, and in both cylindrical- and cuboid-shaped potatoes the POD inactivation was initially achieved at the core of the sample and it gradually extended toward the periphery with the increase of process time, following the temperature profile described in literature for MW heating (Chen, Collins,

Conclusion

During microwave blanching (MWB) of vegetables (potatoes, savoy cabbage and white cabbage), the simultaneous effect of the process conditions (i.e. MW power and process time) and the geometric shape (cylindrical, cubed, parallelepiped and slabbed) was evaluated testing the peroxidase (POD) inactivation and the firmness as indirect measure of the structural modifications.

In all the 3-D geometric shapes, POD inactivation was initially achieved at the core of the sample and gradually extended

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