In order to ensure that Quorn is produced according to approved health regulations, the fermentation process is currently supported on glucose (nearly all of which is obtained from maize) and approved inorganic medium constituents and vitamins. However, the process may also be applied with various sources of starch as the carbon source
(Anderson et al. 1975; Steinkraus 1986). The production of Fusarium venenatum strain A3/5 takes place in turbidostat culture using air-lift fermenters of 155 m3 in volume and 50 m tall, weighing over 250 tons each. The Quorn fermenters are the largest operating air-lift fermentation facility in the world to date (Marlow Foods communication 2002).
Each fermenter operates as a loop where culture medium is circulating (Figure 2). As the liquid flows through the bottom of the loop, air is pumped in. Circulation is induced by a rising column of air bubbles providing good oxygen transfer conditions. This circulation is maintained due to mean density difference between riser and downcomer. The bottom of the downcomer hosts the glucose, biotin, and mineral salts intake. The nitrogen supply is delivered separately in the form of ammonia along with sterile air at the base of the riser. The supply of ammonia to the culture is regulated by a pH monitor set to give a culture pH of approximately 6.0. The dilution rate of the process ranges between 0.17 and 0.2h_1 and is operated so that glucose is always in excess and the fungus always grows at mmax at a biomass concentration of 10 to 15 g L_1 (Trinci 1994). The culture is kept at a temperature of approximately 30°C by a heat exchanger set into the riser.
The dilution rate of the fermenter results in an output of 30 tons of liquid per hour. Harvesting by filtration and RNA reduction ensues. The harvested biomass (Figure 3) containing 8-9% (w/w) RNA is heated to 68°C for 25 min at a pH of 5-6. This results in the reduction of RNA to ca. 1% (w/w), at the expense of losing up to one third of the total mass, including dissolved salts, RNA, internal water, carbohydrates, and protein.
Studies on the incidence of the production phase duration on product cost and commercial viability had indicated that periods above 200 h operation can be necessary to result in consistent unit costs of production (Trilli 1977).
In principle, a continuous culture may run indefinitely, as long as contamination is kept under check. However, cultivation beyond 100 generations (about 400 h) of F. venenatum may result in the appearance of highly branched colonial mutants which can alter the texture of the final product (Wiebe and Trinci 1991; Wiebe et al. 1992; Wiebe et al. 1995).
The details behind the appearance of mutations in continuous culture, with specific reference to F. venenatum strain A3/5 have been studied by external research independent to the production process (Trinci 1992; Trinci 1994). From these studies, the authors obtained the conclusion that spontaneous mutant appearance may be managed by careful manipulation of the selective pressure imposed through culture conditions. The above-described strategies however have been successfully tested in laboratory conditions. These strategies do not conform to the industrial production process and are not necessarily used commercially (Marlow Foods communication 2002).
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