Growth, ethanol production, and inulinase activity on various inulin substrates by mutant Kluyveromyces marxianus strains NRRL Y-50798 and NRRL Y-50799

Abstract

Economically important plants contain large amounts of inulin. Disposal of waste resulting from their processing presents environmental issues. Finding microorganisms capable of converting inulin waste to biofuel and valuable co-products at the processing site would have significant economic and environmental impact. We evaluated the ability of two mutant strains of Kluyveromyces marxianus (Km7 and Km8) to utilize inulin for ethanol production. In glucose medium, both strains consumed all glucose and produced 0.40 g ethanol/g glucose at 24 h. In inulin medium, Km7 exhibited maximum colony forming units (CFU)/mL and produced 0.35 g ethanol/g inulin at 24 h, while Km8 showed maximum CFU/mL and produced 0.02 g ethanol/g inulin at 96 h. At 24 h in inulin + glucose medium, Km7 produced 0.40 g ethanol/g (inulin + glucose) and Km8 produced 0.20 g ethanol/g (inulin + glucose) with maximum CFU/mL for Km8 at 72 h, 40 % of that for Km7 at 36 h. Extracellular inulinase activity at 6 h for both Km7 and Km8 was 3.7 International Units (IU)/mL.

Keywords: Biorefinery platform; Coffee waste fermentation; Inulin; Inulinase; Kluyveromyces marxianus.

Fig. 1

Glucose concentrations and cell growth as a function of incubation time with mutant strains Km7 and Km8 using 4 different media: a YPD, b YPI, c YPDI, and d 1 % inulin

Fig. 2

Fructose concentrations and cell growth as a function of incubation time with mutant strains Km7 and Km8 using 4 different media: a YPD, b YPI, c YPDI, and d 1 % inulin

Fig. 3

Ethanol production and cell growth as a function of incubation time with mutant strains Km7 and Km8 using 4 different media: a YPD, b YPI, c YPDI, and d 1 % inulin

Fig. 4

Thin layer chromatography determination of glucose (dextrose), fructose, and inulin; panel 1: standard glucose, fructose, and inulin solutions; panels 2 and 3: mutant strains Km7 and Km8 incubated in 1 % inulin; panel 4: 1 % inulin incubated without either strain added

Fig. 5

Ethanol production (g/L) and cell growth (CFU/mL) for mutant strain Km7 a or Km8 b in medium with crude inulin extracted from coffee processing waste

Fig. 6

Inulinase activity (International Units (IU)/mL) of mutant strains Km7 and Km8, using HPLC to measure the amount of fructose (area under peak) produced from incubation of cell cultures at 30 °C with 0.2 % inulin solution at 2, 4, and 6 h by these strains compared to a standard inulinase solution. Key: n Km7 = undiluted Km7 culture; n Km8 = undiluted Km8 culture; 10 Km7 = 10-fold dilution of Km7 culture; 10 Km8 is10-fold dilution of Km8 culture