Bioconversion Process

The stillage may be used as a fermentation medium directly after distillation, or it may be pretreated by clarification, dilution, pH adjustment, and sterilization. In certain cases, supplementation with additional sources of nitrogen and/or

Table 2 Products and effects of bioconversion of distillery waste by yeasts

Cultivation

Yeast species

Stillage

Biomass (g/L)

Protein (%)

COD reduction (%)

References

Batch

C. utilis

CMS

9-18

28-50

46

Several references

C. utilis

CMS suppl.

4-25

30-52

16-52

Several references

C. arborea

CMS

7-13

33-43

12-22

Matsuo et al. (1965)

C. brumptii

CMS suppl.

10

53

20

Kim et al. (1976)

C. krusei

CMS suppl.

3-8

31 -48

Tauk (1982) and Tauk and Gambale (1981)

C. membranaefaciens

CMS suppl.

3.6

45

Tauk and Gambale (1981)

C. rugosa

BMS suppl.

14.7

45.5

Lee and Baerwald (1991)

C. steatolytica

HHS

12

Zhang et al. (1982)

C. stellatoidea

CMS suppl.

3.5

48

Tauk and Gambale (1981)

C. tropicalis

WV

48

Karova et al. (1976)

P. tainania

CMS suppl.

19-20

42-49

Lin et al. (1973)

S. cerevisiae

MS suppl.

12.7

Selim et al. (1991)

S. cerevisiae

Rice

6

Yang and Tung (1996)

Fed batch

S. cerevisiae

Rice suppl.

20

Yang and Tung (1996)

C. utilis

CMS suppl.

15

41

Cabib et al. (1983)

Continuous

C. utilis

CMS suppl.

4-24

40-52

30-40

Several references

Hansenula sp.

BMS

5.7

39.6

31

Shojaosadati et al. (1999)

Hansenula sp.

BMS suppl.

8.5

50.6

35.7

Shojaosadati et al. (1999)

K. marxianus

BMS

10-11

60-70

Braun and Meyrath (1981)

S. cerevisiae

Rice suppl.

70

Yang and Tung (1996)

CMS, cane molasses stillage; suppl., supplemented; BMS, beet molasses stillage; WV, wine vinasse; HHS, hemicellulose hydrolyzate stillage; MS, molasses stillage of unknown origin.

CMS, cane molasses stillage; suppl., supplemented; BMS, beet molasses stillage; WV, wine vinasse; HHS, hemicellulose hydrolyzate stillage; MS, molasses stillage of unknown origin.

phosphorus may be necessary. Sometimes enrichment of stillage with an easily metabolizable carbon source can improve growth and substrate utilization (Cabib et al. 1983; Wang et al. 1977). The proportions of elements C:N:P of 50:5:1 were used in growing filamentous fungi in sugarcane stillage (Araujo et al. 1977; De Lamo and De Menezes 1978), while 40:10:1 was optimal for A. fusidioides in the same substrate (Rosalem et al. 1985). Cells from agar slants can be used directly as inocula for stillage fermentation, or the relevant microbes can be precultured under submerged conditions. The usual quantity of inoculum for inoculation is 10% of the broth volume. Previous adaptation of the microbial strain to the stillage may have a positive effect on bioconversion. Submerged aerobic processes are used with laboratory experiments carried out in shake flasks and fermentors. In large-scale experiments culture volumes of 9 m3 (Cabib et al. 1983) and even 80 m3 (Wang et al. 1980) have been reported. Batch, fed batch, or continuous operation modes are employed in trial fermentation. In addition to single fungus inocula, mixed cultures of microorganisms or a two-stage cultivation with successive cultivation of two different microorganisms have been used. Cell recycling proved to be beneficial for higher yields of biomass as well as for better consumption of organic substances (Staheeu et al. 1985; Wang et al. 1980). Cultivation temperatures were kept between 22 and 38°C, suitable for mesophilic microorganisms; pH control at the predetermined value often proved to be beneficial to the microorganism. The fermentation broth required vigorous aeration to ensure good growth. Aeration values of 0.5-1.0vvm generally were employed; exceptions being the very high aeration rates in small bioreactors or lower aeration rates of 1 vvh (Wang et al. 1980) in large scale experiments. Agitation rates range from 200 to 1000 rpm. In continuous cultivation, dilution rates of 0.016 h_1 (Yang and Tung 1996) to 0.383 h"1 (Wang et al. 1980) were employed. The dilution rate could be increased by the use of a cellrecycling technique (Wang et al. 1980). In batch processes the fermentation time is dependent on the microorganism and ranged from several hours to 2 days with yeasts, to several days with molds, and up to 12 days with mushrooms. After fermentation the yeast biomass is generally harvested by centrifugation, whereas with filamentous fungi simple filtration is possible.

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