Biofabrication of Functional Pullulan by Aureobasidium pullulans under the Effect of Varying Mineral Salts and Sugar Stress Conditions

Abstract

Pullulan is a linear exopolysaccharide, produced in the fermentation media of Aureobasidium pullulans, with a variety of applications in the food and pharmaceutical industries. Pullulan derivatives have growing potential for biomedical applications, but the high cost of pullulan biofabrication currently restricts its commercial use. Better control over pullulan yield, molecular weight and melanin production by altering fermentation conditions could improve the economics. In this study, the effects of sugar and mineral salt stresses on the pullulan production of A. pullulans ATCC 42023 were examined in batch processes. The chemical structure of the recovered pullulan was characterized by FTIR and NMR spectroscopy, and the molecular weight distribution was obtained via SEC. Pullulan yield and melanin production varied when the conditions were adjusted, and pullulans with different molar masses were obtained. Higher-yield pullulan production and a lower polydispersity index were observed when CuSO₄ was added to the fermentation in comparison with the control and with the addition of sugars and other salts. Biofabrication of pullulan under stress conditions is a promising strategy to enhance biopolymer yield and to obtain pullulan with a targeted molecular weight.

Keywords: 1H-NMR spectroscopy; FTIR spectroscopy; biofabrication; mineral salt stress; molar mass; pullulan; sugar stress.

Figure 1

Proposed metabolic pathway of pullulan biosynthesis in A. pullulans Y68 by Duan et al. (1 = PGM, 2 = UGP, 3 = FKS). Reprinted with permission from [9] 2023, Elsevier.

Figure 2

Decaying sugar concentration in the fermentation broths of A. pullulans ATCC 42023: (a) Medium containing glucose as the carbon source. This is the control medium; (b) Medium containing (autoclaved) sucrose as the carbon source. Note that part of the sucrose is already hydrolyzed at time zero due to the heat; (c) Medium containing (filtered) sucrose as the carbon source; (d) Medium containing a mixture of glucose and fructose in a ratio of 1:4 as the carbon source.

Figure 3

Decaying sugar concentration in the fermentation broths of A. pullulans ATCC 42023 with mineral salt stress compared to the control.

Figure 4

Molar mass distributions of the commercial and experimental pullulan.

Figure 5

Two types of precipitates were obtained after precipitation in ethanol.

Figure 6

FTIR spectrum of pullulan recovered from the control medium after 120 h (orange) compared to the FTIR spectrum of commercial pullulan (blue).

Figure 7

¹H–NMR spectrum of commercial pullulan.

Figure 8

¹H–NMR spectrum of pullulan from the control medium.