Effects of pulsed-vacuum and ultrasound on the osmodehydration kinetics and microstructure of apples (Fuji)

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Abstract

The influence of pulsed-vacuum (PV) and ultrasound on the osmodehydration kinetics and microstructure of apples (Fuji) was investigated. Apple cylinders (15 mm height × 15 mm diameter) immersed in a 60% (w/w) high-fructose corn syrup solution were subjected to shaking (55 rpm), PV (13 MPa vacuum for 5 min + atmospheric pressure for 5 min + same vacuum for 5 min, then atmospheric pressure), or ultrasound treatment (50/60 Hz and 185 W) for 3 h. Changes in water loss, solid gain, and firmness of apples were measured, and the data were fitted using Weibull and Peleg models. In addition, microstructure was observed using scanning electronic microscopy (SEM). The high regression coefficients (R2 > 0.96) and low percent mean relative deviations (E < 6.37%) indicated the acceptability of Weibull model for predicting both water loss and solid gain under all treatments. The Peleg model well described the sample firmness changes with a R2  0.98 and E  3.24–6.14%. PV resulted in the lowest shape parameter α value (0.74) for solid gain and the greatest rate constant k1 (40.98 s) for firmness loss, indicating the largest amount of solid gain (3.02%) and the least firmness loss of samples, while ultrasound led to the lowest α value (0.45) for water loss and k1 value (33.42 s) for firmness loss: the highest water and firmness losses (56.3% and 22.3%, respectively) in samples among three treatments. SEM showed that cell deformation and cell structure collapse were the most severe in ultrasound treated samples, but moderate in PV samples. SEM also revealed a larger amount of solute uptake in the cells of PV and ultrasound treated samples.

Introduction

Osmotic dehydration (OD) as a pretreatment prior to final drying for partial removal of water from food materials, typically fruits and vegetables can not only save energy cost, but may also improve quality of the final products (Palou et al., 1994, Taiwo et al., 2003). OD is commonly carried out by immersion of the foodstuffs in a concentrated solution of sugar or salt under given time and temperature conditions. The chemical potential gradient between the osmotic solution and the food leads to mass transfer fluxes, in which water and some natural soluble substances outflow from foodstuffs and solutes from osmotic solution infiltrate into product tissues.

Since osmotic pressure is a sole driving force for mass transfer during OD, OD is a low efficient and time-consuming process. For improving processing efficiency and final product quality, pulsed-vacuum (Barat et al., 1999, Moreno et al., 2004, Tapia et al., 1999) and ultrasound (Cárcel et al., 2007, Gabaldón-Leyva et al., 2007, Simal et al., 1998, Taiwo et al., 2003) have been investigated in combination with OD.

Application of vacuum in the osmotic dehydration (VOD) favors the reductions of process time and energy cost (Fito, 1994, Mújica-Paz et al., 2003). Pulsed-vacuum osmotic dehydration (PVOD), as a variation of VOD, consists of the use of a VOD initial process for different times, followed by the application of OD at atmosphere pressure (Tapia et al., 1999). This procedure promotes osmotic medium penetration into the pores of plant tissues by hydrodynamic mechanisms (HDM) (Fito, 1994). HDM is controlled by the presence of the internal gas or liquid occluded in the open pores of a porous product, which is expanded and compressed with pressure changes, allowing the pores to act as outlets or inlets for liquids. PVOD has been reported to increase mass transfer rates in the dehydration of fruits and vegetables (Barat et al., 1999, Moreno et al., 2004, Paes et al., 2007, Tapia et al., 1999) and meat and fish (Collignan, Bohuon, Deumier, & Poligne, 2001) as a technological innovation of the traditional process.

Ultrasound traveling through a solid medium produces a variety of effects that can influence mass transfer (Mason, Paniwnyk, & Lorimer, 1996). Ultrasonic wave can generate minute vapor-filled bubbles to collapse rapidly or generate voids in liquids, a phenomenon called as acoustical cavitation, which is a result of pressure fluctuation (Simal et al., 1998). Ultrasound may also cause other series of effects on mass transfer through structure changes, such as “sponge effect” (Stojanovic & Silva, 2007), acoustic streaming or microstreaming (Mason et al., 1996), and microscopic channels (Cárcel et al., 2007). The use of ultrasound has been considered to improve mass transfer for different products and processes in liquid–solid system, such as osmotic dehydration of apples (Cárcel et al., 2007, Simal et al., 1998) and blueberries (Stojanovic & Silva, 2007), and brining of bell peppers (Gabaldón-Leyva et al., 2007).

The kinetics of mass transfer during OD of fruits and vegetables has been widely studied (Cárcel et al., 2007, Fito, 1994, Khin et al., 2007, Mújica-Paz et al., 2003, Rastogi et al., 2000). In an attempt to simplify the model of mass transfer in apples under OD combined with PV or ultrasound, the Weibull’s model for describing the probabilistic cumulative distribution over time was used to predict water loss and soluble solid uptake kinetics in this study. Water loss and sugar uptake induced by OD also affect the microstructure of plant tissues, such as cell collapse, deformation of cell walls, and ruptures of cellular bonds (Barat et al., 1999, Lewicki and Porzecka-Pawlak, 2005). In osmodehydrated fruits, vacuum impregnation resulted in different tissue structural development from that occurred in non-impregnated fruits due to the substitution of air by an impregnation solution (Barat et al., 1999, Moreno et al., 2004). However, little is reported about the effect of ultrasound on the microstructure changes of osmodehydrated fruits. The changes in microstructure in turn affect texture attributes of products. According to Shewfelt (1993), firmness is the primary textural attribute measured in fruits and vegetables and a combination of cell structure integrity and tissue turgor (Bourne, 2002). During dehydration, the cell wall structure, as a dormitory factor for fruit firmness, is partly modified due to water loss or interaction of middle lamella pectin and osmotic solutes (Moreno et al., 2004). Due to osmotic stress, the state of the cell membrane can change from partial to total permeability, leading to significant changes in tissue architecture (Rastogi et al., 2000). Besides cell wall structure, turgor pressure has major influence on tissue strength and macroscopic fruit firmness. A few reported studies have investigated the kinetics of fruit texture changes during osmodehydration. Among them, Paes, Stringari, and Laurindo (2006) described the stress relaxation of vacuum impregnated apples using Peleg model. Krokida, Karathanos, and Maroulis (2000) developed a simple mathematical model for describing the textural properties, including the maximum stress and strain, the viscoelastic exponent and the elastic parameter of dehydrated apple, banana, carrot and potato.

Understanding the osmotic dehydration kinetics, such as mass transfer, texture and structural alterations associated with various combined treatments is critical for a successful design of a drying system. Therefore, the major objectives of this study were to assess the applicability of the Weibull distribution model to predict the dehydration behavior of apples and the Peleg’s model for describing textural kinetics under osmodehydration with pulsed-vacuum (PV) and ultrasound, and to further investigate the effect of PV and ultrasound on the texture, microstructure, mass transfer efficiency, and some physicochemical properties of apples.

Section snippets

Mathematical models

The Weibull probabilistic distribution function has an interesting potential for applying in engineering processes as a time-to-failure model of particular components and systems because it is relatively simple and generally gives good description of the behaviors of complex processes or systems with high degree of variability (Cunha, Oliveira, & Oliveira, 1998). This model has been used to describe the drying of corns and the rehydration kinetics of dried apples and mushrooms (Cunha et al.,

Raw materials

In this study, Fuji apple was used as a model system. While it is true that some physical and physico-chemical properties of apples, such as porosity, acidity and sugar content may vary depending on the varieties of apples, but the impact of these variations on the dehydration process of apples should be minimal comparing with the processing factors evaluated in this study. Hence, results generated from this study on Fuji apple should have broad applications on other apple varieties.

Apples

Weibull model for fitting water content changes

The Weibull model (Eq. (1)) was fitted to the data from three different osmodehydration treatments (Fig. 1) and the estimated parameters are listed in Table 1. The correlation coefficient (R2) was in a range of 0.97–0.98, χ2 in a range of 0.065–0.0079, RMSE < 0.026, and E% < 5.12% (Table 1), indicating that Weibull mold is suitable and accurate for predicting water content of apples during osmodehydration treatments. The shape parameter α values of water loss models ranged from 0.46 to 0.57, close

Conclusions

Results showed that applying pulsed-vacuum and ultrasound during osmotic treatment had a significant effect on the kinetics of water loss, sugar gain, and firmness loss, as well as microstructure of osmotically dehydrated apples. Weibull and Peleg models adequately described the mass transfer kinetics and firmness changes in the osmodehydration process of apples under all tested conditions. Pulsed-vacuum and ultrasound enhanced the rates of water loss and sugar gain, and caused more noticeable

Acknowledgements

The authors thank Mr. Alfred H. Soeldner, Department of Botany and Plant Pathology, Oregon State University for his assistance in SEM observations.

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