pH-controlled cell release and biomass distribution of alginate-immobilized Lactococcus lactis subsp. lactis

Authors


Klinkenberg Department of Biotechnology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway (e-mail: Geir.Klinkenberg@chem.sintef.no).

Abstract

Aims: To investigate the growth and release of Lactococcus lactis subsp. lactis in gel beads and to affect rates of cell release by changing the growth conditions.

Methods and Results: The rate of release and the distribution of immobilized L. lactis subsp. lactis in alginate beads were studied in continuous fermentations for 48 h. A change in operating pH from 6·5 to 9·25 initially reduced the ratio of the rates of cell release to lactate production by almost a factor of 105. Compared with fermentations at pH 6·5, growth at pH 9·25 also increased the final internal bead biomass concentration by a factor of 5 and increased the final rate of lactate production by 25%. After 48 h, the ratio of the rates of cell release to lactate production was still 10 times lower than in fermentations at pH 6·5.

Conclusions: A change in the operating pH from 6·5 to 9·25 reduced rates of cell release throughout 48 h of fermentation and increased the final rates of lactate production and internal bead biomass concentration.

Significance and Impact of the Study: These data illustrate that diffusional limitations and corresponding pH gradients can be exploited in affecting the distribution of immobilized growing cells and their concomitant release.

INTRODUCTION

Immobilized cell technology has a large potential for use in biotechnological processes and immobilization of bacteria has been shown to be a valuable tool for increasing the performance of microbial bioreactors (Groboillot et al. 1994). A popular method for immobilization is entrapment in gel materials, especially in seaweed gel materials such as carrageenan or alginate (Willaert and Baron 1996). Alginate is a linear heteropolysaccharide of D-mannuronic acid and L-guluronic acid, which can be cross-linked with multivalent cations such as Ca2+ or Ba2+. When dripping a mixture of cells and sodium alginate into a solution containing multivalent cations, the reaction between alginate and multivalent cations forms gel beads with a typical pore size distribution of 5–200 nm (Smidsrød and Skjåk-Bræk 1990). This provides a gentle, simple and cheap method of immobilizing bacteria.

Bacteria immobilized in gels such as alginate are surrounded by a gel network, which strongly limits their movement. When growth occurs, bacteria push the gel network away and colonies containing densely packed bacteria are formed (Willaert and Baron 1993). As the colony expands, it may eventually reach the surface of the gel bead. This leads to an eruption of the colony in which the contents of the colony are released to the surrounding medium (Hüsken et al. 1996). Thus, in fermentations with gel-entrapped lactic acid bacteria there is normally a considerable release of free cells from the immobilization matrix. This release of cells may not always be desirable (Champagne and Clôté 1987). Thus, some effort has been concentrated on controlling rates of cell release in fermentations with immobilized organisms (Champagne et al. 1992; Champagne et al. 1994; Zhou et al. 1998).

The mechanism behind cell release is related to the growth of bacteria in the gel beads. As bacteria are immobilized in gel materials, substrates and waste products must be transported to and from the bacteria by diffusion. In gel beads containing immobilized growing bacteria the situation becomes complex as the growing bacteria are exposed to different local environments throughout the gel beads. (Cachon and Divies 1993) demonstrated that, after 50 h of continuous fermentation of immobilized Lactococcus lactis subsp. lactis in calcium alginate beads, 95% of the biomass is located near the surface of the bead. Mathematical modelling and kinetic studies performed by Cachon et al. (1997) indicate that this effect is mainly due to a decrease in pH in the interior of the beads. This reduction in pH leads to growth inhibition due to an accumulation of undissociated lactic acid in the interior of the beads. The cells in the periphery of the beads are less affected by this effect. According to the model proposed by Cachon et al. (1997), cell concentrations may reach 400 g l–1 in the periphery of the beads.

The aim of this work was to further investigate the growth patterns of immobilized bacteria in gel beads and to apply the knowledge gained by changing the growth conditions in order to affect rates of cell release. Continuous fermentations with controlled pH were used to investigate the development of the biomass distribution within calcium alginate beads and the distribution of metabolic activity in beads containing L. lactis subsp. lactis. According to Cachon et al. (1997), non-homogeneous biomass distributions in gel beads evolve due to a decreased pH in the interior of the beads. We examined, therefore, whether a change in pH of the external medium may affect internal biomass distributions and the subsequent cell release. An increased pH in the growth medium may lead to unfavourable growth conditions in the growth medium and in the periphery of the beads. In the interior of the beads growth may proceed protected by the pH gradient produced by lactic acid production. Thus, an elevated pH of the growth medium might be used as a way to control cell release from the beads and may even allow new areas of application for immobilized cells, in areas previously considered outside the expected operating range.

MATERIALS AND METHODS

Materials

The following chemicals were used: MRS broth, agar, yeast extract and peptone (Oxoid Unipath LDT, Basingstoke, UK); Elliker broth (Difco, Detroit, MI, USA); CaCl2·H2O, K2HPO4, NaH2PO4, gelatin and sodium citrate (Riedel de Haën, Seelze, Germany); glucose monohydrate (Norsk Medisinaldepot, Oslo, Norway); NaCl (Kebo, Oslo, Norway); perchloric acid (Merck, Darmstäd, Germany); sodium alginate (PROTANAL LF 10/60; FMC Biopolymer A/S, Drammen, Norway); chromatography standards of lactic acid and H2SO4 used as eluent for the high performance liquid chromatography (HPLC) analysis were obtained from Sigma Chemical Company (St. Louis, MO, USA).

Organism

Lactococcus lactis subsp. lactis NCIMB 6681 was obtained from The National Collections of Industrial and Marine Bacteria (Aberdeen, Scotland). Stock cultures were maintained at −80°C in MRS broth containing 15% glycerol. The revival of the bacterium was carried out in Elliker broth at 30°C for 12 h.

Culture media

The following culture media were used. Fermentation medium (g l–1): glucose monohydrate, 20·0; yeast extract, 5·0 and CaCl2·2H2O, 0·75. The pH was adjusted to 6·5 prior to autoclaving (121°C, 30 min). Dilution buffer (g l–1): NaCl, 8·5; K2HPO4 , 0·3; NaH2PO4, 0·60 and gelatin, 0·10. The pH was adjusted to 6·5 prior to autoclaving (121°C, 25 min).

Cell immobilization

Lactococcus lactis subsp. lactis cells for immobilization were produced in fermentation medium at a controlled temperature of 30°C for approximately 14 h. The cell production was conducted in a 2·5-l working volume fermenter (Applicon, Schiedam, the Netherlands) at an agitation speed of 250 rev min–1, with the pH maintained at 6·0 using a pH controller with an automatic addition of 3 mol l–1 NaOH. The fermenters were inoculated with 2% (v/v) active culture in Elliker broth.

Cells were harvested aseptically by centrifugation at 8400 g for 20 min at 4°C. The pellets were suspended in dilution buffer to a concentration of 4 g cells l–1. Resuspended cells were mixed with an equal volume of 4% (w/v) sodium alginate solution yielding a final cell concentration of 2 g dry weight cells l–1, which was used in all experiments.

The mixture of alginate and cells was added drop-wise into a sterile solution of sodium chloride (0·2 mol l–1) and calcium chloride (0·05 mol l–1). In order to produce beads with a diameter of 3·3 mm the solution was added through pipette tips with an inner diameter of 0·5 mm. To obtain aseptic operation the beads were made in a 2·5-l working volume fermenter (Applicon) at a stirring rate of 50 rev min–1 and using a single pitch-blade impeller. In order to produce beads with smaller diameters an electrostatic bead generator was used (Kulseng et al. 1998). Sodium chloride was used in the gelling solution in order to ensure a homogeneous polysaccharide concentration throughout the beads (Skjåk-Bræk et al. 1989). To ensure complete gelling, the beads were stirred for at least 40 min in this solution. The entire immobilization procedure was performed at ambient temperatures.

Fermentations

The growth rates of non-immobilized L. lactis subsp. lactis NCIMB 6681 in batch fermentations at different pH values were determined at a controlled temperature of 30°C. The fermentations were conducted in a 2·5-l working volume fermenter (Applicon) at an agitation speed of 250 rev min–1, with pH maintained at 4·5–9·0 using a pH controller with an automatic addition of 3 mol l–1 NaOH. The fermenters were inoculated with 2% (v/v) active culture in Elliker broth.

Continuous fermentations with immobilized L. lactis subsp. lactis in alginate beads were carried out with 100 ml beads in reactors with a total working volume of 550 ml (modified Celstir; Wheaton, Millville, NJ, USA) operated at an agitation rate of 300 rev min–1 and a controlled temperature of 30°C. Fermentation medium was used as feed medium in the continuous fermentations. In order to minimize the contribution from growth of free cells to the concentration of free cells in the reactor, the fermentations were operated at a dilution rate of 4 h–1. Less than 20% of the glucose feed to the reactors was converted to lactate under these conditions. The pH was maintained at 5·5, 6·5, 7·5, 8·5, 8·75, 9·0 and 9·25 using a pH controller with an automatic addition of 1 mol l–1 NaOH. The temperature, pH, outflow of each reactor and addition of NaOH were continuously registered by computer.

Determination of cell concentrations

Cell densities for determination of growth rates of freely suspended cells in batch culture were monitored by measuring the optical density of the culture at 660 nm (1-cm path length, u.v. visible spectrophotometer UV-160; Shimadzu, Kyoto, Japan). Samples were diluted in distilled water to give a final optical density of less than 0·4 and distilled water was used as a blank.

Samples for the determination of free cell concentrations in fermentations with immobilized L. lactis spp. lactis were serially diluted with dilution buffer. Plate counts were conducted in triplicate at each dilution on MRS broth supplemented with 1·5% (w/v) agar. Plates were incubated at 30°C for 2 d. The results were calculated according to guidelines given by Lille et al. (1999) and are reported as cfu ml–1. Tests were performed according to guidelines given by Lille et al. (1999) to assure a sufficient reproducibility (S.D. within 8–30% of the results in test trials).

Samples of beads with immobilized L. lactis subsp. lactis for determination of internal cell density were separated from the fermentation broth and liquefied in a sterile 1% solution of sodium citrate (pH 6·0). Dilution and plate counts were conducted as described above.

Analysis of biomass distributions within calcium alginate beads

The analyses of biomass distribution within calcium alginate beads were performed as described by Cachon and Divies (1993). Assays of biomass concentration inside gel beads were performed by dissolution of alginate beads in a 1% solution of sodium citrate (pH 6·0). At regular intervals, the liquid phase was collected and replaced by fresh citrate solution. Cell densities were determined as described above. Bead size before and during this analysis was measured with a caliper (Tricle bead) after a 5-h incubation in a solution of sodium chloride (0·2 mol l–1) and calcium chloride (0·05 mol l–1). Fifty beads of each type were measured in order to attain sufficient accuracy (S.D. within 12% of measured bead size).

Measurement of pH gradients within alginate beads

Alginate beads containing immobilized L. lactis subsp. lactis were sampled from the reactors and immediately positioned in a chamber which contained medium obtained from the reactor effluent. The effluent was centrifuged (8400 g for 20 min) in order to remove free cells and the pH in the medium was controlled prior to these measurements. The temperature in the measurement chamber was controlled at 30°C by circulating medium through a thermostated water-bath, using a peristaltic pump. This recirculation loop also provided vigorous stirring in the measurement chamber in order to minimize external mass transfer limitations. The measurements were performed with a pH 10 pH microelectrode (Unisense, Aarhus, Denmark) attached to a manual micromanipulator. The micromanipulator provided continuous positioning in lateral movements, with the smallest calibration unit equivalent to 20 μm, and the tip was positioned under observation with a stereomicroscope. A SDR2 reference electrode (WPI-instruments, Aston, UK) was positioned in the surrounding medium as described by Masson et al. (1994). The tip diameter of the pH electrode used was 15 μm. The pH microelectrode was calibrated at three different pH values and the relationship between the measured mV response and pH was established by linear regression.

Determination of lactic acid concentrations

Lactic acid concentrations were determined at intervals throughout the fermentations. The sample (0·8 ml) was added to cold perchloric acid (0·6 mol l–1, 0·2 ml), centrifuged at 13 400 g for 5 min and filtered through 0·2-μm syringe filters (Gelman Sciences, Ann Arbor, MI, USA) before HPLC analysis. A chromatograph was used, equipped with autoinjector (SIL-9 A; Shimadzu) and using an Aminex HPX-87-H (Bio-Rad Laboratories, Hercules, CA, USA) column at 45°C and an RI detector (RID 6 A; Shimadzu). As eluent, 5 mmol l–1 H2SO4 was used (0·6 ml min–1). Commercial standards were used for calibration.

RESULTS

Biomass and activity distribution in alginate beads containing Lactococcus lactis subsp. lactis

The release of immobilized bacteria from calcium alginate beads is related to growth of the bacteria in the beads. As bacteria grow, the distribution of the biomass in the beads changes. In order to investigate these changes, beads containing immobilized L. lactis subsp. lactis NCIMB 6681 were fermented continuously at a controlled pH of 6·5 and harvested for biomass distribution analyses at different times of fermentation. The biomass distributions of beads at the start and after 5·5, 12 and 48 h of continuous fermentation are shown in Fig. 1. Initially, the beads contained an equally distributed biomass. During the first 12 h of continuous fermentation a change in the biomass distribution occurred as the concentration of biomass in the outermost layers increased more than the biomass concentration in the centre of the beads. After 12 h of fermentation a 150-μm thick layer outermost in the beads contained approximately 73% of the biomass. The biomass concentration did not seem to increase further in this layer with prolonged fermentation. In beads analysed after 48 h of continuous fermentation approximately 73% of the biomass was located in the outermost 180-μm of the beads. However, a slight increase in the biomass concentration was also evident in the deeper layers of the beads. In the centre of the beads no changes in the biomass concentration were observed between 5·5 and 48 h of fermentation.

Figure 1.

 Changes in biomass distribution during continuous fermentation of immobilized Lactococcus lactis subsp. lactis in 3·3-mm calcium alginate beads. Beads were fermented continuously and harvested after different times of fermentation: 0 (. . .), 5·5 (- - -), 12 (–––) and 48 (. – . –) h. Biomass concentrations were determined in three layers of the beads. The concentrations are illustrated as mean concentrations in each layer. Radius is given on the basis of bead radius before dissolution

The metabolic activity of the biomass and its distribution within the beads were investigated by monitoring rates of lactate production. Beads with various diameters between 1·4 and 3·5 mm were used in order to investigate the effects of bead size on rates of lactate production. These beads were fermented continuously for 48 h at a pH of 6·5. After an initial increase, the rate of lactate production stabilized. The stabilized rates of lactate production are given in Fig. 2 and are presented as the ratio of the rate of lactate production of each bead size to the rate of lactate production of 3·3-mm beads. Each of the rates is presented both on a basis of total bead volume and on a basis of total bead surface area. Beads with a smaller diameter seemed to have a larger volumetric rate of lactate production than beads with larger diameter. The recorded volumetric rate of lactate production from 1·4-mm beads was more than 90% larger than the volumetric production rate observed from beads with a diameter of 3·3 mm. If, however, the rates of lactate production were calculated on the basis of surface area rather than on total bead volume smaller differences were observed. The recorded rate of lactate production on the basis of bead surface area from 1·4-mm beads was only 14% less than the rate of lactate production observed for reference beads (3·3- or 3·5-mm beads). As shown by the simulations in Fig. 2, these observations are consistent with the hypotheses that primarily cells located near the bead surface contribute to lactate production (Cachon et al. 1995).

Figure 2.

 Rates of lactate production obtained after 38 h of fermentation in fermentations with Lactococcus lactis subsp. lactis immobilized in calcium alginate beads of different size. Rates are illustrated as the ratio of the rate of lactate production on the basis of total bead volume and total bead surface area of each bead type to the rate of lactate production of 3·3- or 3·5-mm beads. Two different trials are shown. ▵, Trial 1, on the basis of total surface; ▴, trial 2, on the basis of total surface; □, trial 1, on the basis of total volume; ▮, trial 2, on the basis of total volume. Lines indicate simulations of relative activity with a 0·180-mm thick active layer. A diameter of 3·4 mm is used as a basis in the simulations

pH-dependent performance of immobilized Lactococcus lactis. subsp. lactis

Since the growth of lactic acid bacteria is accompanied by lactate production, the results from beads of different sizes indicate that most of the growth of lactic acid bacteria is located in the outermost parts of the beads. As the biomass concentration in the outermost layers of the beads seems to stabilize, biomass growth in this area is probably released to the surrounding medium. According to Cachon et al. (1995) the biomass in the interior of the beads is inhibited by undissociated lactic acid and low pH. Thus, an elevated pH in the surrounding medium might be used in order to affect the biomass distribution within the beads and subsequently the cell release from the beads, by favouring growth inside the beads and inhibiting growth in the periphery. In order to investigate this idea, growth rates of L. lactis subsp. lactis in the given culture medium at different controlled pH values were recorded (illustrated in Fig. 8). Based on these experiments the following pH values were used in experiments with continuous fermentations with immobilized bacteria: 6·5 (reference), 8·50, 8·75, 9·00 and 9·25. All fermentations were run at a pH of 6·5 for the first 2 h of fermentation in order to establish a protective pH gradient in the beads. The rate of cell release, monitored as the production rate of free cells, was measured at various intervals throughout the fermentations. The production rates of free cells from each of the fermentations are illustrated in Fig. 3. Biomass concentrations in the beads and lactate concentration in the effluents were measured at the same intervals. The time course of the internal cell density in the beads during the fermentations is illustrated in Fig. 4, while observed rates of lactate production are illustrated in Fig. 5. In order to determine whether an observed reduction in cell release was, in fact, due to reduced cell release and not just a consequence of a decreased metabolic activity, the ratio between rates of cell release and rates of lactate production at intervals throughout the fermentations is illustrated in Fig. 6. The internal pH gradients within alginate beads after 22 h of fermentation at a pH of 6·50, 9·00 and 9·25 are illustrated in Fig. 7.

Figure 8.

 Measured growth rates of Lactococcus lactis subsp. lactis NCIMB 6681 as a function of pH in fermentation medium and averaged stabilized rates of immobilized lactate production, measured at 37–48 h of fermentation at varying pH. ◆, Growth rate of freely suspended L. lactis subsp. lactis in fermentation medium; ▮, stabilized rates of immobilized lactate production

Figure 3.

 Continuous fermentations with immobilized Lactococcus lactis subsp. lactis NCIMB 6681 in calcium alginate beads with different pH values in the growth medium. The production rate of free cells, expressed as cfu ml–1 beads h–1, is shown as a function of fermentation time. ●, pH 6·50; ◆, pH 8·50; ▮, pH 8·75; ▴, pH 9·00; ×, pH 9·25

Figure 4.

 Continuous fermentations with immobilized Lactococcus lactis subsp. lactis NCIMB 6681 in calcium alginate beads with different pH values in the growth medium. The internal cell densities of the beads, expressed as cfu ml–1 beads, are shown as a function of fermentation time. ●, pH 6·50; ◆, pH 8·50; ▮ pH 8·75; ▴, pH 9·00; ×, pH 9·25

Figure 5.

 Continuous fermentations with immobilized Lactococcus lactis subsp. lactis NCIMB 6681 in calcium alginate beads with different pH values in the growth medium. The rate of lactate production, expressed as mmol lactate ml–1 beads h–1, is shown as a function of fermentation time. ●, pH 6·50; ◆, pH 8·50; ▮, pH 8·75; ▴, pH 9·00; ×, pH 9·25

Figure 6.

 Continuous fermentations with immobilized Lactococcus lactis subsp. lactis NCIMB 6681 in calcium alginate beads with different pH values in the growth medium. The ratio of the rate of production of free cells and the rate of lactate production, expressed as cfu mmol–1 lactate, is shown as a function of fermentation time. ●, pH 6·50; ◆, pH 8·50; ▮, pH 8·75; ▴, pH 9·00; ×, pH 9·25

Figure 7.

 Measured local pH as a function of distance from the bead surface in 3·05-mm alginate beads containing Lactococcus lactis subsp. lactis. The beads were harvested after 24 h of fermentation and were fermented at different external pH for the latter 22 h. The values are average values of two independent measurements. At the time of harvest the fermentations had the following ratios of rate of cell release to rate of lactate production: 6·5, 5·5 × 109 cfu mmol–1; 9·00, 9·0 × 109 cfummol–1 and 9·25, 4·9 × 107 cfu mmol–1. ▴, Beads fermented at pH 6·50; ▮, beads fermented at pH 9·00 and ◆, beads fermented at pH 9·25

The rates of cell release increased in the early stages of fermentation. The fermentation run at a pH of 6·50 reached a steady state rate of cell release after 12–15 h of continuous fermentation. Only minor changes in the rate of cell release were observed during the next 33 h of fermentation. This is in good agreement with the results from the analysis of the internal biomass distribution during continuous fermentation. As the pH was increased, changes in the rate of cell release were observed. However, all fermentations, except the fermentation operated at a pH of 9·25, reached a steady-state rate of cell release similar to the fermentation carried out at a pH of 6·50 within 24 h. In the fermentation carried out at a pH of 9·25, an initial sharp decrease in the rate of cell release was seen as the pH was changed, leading to a factor of 105 reduction in the rate of cell release compared with the steady-state level of the other fermentations. During the remainder of the fermentation, the rate of cell release increased steadily, but no steady state was seen. After 48 h of fermentation the rate of cell release was still approximately 10 times lower than the steady-state rate of cell release of the fermentations at lower pHs.

The changed conditions in the surrounding medium also influenced the course of the biomass concentration in the beads during the fermentations. The biomass concentration in the beads fermented at a pH of 6·5, 8·5, 8·75 and 9·00 reached steady-state values within 24 h of continuous fermentation. However, the biomass in the beads fermented at a pH of 9·25 never reached a steady-state concentration within the 48 h of fermentation. Although the increased pH in the surrounding medium did slow the initial increase in internal biomass concentration, these beads reached a biomass concentration approximately five times the steady-state biomass concentration observed for the other beads fermented at pHs of 6·5, 8·5, 8·75 and 9·0. The changed pH in the surrounding medium also seemed to influence the rate of lactate production. The rate of lactate production in the fermentation carried out at pH 9·25 did not increase as much as the other fermentations in the early stages of fermentation but it did, as opposed to the other fermentations, continue to increase during the entire 48 h of fermentation. At the end of the fermentations, a 25% increase in the rate of lactate production was observed in the fermentation run at a pH of 9·25, compared with the other fermentations, even though the rate of cell release was still comparatively lower by an order of magnitude.

The mechanism behind these large changes in fermentation behaviour is assumed to be related to differences in the pH gradients within the beads. As seen from Fig. 7, there is a significant difference between the pH gradients in the outermost parts of those beads fermented at pHs of 9·00 and 9·25 for 22 h. In the inner parts of the beads there seems to be a less significant difference in local pH. There is also a large difference in the pH gradients in beads fermented at a high pH compared with beads fermented at our standard pH of 6·5. In spite of much steeper pH gradients in beads fermented at a high pH, the local pH in the centre of beads fermented at a high pH is only 1·7 units higher than in beads fermented at a pH of 6·5.

Thus, as seen from Figs 6 and 7, it was possible to change the external pH of the medium so that growth was disfavoured in the surrounding medium and in the periphery of the beads, while growth and lactate production continued in the interior of the beads. This situation changed the ratio of the rate of cell release to the rate of lactate production, leading to a product stream containing fewer free cells. These experiments also illustrate, as seen from Figs 7 and 8, that immobilization causes a large shift in the pH operating range, due to diffusional limitations and the concomitant pH gradients within the beads. These data suggest that immobilized cells can be applied with success in applications based on operation at the extreme upper limits of pH tolerance.

DISCUSSION

The present study has investigated phenomena related to the growth and release of L. lactis subsp. lactis NCIMB 6681 immobilized in calcium alginate beads. According to Cachon et al. (1995), lactate accumulates in the interior of beads as immobilized lactic acid bacteria grow. This accumulation leads to a decrease in the pH of the interior of the beads which, in combination with high concentrations of undissociated lactic acid, will eventually inhibit growth inside the beads. In the present study, this development was demonstrated with analysis of biomass distribution of continuously fermented beads, as the concentration in the interior of the beads remained unchanged after the initial period of fermentation. In the periphery of the beads, a high biomass concentration seems to stabilize during the first 12 h of continuous fermentation. Biomass concentrations of 70 g dry weight l–1 were measured in layers with an approximate thickness of 150 μm adjacent to the surface of the beads. These values are somewhat less than those reported by Cachon and colleagues (Cachon and Divies 1993; Cachon et al. 1995) in studying L. lactis subsp. lactis bv. diacetylactis immobilized in calcium alginate beads. However, in the present study, a larger portion of the beads may have been dissolved in each layer. Mathematical simulations performed by Cachon et al. (1997) indicate that extremely high concentrations of biomass are located in a very narrow layer close to the surface of the beads. If a larger portion of the bead is dissolved, the area of high concentration may have been averaged with areas further inside the beads with less biomass density. This may explain the difference between values observed in this study and the previously reported values.

The biomass concentration near the surface seems to stabilize with prolonged continuous fermentation. No changes were observed in the outermost biomass concentrations when comparing beads fermented continuously for 12 and 48 h. These observations are in agreement with observations and simulations reported by Cachon et al. (1995). Results from experiments with continuous fermentations with beads of different sizes indicate that most of the growth and lactate production occurs in a narrow range close to the surface of the beads. Both this area, and the beads overall, seem to reach a steady-state biomass concentration. These observations indicate that, when growth occurs, biomass in excess is released to the surrounding medium. This release of free cells may or may not be desirable. A few methods have been proposed to reduce the release of lactic acid bacteria from alginate beads. Champagne et al. (1992) rinsed alginate beads containing immobilized lactic acid bacteria with distilled water, ethanol, Al(NO2) or hot CaCl2 solutions before fermentation, in order to kill bacteria near the bead surface. Another approach was the application of multiple coats of poly L-lysin and alginate, which reduced cell release by a factor of 10. Yet another approach was demonstrated by Zhou et al. (1998), who investigated the use of chitosan in reducing cell release. Zhou et al. (1998) reported that the number of free cells was reduced in repeated 2-h fermentations of milk with chitosan-coated alginate beads, compared with fermentations with uncoated beads. Klinkenberg et al. (2001) has previously investigated the use of sequential coatings of chitosan and alginate, which reduced the volumetric rate of cell release throughout the investigated 48 h of continuous fermentation. The present study investigates the possibility of exploiting diffusional limitations and the corresponding internal pH gradients in beads containing immobilized growing lactic acid bacteria to affect the development of the immobilized cells and their concomitant release.

Growth rates of lactic acid bacteria are dependent on the pH values in the growth medium. A pH of 6·5 was determined to be the pH optimum for growth of freely suspended L. lactis subsp. lactis NCIMB 6681 in this medium. As pH increases above 7·5 the growth rate decreases rapidly and no growth was observed at a pH of 9·00. An adjustment of the controlled pH of the fermentation medium from 6·50 to 9·25 in fermentations with immobilized L. lactis subsp. lactis caused an initial reduction in the ratio of rate of cell release to rate of lactate production by approximately a factor of 105 compared with fermentations run at the pH optimum for growth of the bacteria. This change in fermentation pH apparently changed the internal pH gradients and the distribution pattern of L. lactis subsp. lactis within the beads. As seen from Fig. 7, there seem to be significant differences in the profile of the pH gradients in the outermost parts of the beads fermented at pH 9·00 and 9·25 when measured after 24 h of fermentation. In the beads fermented at a pH of 9·25 there were only minor reductions in pH in the outermost 0·4 mm of the beads, while the pH gradient seems to fall more rapidly within beads fermented at a pH of 9·00. The apparent threshold layer of pH observed in the outermost parts of beads fermented at a pH of 9·25 is consistent with a layer of little biological activity, as the local pH in this layer is above 9·00. At this pH no growth of freely suspended L. lactis subsp. lactis NCIMB 6681 was observed (Fig. 8). The results from biomass distribution analyses and from fermentations with beads of different sizes indicate that most of the growth and lactate production occurs close to the surface of the beads when the fermentations are carried out at a normal pH. As a consequence, very little gel has to be broken in order to release cells. Increasing the pH seems to move the optimum pH zone of growth toward the centre of the beads, away from the surface. If growth occurs inside the beads, more gel material must be broken in order to release cells. This mechanism probably explains the reduced rate of cell release observed in the fermentation run at a pH of 9·25. In fact, a visual inspection of beads fermented continuously for 46 h at a pH of 9·25 did reveal a layer underneath the surface containing a large amount of loosely bound biomass. Intense growth of immobilized bacteria seems to have weakened the alginate gel in this area. A layer of alginate gel was situated outside this layer, effectively blocking the release of the biomass underneath. This situation might be compared with fermentations with beads coated with an additional cell-free alginate layer or even with capsules of alginate. Several authors have reported reductions in cell release accompanying the use of beads coated with an additional cell-free alginate layer (Champagne et al. 1992; Gòdia et al. 1991; Prévost and Divies 1992). Klinkenberg et al. (2001) have also studied cell release from alginate-coated alginate beads containing immobilized L. lactis subsp. lactis in continuous fermentations. In that study, a significant reduction in the ratio of rate of cell release to rate of lactate production was evident throughout the 48 h of fermentation. However, the effects observed in the earlier stages of those fermentations were not as large as those observed in the present study.

Our results show that a narrow window of operating pH can be selected which promotes a reduction in cell release and changes in the distribution of biomass within the beads. This effect has been demonstrated in several experimental series, although variation in pH measurement and control together with the narrowness of the window gave slightly different values for the exact pH where the effect would appear. The pH of the fermentation medium must be controlled in such a way that the growth rate is severely depressed in free medium, as well as at the periphery of the beads, while growth may continue within the beads protected by a pH gradient. When the pH of the medium is first increased, an initial decrease is observed in the rate of cell release. This initial period is probably critical to the process leading to a longer term reduction in cell release. Growth and lactate production are reduced simultaneously on increase in the pH. Since lactate produced in the interior of the beads must diffuse out, the pH is reduced in the interior of the beads. This may protect cells in this region and allow for continued growth. However, if the increase in pH is too high or abrupt, the production of lactate will also be depressed and the protecting pH gradient will disappear. If the increase in pH is too small, the growth of the cells in the periphery will be temporarily depressed, but lactate production in the interior and on the surface will probably decrease the local pH enough to restore growth and lactate production in the outer areas. In the experiments presented here, a short holding period was applied at pH 6·50 for 2 h before a single step change was made to the chosen operating pH. Other procedures might be possible, for example a series of stepwise changes to attain operating pH, which might extend this window of pH for bringing forth the effects seen here for the case of pH 9·25.

Requirements for special growth conditions may, however, exclude this approach to a reduced cell release in some food applications. However, other possible applications may now be available due to this expanded operation range of pH. It has previously been reported that immobilized lactic acid bacteria can maintain high activities after short exposures to extreme pH values (Dyrset et al. 1998). An interesting observation from Figs 3, 5 and 7 is that high rates of cell release and lactate production were maintained at pH values that normally would cause severely depressed growth rates in free cell cultures. At pH values above 8·5 the growth rates in free cell cultures are negligible, while rates of cell release and rates of lactate production are not significantly affected in the immobilized fermentations. In fact, it seems that the steady-state rates of lactate production and the internal cell densities achieved at pH values of 8·50, 8·75 and 9·00 are greater than those achieved at a pH of 6·50. These differences emerge even though the growth of free cells should contribute here to the measured rate of cell release at a pH of 6·50. At pH 6·50 approximately 18–22% of the free cells originate from growth of free cells in the reactors. At pH values above 8·5, growth of free cells probably does not contribute significantly to the total free cell concentration due to the harsh conditions in the fermentation broth. It may thus be possible to exploit diffusional limitations with the concomitant pH gradients to apply immobilized fermentations to processes involving harsh conditions that provide a certain protective and stabilizing effect to the immobilized biomass, but effectively prevent growth of contaminating organisms.

Acknowledgements

This work was financed by The Research Council of Norway (project no. 110682/420) and is part of SINTEF/NTNU’s strategic institute program Technology for Competitive Food Processing.

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