Effects of the magnetic field on the freezing process of blueberry

Highlights

• Macro and micro experimental systems of the permanent and alternating magnetic field assisted freezing are set up.

• The effect of the magnetic field assisted freezing is evaluated from the aspect of microstructure and freezing parameters.

• The mechanism of different magnetic field assisted freezing is analyzed by the molecular dynamics method.

• The optimal permanent and alternating magnetic field intensities are obtained taking all the indexes into consideration.

Abstract

In this study, the permanent magnetic field (PMF) and alternating magnetic field (AMF) were applied and the effects on the microstructure and freezing parameters (nucleation temperature, freezing temperature, time through the maximum ice crystals formation zone and phase change time) were evaluated during the blueberry freezing process. The magnetic field intensities of PMF ranged from 0 to 10 mT and the peak values of AMF were 0, 0.05, 0.64 and 1.74 mT. The cooling rate and the target temperature were 4 °C/min and −30 °C, respectively. As a result, the minimal average ice crystal areas were 2489 µm² for PMF (10 mT) and 1731 µm² for AMF (0.05 mT). Compared with the control group (3749 µm²), it reduced by 33.6% and 53.8%. Besides, all the freezing parameters were significantly different with PMF (p < 0.05). The nucleation temperature decreased by 5.59 °C with the intensity of 10 mT and led to a great supercooling degree. The optimal PMF intensity was 10 mT for the blueberry from the aspect of the freezing parameters and microstructure. However, the freezing parameters were not significantly different with AMF except the phase change time (p < 0.05). The heat generated by the current coils with AMF should not be ignored. When the AMF intensity increased continuously, the phase change time increased accordingly which was disadvantageous for the food freezing.

Introduction

Blueberry is a kind of healthy, increasingly popular fruit with a relatively short shelf life. The freezing temperature can reduce the rate of chemical reactions as well as the activity of microorganisms and enzymes, thereby extending the storage life of frozen foods. Although freezing causes minimal deterioration of original color, flavor and nutritional values in comparison with thermal processing, food materials are prone to be subjected to irreversible tissue damage due to the solution concentration damage and mechanical damage from the ice crystals (Reid, 1993 Dalvi-Isfahan et al., 2017). The development of freezing methods that could reduce the tissue damage from the ice crystal is of considerable interest.

Blueberry is mainly composed of water. There is no doubt that the magnetic field could influence the properties of water. The strong magnetic field (≥10 T) could levitate water droplets in air (Beaugnon and Tournier, 1991) or deform the water surface (Kitazawa et al., 2001). Weaker magnetic field as low as 1 mT may influence the water properties such as the surface tension force, viscosity, refractive index, electric conductivity and enthalpy of vaporization (Semikhina and Kiselev, 1988; Hosoda et al., 2004; Toledo et al., 2008; Cai et al., 2009; Pang et al., 2012). The essence of food freezing is the conversion of water to ice. Based on the effect of magnetic field on water, the magnetic field assisted freezing attracts extensive attention as an innovative assisted freezing method in potential (James et al., 2015a).

The effects of the magnetic field type and intensity on the different objects have been investigated widely. Mok et al. (2015) explored the impact of attractive (480 mT) and repulsive (50 mT) static magnetic fields on the ice crystals formation of the 0.9% NaCl solution. The results showed that different patterns of the static magnetic field had different influence on the ice crystal formation. Choi et al. (2015) investigated the ultrastructure and sensory characteristics differences of beef between the electromagnetic and air blast freezing. The results showed higher acceptability with the electromagnetic freezing compared with the air blast freezing. However, the freezing conditions of beef were different. Moreover, the electromagnetic field intensity was not measured. Above all, it was difficult to say whether the observed effects were because of the magnetic field or due to the reduced temperature. Yamamoto et al. (2005) investigated the fracture properties and microstructure of chicken breasts frozen by the electromagnetic freezing. The effect of different magnetic field frequency (20, 30, 40 Hz) at the specified magnetic field intensity (1.5–2.0 mT) on the chicken breasts was studied. The results showed that the electromagnetic freezing especially 30 Hz was more suitable for the prolonged preservation of chicken breasts. Otero et al. (2017) indicated that no advantage of electromagnetic freezing (magnetic field intensity ≤2 mT) over air blast freezing was detected during the freezing process of the crab sticks. Purnell et al. (2017) investigated the effects of a range of different OMF freezing settings (0–0.4 mT) using a CAS freezer (ABI Co., Ltd. of Chiba, Japan) on characteristics of apple and potato. The data showed no strong continuum of effect that prevails across all CAS settings and temperatures for any of these products. However, significant effects were seen for some specific situations. The results suggested that weak OMF may not affect all foods in the same manner and that any effect would depend on a complex combination of food, freezing rate, magnetic field frequency and storage conditions. Up to now, the effects of a wide range of magnetic field parameters on the aqueous solution have been explored by Otero et al. (2018). However, only a few investigations with narrow magnetic field parameter range have been carried out on the real food matrices. Therefore, the effects of the magnetic field on the food could not be concluded. A wide range of the magnetic field parameters such as the magnetic field intensity should be considered.

Up to now, magnetic field assisted freezing has been carried out on objects such as aqueous solution (Otero et al., 2017; Mok et al., 2015; Cai et al., 2009; Chang and Weng, 2008; Holysz et al., 2007), meat (Kim et al., 2013), eggs (Fernández-Martín et al., 2017a, b) and biological cell (Abedini et al., 2011). As for fruits, the researches on the other innovative freezing methods such as the ultrasound assisted immersion freezing (Cheng et al., 2014), microwave assisted freezing (Jha et al., 2019), vacuum impregnation treatment (Dymek et al., 2015) and pulsed electric field assisted freezing (Wiktor et al., 2015) were relatively common. And these freezing methods have shown potential in reducing the freeze damage. However, the investigations of the magnetic field assisted freezing on fruits and vegetables were fewer until now (James et al., 2015b; Purnell et al., 2017). Therefore, the effects of magnetic field on the freezing parameters of perishable fruit are worth researching. Moreover, the investigation of the magnetic field assisted freezing may establish a foundation of an innovative freezing technique for blueberry.

Besides, the different indexes are used to evaluate the freezing effects. Otero et al. (2018) described the effects of static magnetic field on the pure water and 0.9% NaCl solutions from the aspects of supercooling and freezing kinetics. Choi et al. (2015) adopted the thawing loss, water holding capacity and sensory characteristics to evaluate the food quality of beef. Mok et al. (2017) evaluated the quality of the chicken breasts by the drip loss, color measurement, texture analysis and lipid oxidation measurement. As for food, plenty of the investigations reflected the freezing quality from the nutrients and sensory. However, the researches on the freezing kinetics of food are relatively fewer. As the investigations conducted by Fallah-Joshaqani et al. (2019), the freezing parameters could also be used to describe the freezing effects except the food indexes.

Although the investigations of the magnetic field assisted freezing have increased in the recent times (Kojima et al., 2015; Koseki et al., 2013; Lin et al., 2013), the effects on the food quality still have some controversies because of the improper experiment method and different investigated objects (Kaku et al., 2012; Wowk, 2012). Otero et al. (2016) indicated that it was not possible to discern whether magnetic fields (MFs) had an appreciable effect on the frozen products. More rigorous experimentation and further evidence were needed to confirm or reject the efficacy of MFs in improving the quality of frozen products.

The aim of this study was to provide rigorous evidence on the efficacy of permanent magnetic field (PMF) and alternating magnetic field (AMF) on the freezing of blueberries. The freezing effects were reflected from the aspects of micro and macro level. The optical microscope and CCD camera were used to observe the whole freezing process of the blueberry cut sections. The ice crystal area was analyzed to compare the effect of magnetic field assisted freezing and the control groups. Besides, the temperature profile of the whole blueberry was measured and recorded during the freezing process. The typical freezing parameters such as the nucleation temperature, the freezing temperature, the time through the maximum ice crystals formation zone ranged from −1 °C to −5 °C and the phase change time were evaluated by the statistical method in the absence and in the presence of PMF and AMF.