Interactions between inhibition factors on microbial stability of fruit-based systems

Use of lemon extract to inhibit the growth of malolactic bacteria

The potential use of lemon extract as a natural preservative to inhibit the growth of Oenococcus oeni and Lactobacillus plantarum, microorganisms involved in the malotactic fermentation of wine, was studied. Growth tests were run at 30 degrees C using laboratory media. Carbon dioxide concentration in the vial headspace was used as metabolic activity index of the investigated microorganisms. The MIC and the noninhibiting concentration (NIC) were calculated for each microorganism. Results suggest that lemon extract was active on each phase of the growth cycle for the tested microorganisms. It was also shown that lemon extract exhibits a nonlinear dose-related inhibitory effect on microbial growth. In particular, the active compound could be added at concentrations slightly higher than the NIC levels in order to appreciably slow down the microbial growth rate as well as to reduce the maximum microbial growth level.

Electrochemical evaluation of the inhibitory effects of weak acids on Zygosaccharomyces bailii

The changes in intracellular redox activity or in mitochondrial electron transport could be taken as indications of the changes in the physiological state of living cells, based on which a mediated electrochemical method was purposed to evaluate the inhibitory effects of weak acids on Zygosaccharomyces bailii, a known food spoilage yeast. The dual mediator systems menadione/ferricyanide and 2,6-dichlorophenolindophenol/ferricyanide were employed as probes to detect the variance in intracellular redox activity and in mitochondrial electron flux, respectively. Measurements were made with a microelectrode voltammetric method to assay the ferrocyanide accumulations arising from menadione- or 2,6-dichlorophenolindophenol-mediated reduction of ferricyanide by Z. bailii suspensions by the presence or absence of increasing concentrations of weak acids. The results obtained from 2 h of incubation showed that the variance in electrochemical response revealed some physiological information underlying the inhibitory effects of weak acid on the yeast. For the first time, it was shown that the mediated electrochemical method provides an adjunct to the conventional method based on respiration inhibition for establishing levels for the utilization of preservatives in the food industry.

Predictive modelling of growth and enzyme production and activity by a cocktail of Pseudomonas spp., Shewanella putrefaciens and Acinetobacter sp

The possibility was examined of developing a predictive model that combined microbial growth (increase in cellular number) with extracellular lipolytic and proteolytic enzyme activity of a cocktail of four strains of Pseudomonas spp. and one strain each of Acinetobacter sp. and Shewanella putrefaciens. Environmental conditions within the following matrix of conditions were examined: temperature 2-20 degrees C, pH value 4.0-7.5 and water activity (a(w)) 0.95-0.995 and a model was constructed, which predicted growth based on increase in cell number. Data on lipase production and protease activity were generated and will be available as a database, but no function could be identified, which was a good fit to these data, since most enzymatic production and activity occurred, as expected, during transition from exponential to stationary phase. Even at lower cell numbers, in more unfavourable conditions, hydrolysing effects were detectable, which made it difficult to construct a model combining both microbiological and enzymatic data.

Effects of weak acid preservatives on the growth and thermal death of the yeast Pichia membranifaciens in a commercial apple juice

Pichia membranifaciens exhibited a dissociative temperature profile (the temperature range of thermal death was distinct from the temperature range of growth) when incubation took place either in a commercial apple juice (AJ) or in a synthetic mineral medium with glucose and vitamins (MGV). In AJ the maximum temperature for growth (Tmax) was 38.6 degrees C, which decreased to 36 degrees C in the presence of either 1 mM sorbic or 1 mM benzoic acid. The minimum temperatures of thermal death (Tmind) were, respectively, 40 and 38 degrees C with either of the acids. The yeast could grow with up to 2 mM sorbic or 3 mM benzoic acid, at 25 degrees C, which is close to the optimum temperature for growth (Top). At temperatures slightly above Tmind, sorbic acid was an actual enhancer of death rather than benzoic, the latter conferring some protection. However, these effects were reversed at higher temperatures (above 43 degrees C), at which benzoic acid was the most operative, in contrast to sorbic which was highly protective of the yeast against thermal death. The addition of acetaldehyde to sulphur-dioxide-containing juice reduced the lag phase and increased the overall specific growth rates. Sporulated or stationary vegetative cultures were more heat-resistant than exponential cultures, particularly at temperatures above 45 degrees C.

Weak acid inhibition of fermentation by Zygosaccharomyces bailii and Saccharomyces cerevisiae

The inhibition kinetics of fermentation by Zygosaccharomyces bailii and Saccharomyces cerevisiae were evaluated for weak carboxylic acids. Several regression equations were tried to fit the experimental data, most cases being best fitted to exponential curves. The following parameters were determined: i) acid concentration responsible for 50% inhibition of fermentation (C50%); ii) area under the regression curve up to that concentration (A50%) and iii) exponential inhibition constant (k(i)). These parameters were compared according to their ability to express the inhibitory effect of each acid. In broad terms, the values of k(i) in association with minimum inhibitory concentrations (x(min)), were found best to express the inhibitory effect of the weak acids. However, C50% values were satisfactorily correlated with k(i). The value of A50% more precisely reflected the occasional stimulatory effect of low concentrations of weak acids. Comparison of inhibition parameters for Z. bailii and for S. cerevisiae revealed a higher resistance of the former to acetic, propionic, butyric and benzoic acids and similar resistance to caproic, caprylic and sorbic acids. Previous cultivation in the presence of acetic, propionic and benzoic acids showed a distinct influence on the resistance of both yeasts, although it did not always induce cellular adaptation. Fermentation inhibition showed a good correlation with the lipid solubility of weak acids suggesting that the acids interact with the hydrophobic regions of cell membranes.

Growth modelling of Listeria monocytogenes and Yersinia enterocolitica in food model systems and dairy products

The growth potential of Listeria monocytogenes, measured with a gaschromatographic method, was evaluated in model systems simulating dairy products in which NaCl concentration, pH and lipid concentration ranged from values corresponding to matured hard cheeses to some light cheeses and other dairy products. The growth data from the model systems, calculated on the basis of the polynomial models obtained, were compared with values observed for dairy products. Growth parameters of Yersinia enterocolitica in traditional and light cheeses were also available. The comparison between predicted and observed data suggested that individual or interactive effects of chemicophysical variables such as pH, salt and lipid concentration are not sufficient to predict the fate of L. monocytogenes in dairy products. Microstructural factors as well as the pH effect on water binding capacity of proteins could also be implicated.

Use of indirect conductimetry to predict the growth of spoilage yeasts, with special consideration of Zygosaccharomyces bailii

In recent years, modeling for the purpose of predicting microbiological spoilage of foods has gained much interest. Predictive modeling requires a concentrated mathematical and experimental approach; to collect data of adequate quality is a technically demanding task when several experimental parameters are involved. Rapid, non-traditional, automated techniques are particularly useful in modeling. Of these, electrometric techniques appear to be most promising. Indirect conductimetry was used to study the effect of temperature, aw, pH and potassium sorbate concentration on the growth of Zygosaccharomyces bailii. The automated Malthus 2000 instrument proved to be convenient for gathering a large amount of data that were then used to develop polynomial models describing the response of the yeast to combinations of experimental factors in terms of conductimetric detection time and maximum rate of change in conductance. Results demonstrated that indirect conductimetry is suitable for monitoring the effect of environmental factors on the growth and activity of Z. bailii and perhaps other food spoilage yeasts.

Relationship between thermal behaviour, fermentation performance and fatty acid composition in two strains of Saccharomyces cerevisiae

Two Saccharomyces cerevisiae strains, one specifically of S. cerevisiae and another belonging to the physiological race S. uvarum, exhibited associative and dissociate thermal profiles, respectively. The S. cerevisiae subsp. uvarum strain, which displayed the dissociative profile, was characterized by a higher aptitude for fermenting glucose in a superoptimal temperature range as well as by a lower fatty acid unsaturation degree. On the other hand, both strains exhibited a similar fatty acid composition modulation pattern with regard to temperature: the unsaturation level presented two relative maxima at 15 and 40 degrees C. However, on the basis of Central Composite Design results, supplementation with an oleic acid source under semi-anaerobic conditions did not improve the fermentative performances in either strain. The modelling of fermentation rate in relation to certain variables indicated that the fermentative performance at superoptimal temperatures, and particularly the optimal temperature (Topt) and maximal temperature (Tmax) of the strain displaying a dissociative profile, could be increased by acting on medium composition.

Spoilage yeasts

Yeasts are best known for their beneficial contributions to society, and the literature abounds with discussions of their role in the fermentation of alcoholic beverages, bread, and other products. Yeasts also cause spoilage, but, with a few exceptions, this unwanted activity often goes unrecognized and underestimated as a major problem in the food and beverage industries. In some cases, there is only a fine line between what is perceived as either a spoilage or beneficial activity. This review examines the occurrence and growth of yeasts in foods and beverages with respect to their spoilage activities, the biochemistry of this spoilage, and technologies for the enumeration and identification of spoilage yeasts.