Как выбрать гостиницу для кошек
14 декабря, 2021
In fed-batch processes (or semi-continuous processes), the substrate and required nutrients are added continuously or intermittently to the initial medium after the start of cultivation or from the point halfway through the batch process. Fed-batch processes have been utilized to avoid utilizing substrates that inhibit growth rate if present at high concentration, to overcome catabolic repression, to demand less initial biomass, to overcome the problem of contamination, and to avoid mutation and plasmid instability found in continuous culture. Furthermore, fed — batch processes do not face the problem of washout, which can occur in continuous fermentation. A major disadvantage of a fed-batch process is the need for additional control instruments that require a substantial amount of operator skill. In addition, for systems without feedback control, where the feed is added on a predetermined fixed schedule, there can be difficulty in dealing with any deviation (i. e., time courses may not always follow the expected profiles) [70]. The fed-batch processes without feedback control can be classified as intermittent fed-batch, constant-rate fed-batch, exponential fed-batch, and optimized fed-batch.
The fed-batch processes with feedback control have been classified as indirect-control and direct-control fed-batch processes [70, 71].
The fed-batch technique is one of the promising methods for fermentation of dilute-acid hydrolyzates of lignocellulosic materials. The basic concept behind the success of this technique is the capability of in situ detoxification of hydrolyzates by the fermenting microorganisms. Since the yeast has a limited capacity for conversion of the inhibitors, the achievement of a successful fermentation strongly depends on the feed rate of the hydrolyzate. By adding the substrate at a low rate in fed — batch fermentation, the concentrations of bioconvertible inhibitors such as furfural and HMF in the fermentor remain low, and the inhibiting effect therefore decreases. At a very high feed rate, using an inhibiting hydrolyzate, both ethanol production and cell growth can stop, whereas at a very low feed rate, the hydrolyzate may still be converted, but at a very low productivity rate, which has been experimentally confirmed. Consequently, there should exist an optimum feed rate [15, 18, 21].
Similar to batch operations, higher optimum dilution rate in fed — batch cultivation can be obtained by (a) high initial cell concentration, (b) increasing the tolerance of microorganisms against the inhibitors, and (c) choosing optimal reactor conditions to minimize the effects of inhibitors. Productivity in fed-batch fermentation is generally limited by the feed rate which, in turn, is limited by the cell-mass concentration [21].