Browsing by Author "Kirby, R."
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- Evidence of membrane damage in Lactobacillus bulgaricus following freeze dryingPublication . Castro, H. P.; Teixeira, Paula; Kirby, R.The mechanism of inactivation of Lactobacillus bulgaricus due to freeze drying was investigated. Cells were freeze-dried in skim milk powder, maltodextrin, glycerol, trehalose and water. Results are presented confirming previous authors’observations regarding membrane damage during freeze drying. In an attempt to define more clearly the nature of this damage, further experiments were carried out. Results show that following freeze drying changes occur in the unsaturated: saturated fatty acid ratio, a decrease in the activity of the membrane-bound enzyme ATPase and a loss of ΔpH.
- 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.
- HACCP in practicePublication . Kirby, R.The hazard analysis critical control point system (HACCP) has been evolving in the food industry since it was first deliberated by the First National Food Protection Conference. Small- and medium-sized enterprises (SMEs) are companies with fewer than 500 full-time employees. This article addresses the implementation of HACCP in SMEs, highlighting some of the problems. The problems can be arranged in four groups: insufficient technical resources, concentration of functions, time and financial power. It is the conclusion of this article however, that there are no barriers to the application of HACCP in all food production operations.
- 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.
- 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.
- Shelf-life of chilled cut orange determined by sensory qualityPublication . Rocha, A. M. C. N.; Brochado, C. M.; Kirby, R.; Morais, A. M. M. B.The shelf-life of chilled cut orange was evaluated in terms of physical and chemical quality characteristics, microbial contamination and sensorial acceptability. After minimal processing (peeling and cutting), fresh orange was stored in air at 4 °C. Evaluations were performed at different times of storage. The respiration rate of the fresh-cut orange was also evaluated. No significant increase was noticed in relation to the whole fruit which means that this was not the factor responsible for an accelerated deterioration of the product. During the first eight days of storage the major quality parameters remain almost unchangeable, except for titratable acidity which decreased around 36% and ascorbic acid content which decreased around 22%. With respect to microbial contamination low temperature determined a considerable shelf-life (15 days). Sensory quality was the parameter which determined the shelf-life of cut orange to five days at 4 °C due to flavour changes.
- 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.
- 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.