The use of a fermenter with very high KLa is not practical in industrial-scale fermentation process because power consumption for agitation will be uneconomically large. Hence, we developed a new culture method using a

conventional-type fermenter under low O2 conditions. In this method, heterotrophic cultivation using fructose as carbon source was first carried out for exponential cell growth. After the fructose in the medium was exhausted, the culture broth was centrifuged. The harvested cells were suspended in sterilized mineral medium and autotrophic cultivation for P(3HB) accumulation was performed by feeding a substrate gas mixture in which the O2 concentration was below 6.9%(second culture stage). This method is referred as a two-stage culture method. The cell and P(3HB) concentrations were 26.3 g«dnr3 and 21.6 gednr3, respectively after 40 h of autotrophic cultivation. The average productivity of P(3HB) in the autotrophic stage was about 0.56 gedm-3»h_1. According to previous reports (9-20) on the fermentative production of P(3HB), the productivity of P(3HB) was 0.08 — 4.00 g-dm-3»^1 in heterotrophic cultures and 0.04 -1.54 g*dm*3»h_1 in autotrophic cultures. Although the KLa of the fermenter used in this experiment was 340 h_1 and the O2

СНз OW CH2 о

1 11 1

о-сн-ст —c 41 о — си — cm — c

‘n4 yn

PoIy-(D-3-hydroxybtyrate-co-D-3-hydroxyvalerate)

Figure 1 Chemical structure of poly-D-hydroxybutyric acid.

1,02 cylinder; 2, H2 cylinder;

З, CO2 cylinder; 4, gas reservoir;

5, differential-pressure type flowmeter; 6, DO controller;

7, alkali feeding pump;

8, pH controller; 9, NH3 solution;

10, fermentor; 11, circulating pump;

12, mixing pump;

13, internal pressure sensor;

14, saline replacement;

15, flowmeter

I

concentration in the gas mixture was very low, a relatively high P(3HB) production rate was obtained. In this culture method, CO2 is evolved from fructose during the heterotrophic stage. However, the two-stage culture works as a CO2 absorption system because the amount of CO2 consumed during the autotrophic stage is about twice that of the heterotrophic stage.