Dehydration of apple slices by sequential drying pretreatments and airborne ultrasound-assisted air drying: Study on mass transfer, profiles of phenolics and organic acids and PPO activity

doi:10.1016/j.ifset.2021.102871

Innovative Food Science & Emerging Technologies

Volume 75, January 2022, 102871

https://doi.org/10.1016/j.ifset.2021.102871 Get rights and content

Highlights

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Pretreatments and air drying with sonication reduced apple drying time by 1.5–68.9%.
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A modified diffusional model with temperature factor predicted apple drying well.
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Water blanching and NaCl immersion enhanced procyanidin B2 content of dehydrated apple.
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Sonication promoted the loss of procyanidin B2 in apple samples with pretreatments.
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Organic acid profile and PPO activity were less affected by ultrasound-assisted drying.

Abstract

Pretreatments of either water blanching or NaCl immersion and airborne ultrasound-assisted air drying were combined to dehydrate apple slices. The drying time was found to be reduced by 1.6–69.0% compared with air drying without any pretreatment and sonication. Meanwhile, it was found that sequential water blanching coupled with air drying with sonication gave the shortest drying time. A modified diffusional model incorporating temperature-dependent moisture diffusivity (D_eff) could simulate the apple drying process well. Water blanching pretreatment and airborne sonication were found to lower the activation energy required for the enhancement of moisture diffusive ability, and intensify water exchange on apple surface to speed up the drying process. Both water blanching and NaCl immersion significantly increased the amounts of procyanidin B2 in dehydrated apple slices. However, airborne sonication promoted the loss of procyanidin B2 in apple slices with pretreatments during air drying. In addition, sonication had no obvious influences on apple PPO activity and individual organic acids throughout drying. Overall, the sequential NaCl immersion pretreatment (or water blanching pretreatment) and drying with airborne sonication are effective to improve the quality of dried apple slices.

Graphical abstract

Introduction

Snack food produced from fruits and vegetables is gaining popularity as it contains less oil and fat contents compared to chips (Akman, Uysal, Ozkaya, Tornuk, & Durak, 2019). Drying is essential to produce dehydrated snack foods. The key problems for apple drying are high energy consumption and quality deterioration. To improve the apple drying process, more efforts are needed in the aspects of novel drying technology development and process optimization.

Hybrid drying technology is more efficient than the individual drying technology in food drying. Particularly, ultrasound, microwave, infrared radiation, radio frequency can be incorporated in different drying processes, helping to reduce drying time and enhance product quality (Chen, Subbiah, Jones, Zhao, & Jung, 2021; Fan, Zhang, & Mujumdar, 2017; Li et al., 2021; Rastogi, 2012). Among them, airborne and contact sonication have been intensively demonstrated to be able to enhance food air drying rate without the overheating problem (García-Pérez, Cárcel, Benedito, & Mulet, 2007). Also, sonication has a promising potential to preserve food flavor and bioactive components during drying (Tao et al., 2021). Apart from drying intensification, many published researches reveal that the pretreatments including osmotic dehydration using either NaCl or sugar as solute, water blanching, immersions in vitamin C (VC) and citric acid solutions can preserve food quality, speed up drying, or both (Ahmed, Qazi, & Jamal, 2016; Miao et al., 2020; Tiganitas, Zeaki, Gounadaki, Drosinos, & Skandamis, 2009; Yao, Fan, & Duan, 2020). Although ultrasound has been previously used to intensify the air drying of apple (Kahraman, Malvandi, Vargas, & Feng, 2021; Nowacka, Wiktor, Śledź, Jurek, & Witrowa-Rajchert, 2012), the combinations of different drying pretreatments with ultrasound-assisted air drying have been rarely applied for the dehydration of apple. For the sake of promoting air drying and manufacturing quality and nutritive dried apple products, it is important to explore the influence of combinations of the aforementioned pretreatments and air drying with sonication on the drying mechanism and apple physicochemical properties.

Food drying involves heat and mass transfer mechanism, as well as the knowledge referring to food chemistry. On one hand, phenomenological modeling is useful to investigate the moisture transport. The physical model based on Fick's law of diffusion and Newton's law of mass balance has been extensively used to simulate individual mass transfer phenomenon under air drying of many agriproducts (Gamboa-Santos, Montilla, Cárcel, Villamiel, & Garcia-Perez, 2014; Magalhães et al., 2017). However, the mass transfer model mentioned above does not consider the effect of temperature. Hence, it cannot accurately predict the moisture transport, since water diffusivity increases significantly with temperature (Tao et al., 2021). To improve the performance of the diffusional model, it is necessary to incorporate the effect of temperature on water diffusivity. In our best knowledge, there are few studies employing the diffusional model containing temperature factor to model the individual mass transfer process under drying. On the other hand, many parameters are used to indicate the quality of dried apple products, such as phenolic composition, polyphenol oxidase (PPO) activity, organic acid profile, VC content, etc. In most research works and reports on fruit and vegetable drying, only the qualities of samples before and after drying are assessed (Guo et al., 2020; Nowacka et al., 2018). To enrich the knowledge on food chemistry during drying, the variations of quality attributes throughout drying are also needed to be investigated.

The goal of the present study was to study the effects of sequential pretreatments (water blanching or NaCl immersion) and air drying coupled with airborne sonication on the drying kinetics and quality variations of apple slices. A diffusional model taking into account the effect of temperature was established to simulate the drying process. The profiles of organic acids and phenolics, VC content and PPO activity in apple slices were analyzed at different stages throughout the entire drying process.

Section snippets

Materials

Fresh and mature apples (Fuji) with pink skin were purchased from the local market in Nanjing. Prior to drying experiment, samples were washed by tap water carefully, peeled, and cut into small pieces (3.0 cm × 3.0 cm × 0.4 cm in dimension). The size and thickness of the apple slices were measured by a caliper with an accuracy of 0.02 mm. The initial dry mass ratio and moisture content were determined by the AOAC method (1995), being 15.6% and 7.285 g water/g DM (dry matter), respectively.

Drying feature

The changes of moisture content under airborne ultrasound-assisted air drying are shown in Fig. 2a. Water blanching pretreatment led to the increase of moisture content in apple slices from 7.285 ± 0.04 g water/g DM to 10.905 ± 0.08 g water/g DM, while NaCl immersion pretreatment decreased the moisture content slightly due to the osmotic forces (Tsamo, Bilame, Ndjouenkeu, & Nono, 2005). Under air drying, moisture content in all samples declined with drying. In the treatment of air drying alone,

Conclusion

The combinations of water blanching pretreatment (or NaCl immersion pretreatment) and airborne ultrasound-assisted drying were successfully proved to intensify the drying of apple slices, as well as protect some bioactive components in apple. The diffusional modeling containing the influence of temperature on D_eff reveals that the treatments of water blanching, airborne sonication and their combinations mainly improved the drying process through lowering the activation energy required for D_eff

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 32072351) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Dehydration of apple slices by sequential drying pretreatments and airborne ultrasound-assisted air drying: Study on mass transfer, profiles of phenolics and organic acids and PPO activity

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Drying feature

Conclusion

Declaration of Competing Interest

Acknowledgements

Food and Bioproducts Processing

Innovative Food Science & Emerging Technologies

LWT-Food Science and Technology.

Biosystems Engineering

LWT – Food Science and Technology

Food Chemistry

Ultrasonics Sonochemistry

Journal of Food Engineering

Innovative Food Science & Emerging Technologies

Food Chemistry

Food Control

Journal of Food Engineering

Ultrasonic – Sonochemistry

Journal of Food Engineering

Food and Bioproducts Processing

Food and Bioproducts Processing

Ultrasonics Sonochemistry

Journal of Food Engineering

Food Chemistry

LWT – Food Science and Technology

Food Chemistry

Food Chemistry

Postharvest Biology and Technology

Journal of Food Engineering

Food Chemistry

Heliyon

Food and Bioproducts Processing

Ultrasonics

Journal of Food Engineering

Food and Bioproducts Processing

LWT – Food Science and Technology

Journal of Food Engineering