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- Microbiological quality of Portuguese yogurtsPublication . Nogueira, C.; Albano, H.; Gibbs, P.; Teixeira, P.The microbiological quality of four brands of natural yogurts and two probiotic yogurts available in the Portuguese market, was evaluated during the shelf-life period. Although the specific flora decreased during storage it was always within the range of recommended values. No coliforms and an insignificant number of fungi were detected.
- Inducible thermotolerance in Lactobacillus bulgaricusPublication . Teixeira, P.; Castro, H.; Kirby, R.The effect of a sublethal heat challenge on the subsequent thermotolerance of Lactobacillus bulgaricus at different stages of growth was investigated along with the effect of heating menstrum on survival. The response of the cells to heat stress was shown to be dependent upon both cell age and heating menstrum. Heat-inducible thermotolerance could be provoked in cells which had been growing exponentially when they were subjected to the sublethal heat stress: pre-incubation at 10°C above the optimum growth temperature. The same effect could not, however, be reproduced in cells taken from the stationary phase. Cells from the stationary phase were shown to always be more thermotolerant as compared to exponential phase cells. Cells showed a greater thermotolerance when heated in milk as compared to buffer.
- Changes in the ceIl membrane of Lactobacillus bulgaricus during storage following freeze-dryingPublication . Castro, Helena P.; Teixeira, Paula; Kirby, RoyThe mechanism of inactivation of freeze-dried Lactobacillus bulgaricus during storage in maltodextrin under controlled humidity was investigated. Evidence is presented supporting the hypothesis that membrane damage occurs during storage. A study on the lipid composition of the cells by gas chromatography showed a decrease in the unsaturated and saturated fatty acid content of the cell. Further evidence indicating membrane damage includes a decrease in membrane bound proton-translocating ATPase activity.
- Lactobacillus: Lactobacillus brevisPublication . Teixeira, P.
- Storage of lyophilized cultures of Lactobacillus bulgaricus under different relative humidities and atmospheresPublication . Castro, H. P.; Teixeira, Paula; Kirby, R.The viability of lyophilized cultures of Lactobacillus bulgaricus in skim milk, during storage at different temperatures, relative humidities, and atmospheres was investigated. Survival was greatest at 11% relative humidity and at 5°C. Indirect and direct evidence is presented supporting the hypothesis that membrane damage occurs during storage. Experiments on the lipid composition of the cell membrane demonstrate that changes occur with time that are probably the result of oxidation. A study on the lipid composition of the cell membrane by gas chromatography showed that the unsaturated/saturated fatty acid index changes with time during storage.
- Identification of sites of injury in Lactobacillus bulgaricus during heat stressPublication . Teixeira, P.; Castro, H.; Mohácsi-Farkas, C.; Kirby, R.Heat resistance of Lactobacillus bulgaricus in skimmed milk at 62°, 64°, 65° and 66°C was studied. The response to increasing temperatures in this range was not linear, with temperatures at 65°C and above giving a lower survival rate than would be predicted from experiments at lower temperatures. To identify sites of injury at these temperatures, chemical markers were used. Heating at 64°C and below resulted in damage to the cytoplasmic membrane. At temperatures of 65°C and above chemical markers also indicated damage in the cell wall and proteins. Using differential scanning calorimetry analysis of whole cells of Lact. bulgaricus seven main peaks were observed (l–51, m1–61, m2–73, n–80, p–89, q–100,r–112°C). Three of these peaks (lr, mr and pr) were the result of reversible reactions. Analysis of cell fractions identified the cell structure involved in giving rise to each of the three reversible peaks; lr, cell membrane lipids, mr, ribosomes, and pr, DNA. The evidence presented in this paper shows that irreversible reactions in the cell ribosomes are a critical site of damage in Lact. bulgaricus during heat stress in liquid media at 65°C and above.
- Evidence of membrane lipid oxidation of spray-dried Lactobacillus bulgaricus during storagePublication . Teixeira, P.; Castro, H.; Kirby, R.Membrane fatty acids of Lactobacillus bulgaricus were analysed by gas-liquid chromatography before and after spray drying. The ratio unsaturated/saturated fatty acids decreased following spray drying, indicating the formation of lesions in cellular lipid-containing structures. The same method was used to analyse membrane lipids of Lact. bulgaricus during storage. Similarly the ratio of unsaturated/saturated fatty acids in dried cells decreased further during storage in air, presenting evidence of lipid oxidation after prolonged storage. The mechanisms of cell death during storage in the dried state are still unknown, but from these results and those presented in the literature, it seems evident that lipid oxidation and survival during storage may be related.
- Lactobacillus: Lactobacillus bulgaricusPublication . Teixeira, Paula
- Spray drying as a method for preparing concentrated cultures of Lactobacillus bulgaricusPublication . Teixeira, P.; Castro, H.; Kirby, R.Spray drying and freeze drying as methods for concentration of Lactobacillus bulgaricus starter cultures were compared in terms of viability,lag phase until onset of pH decrease and total acid production. For the experimental conditions used, no significant differences were detected between the methods. The effect of spray drying on the cell membrane of Lactobacillus bulgaricus was studied. Five separate methods were used to study the theory that spray drying causes cell membrane damage; three relating to leakage of intracellular components from the cell into the surrounding environment (260 and 280 nm absorbing materials, potassium ions and proteins) ; and two relating to increased cell permeability (increased sensitivity to NaCl and increased permeability to o-nitrophenyl-P-D-galactopyranoside (ONPG). Partial loss of some cytoplasmic material from the damaged cells was observed. The dried cells also became sensitive to NaCl and permeable to ONPG. Heat shock increased the survival of exponential cells as compared to controls but did not result in normal levels found with unshocked stationary phase cells. Heat shock had no effect on stationary phase cells. Different rehydration methods and media were investigated : slow rehydration increased survival.
- Death kinetics of Lactobacillus bulgaricus in a spray drying processPublication . Teixeira, Paula; Castro, M. H.; Kirby, R. MSurvival of Lactobacillus bulgaricus during spray drying was studied at various outlet air temperatures. During spray drying the logarithmic survival ratio decreased with increased outlet air temperature with first-order kinetics; the pseudo-z value for Lactobacillus bulgaricus was 17.3°C. Plots of the death-rate constant for Lactobacillus bulgaricus versus reciprocal outlet temperature during spray drying in skim milk show a curve with two different activation energies (Ea). The calculated Ea values were 33.47 kJ/mol above 70°C and 85.77 kJ/mol below 70°C. Thermodynamic quantities for spray drying of Lactobacillus bulgaricus are also presented. Results show that the relationship between the entropy of activation and the enthalpy of activation for both spray drying and heating in liquid medium is linear, with all the data for drying falling in the range of a negative entropy.