Effects of temperature, ammonium and glucose concentrations on yeast growth in a model wine system

In enology, alcoholic fermentation is a complex process involving several mechanisms. Slow and incomplete alcoholic fermentation is a chronic problem for the wine industry and factors leading to sluggish and stuck fermentations have been extensively studied and reviewed. The most studied cause of sluggish and stuck fermentation is the nitrogen content limitation. Nevertheless, other factors, such as temperature of fermentation and sugar concentration can affect the growth of yeasts. In this study we modelled the yeast growth‐cycle in wine model system as a function of temperature, sugar and ammonium concentrations; the individual effects and the interaction of these factors were analysed by means of a quadratic response surface methodology. Cell concentrations and weight loss were monitored in the whole wine fermentation process. The results of central composite design show that lower is the availability of nitrogen, higher is the cell growth rate; moreover, initial nitrogen concentration also influences survival time of Saccharomyces cerevisiae.     

Modelling the survival of starter lactic acid bacteria and Bifidobacterium bifidum in single and simultaneous cultures

The inactivation kinetics at 4 degrees C of Bifidobacterium bifidum, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, cultured alone or consociated in a laboratory medium (modified MRS broth), were modelled through the Weibull model and a second-order polynomial equation. The initial cell number of S. thermophilus and L. delbrueckii subsp. bulgaricus was approximately 6-7log (CFU/ml); the viability loss after 30 days of storage was 2.87 and 1.99log (CFU/l) for L. delbruckii subsp. bulgaricus and S. thermophilus, cultured alone, respectively; whereas the consociation of lactobacilli and streptococci with bifidobacteria reduced viability loss during storage (0.28 and 0.54log CFU/l for lactobacilli and streptococci, respectively). Finally, the consociation of lactobacilli and streptococci with B. bifidum improved their oxygen uptake.     

Whey protein nanoparticles prepared with desolvation with ethanol: Characterization,thermal stability and interfacial behavior

Whey protein isolate (WPI) nanoparticles were prepared by diluting an alkaline solution of protein in ethanol at concentrations varying between 50 and 80%. The nanoparticles were then immediately diluted in buffer. While the nanoparticles were not stable at pH 7, they showed no changes in size when diluted at pH 3. When 75–80% ethanol was added during preparation, the size of the WPI nanoparticles ranged between 10 and 100 nm, with no change in size after dilution and storage at pH 3 for 96 h at 22 °C. When heating was applied, particle aggregation occurred, and large aggregates (>1 μm) were observed at temperatures > 60 °C. The particle size of the heat-induced aggregates could be reduced by homogenization. The nanoparticles prepared by desolvation showed interfacial pressure values similar to those of the corresponding protein solutions, indicating similar interfacial properties and the potential to be used to stabilize emulsions but as supramolecular aggregates of WPI.     

Whey protein aggregate formation during heating in the presence of κ-carrageenan

The effect of a negatively charged polymer, κ-carrageenan, on the aggregation behaviour of whey proteins during heating was studied. Aqueous solutions of whey protein isolate (WPI) at 0.5% were heated in the presence of κ-carrageenan (0.1%) at pH 7.0. This concentration was chosen as optimal in the detection of the intermediate aggregates during chromatographic analysis. The residual unaggregated protein, the intermediate aggregates and the soluble aggregates were all examined as a function of heating time and temperature, using size-exclusion chromatography coupled with light scattering detection. The presence of κ-carrageenan did not affect the aggregation of whey proteins heated at 75 °C; however, a change in the mechanism of aggregation seemed to occur at higher temperatures, and intermediates with higher molecular mass formed at 85 °C. At 90 °C, in the presence of κ-carrageenan, the extent of WPI aggregation was much larger, as soluble aggregates were no longer present and less residual protein was recovered in the unaggregated peak.     

Description of the microflora of sourdoughs by culture-dependent and culture-independent methods

Four types of sourdoughs (L, C, B, Q) from artisanal bakeries in Northern Italy were studied using culture-dependent and culture-independent methods. In all samples, the yeast numbers ranged from 160 to 10⁷ cfu/g, and the numbers of lactic acid bacteria (LAB) ranged from 10³ to 10⁹ cfu/g. The isolated LAB were sequenced, and a similarity was noted between two samples (C, Q), both in terms of the species that were present and in terms of the percentage of isolates. In these two samples, Lactobacillus plantarum accounted for 73% and 89% of the bacteria, and Lactobacillus brevis represented 27% and 11%. In the third sample (B), however, the dominant LAB isolate was Lb. brevis (73%), while Lb. plantarum accounted for only 27%. The fourth sourdough (L) was completely different from the others. In this sample, the most prominent isolate was Weisella cibaria (56%), followed by Lb. plantarum (36%) and Pediococcus pentosaceus (8%). In three out of four samples (L, C and Q), all of the yeasts isolated were identified as Saccharomyces cerevisiae, yet only Candida humilis (90%) and Candida milleri (10%) were isolated in the fourth sample (B). The microbial ecology of the sourdoughs was also examined with direct methods. The results obtained by culture-independent methods and DGGE analysis underline a partial correspondence between the DNA and RNA analysis. These results demonstrate the importance of using a combined analytical approach to explore the microbial communities of sourdoughs.     

Modelling the survival of Enterobacter cloacae in a model olive cover brine solution

The main goal of this research was the evaluation of the survival of Enterobacter cloacae in a model olive brine. Two different assays were run; the first experiment assessed the viability of the target in brines containing NaCl (6–12%) and p‐coumaric acid (0.0–0.05%), adjusted to different pHs (4–10) and stored at 10–30 °C for 9 days. The death rate and cell levels at selected times were modelled with a polynomial equation to highlight the individual and interactive effects of NaCl/p‐coumaric acid/pH/temperature. Then, a second experiment was run for 3 months (temperature, 10 °C; pH, 4.5–5.5; NaCl, 6–8%). The survival of E. cloacae was affected mainly by pH, then by salt and temperature; however, the significance of the variables changed within the time, as salt and temperature acted in a significant way only after 1 day.     

Evaluation of Pseudomonas spp. through O2 and CO2 headspace analysis

This article proposes an approach to determine the level of Pseudomonas spp. in milk, based on the evaluation of the content of oxygen and carbon dioxide produced in the headspace of sealed vials; the research was divided into two phases: model building and preliminary validation. Three different strains of Pseudomonas spp., Ps. putida (wild strain) and Ps. fluorescens (wild and collection isolates), were used as targets. Data of CO₂ and O₂ were modelled through a modified positive (CO₂) or a negative Gompertz equation (O₂) to estimate the Minimum Detection Time (MDT), defined as the time to attain 3% of CO₂ (MDT₁) or a decrease in the content of O₂ by 3% (MDT₂). Then, MDT₁ and MDT₂ were submitted to a linear regression procedure, using cell concentration as independent variable; the correlations ‘MDT₁/cell concentration’ and ‘MDT₂/cell concentration’ showed high determination coefficients (>0.983). Moreover, the regression procedure pointed out that both MDT₁ and MDT₂ decreased by ca. 3 h for an increase in cell count of 1 log cfu mL^?1. Preliminary validation in milk pointed out that the error associated with the regression line ‘MDT₂/cell concentration’ was below 5%.     

Changes of the cell surface hydrophobicity of Lactobacillus acidophilus La‐5 in response to pH,temperature and inulin

The main topic of this study was to study how cell surface hydrophobicity (CSH) could change in response to pH, temperature and inulin; Lactobacillus acidophilus La‐5 was used as a model microorganism. pH, temperature, inulin and incubation time (exposure to prebiotic or incubation at pH 4.0 and 9.0) were combined through a full factorial design and a Central Composite Design; the results were analysed using a multifactorial anova (first step) and a stepwise regression (second step). Temperature and pH significantly affected CSH: an increase in the temperature determined a significant increase in CSH, whereas the correlation pH vs. CSH was negative, as an increase in pH caused a significant decrease in CSH. Inulin played a significant role, but its effect could be influenced by temperature, pH and exposure time. This study is the first approach on the effects of some environmental factors on CSH and suggests that the culturing conditions and/or the exposure to some prebiotics could modify it with positive or negative effects.     

Molecular typing of Staphylococcus aureus isolated from Italian dairy products on the basis of coagulase gene polymorphism, multiple-locus variable-number tandem-repeat and toxin genes

Coagulase gene restriction fragment length polymorphism (RFLP), six-locus variable-number tandem-repeat analysis patterns (MLVA) and detection of enterotoxin genes (se) (sea, sec, sed, seg, seh, sei, sej and sel) were used to determine the phylogenetic relationship among isolates of Staphylococcus aureus isolated from dairy products from different regions of Italy. A total of 25 Staph. aureus were subtyped into 16 coagulase genotypes by RFLP, and MLVA revealed marked genomic variability. Furthermore, 17 of the isolates harboured at least one toxin gene, with the predominance of sea, sed and sej among cow isolates and sec-sel among the goat and sheep strains. Combined RFLP, MLVA polymorphism and se genes were found to be useful techniques for discriminating several genetic variants in Staph. aureus isolates.