Impact of cold plasma processing on quality parameters of packaged fermented vegetable (radish paocai) in comparison with pasteurization processing: Insight into safety and storage stability of products

Highlights

• Four logCFU/g of gas-producing yeasts were selectively eliminated by cold plasma.

• Lactic acid bacteria in paocai were retained by cold plasma.

• Compared with pasteurization, cold plasma alleviated softening of paocai.

• Compared with pasteurization, cold plasma alleviated browning of paocai.

• Cold plasma prevented the increase in nitrite content of paocai during storage.

Abstract

Radish paocai is a representative of fermented vegetables in China. The objective of the present study was to evaluate the potential of cold plasma (CP) treatment as decontamination technology for packaged paocai. By comparing with heat pasteurization, the effect of CP on microbial, physicochemical and organoleptic properties of paocai during storage was investigated. Results showed that CP efficiently eliminated yeasts, especially gas-producing yeast, while allowing retention of lactic acid bacteria. Compared with pasteurization at 70 °C for 30 min which eliminated 5.00 logCFU/g of microorganisms, more reducing sugar was consumed by CP-treated paocai, leading to the increase of total acid content by 72.34% and a lower pH at 3.98 after storage of 7 days. Additionally, nitrite content in CP group was 48.54% lower than that of control and was 22.22% higher than that of pasteurization group, with no significant effect on salt content and amino acid nitrogen. As for organoleptic property, firmness and b* value of CP treated samples was 12.53% higher and 26.26% lower than those of pasteurization group after storage, respectively, indicating that CP could alleviate the softening and browning of paocai. Therefore, compared with control, CP could improve the storage stability of packaged paocai, meanwhile achieving similar safety quality as pasteurization, highlighting its potential in preserving packaged fermented vegetables.

Introduction

Paocai is a kind of traditional fermented vegetable widely consumed in China. To make paocai, vegetables are pretreated and immersed in paocai brine at an ambient temperature (20–25 °C) for 6–10 days in the paocai jar, a spontaneous fermentation procedure which is continuously used in contemporary manufacturing (Xiao et al., 2018). Conventional post-harvest washing and sanitation treatments are incompletely effective for the decontamination of foodborne pathogens and spoilage microorganisms from vegetable (Zhang, Tang et al., 2018). The microbial spoilage and physicochemical changes during storage usually shorten the shelf-life of products, and thus, decontamination should be conducted on paocai product after packaging (Huang, Zhang, & Bhandari, 2019). Due to the susceptibility of vegetable to thermal treatment, thermal pasteurization adversely alters the organoleptic properties of paocai including color, taste, flavor, aroma and texture, leading to softening texture and peculiar flavor (Moussa-Ayoub et al., 2017). To address the aforementioned challenge, novel non-thermal technologies should be investigated as alternative methods of intervention for paocai.

Recently, cold plasma treatment has shown potential in preservation of food as a novel non-thermal processing method (Chizoba Ekezie, Sun, & Cheng, 2017). Cold plasma is an ionized gas characterized by active particles such as electrons, ions, free radicals and atoms that is produced by applying energy to a gas or a mixed system with gas (Silveira et al., 2019). The oxidative species produced during the discharge (reactive oxygen and nitrogen species) could cause oxidation of lipids, protein and DNA, thus damaging microbial cell surface, leading to decontamination of food product (Min et al., 2018). The advantage of cold plasma treatment over conventional methods includes non-thermal, non-toxic, in-package, short time, rare side-effect since cold plasma could be performed at atmospheric conditions. Numerous studies on inactivation of microorganisms by cold plasma on different food matrix have been conducted and a growing body of evidence has demonstrated the effectiveness of cold plasma in decontamination of fruits, vegetables, fruit juice, nuts, grains, legumes, eggs, shrimps and meats (Liao et al., 2018; Mehta, Sharma, Bansal, Sangwan, & Yadav, 2019; Pérez-Andrés, Álvarez, Cullen, & Tiwari, 2019; Zhang et al., 2019). Particularly, cold plasma has recently drawn attention for inactivating endogenous enzyme and spoilage microorganisms of fresh cut vegetable and fruit (Dasan & Boyaci, 2018; Li, Li, Han, et al., 2019). It has been found that a delayed growth of spoilage mesophilic and psychrotropic microflora was achieved for fresh cut melon treated with cold plasma (Tappi et al., 2016). Additionally, E. coli O157:H7 was removed from fresh lettuce by cold plasma (Min et al., 2017). Moreover, texture softening of fresh cut kiwifruit was alleviated by cold plasma treatment, implicating the effectiveness of cold plasma on preservation of vegetable and fruit products (Ramazzina et al., 2015). The effect of cold plasma was found to be highly dependent on operative conditions including plasma-forming gas generated, electrical parameters, exposure time, flow rate and relative humidity (RH) (Huang et al., 2019). Besides, types of microorganism and food matrix exposed to plasma also affect the decontamination outcome of cold plasma. Given its sanitizing effect, cold plasma might have the potential to be applied for preservation of fermented vegetables. However, limited information is available on the effect of cold plasma on quality of fermented vegetable.

In the present study, to evaluate the effectiveness and potential of cold plasma on preservation of fermented vegetable, by comparing with samples treated with pasteurization, the changes in texture, color, physicochemical and microbial properties of fermented vegetable treated with cold plasma during storage was investigated.