Influence of oxygen exposure on fermentation process and sensory qualities of Sichuan pickle (paocai)

Abstract

The physicochemical and microbial changes, volatile profile, texture and appearance were investigated in three groups of Sichuan pickles differing in oxygen exposure during a 64 day fermentation process. At the beginning, all the Sichuan pickles displayed similar sensory and fermentation characteristics regarding lactic acid bacteria domination, sugar utilization, and lactate accumulation. Soon afterwards, continued exposure allowed premature pellicle formation and typical aerobic deterioration of Sichuan pickle, accompanied by texture destruction and an odorous stench characterized by arenes and aldehydes on the 32^nd day. Meanwhile, the facultative anaerobes including Lactobacillus (90 ± 2%), Pediococcus (4.7 ± 0.3%) and Lactococcus (1.2 ± 0.8%) were the dominant genera and probably associated with acetic acid production and arene enrichment under oxygen exposure conditions on the 32^nd day. Until the 48^th day, intermittent oxygen exposure resulted in pellicle formation and an unpleasant flavor characterized by ketones, esters and S-methyl thioacetate. In comparison, Sichuan pickle without oxygen exposure always maintained a relatively favorable fragrant scent and texture. Besides Lactobacillus, anaerobic genera were the main bacteria in Sichuan pickle under closed conditions and accounted for 50 ± 9% of the total bacteria on the 32^nd day. The study indicated the microbial and chemical characteristics of Sichuan pickles during practical production and provided a theoretical guidance for the industrial production of Sichuan pickle.

Fig. 1. Changes in pH value (A), nitrate (B), total sugar (C), and reducing sugar (D) content during the fermentation process of Sichuan pickle in COJ (white square with dotted line), IOJ (grey triangle with dashed line), and CCJ (black round with solid line). Values are the mean ± standard deviation (SD) from the experiments performed in triplicate, and significant difference is analyzed by t-test: (ns) p > 0.05, (*) p ≤ 0.05 and (**) p ≤ 0.01.

Fig. 2. Comparison of bacterial compositions on the 32^nd day's (A) CCJ and (B) COJ at phylum and genus levels. The relative abundances of phylum shown in CCJ and COJ are more than 0.1%. The relative abundances of genera shown in CCJ and COJ are more than 1.0% and 0.1%, respectively. The genera with relative abundances between 0.1% to 1.0% in CCJ were shown in Fig. S3.

Fig. 3. The proportion of aerobes, facultative anaerobes and anaerobes in the Sichuan pickles of COJ and CCJ on the 32^nd day. Significant difference is analyzed by t-test: (*) p ≤ 0.05 and (**) p ≤ 0.01.

Fig. 4. (A) Heatmap of different classes of volatile flavor compounds (VOCs) during the fermentation process of Sichuan pickle. Different color bars indicate the relative abundance of each VOC class; (B) PCA score and loading scatter plots performed on different classes of VOCs in COJ (white square), IOJ (grey triangle), and CCJ (black round). The directions of the curved arrows indicate the routes of the fermentation time of Sichuan pickle.

Fig. 5. Correlation matrix of the Spearman rank correlation between dominant species and chemical characteristics in Sichuan pickles. The Spearman coefficient +1.0 was considered as a strongly positive correlation, while −1.0 was considered as a strong negative correlation. *, p ≤ 0.05.

Fig. 6. Texture changes during the fermentation process of Sichuan pickle in COJ (white square with dotted line), IOJ (grey triangle with dashed line), and CCJ (black round with solid line). Values are the mean ± SD from the experiments performed in triplicate, and significant difference is analyzed by t-test: (ns) p > 0.05, (*) p ≤ 0.05 and (**) p ≤ 0.01.