The use of freeze-dried kefir coculture as a starter in the production of feta-type cheese was investigated. Maturation of the produced cheese at 4°C was monitored for up to 70 days, and the effects of the starter culture, the salting method, and the ripening process on quality characteristics were studied. The use of kefir coculture as a starter led to increased lactic acid concentrations and decreased pH values in the final product associated with significantly higher conversion rates compared to salted rennet cheese. Determination of bacterial diversity at the end of the ripening process in salted kefir and rennet cheeses by denaturing gradient gel electrophoresis technology, based on both DNA and RNA analyses, suggested a potential species-specific inhibition of members of the genera Staphylococcus and Psychrobacter by kefir coculture. The main active microbial associations in salted kefir cheese appeared to be members of the genera Pseudomonas and Lactococcus, while in salted rennet cheese, Oxalobacteraceae, Janthinobacterium, Psychrobacter, and Pseudomonas species were noted. The effect of the starter culture on the production of aroma-related compounds responsible for cheese flavor was also studied by the solid-phase microextraction-gas chromatography-mass spectrometry technique. Kefir coculture also appeared to extend the shelf life of unsalted cheese. Spoilage of kefir cheese was observed on the 9th and 20th days of preservation at 10 and 5°C, respectively, while spoilage in the corresponding rennet cheese was detected on the 7th and 16th days. Microbial counts during preservation of both types of unsalted cheese increased steadily and reached similar levels, with the exception of staphylococci, which were significantly lower in unsalted kefir cheese. All types of cheese produced with kefir as a starter were approved and accepted by the panel during the preliminary sensory evaluation compared to commercial feta-type cheese.
Phosphonoacetate is regarded as an antiviral xenobiotic whose mineralization can be catalysed by an enzyme, phosphonoacetate hydrolase, encoded by the phnA gene. To date the enzyme's activity has been detected in only a limited number of bacteria. Its expression has been shown to occur in a manner independent of the phosphate status of the cell, in direct contrast to the general rule of organophosphonate metabolism being under the control of the pho regulon. In this study the environmental occurrence of the phnA gene was evaluated by polymerase chain reaction amplification of DNA extracts obtained directly from various soil environments. Sensitivity of this method was improved such that a positive result was routinely obtained with soil spiked with as few as 6 colony-forming units (cfu) per gram of soil of Pseudomonas fluorescens 23F (phnA(+)). When total DNA from a variety of Northern Irish, Greek and Bolivian soils was tested, all were positive for phnA. Bacteria capable of utilizing phosphonoacetate as sole carbon, energy and phosphorus source, with the release of essentially equimolar concentrations of phosphate to the culture supernatant, were isolated from all soil samples tested. Analysis of three such isolates revealed all to be species of Pseudomonas sensu stricto, possessing phosphonoacetate hydrolase activity in cell-free extracts. Sequence determination of the phnA gene revealed a similarity of the putative protein sequences at levels of 98.3-99.3% between the Pseudomonas strains. This is the first study to use molecular methods to investigate the distribution of a gene encoding organophosphonate metabolism, and indicates that the phnA gene is ubiquitous within soils from geographically distinct regions. Such an observation supports the proposition that phosphonoacetate is a compound that may also have a biogenic origin.
The aim of the present study was to evaluate the impact of thermal drying of immobilized Lactobacillus delbrueckii subsp. bulgaricus on apple pieces on the use of the derived biocatalyst in whey fermentation. The thermally dried immobilized biocatalyst was compared to wet and freeze-dried immobilized cells, in respect to maintenance of cell viability and fermentation efficiency. The thermal drying process appeared to be more efficient on survival rate as an 84% of the cells used for immobilization survived the process, while the freeze-drying process led to a 78% rate. The thermally dried immobilized biocatalyst was used in 12 repeated batch fermentations of synthetic lactose medium and whey at 37, 45, and 50 degrees C in order to evaluate its metabolic activity. The high number of repeated batch fermentations showed a tendency for high operational stability. Fermentations continued for up to 2 months without any significant loss of metabolic activity. SPME GC/MS analysis of aroma-related compounds revealed the distinctive character of fermented whey produced by the thermally dried immobilized bacterium cells. The effect of storage at 4-6 degrees C for up to 165 days of the biocatalyst, held directly after drying and after repeated batch fermentations, on fermentation activity was also studied. After storage, reactivation in whey was immediate, and the immobilized biocatalyst was able to produce up to 51.7 g/L lactic acid at 37 degrees C. The potential of thermally dried immobilized L. delbrueckii as a starter culture for food production was subsequently evaluated.
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