Weibull probability paper used on highly stressed bimodal components

Under high stress conditions the ‘freak’ and ‘strong’ subpopulations lie very close to each other on the time scale. It is easy to ‘read’ such a time-to-failure distribution as unimodal, especially when probability paper is used. This paper puts forward Parzen'sestimator of the probability density function as a very useful method of indicating the ‘freak’ subpopulation.     

Technological properties of milks fermented with thermophilic lactic acid bacteria at suboptimal temperature

In the present work the synergistic relationship between different strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus was studied at optimal (44 degrees C) and suboptimal temperatures (30 degrees C). Acidification, viscosity, whey syneresis, and bacterial concentration of the final product were evaluated on single-strain and mixed cultures after 24 h at 30 degrees C and 6 h at 44 degrees C. Three pairs of strains (LBB + CP2, LBP + CP2, and LBR + CP2) showed synergistic effect, which was reflected by the viscosity and syneresis of the coagulum. These results were more significant when cultures were incubated at 30 degrees C, reaching apparent viscosity values of 19 to 28 mPa x s. On the other hand, lactobacilli cultures enhanced the growth of two streptococci strains (CP2 and CP4). These results were confirmed by cultures of streptococci supplemented with supernatants of culture of lactobacilli. Those supernatants stimulate the viscosity produced by CP2 and CP4 strains and reduce the syneresis of all cultures of streptococci. Neither the increase of viscosity nor reduction of syneresis could be attributed to a decrease of pH.     

Cyclostationary modelling of rotating machine vibration signals

This paper is dedicated to modelling rotating machine signals as cyclostationary processes, with strong emphasis on the peculiarities and pitfalls that this issue implies. The objective is to demonstrate that machine signals require specific processing that are much more subtle than for communication signals—from which the paradigm of cyclostationarity was originally issued from and developed for. First, different types of cyclostationarity embracing a multitude of rotating machine signals are distinguished. In particular, the importance of considering pure rather than impure cyclostationarity is stressed out. Next, the relationships between angle and time cyclostationarity are investigated and some useful results are derived. It is shown that vibration signals exhibit cyclostationarity if and only if the random speed fluctuation of the machine is periodic, stationary or cyclostationary. Finally, a comprehensive methodology is proposed for processing actual cyclostationary signals: three typical examples dealing with vibration signals of an IC engine, a gearbox and a rolling element bearing are presented, each of them being characterised by a different type of cyclostationarity. The methodology proposed in this paper is general enough and may serve as a guideline for modelling and analysing other types of rotating machine signals, such as pressure signals, electric signals, etc.     

Fatty acid composition and freeze-thaw resistance in lactobacilli

The fatty acid composition and freeze-thaw resistance of eight strains of thermophilic lactobacilli were studied. Seven of these contained the same polar and neutral lipids, the five major components making up 90% of the cellular fatty acid pool being 14:0, 16:0, 16:1, 18:1 and C19 cyclopropane (cyc19:0). Strain comparison by means of cluster analysis based on the fatty acid ratios using the overlap coefficient revealed two well defined clusters. One was formed by three strains of species Lactobacillus delbrueckii subsp. lactis and Lb. delbrueckii subsp. delbrueckii, the other included five strains of the species Lb. delbrueckii subsp. bulgaricus, Lb. acidophilus and Lb. helveticus. Resistance of strains with a high content of unsaturated fatty acids (66-70%) decreased with increasing cyc19:0 concentrations. In contrast, in strains with a low concentration of unsaturated fatty acids (42-49%), increasing cyc19:0 levels were associated with increased freeze-thaw resistance.     

Lactobacilli isolated from kefir grains: evidence of the presence of S-layer proteins

In the present study we report for the first time the presence of S-layer proteins in Lactobacillus kefir and Lactobacillus parakefir isolated from kefir grains. Soluble whole-cell protein profile obtained either by mechanical disruption (X-press) or by a combined treatment with lysozyme and SDS on whole cells, showed a significant band of apparent molecular mass of 66-71 kDa as measured by SDS-PAGE. The intensity of this band was considerably reduced when cells were treated with 5 M-LiCl. The above mentioned proteins were recovered in the LiCl extracts. After dialysis and concentration, the proteins extracted were able to reassemble in a regular array. Negative staining of these protein preparations were analysed by transmission electron microscopy and a paracrystalline arrangement was seen. Thin sections of bacteria analysed by transmission electron micrographs showed an outermost layer over the bacterial cell wall, that was lost after the LiCl treatment. The production of this surface structure under different culture conditions was also evaluated. Finally, the relationship between the presence of S-layer proteins and surface properties (e.g. adhesion to Caco-2 cells, autoaggregation, and hemagglutination) was investigated.     

Inhibition of Bacillus cereus in milk fermented with kefir grains

The effects of kefir-fermented milk were tested against a toxigenic strain of Bacillus cereus. The incubation of milk with B. cereus spores plus 5% kefir grains prevented spore germination and growth of vegetative forms. In contrast, when 1% kefir grains was used, no effects were observed. The presence of metabolically active kefir grains diminished titers of nonhemolytic enterotoxin A, as assessed by enzyme-linked immunosorbent assay. During fermentation, kefir microorganisms produce extracellular metabolites such as organic acids, which could play a role in the inhibition of spore germination and growth of B. cereus, although the effect of other factors cannot be ruled out. Results of the present study show that kefir-fermented milk is able to antagonize key mechanisms involved in the growth of B. cereus as well as interfere with the biological activity of this microorganism.     

Application of power ultrasound on the convective drying of fruits and vegetables: effects on quality

Drying gives rise to products with a long shelf life by reducing the water activity to a level that is sufficiently low to inhibit the growth of microorganisms, enzymatic reactions and other deteriorative reactions. Despite the benefits of this operation, the quality of heat sensitive products is diminished when high temperatures are used. The use of low drying temperatures reduces the heat damage but, because of a longer drying time, oxidation reactions occur and a reduction of the quality is also observed. Thus, drying is a method that lends itself to being intensified. For this reason, alternative techniques are being studied. Power ultrasound is considered as an emerging and promising technology in the food industry. The potential of this technology relies on its ability to accelerate the mass transfer processes in solid–liquid and solid–gas systems. Intensification of the drying process with power ultrasound can be achieved by modifying the product behavior during drying, using pre‐treatments such as soaking in a liquid medium assisted acoustically or, during the drying process itself, by applying power ultrasound in the gaseous medium. This review summarises the effects of the application of the power ultrasound on the quality of different dried products, such as fruits and vegetables, when the acoustic energy is intended to intensify the drying process, either when the application is performed before pretreatment or during the drying process. © 2017 Society of Chemical Industry