Fig. 6.1. The effect of inoculum age on growth and productivity in a streptomycete fermentation, (a) The carbon dioxide production rate (CPR) profile of the inoculum culture showing the points (1, 2 and 3) at which inocula were removed, (b) The effect of inoculum age on the CPR of the production fermentation, (c) The effect of inoculum age on productivity in the production fermentation (Parton and Willis, 1990).
r inoculum fermentation and the points at which inoculum was transferred are shown in Fig. 6.1a. The CPR of the subsequent production fermentations are shown in Fig. 6.1b, from which it may be seen that the three fermentations performed similarly. However, Fig. 6.1c illustrates the very different secondary metabolite production of the three fermentations. Thus, although the lime of transfer had only a marginal influence on biomass in the production fermentation the effect on product formation was critical. It should be emphasized that the amount of biomass transferred was standardized for the three fermentations and, thus, the differences in performance were due to the physiological states of the inocula.
In recent years, probes have been developed for on-line assessment of biomass (see Chapter 8) and these could be invaluable in estimating the time of inoculum transfer. Boulton et al. (1989) reported the use of a biomass sensor (the Bugmeter) to control the yeast pitching rate (inoculum level) in brewing. The probe measures the dielectric permittivity of viable yeast cells and is unaffected by the presence of dead cells, air bubbles or detritus, making it ideal for the routine monitoring of yeast inoculum. Using the probe, these workers developed an automatic inoculum dispenser allowing a preset viable yeast mass to be transferred from a yeast storage vessel to the brewery fermentation.
Alford et al. (1992) reported the use of a real-time expert computer system to predict the time of inoculum transfer for industrial-scale fermentations. The system involves the comparison of on-line fermentation data with detailed historical data of the process. A problem with the interpretation of carbon dioxide production rate figures is that the data are not available continuously because the analyser is not dedicated to any one fermenter, but is analysing process streams from a large number of vessels via a multiplexer system (see Chapter 8). Thus, a fermentation may have passed a critical stage between monitoring times. Also, occasional false readings may be generated. The expert system enabled the verification of data points as well as prediction of the outcome of the fermentation from early information. Data from seed fermentations were analysed by the expert system and the transfer time predicted. As a result of this approach operators were able to plan their work more effectively, the need for manual sampling was reduced and early warning of contamination was provided if the seed-culture profile predicted from early readings was abnormal.
Smith and Calam (1980) compared the quality and enzymic profile of differently prepared inocula Penicil-
Hum patulum (producing griseofulvin) and demonstrated that a low level of glucose 6-phosphate dehydrogenase was indicative of a good quality inoculum. The enzyme profile of good quality inoculum was established early in the growth of the seed culture. Thus, this approach could be used to assess the cultural conditions giving rise to satisfactory inoculum, but would be of less value in determining the time of transfer.
Yeast, unicellular bacterial, fungal and strep-tomycete fermentations have different requirements for inoculum development and these are dealt with separately.
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