PRODUCTION Of NUTRIENT SOURCES FOR RHIZOBIUM

Starch fermentation was conducted in lab scale by using simple process, noncontrolled
pH, the flask was shaking with 200 rpm, at 28 o C. The forth levels: 1, 2, 3 and 4% of
inoculum sizes were used in this study. Figure 40 and 41 showed the time courses of mannitol
production in 2% Khao-klong starch medium. The effects of inoculum sizes were demonstrated in the
same pattern of fermentation. The pH level was rapidly decreased from initial pH at 6.0 to 5.3 in the
first day of cultivation and again increased to 6.4 throughout fermentation period. It was suggested
that when the reducing sugars was consumed, the main product was organic acids that decreased pH
value. Relatively, the reducing sugars was reached maximum at the first day of cultivation. The high
amounts of starch were rapidly hydrolysed from the initial culture to the second day of incubation.
Mannitol in the yeast cell were observed for the high amounts at the first day and gave the maximum
yield at the second day of fermentation. After that it gradually decreased at the end of fermentation.
The results clearly suggested that at the first day of cultivation, starch was vigorously
hydrolysed by the amylase-producing yeast KAY1. At the same time, the high amounts of reducing
sugars flow into the medium, and was absorbed by yeast cell for its energy sources. The
concentrations of reducing sugars in the medium which defined as osmotic pressure (Reed et al.,
1987) were supported the construction of polyols in the yeast cell. For this experiment, the yeast
isolate KAY1 could produced the mannitol inside the cells. However, after the reducing sugars
gradually declined, consequently the mannitol in the cell was also decreased. It was noticed that
mannitol is a common reserve product of fungi (Lee, 1907). When the osmotic pressure in the
cultured medium reduced. Mannitol intracellular was converted to the energy sources. Low molecular
mass compounds were accumulated intracellularly in most organisms when exposed to osmotic stress
to equilibrate the cytoplasmic water activity (a w) with the a w of the surrounding environment (Yancey
et al., 1982; Csonka, 1989). The main solutes accumulated in yeasts exposed to osmotic stress are
polyhydroxy alcohols (polyols) such as glycerol, D-arabitol, D-mannitol, and meso-erythritol
(Spencer, 1978), and are compatible with metabolic activity (Brown, 1978). The polyol accumulated
is related to the yeast species, the growth phase of the yeast (Nobre and da Costa, 1985) and the
carbon source used for growth (Van Eck et al., 1989). The terms "osmophilic" and "osmotolerant"
have been widely used to described the water relations of yeasts.
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