Fermentor productivity

Mehala, M. A., Cheryan, M. and Argondelis, A. 1985. Conversion of whey permeate to ethanol. Improvement of fermentor productivity using membrane reactors. Cult. Dairy Prod. J. 20, 9-12. 

Mobile Cells. Figure 3.10275 shows one of the earliest concepts of a continuous membrane fermentor. The membrane retains the biomass while products of the fermentation are continuously withdrawn through the membrane. In addition, metabolic waste products, which inhibit cell growth, are removed continuously through the membrane, thereby improving cell growth and fermentor productivity. 

In 1970, Porter and Michaels84 first suggested the use of membranes to enhance fermentor productivity traditional fermentors could in fact be coupled to a membrane separation unit in a configuration called a "membrane fermentor" where flux through the membrane was pressure driven. Since then a number... 

Figure 7.44 shows the typical dependence at steady state of substrate and product concentrations, and of productivity on permeate flow rate, i.e., dilution rate, for lactose fermentation to ethanol by Kluyveromices fragilis in a cell recycle membrane fermentor.89 As dilution rate increases fermentor productivity increases, attains a maximum value and then decreases. Product and substrate concentrations in the permeate, instead, steadily decrease and increase, respectively, as dilution rate increases. A compromise generally has to be made between production rate and product concentration in the effluent. When the absence of substrate in the permeate is required, it obviously limits fermentor productivity, as in the case of wastewater treatment in the dairy industry. On the other hand, low substrate concentrations in the permeate keep recovery... 

Candida boidinii (18) or Pichia pastoris (19) grown on methanol are useful sources of FDH. Expression of T. intermedins PDH in P. pastoris, inducible by methanol, yielded both enzymes from a single fermentation. Formate dehydrogenase activity per gram wet cells in P. pastoris was 2.7-fold greater than for C. boidinii and fermentor productivity was... 

Fig. 42. Integrated distillation/pervaporation plant for ethanol recovery from fermentors. The distillation columns concentrate the ethanol—water mixture from 5 to 80%. The pervaporation membrane produces a 99.5% ethanol product stream and a 40—50% ethanol stream that is sent back to the distillation...

Bakers Yeast Production. Bakers yeast is grown aerobicaHy in fed-batch fermentors under conditions of carbohydrate limitation. This maximizes the yield of yeast biomass and minimizes the production of ethanol. Yeasts grown under these conditions have exceUent dough leavening capabHity and perform much better in the bakery than yeast grown under anaerobic conditions. 

Continuous processes may be used for the production of yeast biomass. Raw Hquid feed is added continuously to the fermentor and an equal volume of fermentor Hquid is removed to harvest the yeast ceUs. These may be a single homogeneous fermentation in a stirred fermentor or two fermentors in series. Growth rates are high a typical dilution rate in the production of C. utilis on sulfite waste Hquor is 0.25, ie, one-fourth of the fermentor volume is harvested hourly. 

The dried malted barley is ground and mashed in a tub, after which the Hquid portion is drained off, cooled, and placed in the fermentor. After fermentation, a batch distillation system is usually used to separate the whisky from the fermented wort. The stiU consists of a copper ketde with a spiral tube or "worm" leading from the top. The dimensions and shape of the stills have a critical effect on the character of the whisky. The product taken off in the first part of the distillation is called foreshots (heads). The middle portion is the high wines and the last portion is the feints (tails). The middle portion is redistilled at the 140—160° proof (70—80%) range and matured in used oak cooperage. 

After 30 hours, the maximum and critical fermentation is underway and the pH must remain above 4.0 for optimal fermentation. However, accompanying bacterial contamination from various sources such as yeast contamination, improper cleaning procedures, slow yeast growth, or excessive temperatures can result in a pH below 4.0. The remaining amylase enzymes, referred to as secondary conversion agents, are inactivated and can no longer convert the dextrins to maltose. Under these circumstances, the fermentor pH continues to drop because of acid production of the bacteria, and the pH can drop to as low as 3.0. The obvious result is a low ethanol yield and quaUty deterioration. 

A commercial bacterial cellulose product (CeUulon) was recently introduced by Weyerhaeuser (12). The fiber is produced by an aerobic fermentation of glucose from com symp in an agitated fermentor (13,14). Because of a small particle diameter (10 P-m), it has a surface area 300 times greater than normal wood cellulose, and gives a smooth mouthfeel to formulations in which it is included. CeUulon has an unusual level of water binding and works with other viscosity builders to improve their effectiveness. It is anticipated that it wiU achieve GRAS status, and is neutral in sensory quaUty microcrystaUine ceUulose has similar attributes. 

The alternative to batch mode operation is continuous operation. In the continuous mode there is a continuous flow of medium into the fermentor and of product stream out of the fermentor. Continuous bioprocesses often use homogenously mixed whole cell suspensions. However, immobilised cell or enzyme processes generally operate in continuous plug flow reactors, without mixing (see Figure 2.1, packed-bed reactors). 

Studies show that the production of 1kg dry biomass requires 2.0 kg sugar, 0.7 kg oxygen, 0.1 kg ammonia, with the liberation of 12,300 k Joules heat. A typical continuous fermentation operates at a dilution rate (D) = 0.2 h 1, with sugar concentration of 3% (w/v) in the incoming medium. With a fermentor of 50 m3 capacity and 90% utilisation of carbohydrate [ie 0.3% (w/v) sugar in the outgoing medium] what would be ... 

Figure 4.5 Fermentor types used in the production of SCP from sugar sources, (a) Stirred baffled system, (b) and (c) air lift systems...

Figure 4.7 fermentor types used In the production of SCP from n-parafflns... 

True. Batch cultures give lower overall outputs than continuous cultures, as they suffer from non-productive down-time (the time taken to empty, clean, re-sterilise and re-fill the fermentor). After inoculation, considerable time can be taken for biomass to build up to a level where substrates are effectively utilised. Continuous cultures do not suffer such drawbacks once they are in operation. 

A. niger remains the organism of choice for the production of citric acid. In a submerged fermentor, either purified compressed ah or oxygen and agitation are used. Molasses are a desirable raw material for citric acid fermentation because of their availability and relatively low price. 

Fig. 5. Growth curve and PHAs production during batch fermentation of sweet sorghum by Bacillus arybhattai in 3 L fermentor (Tanamool et al., 2011)...

Fed-batch fermentation process is a production technique between batch and continuous fermentation. A proper medium feed rate is required to add sequentially into the fermentor during the process and the product is harvested at the end of fermentation just like a batch type. 

Fig. 6 shows a fed batch fermentation of sweet sorghum juice (SSJ) by Bacillus aryabhattai in 3 L fermentor under cultivating condition with agitation rate at 200 rpm, air rate of 1.5 1/min, at 30° C and feeding time at 18 and 24 hr during log phase of the culture. It was found that the cell could continuously produce both biomass and PHAs. Maximum cells were obtained at about 14.20 g/1 at 54 hr when PHAs content reached 4.84 g/1 after 66 hr (Tanamool et al., 2011). In addition, in Table 2, fed batch fermentation by A, latus was used for the production of PHAs (Yamane et al, 1996 Wang Lee, 1997). It could yield high productivity with the use of cheap carbon sources. 

---