Effect of ultrafiltration of yeast extracts on their ability to promote lactic acid bacteria growth

Five yeast extracts (YE) were fractionated by ultrafiltration (UF) with 1, 3, and 10 kDa molecular weight cutoff membranes, concentrated by freeze-drying, and the resulting powders of yeast extract filtrates (YEF) were evaluated for their growth-promoting properties on nine cultures of lactic acid bacteria (LAB). There was an increase in alpha-amino nitrogen content of the YEF powders as the pore size of the UF membranes used to filter the YE solutions decreased. The source of YE had a much greater effect than UF on the growth of LAB. This was also the case for the YEF contents in total and alpha-amino nitrogen. Growth curves of the LAB showed that maximum growth rate (mumax) data were on average 30% higher with bakers' YE than with brewers' YE, while maximum optical density (ODmax) values were on average 16% higher with bakers' YE. This could be related to the higher nitrogen content of the bakers' YE used in this study. Modification by UF of the YE had no significant influence on the growth of 4 of the 9 LAB strains. The three strains of Lactobacillus casei were negatively influenced by UF, as they did not grow as well in the media containing the YEF obtained after filtering with 1 and 3 kDa membranes. On a total solids basis, the 2.5 x retentates from the 10 kDa membrane gave, on average, 4% lower mumax and 5% lower ODmax values as compared to cultures where the corresponding YEF was used as medium supplement. This could also be partially related to the different nitrogen contents of the filtrates and retentates.     

Development of bacterial contamination during production of yeast extracts.

Baker's yeast suspensions having bacterial populations of 10(6) and 10(8) CFU/ml were subjected to autolysis processes designed to obtain yeast extracts (YE). The bacterial contaminants added to the yeast cell suspensions were produced with spent broths obtained from a commercial yeast production plant and contained 59% cocci (Leuconostoc, Aerococcus, Lactococcus) as well as 41% bacilli (Bacillus). Autolyses were conducted at four different pH levels (4.0, 5.5, 7.0, and 8.5) and with two autolysis-promoting agents (ethyl acetate and chitosan). Processing parameters were more important than the initial bacterial population in the development of contaminating bacteria during manufacture of YE. Drops in the viable bacterial population after a 24-h autolysis were observed when pH was adjusted to 4.0 or when ethyl acetate was added. A significant interaction was found between the effects of pH and autolysis promoters on the bacterial population in YE, indicating that the activity of ethyl acetate, as opposed to that of chitosan, was not influenced by pH.     

Effect of high pressure homogenization and papain on the preparation of autolysed yeast extract

The effect of high pressure homogenization (600 and 1000 bar) prior to autolysis of a commercial pressed baker's yeast was examined. High pressure homogenization released a maximum of 30% of the solids and 34% of the total nitrogen (TN). After autolysis of the whole homogenized slurry, high yields of solids and TN (up to 81 and 85%, respectively) were obtained. Autolysis of non-homogenized controls yielded much lower yield values (30 and 39%, respectively), whereas autolysis in the presence of papain but without prior disruption gave intermediate values (50 and 61%, respectively). The various treatments led to changes in the extract composition: standard autolysates had the highest total nitrogen and true protein weight contents and the lowest carbohydrate content, whereas this trend was reversed when cells were first disrupted before autolysis. In contrast to controls obtained by standard autolysis without or with papain, centrifuged autolysates from pre-homogenized fractions were not clear. Treatment with a combination of a flocculation and a weighting agent clarified the extracts but resulted in a loss of solids (approximately 20%), including nitrogen and carbohydrates.     

Production of bakers' yeast in cheese whey ultrafiltrate

A process for the production of bakers' yeast in whey ultrafiltrate (WU) is described. Lactose in WU was converted to lactic acid and galactose by fermentation. Streptococcus thermophilus was selected for this purpose. Preculturing of S. thermophilus in skim milk considerably reduced its lag. Lactic fermentation in 2.3x-concentrated WU was delayed compared with that in unconcentrated whey, and fermentation could not be completed within 60 h. The growth rate of bakers' yeast in fermented WU differed among strains. The rate of galactose utilization was similar for all strains, but differences in lactic acid utilization occurred. Optimal pH ranges for galactose and lactic acid utilization were 5.5 to 6.0 and 5.0 to 5.5, respectively. The addition of 4 g of corn steep liquor per liter to fermented WU increased cell yields. Two sources of nitrogen were available for growth of Saccharomyces cerevisiae: amino acids (corn steep liquor) and ammonium (added during the lactic acid fermentation). Ammonium was mostly assimilated during growth on lactic acid. This process could permit the substitution of molasses by WU for the industrial production of bakers' yeast.     

The evaluation of mixtures of yeast and potato extracts in growth media for biomass production of lactic cultures

The effectiveness of yeast extracts (YE) and potato extracts (PE) to promote growth of seven lactic cultures was evaluated by automated spectrophotometry (AS). Two aspects of the growth curve were analysed: (1) maximum biomass obtained (using ODmax) and (2) highest specific growth rate mu(max)) Eleven lots from the same PE-manufacturing process were examined for lot-to-lot variability. The ODmax values of three of the seven strains were significantly affected by lot source, but mu(max) was not significantly affected. The growth of bacteria was systematically lower in base medium containing 100% PE than in base medium containing 100% YE for both ODmax or mu(max) data, which could be related to the lower content in nitrogen-based compounds in PE. In AS assays, highest OD values for Lactobacillus casei EQ28, Lactobacillus rhamnosus R-011, Lactobacillus plantarum EQ12, and Streptococcus thermophilus R-083 were obtained with a mixture of PE and YE. Fermentations (2 L) were also carried out to determine the accuracy of AS to predict biomass levels obtained under fermentation trials. In these fermentations, replacement of 50% YE with PE was shown to enable good growth of S. thermophilus. With L. rhamnosus R-011, a high correlation (R2 = 0.95) was found between ODmax data obtained in the AS assays and that of the 2-L bioreactor when the same growth medium was used for both series of fermentations. However, AS was not as efficient when industrial media were used for the bioreactor assays. The relationship was still good for ODmax between AS data and that of the bioreactor data with L. rhamnosus R-011 in industrial LBS medium (R2 = 0.87), but was very poor with the S. thermophilus R-083 on Rosell #43 industrial medium (R2 = 0.33). Since PE cost 40% less than YE, there are strong economic advantages in considering such a partial replacement of YE by PE.     

Development of Bacterial Contamination during Production of Yeast Extracts

Baker’s yeast suspensions having bacterial populations of 10⁶ and 10⁸ CFU/ml were subjected to autolysis processes designed to obtain yeast extracts (YE). The bacterial contaminants added to the yeast cell suspensions were produced with spent broths obtained from a commercial yeast production plant and contained 59% cocci (Leuconostoc, Aerococcus, Lactococcus) as well as 41% bacilli (Bacillus). Autolyses were conducted at four different pH levels (4.0, 5.5, 7.0, and 8.5) and with two autolysis-promoting agents (ethyl acetate and chitosan). Processing parameters were more important than the initial bacterial population in the development of contaminating bacteria during manufacture of YE. Drops in the viable bacterial population after a 24-h autolysis were observed when pH was adjusted to 4.0 or when ethyl acetate was added. A significant interaction was found between the effects of pH and autolysis promoters on the bacterial population in YE, indicating that the activity of ethyl acetate, as opposed to that of chitosan, was not influenced by pH.     

Induction of rosmarinic acid biosynthesis in Lithospermum erythrorhizon cell suspension cultures by yeast extract

Summary A transient increase in rosmarinic acid (RA) content in cultured cells of Lithospermum erythrorhizon was observed after addition of yeast extract (YE) to the suspension cultures, reaching a maximum at 24 hr. The highest increase of the RA content (2.5-fold) was obtained when 6-day-old cells in the exponential growth phase were treated with YE. Preceding the induced RA accumulation, phenylalanine ammonia-lyase (PAL) activity increased rapidly, whereas tyrosine aminotransferase (TAT) activity was largely unaffected by the treatment. The incorporation of both ¹⁴C-phenylalanine and ¹⁴C-tyrosine into RA was enhanced in the YE-treated cells, consistent with increased synthesis of the ester.Abbreviations 2,4-D 2,4 dichlorophenoxyacetic acid - PAL phenylalanine ammonia-lyase - TAT tyrosine aminotransferase - RA rosmarinic acid - YE yeast extract     

Barley pathogenesis-related proteins with fungal cell wall lytic activity inhibit the growth of yeasts.

Proteins from intercellular fluid extracts of chemically stressed barley (Hordeum vulgare L.) leaves were separated by native polyacrylamide gel electrophoresis at alkaline or acid pH. Polyacrylamide gels contained Saccharomyces cerevisiae (bakers' yeast) or Schizosaccharomyces pombe (fission yeast) crude cell walls for assaying yeast wall lysis. In parallel, gels were overlaid with a suspension of yeasts for assaying growth inhibition by pathogenesis-related proteins. The same assays were also performed with proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. In alkaline native polyacrylamide gels, only one band corresponding to yeast cell wall lytic activity was found to be inhibitory to bakers' yeast growth, whereas in acidic native polyacrylamide gels one band inhibited the growth of both yeasts. Under denaturing nonreducing conditions, one band of 19 kD inhibited the growth of both fungi. The 19-kD band corresponded to a basic protein after two-dimensional gel analysis. The 19-kD protein with yeast cell wall lytic activity and inhibitory to both yeasts was found to be different from previously reported barley chitosanases that were lytic to fungal spores. It could be different from other previously reported lytic antifungal activities related to pathogenesis-related proteins.     

Fed-batch cultivation of bakers’ yeast: Effect of nutrient depletion and heat stress on cell composition

The physiology of a commercial strain of bakers' yeast was studied in terms of the cell composition under different growth conditions and of its response to stress. The study comprised fed-batch experiments since this is the system used in bakers' yeast industry. The classical fed-batch fermentation procedure was modified in that the yeast cells were continuously grown to a steady-state at a dilution rate of 0.1/h in order to achieve more or less the same initial starting point in terms of cell composition. This steady-state culture was then switched to fed-batch concomitantly with exposure to stress. The highest amount of trehalose accumulation was achieved when nutrient depletion and heat stress were applied concomitantly. The highest amount of trehalose, 12%, was attained in cells stressed by both nitrogen depletion and heat stress. The protein content remained constant, although with some oscillations, at a value of 30% throughout this dual stress experiment.     

Evaluation of liquid residues from beer and potato processing for the production of docosahexaenoic acid (C22:6n-3, DHA) by native thraustochytrid strains

Liquid residues from beer (RB) and potato (RP) processing were evaluated as carbon sources for the production of docosahexaenoic acid (C22:6n-3, DHA) by two native Thraustochytriidae sp., M12-X1 and C41, in shaking flask experiments. Results were compared with those obtained in the fermentations of glucose, maltose, soluble starch and ethanol. Both strains produced the highest biomass concentration (2.3 g/L) in the fermentation of RB supplemented with nitrogen sources [yeast extract (YE) and monosodium glutamate (MSG)]. DHA content in the fatty acids produced by the native thraustochytrids was dependent on the fermented carbon source; the fatty acids from biomass grown on carbon sources that permitted a lower growth rate contained more DHA. The highest DHA productivity [55.1 mg/(day L)] was obtained in the fermentation of RB-YE-MSG by M12-X1 strain. In this medium, M12-X1 strain grew at a specific growth rate of 0.014 h^?1 and total fatty acid content in the biomass was 41.3%. Production of DHA by M12-X1 strain followed a non-growth rate associated pattern and DHA content in the biomass decreased significantly after growth ceased.     

Characteristics of the inhibition and metabolic inactivation of the yeast TRNA nucleotidyl transferase.

1. Yeast tRNA nucleotidyl transferase is inhibited by low molecular weight compounds present in cell-free extracts. The inhibition produced by the main component(s) is competitive with respect to ATP and is not prevented by metal chelating agents. The major component(s) has been partially purified. It is resistant to heat (90 degrees C, 5 min) and insensitive to digestion by alkaline phosphatase, snake venom phosphodiesterase and inorganic pyrophosphatase, indicating that it is not a nucleotide. 2. Besides the masking of the transferase activity in the crude extracts by the inhibitors, the enzyme is inactivated in nitrogen starved cells. The inactivation also occurs in yeast mutants lacking several proteases and is not prevented by inhibitors of yeast proteases. These results rule out extracellular proteolysis as the cause of inactivation and strength our previous observations on the metabolic inactivation of the transferase in response to nitrogen starvation.     

Improving the freeze tolerance of bakers' yeast by loading with trehalose

We examined the freeze tolerance of bakers' yeast loaded with exogenous trehalose. Freeze-tolerant and freeze-sensitive compressed bakers' yeast samples were soaked at several temperatures in 0.5 M and 1 M trehalose and analyzed. The intracellular trehalose contents in both types of bakers' yeast increased with increasing soaking period. The initial trehalose-accumulation rate increased with increasing exogenous trehalose concentration and soaking temperature. The maximum trehalose content was almost identical (200-250 mg/g of dry cells) irrespective of the soaking temperature and the type of bakers' yeast, but depended on the exogenous trehalose concentration. The leavening ability of both types of bakers' yeast loaded with trehalose was almost identical to that of the respective original cells, irrespective of the soaking conditions. The freeze-tolerant ratio (FTR) of both types of bakers' yeast increased with increasing intracellular trehalose content. However, FTR decreased during over-soaking after the maximum amount of trehalose had accumulated. FTR of the freeze-sensitive bakers' yeast was more efficiently improved than that of the freeze-tolerant type.     

Nutritional evaluation of Cyanobacterium (Nostoc sp.) extract in Rhizobium cultures

Extracts from Nostoc sp. biomass (CE) were compared with yeast extract (YE) in the formulation of media for cultivation of Rhizobium etli and Bradyrhizobium japonicum. Growth depended on the rhizobium strain and also on the type and concentration of the extract. The growth kinetics of R. etli were substantially influenced by the addition of CE an YE at concentrations between 0.3 and 1% (w/v); cultures containing YE showed shorter lag periods and higher growth rates and those with CE presented higher growth yields. Depending on the extracts, a different behaviour was displayed by the cultures at the stationary phase; cell viability increased or remained fairly constant in cultures developed on CE, and those developed on high concentrations of YE showed a decline in viable cell counts.     

Nutritional evaluation of Cyanobacterium (Nostoc sp.) extract in Rhizobium cultures

Extracts from Nostoc sp. biomass (CE) were compared with yeast extract (YE) in the formulation of media for cultivation of Rhizobium etli and Bradyrhizobium japonicum. Growth depended on the rhizobium strain and also on the type and concentration of the extract. The growth kinetics of R. etli were substantially influenced by the addition of CE an YE at concentrations between 0.3 and 1% (w/v); cultures containing YE showed shorter lag periods and higher growth rates and those with CE presented higher growth yields. Depending on the extracts, a different behaviour was displayed by the cultures at the stationary phase; cell viability increased or remained fairly constant in cultures developed on CE, and those developed on high concentrations of YE showed a decline in viable cell counts.     

The autolysis of industrial filamentous fungi

Fungal autolysis is the natural process of self-digestion of aged hyphal cultures, occurring as a result of hydrolase activity, causing vacuolation and disruption of organelle and cell wall structure. Previously, authors have considered individual aspects of fungal lysis, in terms of either an enzyme, a process or an organism. This review considers both the physiology and morphology of fungal autolysis, with an emphasis on correlations between enzymological profiles and the morphological changes occurring during culture degeneration. The involvement of the main groups of autolytic hydrolases is examined (i.e., proteases, glucanases, and chitinases), in addition to the effects of autolysis on the morphology and products of industrial bioprocesses. We call for a concerted approach to the study of autolysis, as this will be fundamental for research to progress in this field. Increased understanding will allow for greater control of the prevention, or induction of fungal autolysis. Such advances will be applicable in the development of antifungal medicines and enable increased productivity and yields in industrial bioprocesses. Using paradigms in existing model systems, including mammalian cell death and aging in yeast, areas for future study are suggested in order to advance the study of fungal cell death.     

Effects of yeast extract and glucose on xanthan production and cell growth in batch culture of Xanthomonas campestris

Although available kinetic data provide a useful insight into the effects of medium composition on xanthan production by Xanthomonas campestris, they cannot account for the synergetic effects of carbon (glucose) and nitrogen (yeast extract) substrates on cell growth and xanthan production. In this work, we studied the effects of the glucose/yeast-extract ratio (G/YE) in the medium on cell growth and xanthan production in various operating modes, including batch, two-stage batch, and fed-batch fermentations. In general, both the xanthan yield and specific production rate increased with increasing G/YE in the medium, but the cell yield and specific growth rate decreased as G/YE increased. A two-stage batch fermentation with a G/YE shift from an initial low level (2.5% glucose/0.3% yeast extract) to a high level (5.0% glucose/0.3% yeast extract) at the end of the exponential growth phase was found to be preferable for xanthan production. This two-stage fermentation design both provided fast cell growth and gave a high xanthan yield and xanthan production rate. In contrast, fed-batch fermentation with intermittent additions of glucose to the fermentor during the stationary phase was not favorable for xanthan production because of the relatively low G/YE resulting in low xanthan production rate and yield. It is also important to use a moderately high yeast extract concentration in the medium in order to reach a high cell density before the culture enters the stationary phase. A high cell density is also important to the overall xanthan production rate. Received: 30 September 1996 / Received revision: 21 January 1997 / Accepted: 10 February 1997     

Evaluation of yeast extracts as growth media supplements for lactococci and lactobacilli by using automated spectrophotometry

An automated spectrophotometric (AS) method was used to evaluate the growth-promoting ability of yeast extracts (YE) on cultures of Lactobacillus acidophilus and Lactococcus lactis subsp. cremoris. The AS data were compared to that obtained from classical shake flask fermentations and from 250 ml bioreactors equipped with pH control. In assays involving the evaluation of 26 different commercial YE, maximum growth rate (&mgr;(max)) values determined with the AS unit ranged from 0.25 to 0.45 h(-1) for Lb. acidophilus and from 0.10 to 0.40 h(-1) for Lc. cremoris. Good correlations were obtained between AS data and manual sampling from the shake flasks or the bioreactors for mmax, as well as maximum optical density (OD(max)). The AS method is thus useful as a screening tool for the selection of YE lots in media formulation. Species reacted differently to the 26 YE, but less variation was observed between strains of the same species. This suggests that a producer of various lactococci or lactobacilli can expect a relatively constant response to a given YE lot between strains of the same species. However, it should not be assumed that the YE having the best growth-promoting properties for Lb. acidophilus will also be the best media supplements for the growth of Lc. cremoris.     

Increased yields of Rhizobium japonicum by an extract of the green alga,Scenedesmus obliquus (276-3a)

Summary The effects of algal extract (AE) and yeast extract (YE) on the growth of Rhizobium japonicum were studied. YE concentrations above 0,1% inhibited growth. Increased concentrations of AE in growth medium increased bacterial yield up to an extract concentration of 1%. At 1% AE concentration maximum yields were achieved which were 3 times greater than those achieved with YE.     

A method for the detection of Issatchenkia orientalis in a baker's yeast factory

Summary A method for the detection of Issatchenkia orientalis (Candida krusei), a contaminant of bakers' yeast, is described. Nine different liquid medium types were compared and maximum specific growth rates for the yeast contaminant were determined in each medium. Issatchenkia orientalis grew fastest in malt extract broth (0,37 h^-1) and potato dextrose broth (0,36 h^-1). Five antibiotics were tested for selective inhibition of seven different bakers' yeast strains in malt extract broth. Nystatin was the only antibiotic tested that inhibited the growth of all the bakers' yeast strains used, but did not affect the growth of I. orientalis.     

Optimal quality control of bakers' yeast fed-batch culture using population dynamics

An optimal quality control policy for the overall specific growth rate of bakers' yeast, which maximizes the fermentative activity in the making of bread, was obtained by direct searching based on the mathematical model proposed previously. The mathematical model had described the age distribution of bakers' yeast which had an essential relationship to the ability of fermentation in the making of bread. The mathematical model is a simple aging model with two periods: Nonbudding and budding. Based on the result obtained by direct searching, the quality control of bakers' yeast fed-batch culture was performed and confirmed to be experimentally valid.