Bonita A. Glatz

Reduction of Escherichia coli O157:H7 Counts on Whole Fresh Apples by Treatment with Sanitizers

The objectives of this study were to determine if washing of whole apples with solutions of three different sanitizers (peroxyacetic acid, chlorine dioxide, or a chlorine-phosphate buffer solution) could reduce a contaminating nonpathogenic Escherichia coli O157:H7 population by 5 logs and at what sanitizer concentration and wash time such a reduction could be achieved. Sanitizers were tested at 1, 2, 4, 8, and 16 times the manufacturers recommended concentration at wash times of 5, 10, and 15 min. Whole, sound Braeburn apples were inoculated with approximately 1 x 108 or 7 x 106 CFU per apple, stored for 24 h, then washed with sterile water (control) or with sanitizers for the prescribed time. Recovered bacteria were enumerated on trypticase soy agar. Washing with water alone reduced the recoverable population by almost 2 logs from the starting population; this can be attributed to physical removal of organisms from the apple surface. No sanitizer, when used at the recommended concentration, reduced the recovered E. coli population by 5 logs under the test conditions. The most effective sanitizer, peroxyacetic acid, achieved a 5-log reduction when used at 2.1 to 14 times its recommended concentration, depending on the length of the wash time. The chlorine-phosphate buffer solution reduced the population by 5 logs when used at 3 to 15 times its recommended concentration, depending on wash time. At no concentration or wash time tested did chlorine dioxide achieve the 5-log reduction.

Propionic acid production by extractive fermentation. I. Solvent considerations

Solvent selection for extractive fermentation for propionic acid was conducted with three systems: Alamine 304-1 (trilaurylamine) in 2-octanol, 1-dodecanol, and Witcohol 85 NF (oleyl alcohol). Among them, the solvent containing 2-octanol exhibited the highest partition coefficient in acid extraction, but it was also toxic to propionibacteria. The most solvent-resistant strain among five strains of the microorganism was selected. Solvent toxicity was eliminated via two strategies: entrapment of dissolved toxic solvent in the culture growth medium with vegetable oils such as corn, olive, or soybean oils; or replacement of the toxic 2-octanol with nontoxic Witcohol 85 NF. The complete recovery of acids from the Alamine 304-1/Witcohol 85 NF was also realized with vacuum distillation.

Biomodification of fats and oils: Trials withCandida lipolytica

Various oil-accumulating yeasts were tested for their ability to produce lipase and live on fats and oils as carbon sources. Of these,Candida lipolytica seemed most promising, and the possibility was explored of modifying fats and oils by fermenting them withC. lipolytica and extracting the modified oil deposited in the yeast cells. Oxygen was required for the growth of yeast on fats and oils, but unless the oxygen level was controlled at a low value after cell populations peaked, most of the substrate oil was converted to citrates rather than accumulating as oil. Oil accumulation byC. lipolytica from a corn oil substrate was slightly depressed by excess nitrogen in the medium. The yeasts were able to use about 18 g/l of oil in 72 hr. At substrate oil levels greater than 18 g/l, the dry yeasts were 60% oil, and about 45–57% of the substrate oil was recovered as yeast oil. The fatty acid composition of the yeast oil was quite similar to that of the substrate oil under optimum conditions of deposition. Sterols, but not tocopherols, were transferred from the substrate to the yeast oil.Candida lipolytica oil was high in free fatty acids. The greatest potential for biomodification by fermentation withC. lipolytica seems to be in altering glyceride structure.

Effects of propionic acid on propionibacteria fermentation

Various amounts of propionic acid were added to the fermentation medium of Propionibacterium thoenii strain P20 to examine the effects of propionic acid on cell growth, product formation, and substrate consumption and yield. The fermentation kinetics obtained from the cell-immobilized reactor were free of mass transfer limitations. Such inherent kinetics were described by exponential models in terms of specific propionic acid productivity and the glucose consumption rate. As the propionic acid concentration in the fermenter increased from 2.77 to 30.41 g/l−1, cell growth declined by two-thirds, specific propionic acid productivity and the glucose consumption rate also decreased from 0.059 to 0.015 g propionic acid g−1 cell · h−1, and 0.11 to 0.04 g glucose g−1 cell · h−1, respectively. The excess of propionic acid also altered the bacterial metabolism to produce more by-products such as acetic, lactic and succinic acids which resulted in the decrease of substrate yield on propionic acid from 0.52 to 0.41 g propionic acid g−1 glucose. To overcome such effects, it is desirable to maintain the acid concentration as low as possible which can be achieved by extractive fermentation. At propionic acid concentrations below 3 g l−1, a propionic acid volumetric productivity of 3.9 g l−1 · h−1 can be attained with 40% (wv) bead load.

Preservation of High-Moisture Corn by Microbial Fermentation

High-moisture corn samples (27% moisture) were inoculated with Lactobacillus plantarum and/or Propionibacterium shermanii and stored in sealed containers or under carbon dioxide atmosphere for 60 d at 26°C. Growth of the inoculated organisms was observed in the corn, and the final pH of inoculated samples was significantly lower than the final pH of uninoculated samples. Mold growth was prevented in all samples, and the initial yeast population was drastically reduced in those samples inoculated with P. shermanii . Inoculation with L. plantarum accelerated acid production in the early stages of the fermentation. Flushing with carbon dioxide did not alter the fermentation but resulted in a slightly lower final moisture content.

Improved Organic Acid Production by Calcium Alginate-Immobilized Propionibacteria☆

Abstract Propionic and acetic acid production by Propionibacterium cells immobilized in calcium alginate beads was evaluated in 12-h repeated-batch fermentations on glucose and lactate as primary carbon sources. Beads contained approximately 2 × 10 11 cells g −1 (wet weight) and were added at 40% of broth volume. High initial substrate concentrations (up to 115 gl −1 glucose and up to 78 gl −1 lactate) were utilized by the immobilized cells. The best substrate utilization rates and acid yields were obtained at 75 gl −1 initial glucose and 42 gl −1 initial lactate; 34 gl −1 and 22 gl −1 propionate were produced, respectively, at these substrate concentrations in 12 h. The ratio of propionic to acetic acid increased to 9:1 with increasing glucose concentration, but remained at 2:1 at the different initial lactate concentrations tested. Glucose consumption and propionate production rates were higher in fermentations controlled at pH 6.0 than at pH 7.0, but yields of acids from glucose were highest at pH 7.0.

A relationship between growth and lipid accumulation in Candida curvata D

Abstract In batch fermentation of the lipid-accumulating yeast Candida curvata D the accumulation of cellular nitrogen, nonlipid biomass, and lipid were sequential. The specific rate of lipid accumulation (with respect to the cellular nitrogen concentration) increased dramatically during batch fermentation and with the dilution rate in the continuous fermentations, at the same time in which there was a decrease in the specific rate of non-nitrogenous nonlipid biomass accumulation. The rate of lactose use remained constant during the change in fermentation products.

Growth and oil production of Apiotrichum curvatum in tomato juice

The lipid-accumulating yeast Apiotrichum curvatum ATCC 20509 (formerly Candida curvata D) grew in shake flask culture in freshly prepared tomato juice or tomato pulp; growth was improved when the medium was supplemented with sucrose and a basal salts mixture. Under controlled conditions in the fermentor, biomass and oil production reached a maximum at 96 h, compared with 130 h in shake flasks. Biomass production increased with the addition of 0.3% (wt/vol) urea to the juice, but decreased with the addition of 0.5% (wt/vol) yeast extract or Casamino Acids. The total amount of lipid produced and the percent of intracellular oil per cell dry weight were greatest in juice supplemented with 0.3% urea.

Response of cultures of Propionibacterium to acid and low pH: Tolerance and inhibition

Seventeen Propionibacterium strains were tested for acid production and final pH achieved on glucose, fructose, or maltose as the primary carbon source. On average, strains of Propionibacterium acidipropionici produced more acid and reached lower final pH values than did strains of the other species. Three strains of P. acidipropionici, one Propionibacterium jensenii, and two Propionibacterium thoenii strains were tested further for the ability to survive and/or grow at low pH with lactic, hydrochloric, or propionic acid as acidulant. The organic acids were more inhibitory than hydrochloric acid; propionic acid was most inhibitory. In all cases, the P. jensenii and P. thoenii strains initiated growth and survived at lower pH values than did the p. acidipropionici stains. The ability to produce large amounts of acid or achieve low final pH values did not coincide with the ability to initiate growth or survive in low-pH conditions. Strains could not initiate growth below pH 5.0, but cultures started at neutral pH reached final pH values of less than 4.4. At neutral pH, strains could grow in the presence of increased lactate concentrations (up to 180 mM) or propionate concentrations (150 mM) that were inhibitory at acid pH. attempts to isolate variants able to initiate growth below pH 5.0 were unsuccessful.

Enhanced bacteriocin production by Propionibacterium thoenii in fed-batch fermentation

Synthesis of the bacteriocin propionicin PLG-1 as well as culture growth and organic acid production by Propionibacterium thaenii P127 were followed in fed-batch fermentations conducted for 504 h in a sodium lactate broth. Average concentrations of viable cells were higher in two small-scale fed-batch fermentations than in batch fermentations: 2.2 × 109 cells per ml versus 3.7 × 108 cells per ml. Propionic acid concentration averaged 35.8 g/liter at the end of fed-batch fementation, and maximum bacteriocin titers were 184 and 146 AU/ml in the two fed-batch fermentations. After reaching the maximum value, bacteriocin activity dropped sharply upon continued incubation. Large quantities of propionicin PLG-I could be obtained in large-scale fed-batch fermentation, but the concentration (100 AU/ml) was lower than in the small-scale fermentations. Fed-batch fermentation shows promise as a method to obtain high concentrations of bacteriocin from the propionibacteria.