Yeast permeation

Table 5 presents typical operating conditions and cell production values for commercial-scale yeast-based SCP processes including (63) Saccharomjces cerevisae ie, primary yeast from molasses Candida utilis ie, Torula yeast, from papermiU. wastes, glucose, or sucrose and Klujveromjces marxianus var fragilis ie, fragihs yeast, from cheese whey or cheese whey permeate. AH of these products have been cleared for food use in the United States by the Food and Dmg Administration (77). 

K. marxianus var. fragilis which utilizes lactose, produces a food-giade yeast product from cheese whey or cheese whey permeates collected from ultrafiltration processes at cheese plants. Again, the process is similar to that used with C. utilis (2,63). The Provesteen process can produce fragiUs yeast from cheese whey or cheese whey permeate at cell concentrations ia the range of 110—120 g/L, dry wt basis (70,73). 

Lactose is readily fermented by lactic acid bacteria, especially Lactococcus spp. and Lactobacillus spp., to lactic acid, and by some species of yeast, e.g. Kluyveromyces spp., to ethanol (Figure 2.27). Lactic acid may be used as a food acidulant, as a component in the manufacture of plastics, or converted to ammonium lactate as a source of nitrogen for animal nutrition. It can be converted to propionic acid, which has many food applications, by Propionibacterium spp. Potable ethanol is being produced commercially from lactose in whey or UF permeate. The ethanol may also be used for industrial purposes or as a fuel but is probably not cost-competitive with ethanol produced by fermentation of sucrose or chemically. The ethanol may also be oxidized to acetic acid. The mother liquor remaining from the production of lactic acid or ethanol may be subjected to anaerobic digestion with the production of methane (CH4) for use as a fuel several such plants are in commercial use. 

Figure 7.16 An illustration of the efficiency of back-pulsing in removing fouling materials from the surface of microfiltration membranes. Direct microscopic observations of Mores and Davis [9] of cellulose acetate membranes fouled with a 0.1 wt% yeast suspension. The membrane was backflushed with permeate solution at 3 psi for various times. Reprinted from J. Membr. Sci. 189, W.D. Mores and R.H. Davis, Direct Visual Observation of Yeast Deposition and Removal During Microfiltration, p. 217, Copyright 2001, with permission from Elsevier...

Marcinkowsky et al. (16) were the first to use dynamic secondary membranes in reverse osmosis for rejection of salts. Giiell et al. (17) later investigated protein transmission and permeate fluxes in microfiltration of protein mixtures using yeast to form a predeposited secondary membrane, and they observed higher flux and protein transmission in the presence of the secondary layer. Kuberkar and Davis (18) also observed higher flux and transmission of BSA in the presence of a cake layer of yeast,... 

Fig. 5. Permeate flux during microfiltration of mixture of 2.0 g/L of BSA and 1.34 g/L of yeast with deposition of SMY and with backflushing (A), without SMY but with backflushing (x), and without deposition of SMY and without backflushing ( ) permeate flux for filtration of BSA without deposition of SMY and with backflushing...

Average permeate fluxes (normalized by the clean membrane water flux, J0) for yeast-BSA mixtures are plotted vs time in Fig. 6 for different concentrations of yeast in the primary and secondary feed reservoirs. The average flux, , was calculated by dividing the amount of net permeate collected by the time required to complete one cycle of yeast deposition, feed filtration, and backflushing (fs/+ tf + tb) the net permeate collected is... 

Fig. 6. Normalized microfiltration permeate flux vs time for different concentrations of yeast in secondary feed reservoir ( ) 1.36 g/L ( ) 0.68 g/L ( ) 0.34 g/L. The cycle conditions were tf = 280 s, = 10 s, and th = 10 s, with Pf=Ph = 7.5 psi. The primary feed contained a mixture of yeast (with the same concentration of yeast as in the secondary feed) and 2.0 g/L of BSA. The dashed line represents the normalized permeate flux during filtration of protein alone without deposition of SMY and without backflushing. Error bars represent SD for two to three repeats.

Protein transmission data (Fig. 8) indicate nearly 100% transmission of protein initially, but the fraction of protein transmitted decreased rapidly for forward filtration of a BSA-only solution and appeared to reach a steady value of only 35% transmission after 10,000 s. When a secondary membrane of yeast was deposited at the beginning of each cycle, and then filtration of a BSA solution was carried out, the protein transmission values remained at nearly 100% for about 4000 s and subsequently decreased gradually to about 60% after 18,000 s of filtration. With SMY, the amount of protein recovered in the permeate is more than two times that recovered after filtration of the BSA-only solution after 18,000 s. 

Fig. 9. Permeate flux vs time during ultrafiltration of 5.0 g/L of cellulase. The solid line represents SMY and backflushing with Pf= 30 psi, Ph = 15 psi, f( = 300 s, tsf= 5 s, and tb = 2s the line with short and long dashes represents SMY and backflushing under the same conditions but with t = 10 s. The yeast concentration in the secondary feed was 4.0 g/L. The dashed line is the permeate flux obtained without deposition of a secondary membrane or backflushing. A wall shear rate of 1300 s 1 was used. LMH = L(m2 h).

For the conditions in Fig. 9, improvements of 35-50% in the permeate flux were observed when a secondary membrane was used, owing to a reduction in the protein fouling of the primary membrane. Little or no flux recovery was observed with each backpulse, as might be expected from the relatively low resistance of the yeast layer and the irreversible nature of the protein fouling. The flux continuously declined with time owing to irreversible fouling, though the rate of decline was reduced by the SMY. 

Fig. 10. Average permeate flux after 6000 s of ultrafiltration of 5.0 g/L of cellulase for different concentrations of yeast in secondary feed for wall shear rates of 400 s 1 (— —) and 100 s x with Pf = Ph = 7.5 psi, tf=300 s, f(, = 3s,and tsf= 30 s. Also...

Alumina and other ceramic membranes of various microfiluaiion pore sizes have been used for the separation of yeast (saccharomyces cerevisiae) from the broth and the clarification of thin stillage [Cheryan, 1994]. A typical flux of 110 L/hr-m can be obtained with a crossflow velocity of 4 m/s and a transmembrane pressure of 1.7 bars. The crossflow velocity is found to markedly affect the membrane flux. Concenuation factors (ratios of final to initial concentrations) of 6 to 10 for both the broth and the stillage can be achieved. Backflushing with a frequency of every 5 minutes and a duration of 5 seconds helps maintain the flux, particularly in the initial operating period. The permeate flux for both types of separation reaches steady state after 30 to 90 minutes. 

Yeast can be separated from an ethanol fermentation broth by porous ceramic membranes with backflushing [Matsumoto et al., 1988]. Tubular alumina membranes with a nominal pore diameter of 1.6 pm were demonstrated to be effective for this application with a maximum permeate flux of 1,1(X) IVhr-m with backflushing. The permeate flux increases with increasing feed rale (or crossflow velocity) and TMP and with decreasing yeast concentration. Various backflushing techniques were investigated and the reverse flow of filtrate (instead of air) either by pressure from the permeate side... 

Figure 6.11. Effect of crossflow velocity on permeate flux of a bakers yeast suspension through a ceramic/metal mesh membrane [Gallagher, 1990]...

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