Undissociated Lactic Acid Concentration Research Articles

Growth Potential of Listeria monocytogenes in Chef-Crafted Ready-to-Eat Fresh Cheese-Filled Pasta Meal Stored in Modified Atmosphere Packaging

This study evaluated the growth of lactic acid bacteria (LAB) in a fresh, filled-pasta meal, stored in modified atmosphere packaging and the influence of lactic acid (LA) and pH on the growth of Listeria monocytogenes (Lm). Samples were taken from three lots manufactured by a local catering company and stored at both 6 and 14°C. LAB numbers, LA concentration, pH, and the presence of Lm were evaluated at 1, 4, 6, 8, 10, 12, and 14 days of shelf life and the undissociated LA concentration ([LA]) was calculated. The LAB maximum cell density was greater in the products stored at 14°C than those stored at 6°C (10.1 ± 1.1 versus 5.6 ± 1.5 log CFU/g) and [LA] at 14 days was 9 to 21 ppm at 6°C and 509 to 1,887 ppm at 14°C. Challenge tests were made to evaluate the interference of LAB and [LA] on Lm growth. Aliquots of the samples (25 g) were inoculated at 1 to 10 days of shelf life and incubated at 9°C for 7 days, and the difference between Lm numbers at the end and at the beginning of the test (δ) was calculated. Logistic regression was used to model the probability of growth of Lm as a function of LAB and [LA]. The products inoculated at 1 day of shelf life had δ values between 4.2 and 5.6 log CFU/g, but the growth potential was progressively reduced during the shelf life. Lm growth was never observed in the products stored at 14°C. In those stored at 6°C, it grew only in the samples with LAB <5.7 log CFU/g. LAB interaction might thus inhibit the growth of Lm in temperature-abused products and limit its growth in refrigerated products. Logistic regression estimated that the probability of Lm growth was <10% if LAB was >6.6 log CFU/g or log[LA] was >2.2 ppm. The growth or inactivation kinetic of Lm was investigated with a homogenate of three samples with LAB numbers close to the maximum population density. After an initial growth, a subsequent reduction in the number of Lm was observed. This means that the maximum numbers of Lm might not be detected at the end of the product shelf life.

Factors that inhibit growth of Listeria monocytogenes in nature-ripened Gouda cheese: A major role for undissociated lactic acid

In this study, factors relevant to nature-ripened Gouda cheese were evaluated for their potential to inhibit growth of Listeria monocytogenes. Factors included water activity, pH, undissociated acetic and lactic acid, diacetyl, free fatty acids, lactoferrin, nitrate, nitrite and nisin. In addition, the effect of temperature was evaluated. For each factor, the actual concentrations and values relevant to Gouda cheese were obtained and the inhibitory effect of these individual factors on growth of L. monocytogenes was assessed. This evaluation revealed that undissociated lactic acid is the most important factor for growth inhibition of L. monocytogenes in Gouda cheese and that, additionally, low water activity as present in the cheese rind and after prolonged ripening times can also cause full growth inhibition. Gouda cheeses have a typical total lactic acid content of 1.47% w/w. In a 2-week old Gouda cheese, with a pH value of 5.25 and a moisture content of 42% w/w, the concentration of undissociated lactic acid in the water phase is 10.9 mM. Growth of L. monocytogenes is not supported when the undissociated lactic acid concentration is >6.35 mM. Concentrations of undissociated lactic acid in the water phase of Gouda cheese will be higher than this value when the total lactic acid content is >0.86% w/w at a pH < 5.25 (relevant to young Gouda cheese), or >1.26% w/w at a pH < 5.50 for mature Gouda cheese (moisture content of 35% w/w). This study underlines the importance of undissociated lactic acid as growth inhibitor for L. monocytogenes in Gouda cheese.

Modeling Lactic Fermentation of Gowé Using Lactobacillus Starter Culture.

A global model of the lactic fermentation step of gowé was developed by assembling blocks hosting models for bacterial growth, lactic acid production, and the drop of pH during fermentation. Commercial strains of Lactobacillus brevis and of Lactobacillus plantarum were used; their growth was modeled using Rosso’s primary model and the gamma concept as a secondary model. The optimum values of pH and temperature were 8.3 ± 0.3, 44.6 ± 1.2 °C and 8.3 ± 0.3, 3.2 ± 37.1 °C with μmax values of 1.8 ± 0.2 and 1.4 ± 0.1 for L. brevis and L. plantarum respectively. The minimum inhibitory concentration of undissociated lactic acid was 23.7 mM and 35.6 mM for L. brevis and L. plantarum, respectively. The yield of lactic acid was five times higher for L. plantarum than for L. brevis, with a yield of glucose conversion to lactic acid close to 2.0 for the former and 0.8 for the latter. A model was developed to predict the pH drop during gowé fermentation. The global model was partially validated during manufacturing of gowé. The global model could be a tool to aid in the choice of suitable starters and to determine the conditions for the use of the starter.

The effect of different matrices on the growth kinetics and heat resistance of Listeria monocytogenes and Lactobacillus plantarum

Microbial growth and inactivation kinetics in food can be predicted when the effects of food properties and environmental conditions on microbial responses are available. However the effects of these intrinsic and extrinsic variables on microbial kinetics are often obtained using laboratory media, and deviations between predictions and true behaviour might occur if the specific effect of a food product is not known or considered in the prediction. Therefore, knowing the food specific effect on microbial kinetics might not only result in a more realistic growth and inactivation prediction, but also extend the knowledge on factors influencing growth and heat resistance. In this study, growth predictions of Listeria monocytogenes and Lactobacillus plantarum were validated in laboratory media and in milk and ham as model food products. A good agreement between the predicted and observed growth kinetics in laboratory media highlighted the possibility to predict μmax based on cardinal growth parameters obtained from OD-based measurement in laboratory media. Only in two conditions (BHI pH5.5 at 7°C; and BHI pH5.5, undissociated lactic acid concentration of 1mM at 7°C) a possible interaction between growth limiting factors was observed, yet existing interaction models were not better in predicting growth. Growth validation in the two model foods showed that the food specific effects were strain dependent, which might further complicate accurate prediction. For both species the effect of strain variability on thermal inactivation was similar to the food specific effects, and the latter was mainly determined by the effect of ham as heating medium. The combination of both effects explained (almost) all variability found in literature, however, with some bias.

Quantifying Variability in Growth and Thermal Inactivation Kinetics of Lactobacillus plantarum.

The presence and growth of spoilage organisms in food might affect the shelf life. In this study, the effects of experimental, reproduction, and strain variabilities were quantified with respect to growth and thermal inactivation using 20 Lactobacillus plantarum strains. Also, the effect of growth history on thermal resistance was quantified. The strain variability in μmax was similar (P > 0.05) to reproduction variability as a function of pH, aw, and temperature, while being around half of the reproduction variability (P < 0.05) as a function of undissociated lactic acid concentration [HLa]. The cardinal growth parameters were estimated for the L. plantarum strains, and the pHmin was between 3.2 and 3.5, the aw,min was between 0.936 and 0.953, the [HLamax], at pH 4.5, was between 29 and 38 mM, and the Tmin was between 3.4 and 8.3°C. The average D values ranged from 0.80 min to 19 min at 55°C, 0.22 to 3.9 min at 58°C, 3.1 to 45 s at 60°C, and 1.8 to 19 s at 63°C. In contrast to growth, the strain variability in thermal resistance was on average six times higher than the reproduction variability and more than ten times higher than the experimental variability. The strain variability was also 1.8 times higher (P < 0.05) than the effect of growth history. The combined effects of strain variability and growth history on D value explained all of the variability as found in the literature, although with bias. Based on an illustrative milk-processing chain, strain variability caused ∼2-log10 differences in growth between the most and least robust strains and >10-log10 differences after thermal treatment. Accurate control and realistic prediction of shelf life is complicated by the natural diversity among microbial strains, and limited information on microbiological variability is available for spoilage microorganisms. Therefore, the objectives of the present study were to quantify strain variability, reproduction (biological) variability, and experimental variability with respect to the growth and thermal inactivation kinetics of Lactobacillus plantarum and to quantify the variability in thermal resistance attributed to growth history. The quantitative knowledge obtained on experimental, reproduction, and strain variabilities can be used to improve experimental designs and to adequately select strains for challenge growth and inactivation tests. Moreover, the integration of strain variability in prediction of microbial growth and inactivation kinetics will result in more realistic predictions of L. plantarum dynamics along the food production chain.

Quantifying strain variability in modeling growth of Listeria monocytogenes

Prediction of microbial growth kinetics can differ from the actual behavior of the target microorganisms. In the present study, the impact of strain variability on maximum specific growth rate (μmax) (h−1) was quantified using twenty Listeria monocytogenes strains. The μmax was determined as function of four different variables, namely pH, water activity (aw)/NaCl concentration [NaCl], undissociated lactic acid concentration ([HA]), and temperature (T). The strain variability was compared to biological and experimental variabilities to determine their importance. The experiment was done in duplicate at the same time to quantify experimental variability and reproduced at least twice on different experimental days to quantify biological (reproduction) variability. For all variables, experimental variability was clearly lower than biological variability and strain variability; and remarkably, biological variability was similar to strain variability. Strain variability in cardinal growth parameters, namely pHmin, [NaCl]max, [HA]max, and Tmin was further investigated by fitting secondary growth models to the μmax data, including a modified secondary pH model. The fitting results showed that L. monocytogenes had an average pHmin of 4.5 (5–95% prediction interval (PI) 4.4–4.7), [NaCl]max of 2.0mM (PI 1.8–2.1), [HA]max of 5.1mM (PI 4.2–5.9), and Tmin of −2.2°C (PI (−3.3)–(−1.1)). The strain variability in cardinal growth parameters was benchmarked to available literature data, showing that the effect of strain variability explained around 1/3 or less of the variability found in literature. The cardinal growth parameters and their prediction intervals were used as input to illustrate the effect of strain variability on the growth of L. monocytogenes in food products with various characteristics, resulting in 2–4 logCFU/ml(g) difference in growth prediction between the most and least robust strains, depending on the type of food product. This underlined the importance to obtain quantitative knowledge on variability factors to realistically predict the microbial growth kinetics.

The effect of undissociated lactic acid on Staphylococcus aureus growth and enterotoxin A production

The potential of Staphylococcus aureus cheese isolates to grow and produce staphylococcal enterotoxin A under conditions typical for cheese making was investigated in three broth experiments. The effect of the concentration of undissociated lactic acid (HLac) in conjunction with specific pH values was studied by adjusting pH at a single concentration of lactic acid. First, the time-to-growth of S. aureus was modelled by using survival analysis and absorbance data obtained from an automated turbidity reader. The fitted model describes the time to growth and indicates the growth ⁄ no growth boundary of S. aureus as a function of HLac concentration, temperature and water activity. Second, growth rates and lag times of S. aureus were estimated after two different pre-treatments in skim milk at three HLac concentrations and two temperatures based on optical detection times of serial dilutions of bacterial solutions. Growth rates differed between strains, and increased with increasing temperature and decreasing HLac concentration. Preliminary results indicate that lag times were dependent on pre-treatment suggesting that the growth potential of S. aureus in cheese curd may be greater if milk is used immediately after milking compared to holding at 4°C after milking. Third, growth, inactivation, and enterotoxin A production of S. aureus strains were investigated at twelve combinations of HLac concentration and temperature. Concentrations of enterotoxin A increased linearly during the first four days, with a production rate increasing with increasing temperature and decreasing HLac concentration. Significant amounts of enterotoxin A were produced during extended incubation, up to 14days, but then initial pH had changed. This highlights a potential limitation of modelling based on the initial environmental conditions in batch experiments.In summary, ranges of time-to-growth, growth rates, lag times and enterotoxin A production rates of S. aureus in the presence of HLac were estimated. The results can be used together with process data to indicate the range and magnitude of growth and enterotoxin A production during initial stages of cheese production.

Lactic acid production by Enteroccocus faecium in liquefied sago starch.

Enterococcus faecium No. 78 (PNCM-BIOTECH 10375) isolated from puto, a type of fermented rice in the Philippines was used to produce lactic acid in repeated batch fermentation mode. Enzymatically liquefied sago starch was used as the sole carbon source, since sago (Metroxylon spp) is a sustainable crop for industrial exploitation. Liquefied sago starch was inoculated with E. faecium to perform the saccharification and fermentation processes simultaneously. Results demonstrated that E. faecium was reused for 11 fermentation cycles with an average lactic acid yield of 36.3 ± 4.71 g/l. The lactic acid production was superior to that of simple batch mode and continuous fermentation in terms of lactic acid concentration. An un-dissociated lactic acid concentration of 1.15 mM affected the productivity of the cells. Work is in progress to maintain and increase the usability of the cells over higher fermentation cycles.

Time to growth and inactivation of three STEC outbreak strains under conditions relevant for fermented sausages

Published models predicting growth and survival capabilities of shiga-toxin-producing Escherichia coli (STEC) under a w and lactic acid stress were validated by performing experiments with fermented sausage associated outbreak strains. Strain variation in inactivation and time to growth (TTG) were investigated for strains representing three serotypes (O103, O111, and O157). The TTG and growth boundaries of each strain were compared with predictions of a model for generic acid adapted E. coli and survival with predictions of two inactivation models. In addition, the influence of strain variation on the performance of the inactivation models, in terms of bias and accuracy factors, was illustrated. Strains with induced acid tolerance were used in broths containing 50 or 110 mM total lactic acid. The concentration of undissociated lactic acid (HLac) was adjusted by setting the pH-value, and water activity (0.900 to 0.995 depending on experiment) was adjusted by adding NaCl. The survival capabilities of the outbreak strains were good compared to the model predictions. The average bias factors of inactivation model predictions were within a factor of 2.2 depending on the strain used to validate the model indicating that inactivation rates of outbreak strains were slower than predicted. However, the observed rates were similar to the rates of a previously studied acid tolerant generic E. coli strain. Similarly, the time to growth of two of the strains (O103 and O157) was comparable with model predictions, whereas the growth capability of the third strain (O111) was lower than predicted. These results suggest that the properties of the most tolerant sausage outbreak strains are comparable to tolerant generic E. coli strains, which imply that suitable non-pathogenic E. coli strains are valid surrogates for fermented sausage outbreak strains. The relative sensitivity of strains depended on the environmental parameters and the response evaluated. The strain with the smallest log reduction at 20 °C was O157, whereas it was strain O103 at 8 °C. Under conditions unfavorable for growth, the time to growth was much shorter for strains O103 and O157 than for strain O111, whereas differences between strains were negligible under conditions favorable for growth. Depending on the response variable and the specific application the limitation of not addressing strain variation may lead to biased, fail-dangerous, predictions. Thus, solutions on how to best address strain variation in the development and validation of predictive models are needed.

Modelling time to growth ofEscherichia colias a function of water activity and undissociated lactic acid

To model the effect of water activity (a(w)) and concentration of undissociated lactic acid (HLac) on the time to growth (TTG) and the growth/no growth boundary of acid-adapted generic Escherichia coli, used as model organisms for Shiga toxin-producing E. coli (STEC). For each of two E. coli strains, the TTG in brain heart infusion broth at 27 degrees C was estimated at 30 combinations of a(w) (range 0.945-0.995) and concentration of HLac (range 0-6.9 mol m(-3)) by using an automated turbidity reader. Survival analysis was used to develop a model predicting the TTG and the growth/no growth boundary. The present model can be used to predict the TTG and to indicate the growth/no growth boundary of acid-adapted E. coli strains as a function of a(w) and concentration of HLac. Fermented food products have been implicated as sources of STEC in several outbreaks. The study results are relevant for modelling of growth of STEC in fermented food and can be used in microbiological risk assessments or in the design and validation of food-production processes.

Modelling the combined effects of structured food model system and lactic acid on Listeria innocua and Lactococcus lactis growth in mono- and coculture

A new class of predictive model developed in a liquid system is extended in order to quantify gelatin gel matrix structure effects on the growth of Listeria innocua and Lactococcus lactis (both in mono- and coculture, and both producing mainly lactic acid). It was observed that gelatin does not only act as a structuring agent but also alters the buffering capacity of the medium. Model extension occurs in two stages, describing chemical and microbiological processes, respectively. Firstly, equations relating undissociated lactic acid concentration and total lactic acid concentration on the one hand, and undissociated lactic acid concentration and pH on the other hand, are extended to account for the effects of gelatin concentration. Secondly, these equations are incorporated into the growth model to describe the combined effect of gelatin concentration, (undissociated) lactic acid and pH on the growth of either microorganism. The description of the model is in good agreement with the experimental data acquired in monoculture conditions. In a subsequent model validation step, when gelatin concentration and total lactic acid profile of the coculture experiments are used as inputs, the developed growth model consisting of condensed knowledge extracted from the monoculture experiments, is able to predict accurately the interaction effect occurring in coculture. The study suggests that, on the one hand, the extent of the effects of undissociated lactic acid and pH on microbial growth in structured food systems can be modified by the increase in buffering capacity, which can protect microorganisms and eventually promote higher levels of cell growth in comparison with liquid culture conditions. On the other hand, food matrix structure, in casu the gelatin, reduces the rate of microbial multiplication. Both effects are incorporated in the growth model developed in this research.

Inhibition of yeast by lactic acid bacteria in continuous culture: nutrient depletion and/or acid toxicity?

Lactic acid was added to batch very high gravity (VHG) fermentations and to continuous VHG fermentations equilibrated to steady state with Saccharomyces cerevisiae. A 53% reduction in colony-forming units (CFU) ml(-1) of S. cerevisiae was observed in continuous fermentation at an undissociated lactic acid concentration of 3.44% w/v; and greater than 99.9% reduction was evident at 5.35% w/v lactic acid. The differences in yeast cell number in these fermentations were not due to pH, since batch fermentations over a pH range of 2.5-5.0 did not lead to changes in growth rate. Similar fermentations performed in batch showed that growth inhibition with added lactic acid was nearly identical. This indicates that the apparent high resistance of S. cerevisiae to lactic acid in continuous VHG fermentations is not a function of culture mode. Although the total amount of ethanol decreased from 48.7 g l(-1) to 14.5 g l(-1) when 4.74% w/v undissociated lactic acid was added, the specific ethanol productivity increased ca. 3.2-fold (from 7.42 x 10(-7) g to 24.0 x 10(-7) g ethanol CFU(-1) h(-1)), which indicated that lactic acid stress improved the ethanol production of each surviving cell. In multistage continuous fermentations, lactic acid was not responsible for the 83% (CFU ml(-1)) reduction in viable S. cerevisiae yeasts when Lactobacillus paracasei was introduced to the system at a controlled pH of 6.0. The competition for trace nutrients in those fermentations and not lactic acid produced by L. paracasei likely caused the yeast inhibition.

Effects of physical properties of feed on microbial ecology and survival of Salmonella enterica serovar Typhimurium in the pig gastrointestinal tract.

A two-by-two factorial experiment with pigs was conducted to study the effect of feed grinding (fine and coarse) and feed processing (pelleted and nonpelleted) on physicochemical properties, microbial populations, and survival of Salmonella enterica serovar Typhimurium DT12 in the gastrointestinal tracts of pigs. Results demonstrated a strong effect of diet on parameters measured in the stomachs of the pigs, whereas the effect was less in the other parts of the gastrointestinal tract. Pigs fed the coarse nonpelleted (C-NP) diet showed more solid gastric content with higher dry matter content than pigs fed the fine nonpelleted (F-NP), coarse pelleted (C-P), or fine pelleted (F-P) diet. Pigs fed the C-NP diet also showed significantly increased number of anaerobic bacteria (P < 0.05), increased concentrations of organic acids, and reduced pH in the stomach. In addition, pigs fed the C-NP diet showed increased in vitro death rate of S. enterica serovar Typhimurium DT12 in content from the stomach (P < 0.001). Pigs fed the C-NP diet had a significantly higher concentration of undissociated lactic acid in gastric content than pigs fed the other diets (P < 0.001). A strong correlation between the concentration of undissociated lactic acid and the death rate of S. enterica serovar Typhimurium DT12 was found. In the distal small intestine, cecum, and midcolon, significantly lower numbers of coliform bacteria were observed in pigs fed the coarse diets than in pigs fed the fine diets (P < 0.01). Pigs fed the C-NP diet showed the lowest number of coliform bacteria in these segments of the gastrointestinal tract. Pigs fed the coarse diets showed increased concentration of butyric acid in the cecum (P < 0.05) and colon (P < 0.10) compared with pigs fed the fine diets. It was concluded that feeding a coarsely ground meal feed to pigs changes the physicochemical and microbial properties of content in the stomach, which decreases the survival of Salmonella during passage through the stomach. In this way the stomach acts as a barrier preventing harmful bacteria from entering and proliferating in the lower part of the gastrointestinal tract.

Seed culture and its effect on the growth and lactic acid production of Lactobacillus helveticus.

Effects of seed culture medium on the subsequent culture of L. helveticus growing on whey supplemented with yeast extract and peptones have been examined. Cells were inoculated when the maximum cellular activity was achieved, i.e. the maximum for the target function, the product of the specific growth rate with the undissociated lactic acid concentration. This function decreased when the nitrogen supplementation of the preculture medium was lowered, resulting only in an increase of the lag phase length of the culture, corresponding to a cellular adaptation to the new medium. On the contrary, when cells were not in the same physiological state, growth and production parameters were affected, maximum rates, as well as maximum biomass concentration. This occurred in case of the comparison of inocula carried out under pH control or not, or when various preculture lengths were tested.

Unstructured model for seed cultures without pH control ofLactobacillus helveticus growing on supplemented whey permeate

In cultures of Lactobacillus helveticus with pH controlled at 5.9, growth and lactic acid production were under the control of nutritional limitations (carbon and nitrogen). In the absence of pH control, as was the case for seed cultures, an inhibitory effect on growth of both pH and the undissociated lactic acid concentration were evident before nutritional limitations. The variations of the specific growth rate with the undissociated lactic acid concentration followed exponential decay. Then, the inhibitory effect of pH was also taken into account through the Henderson–Hasselbach equation. The specific growth rate history has been previously found to follow sigmoidal variation. Integration of these relationships gave the theoretical biomass and lactic acid production kinetics. The model fitted experimental kinetic data well and allowed fine analysis of the various growth phases, since the growth-associated, the deceleration and the stationary states can be characterized by means of the model parameters. © 2002 Society of Chemical Industry

Modelling the growth limits (growth/no growth interface) of Escherichia coli as a function of temperature, pH, lactic acid concentration, and water activity.

The form of a previously developed Bĕlehrádek type of growth rate model was used to develop a probability model for defining the growth/no growth interface as a function of temperature (10 to 37 degrees C), pH (pH 2.8 to 6.9), lactic acid concentration (0 to 500 mM), and water activity (0.955 to 0.999; NaCl was used as the humectant). Escherichia coli was unable to grow in broth in which the undissociated lactic acid concentration exceeded 11 mM or, with two exceptions, at a pH of 3.9 or less with no lactic acid present. Under experimental conditions at which the pH and the undissociated acid concentrations were the major growth-limiting factors, the growth/no growth interface was essentially independent of temperature at temperatures ranging from 15 to 37 degrees C. The interface between conditions that allowed growth and conditions at which growth did not occur was abrupt. The inhibitory effect of combinations of water activity and pH varied with temperature. Predictions of the model for the growth/no growth interface were consistent with 95% of the experimental data set.

Modelling the growth rate of Escherichia coli as a function of pH and lactic acid concentration.

The growth rate responses of Escherichia coli M23 (a nonpathogenic strain) to suboptimal pH and lactic acid concentration were determined. Growth rates were measured turbidimetrically at 20 degrees C in the range of pH 2.71 to 8.45. The total concentration of lactic acid was fixed at specific values, and the pH was varied by the addition of a strong acid (hydrochloric) or base (sodium hydroxide) to enable the determination of undissociated and dissociated lactic acid concentrations under each condition. In the absence of lactic acid, E. coli grew at pH 4.0 but not at pH 3.7 and was unable to grow in the presence of > or = 8.32 mM undissociated lactic acid. Growth rate was linearly related to hydrogen ion concentration in the absence of lactic acid. In the range 0 to 100 mM lactic acid, growth rate was also linearly related to undissociated lactic acid concentration. A mathematical model to describe these observations was developed based on a Bĕlehrádek-like model for the effects of water activity and temperature. This model was expanded to describe the effects of pH and lactic acid by the inclusion of novel terms for the inhibition due to the presence of hydrogen ions, undissociated lactic acid, and dissociated lactic acid species. Preliminary data obtained for 200 and 500 mM total lactic acid concentrations show that the response to very high lactic acid concentrations was less well described by the model. However, for 0 to 100 mM lactic acid, the model described well the qualitative and quantitative features of the response.

Recovery of lactic acid by sorption

Weak-, moderate-, and strong-base resins were evaluated for their sorption capacities of lactic acid from solutions with different pHs. Composite isotherms and breakthrough curves indicated that the sorption capacities of weak- and moderate-base resins decreased markedly as the pH of the feed exceeded the pKa of lactic acid. The decrease in capacity was mainly the result of the decrease in concentration of undissociated lactic acid, the species preferentially adsorbed by the resin, as the pH of the feed increases. The sorption capacity of the strong-base resin remained constant in the same pH range (pH 2–6).

Limitation of growth and lactic acid production in batch and continuous cultures of Lactobacillus helveticus

Continuous and batch cultures of Lactobacillus helveticus operated under different conditions were studied with respect to the limitation of growth and lactic acid production by increasing undissociated lactic acid and hydrogen ion concentrations, respectively. In a single-stage continuous culture without pH control a final pH of 3.8 and 65 mm undissociated lactic acid was obtained. In two-stage continuous cultures provided with different growth media and run at different pH values, 65–70 mm free acid was obtained in the second stage. Further batch-culture experiments showed growth limitation at 60–70 mm lactic acid. After growth ceased, production of lactate continued until a lactic acid concentration of about 100 mm was reached; obviously an uncoupling of growth and acid production had occurred. Examining the effect of different concentrations of either lactic acid or hydrochloric acid, added to growing batch cultures of L. helveticus, it was shown that the undissociated lactic acid concentration was responsible for growth limitation and lactic acid production in this organism, whereas the pH value had only an indirect effect.

Growth, Inhibition, and Survival of Listeria monocytogenes as Affected by Acidic Conditions

Growth and survival of four strains of Listeria monocytogenes under acidic conditions were investigated. Tryptic soy broth with yeast extract (TSBYE) was acidified with acetic, citric, hydrochloric, lactic, or propionic acid to pH 4.0–6.0, inoculated with L. monocytogenes and incubated at 30 or 4°C. The minimum test pH at which L. monocytogenes did not grow (inhibitory pH) was determined for each acid. In the pH range tested, this inhibitory pH was 5.0 for propionic acid, 4.5 for acetic and lactic acids, and 4.0 for citric and hydrochloric acids. All four strains gave similar results. Subsequent studies were conducted at 10 and 30°C to determine changes in cell populations in TSBYE adjusted to each inhibitory pH. Initial populations of viable cells (104 CFU/ml) were reduced to <10 CFU/ml within 1–3 weeks at 30°C, whereas at 10°C, L. monocytogenes survived for 11–12 weeks in acetic, citric, or propionic acid-adjusted media and for 6 weeks in media adjusted with hydrochloric or lactic acid. The concentration of undissociated lactic acid was 0.002M at pH 4.5.