International Journal of Environmental Monitoring and Analysis
Volume 3, Issue 4, August 2015, Pages: 233-237

Osmotic stress Response of Saccharomyces cerevisiae under HG and Elevated Temperature Environment

Ambreen Gul*, Asma Siddique, Quratulain Syed, Muhammad Nadeem, Shahjahan Baig

Food and Biotechnology Research Centre, Pakistan Council of Scientific and Industrial Research Laboratories Complex, Lahore, Pakistan

Email address:

(A. Gul)

To cite this article:

Ambreen Gul, Asma Siddique, Quratulain Syed, Muhammad Nadeem, Shahjahan Baig. Osmotic stress Response of Saccharomyces cerevisiae under HG and Elevated Temperature Environment. International Journal of Environmental Monitoring and Analysis. Vol. 3, No. 4, 2015, pp. 233-237. doi: 10.11648/j.ijema.20150304.15


Abstract: The High Gravity (HG) ethanol fermentation at high temperature is very attractive and promising technology for fuel ethanol production. This study was designed to improve the osmotic as well as thermal behavior of the Saccharomyces cerevisiae strain isolated from distillery waste. Therefore, initial pH and substrate concentrations were optimized for this strain. The S. cerevisiae was subjected to thermal treatment to improve its fermentation ability without significant yield losses. At pHo 5.0, 95g/L ethanol was produced with the productivity (Qp) value of 1.02. The activation energy Ea value calculated at 30-40oC was 16.48kcal/mol indicating the thermal tolerance of the strain SC36. The results of glucose optimization revealed that at 250g/L glucose concentration, Qp, Yp/s and Yp/x value of 1.53g/Lh, 0.443g/g substrate and 41.4g/g biomass were obtained. The strain’s potential to be able to ferment very high gravity medium is very promising for fuel ethanol production.

Keywords: Thermal Treatment, Saccharomyces cerevisiae, Monod Model, High Gravity, Thermotolerant


1. Introduction

The evolutionary increased energy demand in chemical industry and automobiles has modulated the direction of research towards new renewable energy production approaches for sustainable energy supply. One among many approaches is the bioethanol production from cellulosic substrates, which is being considered an economic substitute for growing worldwide biofuel demand (Jones and Ingledew, 1994). Since 1990s, High Gravity (HG) and Very High Gravity (VHG) ethanol fermentation has revolutionized the field of biofuel industry to make the production of bioethanol an attractive alternative for sustainable energy supply. The VHG fermentation for maximum ethanol production (above 15% v/v) poses severe stresses on yeast growth, cell viability (Zhao and Bai, 2009), high osmotic pressure and substrate inhibition but the advantages do include the high ethanol yield, reduced labor, shorter ethanol recovery time, reduced capital costs and energy consumption (Casey et al., 1984). The high-temperature requirement is an essential requirement for industrial ethanol production though it can pose severe damage to S. cerevisiae growth and reduced fermentation rate due to glycolytic shutdown. Similarly, low pH tolerance is a fundamental requirement of ethanol fermentation to avoid the bacterial contamination (Edgardo et al., 2008; OrtizMuñiz et al., 2010). Temperature tolerance is closely related to pH tolerance as both factors affect the membrane fluidity, mitochondria and cell viability. High temperature as well as high ethanol as a result of VHG media leads to the changes in membrane fluidity, and ultimately the cell death of S. cerevisiae (Lei et al., 2007; Piper, 1995).

The present research was attributed to study kinetics of ethanol fermentation from dextrose by S. cerevisiae SC36 isolated from distillery waste. The effect of fermentation parameters such as initial pH and substrate on kinetics of ethanol fermentation were investigated. The strain was then subjected to thermal treatment to improve its temperature tolerance during VHG fermentation. For microbial growth, substrate consumption and product formation, kinetic models by Monod and Leudeking-Piret were also evaluated in batch fermentation by S.cerevisiae.

2. Materials and Methods

2.1. Strain, Media and Culture Conditions

S. cerevisiae was isolated from the distillery plant and maintained on the culture medium containing (g/l): glucose, 20; yeast extract, 5; MgSO4, 5 and agar, 20 at a pH of 5.0 and then preserved at 4oC. The fermentation media was (g/l): yeast extract, 5; MgSO4, 3; dextrose, 50-300, at 50 interval and initial pH, 4.0-5.5 at 0.5 intervals. The fermentation medium was sterilized for 15 min at 121oC. Both the pH and osmotolerance optimizations were carried out at 30oC.

2.2. Development and Selection of Thermotolerant Yeast Strain

To improve the thermotolerant ability of the yeast strain, two different approaches were adopted based on the methodology of Edgardo et al. (2008); 1) direct thermal shock and 2) progressive thermal shock. Direct thermal shock was given in four cycles, first at 45oC for 1hr and rest of all at 30oC for 30 minutes to the previously optimized strain for osmotolerance and pHo. In the progressive thermal treatment, thermal shock was given from 35 to 42oC with a 1oC rise in temperature after every 24 hr. Thermotolerance proof tests were conducted by inoculating the thermally treated yeast cells from each treatment on YMG agar plates and incubated at 36 to 42oC. After the selection of the thermotolerant strain, the ethanol fermentation profile of the selected strain at different temperatures (30 to 40oC) was conducted to confirm the fermentative ability of the isolated strain.

2.3. Analytical Methods

Ethanol concentration was determined spectrophotometrically by an acid-dichromate method developed by (Bennett, 1971; Pilone, 1984). Gravimetric method was used for cell mass determination by centrifuging the 50ml culture medium and drying the pellet at 60oC for 24hr. Reducing sugar content was determined following the method described elsewhere (Miller, 1959).

2.4. Kinetic Modeling

To describe the kinetics of fermentation process, rate equations were employed for biomass (X), glucose (S) and ethanol (P).

The equation to describe the growth rate during exponential phase of fermentation process can be described by the equation developed by Monod kinetic model.

  = µX                                  (1)

Where, the specific growth rate µ is given by the Monod as

µ = µmax.                                            (2)

Where, µmax is the maximum specific growth rate and ksis the substrate saturation constant.

An unstructured model that combines both the growth-associated and non-growth associated product formation was based on the Leudeking-Piret, who originally developed it for lactic acid production(Leudeking, 1959), is as follows

(3)

Where α is for growth associated and β is for non-growth associated product formation.

The substrate glucose is used both for the maintenance of cell growth and for the production of cell metabolites. So the substrate consumption can be described by the equation,

(4)

Where Yx/s and Yp/s are yield constants and ms is the maintenance coefficient. By simplifying and substituting eq 3. ineq4. we get,

(5)

Where γ and λ are constants for growth and non-growth associated substrate consumption. Both of these are represented by eq 6; 

and        (6)

3. Results and Discussion

The effects of pHo, dextrose and temperature were observed on the ethanol production behavior of S. cerevisiae SC36 strain.

pH effect on growth and fermentation

Table 1. Kinetic analysis at various initial pH on ethanol production.

pH  µmax h-1 Yp/x g/g Yp/s g/g Qp g/Lh qp g/gh
4.0 0.0255 40.875 0.4496 0.937 0.426
4.5 0.0237 38.705 0.4064 0.847 0.40
5.0 0.0341 44.72 0.489 1.019 0.466
5.5 0.0176 35.904 0.454 0.945 0.374

*Terms used: µmax, Growth rate; Yp/x ,product yield in g ethanol/g biomass; Yp/s , Product Yield (g ethanol/gsubstrate);Qp, volumetric productivity (g ethanol /L.hour); Specific productivity (g ethanol produced/g biomass/h).The data is presented as mean of 3 replicates.

The results of ethanol fermentation performance of the S. cerevisiae SC36 strain revealed that at pHo 5.0, the specific growth rate (µmax), yield coefficients (Yp/s, Yx/s) and volumetric productivity (Qp) were highest. The biomass production was higher at pHo 4.0 than pHo 5.0 but on the basis of ethanol yield and the qualitative productivity, pHo 5.0 was selected as the optimum value (Table 1). The results of our study are in aggrement with the findings by Wong and Sanggari (2014) and Paramanik and Rao(2005)(Pramanik and Rao, 2005; Wong and Sanggari, 2014). The S. cerevisiae is acidophilic in nature and grows best at pHo range of 4.0 to 5.0 depending upon the strain.  Dechant et al. (2014) observed that cytosolic pH acts as nutrient sensor and activates signaling pathways for promoting cell growth(Dechant et al., 2014). The glycolytic and alcohlogenic enzyme of the yeast S. cerevisiae are active at neutral pH and a rapid decline in external pH triggers a rise in internal pH of the cell and the failure of which results in decline of fermentative ability of the S. cerevisiae (Dombek and Ingram, 1987). Paramanik and Rao  reported that S. cerevisiae was more active at pH 4.5 in grape juice fermentation(Pramanik and Rao, 2005), while various other authors reported it to be at 5.0 (Magesh et al., 2011; Oghome and Kamalu, 2012) and 3.5 (OrtizMuñizet al., 2010) for S. cerevisiae.

Thermal treatment for strain improvement

Direct Thermal Shock: When the strain was subjected to direct thermal shock at 4oC in the first cycle for 1 hr and 30oC for the rest of cycles, no viable cell count was observed. Sudden exposure to very higher temperature may have reduced the cell’s ability to adapt with the stressed environment and may have resulted in protein denaturation, loss of membrane integrity and the cell death.

In the second strategy, progressive acclimatization (35-42oC with 1oC interval) was adopted. The viable strain (isolated after treatment) which was able to grow at 41oC produced very little ethanol yield at the same temperature. This SC36 strain was further optimized for thermal fermentation profile between 30 to 40oC. Highest ethanol production was achieved at 36oC. Therefore, the strain was named as SC36 and will be referred as such hereafter in the document. Above 36oC, both the biomass and the ethanol production potential of the SC36 strain showed a sharp decline. Fermentation efficiency of SC36 decreased due to changes in membrane fluidity with increase in temperature. Any change in the fluidity of membrane is due to change in the fatty acid composition which is necessary to maintain the cell viability (Ohta et al., 1988; Suutari et al., 1990; Van Uden, 1985). It was observed that progressive increase in stress (temperature) could stimulate the modulation of enzymatic activities and other cell proteins to adjust the changing environment, hence progressive acclimatization proved the best methodology for strain development.

Table 2. Effect of temperature on kinetics of ethanol production.

Temp. oC µmax h-1 Yp/x  g/g Yp/s  g/g Qp g/lh qp g/gh
30 0.0268 0.490 1.021 0.155 0.155
32 0.0274 0.492 1.026 0.156 0.156
34 0.0278 0.499 1.039 0.144 0.144
36 0.0262 0.505 0.82 0.437 0.437
38 0.0205 0.287 0.598 0.374 0.374
40 0.00172 0.036 0.074 0.149 0.149

*Terms used: µmax, Growth rate; Yp/x ,product yield in g ethanol/g biomass; Yp/s , Product Yield (g ethanol/gsubstrate);Qp, volumetric productivity (g ethanol /L.hour); Specific productivity (g ethanol produced/g biomass/h).The data is presented as mean of 3 replicates.

Thermal Effect

Activation energy was used to evaluate the effect of temperature on SC36 strain. Arrhenius model is recognized very well for thermodynamic study of bioprocesses. Arrhenius equation describes the temperature dependent growth rate.

The activation energy value was calculated between 30-40oC temperature ranges at pH 5.0. Value of activation energy determined for this strain of S. cerevisiae was 16.48kcal/mol with 3.6 x 1012pre-exponential factor. Ea value of this strain is little higher than S. cerevisiae ITV-01 (15.6kcal/mol) indicating that it is sensitive to temperature but still less sensitive than Schizosaccharomycespombe (26.2kcal/mol) (OrtizMuñizet al., 2010). This value of activation energy warrants the thermal resistance of the SC36 strain. However, our strain produced more ethanol at 36oC than the S. cerevisiae ITV-01 which was best at 30oC.The activation energy (Ea) value categorizes the process in biological or diffusional range.  Higher activation energy value than 12 kcal/mol mean the process is within the biological range, while below this value determines the process in diffusional range.

Osmotolerance Effect

The six different initial substrate concentrations (50-300g/L dextrose) were evaluated for ethanol fermentation efficiency on VHG medium by SC36 strain at 36oC and pH5.0. The biomass and product were increased with increase in dextrose concentration from 50 to 300 g/L. A slightly fast substrate consumption was observed at 300g/L substrate in the first 48hr with a gradual sharp decline in the residual substrate. At highest substrate concentration (300g/L), time taken to complete the fermentation was increased with a simultaneous decrease in the specific growth rate (µmax). The µmax at 250g/L substrate was higher both above and below this concentration (Table 3). This confirms the validation of the strain SC36 to be osmotolerant strain capable of fermentation in VHG at 250g/L substrate.  The decrease in µmaxat 300g/L substrate concentration indicates the appearance of substrate saturation and lower water activity caused by VHG. Lower water activity and increased osmotic pressure can cause plasmolysis of the cell and affects the ethanol yield(Bai et al., 2008; Roukas et al., 1991). It was also observed that increase in initial substrate resulted in increase in fermentation time at the same inoculum concentration as expected. Substrate inhibition by S.cerevisiae has been observed even at 150 g/L initial glucose (OrtizMuñizet al., 2010) and at 200g/L glucose; only 32g/L ethanol was produced in the study by Pramanik and Rao (Pramanik and Rao, 2005).

Table 3. Kinetics of Initial sugar concentration on specific growth rate, yield and productivity at initial pH 5, temperature 36oC.

Sugar g.L-1 µmax h-1 Yp/x  g/g Yp/s  g/g Qp g/lh qp g/gh
50 0.0343 12.250 0.480 0.500 0.170
100 0.0420 14.088 0.496 1.033 0.196
150 0.0366 17.416 0.466 0.970 0.242
200 0.0377 20.392 0.409 1.136 0.283
250 0.0420 27.873 0.443 1.533 0.3871
300 0.0311 27.654 0.443 1.383 0.3841

*Terms used: µmax, Growth rate; Yp/x ,product yield in g ethanol/g biomass; Yp/s , Product Yield (g ethanol/gsubstrate);Qp, volumetric productivity (g ethanol /L.hour); Specific productivity (g ethanol produced/g biomass/h).The data is presented as mean of 3 replicates.

Kinetic Modeling for Process Optimization

Growth Kinetics

The Monod model explains the relation between specific growth rate of microorganism and the substrate concentration. The values of Ks and µmax were determined using Monod model by plotting µ and substrate concentration with a slope of µmax and intercept of Ks and are shown in Table 4.

Product formation kinetics

The eq. 3 describes the product formation kinetics as described by Leudeking-Piret. The product formation rate is dependent on the instantaneous biomass (X) and the growth rate dX/dt in a linear way.

A plot of 1/X.dP/dt versus 1/X.dX/dt was linear plot with a slope of  and intercept of  which may vary depending on the fermentation conditions. The values of both parameters are given in Table 4.

Substrate consumption kinetics

The substrate consumption can be described by the eq 4. In which the constants γ and λ are dependent on fermentation. A plot of 1/X.dX/dt versus 1/X.dS/dt gave a straight line in which the slope provides the value of γ and intercept is the value of λ. The values of both constants are given in Table 4.

Table 4.  Kinetic model parameter values for growth kinetics, product formation and substrate consumption.

Model Parameters
Monod Gowth Kinetics
µmax 0.4165 h-1
Ks 213.81g/L
Leudeking-Piret Product Formation kinetics
Α 0.038
Β 11.2
Substrate consumption kinetics
Λ 1.778
Γ 0.174

4. Conclusion

S.cerevisiae SC36 is an osmotolerant and thermotolerant yeast strain which is able to grow and produce ethanol under highly stressed environment at 250 g/L dextrose concentration and 36oC fermentation temperature at pH 5.0. Kinetic parameters clearly indicate that the SC36 strain when subjected to >150g/L initial substrate concentration, exhibit higher growth rate at 250g/L. The thermal treatment of the strain and the value of activation further demonstrated that the SC36 strain is thermotolerant strain capable to produce ethanol at 36oC. The Monod model for growth kinetics and Leudeking-Piret model for product formation and substrate consumption further highlighted the kinetic characteristics of the strain SC36.

Acknowledgement

The author acknowledge the PCSIR Laboratories complex for the entire work facility.


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