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Can. J. Microbiol./Rev. Can. Microbiol. 48(7): 626-634 (2002)

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

H. Gaudreau, N. Renard, C.P. Champagne, and D. Van Horn

ABSTRACT

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 OD_max) and (2) highest specific growth rate (μ_max). Eleven lots from the same PE-manufacturing process were examined for lot-to-lot variability. The OD_max values of three of the seven strains were significantly affected by lot source, but μ_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 OD_max or μ_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 OD_max 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 OD_max 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.

Key words: yeast extracts, potato extracts, lactic acid bacteria.

*Subsidiaries of Lallemand Inc. (Montreal, Canada).
^†MRS and M17 broth were from BDH (Toronto, Canada).
^‡Fermentation time, which represents the time at which the stationary phase is reached.

Many ingredients are found in laboratory media (MRS, Elliker, or M17) or commercial starter media that are des­tined for LAB. Amino acids and vitamins are principally added through peptones, meat extracts, or yeast extracts (YE). A number of studies have shown the growth­promoting properties of YE on microorganisms (Bibal et al. 1989; Jensen and Hamer 1993; Milton et al. 1991; Ibrahim and Bezkorovainy 1994; Olmos-Dichara et al. 1997), and they are the ingredients of choice in many media. Francou (1986) has shown that lactic yeast autolysate may be used instead of YE. Meat extracts and meat-derived peptones are not used very much on a large scale because of costs, and lately they have been a health concern. Plant extracts are generally more economical than meat extracts, and there are reports of their use with LAB (e.g., tomato juice (Difco Lab­oratories 1984), corn steep liquor (Amrane and Prigent 1993; Cox and MacBean 1977), and malt extracts (Vahvaselka and Linko 1987)). There is always a need for novel, inexpensive media ingredients, and this study exam­ines the effectiveness of a potato extract (PE) novel to LAB (FNI-400; Lallemand Inc., Montr6al, Que.). PE is another product of plant origin, which has traditionally been used in growth media for yeasts (Admassu et al. 1984; Pasari et al. 1987) or molds (Chrzanowska et al. 1993; El-Banna and Leistner 1988; Murali et al. 1987). There are reports of the use of PE in media designed for bacteria (Kajimura and Kaneda 1996; Ulezlo et al. 1987), but we have found no published literature on the usefulness of PE in media de­signed for lactic cultures.

In traditional experimental plans, the nitrogen sources were studied one at a time. However, commercial media of­ten contain two nitrogen sources, and the study of multiple components is preferable to optimize growth. YE are recog­nized as being among the best nitrogen sources for LAB (Amrane and Prigent 1993; Chiarini et al. 1992; Lee et al. 1991; Aeschlimann and von Stockar 1990). Although less expensive than many peptones, YE nevertheless constitute a significant portion of media costs (Neklyudov et al. 1993; Chiarini et al. 1992), and their partial replacement with lower costing sources (some PE cost 40% less than YE) is of commercial interest. In this study, mixtures of YE and PE were added to a base medium in the aim of partially substi­tuting YE.

The optimization of growth media with multiple components is a tedious process when using fermentors or flasks; ingredients and their concentrations must be changed, which requires a great number of assays and considerable person­nel time. To answer these problems, automated spectro­photometry (AS) has been used for the determination of the growth-promoting value of YE with LAB (Potvin et al. 1997; Champagne et al. 1999) and enables the simultaneous evaluation of up to 20 media. Results from these studies have shown a good correlation between the AS data and flask assays. However, little information has been obtained on the correlation between AS data and fermentations con­ducted under pH control, particularly when the commercial media contain turbid ingredients that cannot be studied with AS.

The aim of this study is to evaluate a novel PE for LAB biomass production, in partial replacement of YE, and to further evaluate the effectiveness of AS in selecting nitrogen sources for media formulation.

Materials and methods

Organisms and growth conditions

The sources of the various lactic cultures are presented in Table 1. Stock cultures were prepared by mixing 10 mL of fresh cultures with 25 mL of 20% skim milk and 25 mL of a 20% glycerol solution (glycerol and milk were sterilized separately). The milk and glycerol cell suspensions were di­vided into 1-mL fractions, added to sterile cryovials (Nalgene, Rochester, N.Y., U.S.A.), and stored at –70°C un­til used. The inocula were prepared by adding 1 mL of thawed stock culture to the appropriate growth medium and incubating in precise conditions (Table 1). Incubations were carried out in a programmable water bath (Cole Palmer, Chi­cago, Ill., U.S.A.), and the temperatures were brought to 4°C when the desired incubation period was reached.

Yeast or potato extracts

Bakers’ YE used in this study were obtained from BioSpringer (BioSpringer, Maisons-Alfort, France) and Lallemand Inc., while brewers’ YE (X-803) were obtained by Lallemand Inc. For identification purposes, they were coded YE-BS, YE-LA, and YE-X803, respectively. The PE (FNI-400) was also supplied by Lallemand Inc. The a-amino nitrogen of PE extracts was determined by titration follow­ing reaction with formaldehyde (USP 1985). The 5% PE so­lutions were adjusted at pH 7.0 with 0.1 M NaOH or 0.1 M

HCl. Total nitrogen determination was done using a FP-428 LECO apparatus (LECO Corporation, Saint-Joseph, Mich.), operated under the following conditions: sample size, 150 mg; oxidation furnace temperature, 900°C; oxidation standby temperature, 650°C; purge cycles, 3; minimum timeout, 30 s; comparator level, 1.00; loop select low range; flow constants at high; gases, oxygen 99.99% and helium 99.99%. The calibration standard was composed of 150 mg EDTA (No. 502-092) (LECO Corporation). Starch content of PE was evaluated using AOAC (1996) method 996.11.

Automated spectrophotometry (AS) assays

An AS instrument (Bioscreen C, Labsystems Corp., Hel­sinki, Finland) was used to assess the effect of YE and PE concentrations on strain growth, with the method described by Champagne et al. (1999). The base medium (ASm) used for the growth of lactobacilli and Pediococcus pentosaceus strains had the following composition per litre of medium: glucose, 20 g; K₂HPO₄, 3 g; KH₂PO₄, 3 g; sodium citrate, 3 g; Tween 80, 1 g; MgSO₄·7H₂O, 0.2 g; MnSO₄·2H₂O, 0.05 g, and PE and (or) YE at set concentrations. Glucose and magnesium sulfate were mixed together and sterilized for 15 min at 121°C separately from the remaining ingredi­ents. For Lactococcus lactis and Streptococcus thermophilus, ASm was the same except that glucose was substituted by lactose.

In the assays aimed at evaluating mixtures of YE and PE on growth of Lactobacillus acidophilus R-052, 0.05% cysteine hydrochloride and 0.1% ascorbic acid were added to ASm. The ascorbic acid solution was filtered through a 0.22- m filter (Millipore, Bedford, Mass. U.S.A.) and added after sterilization of the growth medium. The data that justi­fied the addition of these compounds to the growth medium are presented in the Results and discussion section.

Sterile media containing various YE or PE were prepared in test tubes and inoculated at 1% (v/v) from a fresh culture, and 350- L samples were added aseptically to each micro­well. Microplates were incubated at 37°C for 24 h for the lactobacilli, P. pentosaceus EQ44 and S. thermophilus R-083 and at 30°C for the strains Lactococcus lactis R-058 and Lactobacillus plantarum EQ12. The absorbency at 600 nm of each microwell was automatically recorded every 15 min. Prior to each reading, the plates were shaken at low intensity for 5 s. The various media had different initial optical den­sity (OD) values. The initial OD of the medium was sub­tracted from the OD values obtained subsequently during the fermentation. Therefore, values reported are increases in OD.

Previous studies have shown that the appropriate concen­tration of YE in the base medium, used to evaluate the bio­logical value of various YE, must be defined for each strain (Potvin et al. 1997; Champagne et al. 1999). Preliminary as­says were thus carried out using the method of Champagne et al. (1999) to determine the appropriate concentrations of PE or YE for the determination of maximum growth rates ( tmax) or maximum optical density _(ODmax). Thus, YE were added at 1 g/L for strains EQ28, EQ12, and R-058 and at 2 g/L for others cultures. Different levels were selected for media containing only PE. A concentration of 2 g/L of PE was chosen for strains EQ12 and R-083 and 3 g/L for other strains. In AS assays involving mixtures of YE and PE, con­centrations determined for YE alone were used.

Fermentations with Lactobacilus rhamnosus R-011

LBS broth (propietary formulation, Rosell-Lallemand, Montr6al, Canada) was used to evaluate the growth­promoting properties of various YE, PE, or a mix of YE and PE on L. rhamnosus R-011. Extracts were added to the LBS broth at a concentration of 5 g/L. The media were pasteur­ized at 90°C for 30 min and the pH was adjusted to 7.0 with 5 M NaOH before inoculation.

Some fermentations were carried out in the same base me­dium rather than in AS assays to evaluate the capacity of AS analyses in predicting data of 2-L fermentations.

Fermentations of L. rhamnosus R-011 were carried out at 37°C in 2.5-L New Brunswick BioFlo 3000 U (New Bruns­wick, Edison, U.S.A.). Two litres of medium was inoculated at 1.5% (v/v) from a fresh culture, and pH was measured ev­ery 12 min. OD (600 nm) of fermented media were read when the pH reached 4.3. Viable counts were also carried out when pH reached 4.3 by plating appropriate dilutions (0.1% sterile peptone) in Lactobacillus MRS agar (BDH, Toronto, Canada). Chain lengths were determined by direct microscopic examination after methylene blue staining ac­cording to Packard and Ginn (1985). No step for breaking chains prior to CFU analyses was included; thus, the total bacterial population was estimated by multiplying the viable count by the average number of cells per chain (Champagne et al. 1995).

Fermentations with Streptococcus thermophilus R-083

Broth medium #43 (Rosell-Lallemand) was used to evalu­ate the growth-promoting properties of various types of YE and mixtures of YE and PE on S. thermophilus R-083. Ex­tracts were added to the media at a concentration of 5 g/L. The pH of the media was adjusted at 5.0 with 85% phospho­ric acid before pasteurization at 90°C for 30 min. The pH was adjusted to 6.8 with 5 M NH₄OH before inoculation.

Fermentations were carried out at 37°C in 1.5-L Biostat M bioreactors (B. Braun Instruments, Munster, Ger­many). One litre of medium was inoculated at 1.0% (v/v) with a fresh culture, and OD (600 nm) was measured every hour. The pH was maintained at 5.9 with 5 M NH₄OH after the pH of the media reached this value. Viable counts on fi­nal samples taken from fermentation were obtained by plat­ing appropriate dilutions (0.1% sterile peptone) in M17 agar (BDH).

Statistical analyses

Three independent replicates were carried out for each treatment, and data are the average of these three assays. Statistical analyses were performed using SAS Institute Inc. (1989) software. Analysis of variance was performed using Duncan’s test and significance was defined at P <_ 0.05.

Biomass values in AS assays system were transformed into ln values of OD and plotted against time for the expo­nential growth part of the curve. The slope of the linear por­tion of the curve, giving the _gmax, was obtained by simple linear regression.

Table 2. The effect of FNI-400 PE production lot on _ODmax ^and _µmax values obtained in the base medium with 2 g/L PE (EQ12 and R-083) or 3 g/L (all other strains).

*Mean _ODmax obtained with 11 lots of PE.

^†Statistical analysis of variance have shown a significant difference between _ODmax obtained with the 11 lots.

Results and discussion

Two aspects of the growth curve were analysed: (1) maxi­mum biomass obtained (using _ODmax) and (2) _µmax. We con­sidered that these values present different properties of the growth supplements. Since _µmax occurred at the beginning of the growth curve, the effect on growth is related to the mere presence of essential as well as stimulatory components, and high concentrations are not required at this point. However, for OD_max, the concentration of all the essential growth fac­tors is of prime importance, since growth will stop when only one essential factor has been depleted in the medium.

Variability in the growth-promoting properties of PE lots

Lot-to-lot variability in the composition of YE has been demonstrated by Peppler (1982) and confirmed by Potvin et al. (1997). Variability in ingredient composition used in cul­ture medium for the growth of LAB may impart important yield differences. Evaluation of lot-to-lot variability of the PE was thus an important concern to evaluate the potential of this ingredient for large-scale fermentations.

Eleven lots from the same manufacturing process were evaluated for their growth-promoting properties. For the 11 lots, presented in Table 2, the standard deviation values gen­erally represented less than 10% of the average ODmax or _µmax values. Strains reacted differently to the variability in composition of PE lots and, with regard to _ODmax, three of the seven cultures (L. plantarum EQ12, P. pentosaceus EQ44, and Lactococcus lactis R-058) were significantly af­fected by lot source (Table 2). However, _µmax data were not significantly affected by lot source. The variable reaction of strains to lot might be related to the fact that LAB have vari­able requirements in growth factors (Desmazeaud and de Roissard 1994; Hugenholtz et al. 1987). These results add to the literature with respect to the significance of lot variabil­ity of media ingredients on growth of LAB and are the first report on PE in this respect.

Poor growth of L. acidophilus R-052 was obtained in the base medium containing PE, and the growth rates were diffi­cult to measure. It was our concern that sensitivity to oxygen might affect the growth of this strain in the multiwell plates. Some additional media formulations were then examined. In the standard base medium containing 3 g/L of PE, ^ODmax reached 0.087 on average (data not shown). When 0.05% cysteine hydrochloride or 0.1% ascorbic acid were added, the _ODmax values were 0.070 and 0.060, respectively. When ascorbic acid and cysteine were both added, growth was better, with _ODmax reaching 0.400. Consequently, these two compounds were added to the base medium for the subse­quent experiments with L. acidophilus R-052.

Mixtures of PE and YE in the basal medium

Growth of bacteria in base medium containing 100% PE was almost always lower than growth in base media contain­ing a mixture of PE and YE or 100% YE, with both ^ODmax (Table 3) or _µmax data (Table 4). Thus, PE could not be used alone for the satisfactory production of the lactic cultures tested. One hypothesis for this observation could be the lower content in nitrogen-based compounds in PE. Indeed, YE typically contain between 50 and 71% “protein” (nitro­gen × 6.25; Peppler 1982), which represents, respectively, 7 and 11% nitrogen, while FNI-400 PE contains only 3.97% total nitrogen, of which 2.25% is a-amino nitrogen. The ni­trogen content of YE, which includes amino acids, peptides, and nucleic fractions, has been shown to be a major contrib­utor to the growth-promoting property of YE (Smith et al. 1975; Gaudreau et al. 1999; Conway et al. 2000). However, mixtures of 50:50 YE-BS:FNI 400 PE gave higher OD lev­els than YE-BS alone for five out of seven strains. Therefore other compounds than nitrogen contribute to the biological values of PE. Vitamins could be implicated, since PE con­tains vitamins essential for growth of the LAB (Desmazeaud and de Roissard 1994). The reason for low nitrogen level in PE is the high starch content. The PE contains 32% starch. Starch is not an essential growth factor for lactic cultures. Removal of starch from PE, by ultrafiltration for example, would thus potentially enable a 32% concentration of growth factors and improve the value of PE as a microbiological medium ingredient. Ultrafiltration of YE has been shown to modify their content in nitrogen (Conway et al. 2000), and studies are currently under way to determine if this is also the case with the FNI-400 PE.

For L. casei EQ28, L. rhamnosus R-011, and S. thermophilus R-083, the highest OD values were obtained with a mixture of PE and YE-BS. This suggests that YE and PE complement themselves for these strains with respect to nutrient compounds. Therefore, producers of starter cultures could find an economic interest in substituting YE with PE. Furthermore, for S. thermophilus R-083, mixing PE and YE had a marked benefit on growth rate. The AS data enable the selection of PE and YE mixtures that would benefit biomass

Table 3. The effect of combination of YE and PE on OD,. values obtained in base medium in automated spectrophotometry assays. 

Table 4. The effect of combination of YE and PE on µ,„_a,, values obtained in base medium in automated spectrophotometry assays.

Table 5. Composition of the various media used for the 2-L fermentation assays of Lactobacillus rhamnosus R-011 and biomass obtained (OD or CFU/mL) once pH had reached 4.3.

Note: For a given column, means that are followed by the same letter are not significantly different (P < 0.05). —, No addition.

*Fermentation time, which represents the time at which the stationary growth phase is reached. ^†The average number of cells per chain, as determined by microscopic examination.

yields while reducing costs. Since S. thermophilus appears to benefit so much from the PE–YE mixtures, it was a logi­cal candidate for the studies in bioreactors.

When mixtures of YE and PE gave the highest _ODmax val­ues of all blends, it was always with the YE-BS brand. This suggested that YE-BS was better suited than the other YE brands for the LAB, but closer examination showed that this was not the case. Indeed, 100% YE-X803 gave the highest OD values for P. pentosaceus EQ44, while 100% YE-BS was best for Lactococcus lactis ssp. lactis R-058 and 100% YE-LA was best for L. acidophilus R-052 (Table 3). This study thus adds to the literature with respect to the variabil­ity of YE growth-promoting properties and shows that there is a YE–strain interaction in this respect. This points to the usefulness of evaluating the ingredient source when impor­tant quantities of a given strain need to be produced. To pre­vent such source-related variations, Porubcan and Sellars (1979) have suggested to mix the nitrogen sources. The pres­ent study, aimed at evaluating such blends, is in line with this approach. It is noteworthy that in media containing 100% YE, YE-BS gave the highest _µmax for all strains except S. thermophilus R-083 (Table 4), which points to the differ­ent pictures that _ODmax ^and _µmax provide.

Bioreactor fermentations of Lactobacilus rhamnosus R-011

One of the goals of this study was to examine if data ob­tained with AS would be in correlation with those obtained in bioreactors. The growth rates were not measured during fermentation in the bioreactors, but the fermentation time does provide a picture. With L. rhamnosus R-011, the media that generated the highest biomass levels were also those in which the fermentation was the shortest (Table 5). In LBS medium, the addition YE and PE supplements reduced the fermentation time (tf; Table 5). There was good correlation (R² = 0.84) between the tf values (Table 5) and the corre­sponding _µmax data (Table 4). This suggests that the growth­rate data obtained in AS assays can be helpful to predict the fermentation time.

The optical density (600 nm) and viable counts (CFU/mL) in samples were measured at the end of the fermentation and the results are shown in Table 5. There is little information of biomass levels in CFU/mL of L. rhamnosus in the litera­ture. Arasaratnam et al. (1996) obtained 3.8 × 10⁹ CFU/mL in a whey medium supplemented with YE. Viable popula­tions obtained in this study, up to 7.5 × 10⁹ CFU/mL, were thus high and show that the PE and YE ingredients were ef­fective in sustaining good growth of lactobacilli.

Bioreactor viable counts and OD obtained with the ex­tracts added in ASm were not significantly different, pre­sumably because of the small differences in the values obtained, the low supplementation level of YE and PE, as well as because of the inherent error (typically between 10 and 30%) of CFU analyses. The relationship between ^ODmax data obtained in the AS assays and that of the bioreactor in the ASm showed high correlation (R² = 0.93; Table 6). This confirms previous data with L. acidophilus (Champagne et al. 1999) and shows that AS is a good tool to predict bio­mass levels at larger scales if the same media and fermenta­tion conditions are used. Biomass levels reached in the AS medium were significantly lower than those in the industrial LBS medium (Table 5). This is probably related to the lower YE and PE quantities added to the AS media.

The viable counts and OD values obtained in the bioreactors with LBS media showed significant differences (Table 5). As was found in the AS assays, OD values in the fermentations were highest when a mixture of YE and PE was used. Indeed, correlations between AS and bioreactor-LBS data for OD were good (R² = 0.87; Table 6). Unfortu­nately, AS data seemed less effective in predicting the via­ble counts in LBS-based media (R² = 0.37). One reason was found to be variable bacterial chain length (Table 5). When the viable counts were corrected by multiplying CFU/mL values by the average number of cells per chain (evaluated by direct microscopic counts), thus giving the total populations, the correlation with AS data improved. When the corrected CFU values were themselves trans­formed into their log values, correlations with AS data improved again (Table 6). The question of biomass determi­nation method is an important feature of growth studies. In this study, it was felt that using two methods would provide a more complete picture. Every method has its advantages

Table 6. Relationship between data obtained with automated spectrophotometry (AS) and bioreactor fermentations.

Table 7. Composition of the various media used for the pH-controlled fermentation assays of Streptococcus thermophilus R-083 and biomass obtained (optical density or CFU/mL).

and disadvantages. OD reflects the total biomass level, but has the disadvantage of not distinguishing between dead and live cells. In fresh cultures such as these, it can be ex­pected that the content of dead cells is low and that OD is a good indicator of live biomass levels. CFU counts provide a better picture of the live biomass level. However, CFU counts are less precise than OD because of the experimen­tal error that accumulates in the successive dilutions. Fur­thermore, the presence of chains introduces variations, which we tried to quantify by carrying out microscopic ex­amination of the cultures. By multiplying the CFU counts (which basically represents the number of chains) by the average number of cells per chain, we had a better picture of the total viable population. It is difficult to say which method is best. However, most teams present the biomass levels with CFU counts to indicate the level of live popula­tion.
 

Bioreactor fermentations of Streptococcus thermophilus R-083

Some assays were conducted with S. thermophilus R-083, since this strain was the one most positively affected by the PE during AS assays, and it was an opportunity to verify these data in bioreactor assays. Furthermore, the in­dustrial production of S. thermophilus R-083 is carried out under pH control, which was not the case for L. rhamnosus R-011. Thus, the bioreactor fermentations differed from the AS assays in the following three aspects: (1) concentrations of PE and (or) YE, (2) growth medium ingredients other than PE or YE, and (3) pH control.

The final biomass values obtained in S. thermophilus R-083 fermentations are shown in Table 7. Viable counts (CFU/mL) obtained in media with all supplements were not judged to be significantly different. _ODmax obtained in broth medium #43 with 100% PE was lower than with 100% YE, as predicted by AS data. The 50% substitution of YE-BS with PE (FNI-400) was successful in sustaining good growth because viable counts and OD were not sig­nificantly different than with 100% YE. Therefore, as the AS assays had suggested, partial substitution of YE with PE is worth considering in the industrial production of S. thermophilus cultures.

As with L. rhamnosus R-011, relationships between ^ODmax values obtained with AS assays and bioreactor data were examined (Table 6). When AS and fermentation as­says with L. rhamnosus R-011 were conducted in the same medium and with the same fermentation parameter (no ex­ternal pH control), the correlation was very high (R² = 0.98) between the two sets of OD data. When the industrial medium was used in the fermentation assays, there was a decrease in correlation between the AS and bioreactor OD data. The relationship was still good with the lactobacilli, but was very poor with the streptococci. The industrial me­dia have a much higher content in the growth supplements and many sources of supplementation are often used. Since data from this study show that interactions occur between YE and PE in AS conditions (Tables 3 and 4), it is possible that such a phenomenon also occurs between YE, PE, and the other ingredients of the industrial media.
 

Conclusion

Many growth media contain mixtures of YE and peptones. However, casein, meat, and soy peptones are generally more expensive than YE, and partially replacing YE with such peptones may not be economically advantageous. New

sources must thus be examined. Results showed that PE can­not solely replace YE as ingredients in growth media for lac­tic cultures. However, in some instances a partial substitution of YE with PE was beneficial, and this could be economically attractive, since some PE cost 40% less than YE. However, the beneficial effect of YE replacement with PE varies between strains and YE source, and producers of lactic starters must carry out assays to determine if such a strategy is useful for their particular strains.

AS is a powerful tool for the screening of ingredients aimed for use in microbiological media. When the same me­dium used for AS is used for fermentation assays, this study, as well as previous studies (Champagne et al. 1999), have shown that good correlations are found between AS-biomass and fermentor-biomass data. However, this study also pointed to limits of the current AS-prediction method with respect to industrial media. Further studies are currently un­der way to determine if the dilution of complete industrial media, as suggested by Potvin et al. (1997), could provide better AS predictions of the growth of the cultures in bioreactors.

Acknowledgements

Gratitude is expressed to Nancy Gardner, Thomas Tompkins, and John Conway for technical support.

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