Journal of Scientific & Industrial Research Vol. 65, January 2006, pp. 72-76
Baker’s yeast production under fed batch culture from apple pomace Shashi Bhushan1 and V K Joshi2,* 1
Institute of Himalyan Bioresource Technology, CSIR, Palampur 176 061 Department of Postharvest Technology, Dr Y S Parmar University of Horticulture and Forestry, Nauni-Solan, 173 230 Received 31 December 2004; revised 30 September 2005; accepted 10 October 2005
2
Apple pomace extract (APE) in aerobic variable fed batch culture under standardized fermentation parameters was evaluated for the production of baker’s yeast. The substrate was fed at a flow rate of 0.39 dm3 h-1 to regulate fermentable sugar concentration between 1-2 % in bioreactor. An average specific growth rate of 0.24 h-1 and cellular yield coefficient of 0.48 g/g sugar was obtained during baker’s yeast production. Under variable fed batch aerobic baker’s yeast fermentation, yield obtained with APE was 96 % to that of expected theoretical yield and thus, qualified as an alternative to molasses, the traditional baker’s yeast production carbon source. The dough raising capacity of experimentally produced yeast cells revealed no apparent difference from that of commercial sample. Keywords: Aerobic fermentation, Apple pomace extract, Bakery products, Baker’s yeast, Fed batch culture, Molasses, Saccharomyces cerevisiae, Specific growth rate IPC Code: C12N1/18
Introduction Apple pomace (AP), a leftover residue after apple juice extraction, poses a lot of problem of environmental pollution and a loss of nutritive natural wealth1. High biochemical oxygen demand (BOD, 395-37000) of AP makes it easily vulnerable to fermentation / biodegradation, consequently if not disposed-off immediately, would generate foul smell2,3. The nutritive value of AP rests mainly on its total carbohydrates, crude protein, crude fat, fibres and minerals. AP can be used for the preparation of jam, sauce, soft drinks, animal feed along with direct incorporation in bakery products4-6, besides production of citric acid, ethanol, acetone, butanol and microbial colour7-9. Due to high nutritive value and easy fermentability, it is suitable for the production of baker’s yeast. The fermentable sugar was found enough in apple pomace extract (APE) to support the growth of baker’s yeast along with crude protein contents as well as a few trace elements3,10,11. Besides type and concentration of carbon sources, other factors such as dissolved oxygen (DO) agitation, pH and temperature, are also known to influence the fermentation behaviour and overall biomass production12-15. The fermentation parameters such as temperature, pH and DO, standardized during earlier __________ *Author for correspondence
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investigation16, were employed in the present fermentation. This paper presents the results of substitution of molasses (a traditional substrate) by APE and its effect on the fermentation characteristics such as specific growth rate (SGR), total biomass yield, cellular yield coefficient (CYC) and other related fermentation characteristics. Materials and Methods Apple Pomace Extract (APE)
The dried AP, procured from HPMC (Horticultural Produce Processing and Marketing Corporation), Fruit processing unit, Parwanoo (HP), was ground into fine powder, diluted with water in 1:6 ratio and boiled for ½ h before extraction.The boiled material was pressed in hydraulic press and the liquid filled into conical flasks (5 l), was sterilized at 121°C for 20 min. Inoculum
Preserved slant of commercial baker’s yeast (Saccharomyces cerevisiae var. diastatic) was used to inoculate in yeast malt extract broth and incubated at 30°C for 24 h and then, transferred to APE medium and again incubated at the same temperature for another 24 h. Inoculum (102-103 cells/ml) was added at the rate of 1 percent to initiate the fermentation.
BHUSHAN & JOSHI: BAKER’S YEAST PRODUCTION UNDER FED BATCH CULTURE FROM APPLE POMACE
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Fermentation Kinetics
Fermentation kinetics parameters (SGR, CYC and total biomass) were determined20,21, whereas generation time was calculated22. The biomass at any point of time in a variable fed batch system is: xt = x0+Y (SR -S)
…(1)
where xt and x0 are the total amount of biomass in bioreactor at any time ‘t’ and zero time, respectively. SR is the initial substrate concentration; S is the residual substrate concentration and Y is the yield factor (g biomass/g substrate consumed). The specific growth is determined by Fig. 1 — Schematic representation of baker’s yeast production 1) Exhaust port; 2) Head plate; 3) Impeller; 4) Cooling; 5) Aeration supply; 6) Heating pad; 7) Air flow meter; 8) Water inlet; 9) Temperature sensor; 10) Incremental feeding; 11) Water outlet; and 12) DO probe
F.SR = u (X/Y)
…(2)
F.SR. Y u = ————— X
…(3)
Fed-batch Cultivation of Baker’s Yeast
where X = x v, cell concentration and volume at time ‘t’, respectively. In the equation, ‘F’ is the flow rate of medium (dm3 h-1) incrementally fed to the bioreactor, ‘X’ the total biomass in a bioreactor, ‘Y’ the yield factor and ‘u’ is the specific growth rate. The generation time (G) was calculated as:
Replicated fermentations were performed in a variable fed batch mode in a 5 l bioreactor (Bioflo, 2000, New Brunswick Sci. Co., New Jersy), which was computer controlled having advanced fermentation software (Fig. 1). APE (1:6) contained fermentable sugar (40-45 g l-1), supplemented with yeast extract and beef extract (0.3% each), peptone (0.5%) and ammonium sulphate (1.8 %). The fermentation conditions were as per the standardized parameters used in an earlier experiments12. The initial volume in the vessel of fermenter was kept 1.5 l and rest of the medium of same concentration was fed incrementally at a flow rate of 0.39 dm3 h-1 to get the final fermentation volume at 4.5 l. After inoculation, the fermentation was run in a batch mode for 1 h as a terminal cell maturity (TCM) step whereas, agitation and continuous oxygen supply as per the standardized conditions, was provided. The same was carried out to mature the cells prior to running the fermentation in the fed batch mode. Analytical Methods
The AP medium was initially analyzed for pH and fermentable sugar. The samples were drawn after an interval of 2 h from the vessel for the off-line determination of sugar, pH, cell biomass etc. The reducing sugar was determined by Nelson Somogyi method17 while pH was estimated using digital pH meter (3030 ELTOP). The weight of fresh biomass18 after drying at 70+1°C was determined.
G = t/n = 1/v
…(4)
where ‘t’ is the time, ‘n’ number of divisions and ‘v’ is number of division per hour. Quantity of CYC was calculated on the basis of (g) biomass produced at a time ‘t’ from the total sugar consumed at that time and volumetric yield obtained (g biomass/100 ml medium). The total biomass yield (X) was the cumulative gram biomass produced during the fermentation from the total medium or substrate fed to the bioreactor at that particular time as: X = xt x Vt
…(5)
where xt and Vt are the cell biomass and volume of the medium respectively at a time ‘t’. Sugar consumed at any time was calculated from a material balance considering sugar fed, sugar present in the bioreactor and sugar withdrawn during sampling, and expressed as glucose equivalent. Dough Raising Capacity (DRC)
A known volume of dough (made with respective yeasts) was taken in a 250 ml measuring cylinder,
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Table 1 — Effect of optimized parameters on the growth characteristics** of baker’s yeast under fed batch culture Time (h) Residual sugar % glucose
Ethanol %
Volumetric yield %
Specific growth rate h-1
Cellular yield coefficient g/g sugar
Total biomass Theoretical Deviation yield biomass yield g g
0
4.34+0.130
0.00
0.05+0.001
-
-
-
-
-
1*
2.15+0.171
0.03+0.018
1.03+0.091
0.32+0.010
0.52+0.019
15.46+1.164
16.42+1.067
0.097
3
1.87+0.044
0.08+0.006
1.15+0.050
0.28+0.021
0.47+0.021
26.20+1.199
28.12+0.725
1.924
5
1.76+0.052
0.24+0.087
1.26+0.035
0.25+0.029
0.46+0.016
38.60+1.143
39.43+0.310
0.833
7
1.56+0.140
0.34+0.054
1.30+0.059
0.24+0.016
0.45+0.022
49.77+2.243
53.44+1.423
3.669
9*
1.30+0.041
0.46+0.032
1.47+0.084
0.21+0.024
0.47+0.017
67.91+1.679
70.22+0.630
2.309
10
0.46+0.160
0.54+0.047
1.82+0.090
0.18+0.033
0.48+0.009
85.23+1.855
89.01+1.227
3.782
Mean
0.94
0.41
1.34
0.24
0.48
47.03
49.11
2.248
CD(p
