Responses of Issatchenkia terricola WJL-G4 upon Citric Acid Stress

Effects of an inoculation dose of Issatchenkia terricola WJL-G4 on physicochemical properties, active substances, and antioxidant capacity of black, red, and white currant juice

BACKGROUND

Due to the high level of organic acids - primarily citric acid - black, red, and white currants have an excessively sour taste, making taste adjustment during processing challenging. This study investigated and evaluated the effects of an inoculation dose of the acid-reducing yeast Issatchenkia terricola WJL-G4 on several aspect such as physicochemical properties, chromaticity, active substances, and antioxidant capacity. A sensory evaluation was also conducted.

RESULTS

The results indicated that, when the inoculation dose increased from 2% to 12%, the total phenol, total flavonoid, and total anthocyanin content, and antioxidant capacity in currant juice decreased. A low inoculation dose (2-4%) was beneficial for preserving the total phenol and total flavonoid content. Although the levels of most phenolic compounds decreased, the concentrations of caffeic acid, p-coumaric acid, ferulic acid, rutin, and epicatechin were significantly higher than the control after fermentation. Overall acceptability and taste scores of fermented currants improved compared with those of the control group.

CONCLUSION

This experiment provided an effective solution, with a theoretical basis, to the problems of the sour taste and harsh flavor of currant juice. © 2024 Society of Chemical Industry.

Effect of citric acid on cell membrane structure and function of Issatchenkia terricola WJL-G4

AIMS

This study aimed to investigate the changes of cell membrane structure and function of Issatchenkia terricola under citric acid by performing physiological analysis.

METHODS AND RESULTS

The membrane integrity, surface hydrophobicity, structure, fluidity, apoptosis, and fatty acid methyl esters composition of I. terricola WJL-G4 cells were determined by propidium iodide staining, microbial adhesion to hydrocarbon test, transmission electron microscopy analysis, fluorescence anisotropy, flow cytometry, and gas chromatography-mass, respectively. The results showed that with the increasing of citric acid concentrations, the cell vitality, membrane integrity, and fluidity of I. terricola reduced; meanwhile, apoptosis rate, membrane permeable, hydrophobicity, and ergosterol contents augmented significantly. Compared to control, the activities of Na+, K+-ATPase, and Ca2+, Mg2+-ATPase increased by 3.73-fold and 6.70-fold, respectively, when citric acid concentration increased to 20 g l-1. The cells cracked and their cytoplasm effused when the citric acid concentration reached 80 g l-1.

CONCLUSIONS

I. terricola could successfully adjust its membrane structure and function below 60 g l-1 of citric acid. However, for citric acid concentrations above 80 g l-1, its structure and function were dramatically changed, which might result in reduced functionality.

Metabolomics analysis of the metabolic effects of citric acid on Issatchenkia terricola WJL-G4

In current research, yeast species Issatchenkia terricola WJL-G4 was shown to be capable of degrading citric acid, especially in the processing of fruit juice and wine. I. terricola WJL-G4 was able to use citric acid as a carbon source, but the metabolic effects of citric acid on yeast remained unclear. In this study, the metabolic effects of citric acid on I. terricola WJL-G4 were studied using liquid chromatography-mass spectrometry metabolomics technology, with glucose treatment as the control. Results showed that organic acid contents related to the extracellular tricarboxylic acid cycle (TCA) varied greatly. The metabolomics results indicated that I. terricola WJL-G4 might metabolize citric acid through the TCA pathway, and the glycolysis pathway might be inhibited; however, gluconeogenesis proceeded normally during citric acid treatment. Some fatty acids and phospholipids, along with the metabolic pathways of amino acids, vitamins, purines and nicotinamide in I. terricola WJL-G4 were also affected by the citric acid treatment. This work provided a theoretical basis for further study of the mechanism of yeast metabolism of citric acid.

Regulation of cell differentiation to promote pullulan synthesis in Aureobasidium pullulans NG

Pullulan is a polymer produced by Aureobasidium spp. The yield of pullulan production can be impacted by the cellular differentiation of Aureobasidium spp., which changes with alterations in the growth environment. To improve pullulan yield, identifying key factors that regulate cellular differentiation is crucial. In this study, the main form of pullulan synthesis in Aureobasidium pullulans NG was through swollen cells (SC). The results showed that citric acid (CA) can regulate the cellular differentiation of Aureobasidium pullulans NG by accumulating higher levels of CA in the cells to maintain growth in SC form and increase pullulan production. The addition of 1.0% CA to Aureobasidium pullulans NG for 96 h resulted in a significant increase in pullulan production, producing 18.32 g/l compared to the control group which produced 10.23 g/l. Our findings suggest that controlling cellular differentiation using CA is a promising approach for enhancing pullulan production in Aureobasidium pullulans. KEY POINTS: • The regulation of cell differentiation in Aureobasidium pullulans NG is demonstrated to be influenced by citric acid. • Intracellular citric acid levels in Aureobasidium pullulans NG have been shown to support the growth of swollen cells. • Citric acid has been found to increase pullulan production in Aureobasidium pullulans NG.