Metabolite production of yeasts on a strawberry-agar during storage at 7°C in air and low oxygen atmosphere

Survey on technical management of strawberries in Morocco and evaluation of their post-harvest microbial load

The climatic conditions of the growing regions influence the fruit's microbiological quality and their tolerance to post-harvest pathogens. The present work aims to identify the prevalence of bacterial and fungal strains of strawberries (Fragaria × ananassa) in the Gharb and Loukkos regions of Morocco. Thus, to establish a correlation between the microbial load and the climatic conditions of the two targeted regions. The bacteriological analyses were studied according to the International Organization for Standardization (ISO) methodologies. Regarding the mycological study, fungal species determination was performed using identification keys. Yeast species determination was done using genus analysis, assimilation, and fermentation tests. Emberger bioclimatic quotients (Q2) were calculated for the Gharb and Loukkos regions and bioclimatic stages were determined. Salmonella spp. was not detected in the studied samples. However, Listeria monocytogenes and Escherichia coli were isolated from the Gharb samples. Sulfite-reducing clostridia spores were found in two Gharb samples versus one Loukkos sample. Coagulase-positive staphylococci were negative in all samples analyzed. Loukkos and Gharb regions were contaminated by Bacillus cereus with percentages of occurrence of 5.2 and 7.8%, respectively. The fungi found in strawberries from both regions were Aspergillus niger, Botrytis cinerea, Fusarium spp., Penicillium spp., Rhizopus spp., and Alternaria alternata with a significant predominance in the Loukkos samples. Indeed, a marked presence is noted for Candida sake and Rhodotorula glutinis in strawberries from Loukkos. Gharb is located on the semi-arid stage, while Loukkos is located on the sub-humid stage. Climatic conditions have a strong influence on plant microbial load, which explains the prevalence of bacteria in strawberries from Gharb and the prevalence of fungi in strawberries from Loukkos.

Growth Prediction of the Total Bacterial Count in Freshly Squeezed Strawberry Juice during Cold Storage Using Electronic Nose and Electronic Tongue

The growth models of total bacterial count in freshly squeezed strawberry juice were established by gas and taste sensors in this paper. By selecting the optimal sensors and fusing the response values, the Modified Gompertz, Logistic, Huang and Baranyi models were used to predict and simulate the growth of bacteria. The results showed that the R² values for fitting the growth model of total bacterial count of the sensor S7 (an electronic nose sensor), of sweetness and of the principal components scores were 0.890-0.944, 0.861-0.885 and 0.954-0.964, respectively. The correlation coefficients, or R-values, between models fitted by the response values and total bacterial count ranged from 0.815 to 0.999. A single system of electronic nose (E-nose) or electronic tongue (E-tongue) sensors could be used to predict the total bacterial count in freshly squeezed strawberry juice during cold storage, while the higher rate was gained by the combination of these two systems. The fusion of E-nose and E-tongue had the best fitting-precision in predicting the total bacterial count in freshly squeezed strawberry juice during cold storage. This study proved that it was feasible to predict the growth of bacteria in freshly squeezed strawberry juice using E-nose and E-tongue sensors.

Hermetia illucens (L.) (Diptera: Stratiomyidae) Odorant Binding Proteins and Their Interactions with Selected Volatile Organic Compounds: An In Silico Approach

The black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae), has considerable global interest due to its outstanding capacity in bioconverting organic waste to insect biomass, which can be used for livestock, poultry, and aquaculture feed. Mass production of this insect in colonies requires the development of methods concentrating oviposition in specific collection devices, while the mass production of larvae and disposing of waste may require substrates that are more palatable and more attractive to the insects. In insects, chemoreception plays an essential role throughout their life cycle, responding to an array of chemical, biological and environmental signals to locate and select food, mates, oviposition sites and avoid predators. To interpret these signals, insects use an arsenal of molecular components, including small proteins called odorant binding proteins (OBPs). Next generation sequencing was used to identify genes involved in chemoreception during the larval and adult stage of BSF, with particular attention to OBPs. The analysis of the de novo adult and larval transcriptome led to the identification of 27 and 31 OBPs for adults and larvae, respectively. Among these OBPs, 15 were common in larval and adult transcriptomes and the tertiary structures of 8 selected OBPs were modelled. In silico docking of ligands confirms the potential interaction with VOCs of interest. Starting from the information about the growth performance of H. illucens on different organic substrates from the agri-food sector, the present work demonstrates a possible correlation between a pool of selected VOCs, emitted by those substrates that are attractive for H. illucens females when searching for oviposition sites, as well as phagostimulants for larvae. The binding affinities between OBPs and selected ligands calculated by in silico modelling may indicate a correlation among OBPs, VOCs and behavioural preferences that will be the basis for further analysis.

Relationship between carbohydrate composition and fungal deterioration of functional strawberry juices preserved using non-thermal treatments

BACKGROUND

The quantification of the main carbohydrates present in strawberry juices enriched with inulin and fructo-oligosaccharides (FOS) and preserved by non-thermal techniques (vanillin and ultrasound) was conducted, in addition to an investigation of the evolution of these compounds and their relationship with fungal deterioration over 14 days of refrigerated storage.

RESULTS

A simple and environmentally friendly analytical approach based on high-performance liquid chromatography with a reflection index detector was developed for simultaneous determination of inulin, FOS and mono- and disaccharides present in the juices. When analyzing the evolution of carbohydrates during storage, a direct relationship between the consumption of sucrose and the growth of yeasts and molds (main spoilage flora in strawberry) was observed, especially in untreated samples (control). By contrast, no sucrose consumption was observed during storage of the treated sample, thus demonstrating the efficiency of the non-thermal treatments for controlling yeasts and mold growth. In turn, inulin and FOS added to juices were not degraded during storage.

CONCLUSION

The results obtained in the present study demonstrate that non-thermal treatments are adequate for preventing the growth of deteriorative flora in strawberry juices and that the addition of inulin and FOS can be a good strategy for functionalizing them, as well as improving their nutritional properties. © 2017 Society of Chemical Industry.

Novel biopreservatives to enhance the safety and quality of strawberry juice

Pomegranate extract, vanillin and geraniol were studied as natural antimicrobials on strawberry juice. Strawberry juice was treated with each agent at two concentrations: pomegranate extract at 180 and 360 μg/mL; vanillin at 2.5 and 5 mg/mL; and geraniol at 0.6 and 1.2 μL/mL. After being treated, juices were stored at 5 °C and microbiological, physicochemical and sensory studies were carried out. Also, a second batch of juice was inoculated with Escherichia coli O157:H7 (10(5) CFU/mL) before being treated, to safety study. Geraniol and vanillin, at both concentrations tested, were highly effective in reducing the native microflora on strawberry juice (more than 3 log cycles), extending the microbiological shelf-life of the product. Moreover, both antimicrobials improved the product safety by reducing inoculated E. coli O157:H7. Furthermore, vanillin showed a significant increase in polyphenol content compared to untreated juice. On the other hand, pomegranate extract applied at the highest concentration showed important reductions on mesophilic and psychrophilic bacteria, but no effect on yeast and molds and inoculated E. coli. Even though vanillin and geraniol incorporation on strawberry juice had a negative effect on its sensory quality, pomegranate extract had no impact on the sensory attributes evaluated. Combinations of the biopreservatives could be studied in order to decrease the concentration of the antimicrobials, reducing the effects on strawberry juice sensory characteristics.

Effect of high oxygen and high carbon dioxide atmosphere packaging on the microbial spoilage and shelf-life of fresh-cut honeydew melon

This study evaluated the potential of modified atmospheres (MAs) combining high oxygen (O₂) and high carbon dioxide (CO₂) levels to extend the shelf-life of fresh-cut honeydew melon. Firstly, the effect of MA on the growth and volatile organic metabolite production of Candida sake, Leuconostoc mesenteroides and Leuconostoc gelidum, which had all been previously isolated from spoiled commercial fresh-cut honeydew melon, was evaluated separately on honeydew melon agar at 7 °C. Additionally, the effect of selected MAs on the microbial, physico-chemical and sensory quality of commercial fresh-cut honeydew melon cubes was evaluated at 7 °C. The results showed that MAs with high O₂ and high CO₂ levels greatly retarded the growth, CO₂ and volatile metabolite production (i.e. ethanol, 2-methyl-1-butanol, ethyl acetate, phenylacetic acid, nonanal) of C. sake on honeydew melon agar; especially MAs consisting of 50% O₂+50% CO₂ and 70% O₂+30% CO₂. In contrast, the MAs evaluated only had a minor effect on the growth and volatile metabolite production of L. mesenteroides and L. gelidum. Overall, the effect of MAs on colour, juice leakage, juiciness, and firmness of fresh-cut honeydew melon was small during storage. Sensory preference was generally for fresh-cut honeydew melon cubes packaged in MA of 50% O₂+50% CO₂. These were still acceptable on day five of storage and had appreciably lower populations of yeasts and lactic acid bacteria, lower quantities of volatile organic compounds, but slightly stronger colour oxidation compared to honeydew melon that was packaged in air. Additionally, most of the samples packed in air had blown by day five due to the large quantity of CO₂ production during storage. Therefore, 50% O₂+50% CO₂ is a potential MA solution for extending the shelf-life of fresh-cut honeydew melon.

Effect of inoculation on strawberry fermentation and acetification processes using native strains of yeast and acetic acid bacteria

The aim of this work was to analyze the microbiota involved in the traditional vinegar elaboration of strawberry fruit during a spontaneous and inoculated process. In the spontaneous processes, low biodiversity was detected in both alcoholic fermentation (AF) and acetification. Nevertheless, a strain of Saccharomyces cerevisiae and of Acetobacter malorum were selected and tested as starter cultures in the inoculation study. The inoculated processes with these strains were compared with another spontaneous process, yielding a significant reduction in time for AF with a total imposition of the S. cerevisiae strain. The resulting strawberry wine was acetified in different containers (glass and wood) yielding an initial imposition of the A. malorum inoculated strain, although displacement by Gluconacetobacter species was observed in the wood barrels.

Volatile metabolite production of spoilage micro-organisms on a mixed-lettuce agar during storage at 7 degrees C in air and low oxygen atmosphere

This paper describes the volatile metabolite production of spoilage bacteria (Pantoea agglomerans and Rahnella aquatilis) and spoilage yeasts (Pichia fermentans and Cryptococcus laurentii), previously isolated from mixed lettuce, on a simulation medium of shredded mixed lettuce (mixed-lettuce agar) both under air conditions and modified atmosphere (MA)-conditions at 7 degrees C. These latter conditions simulated the equilibrium modified atmosphere packaging, which is used to extend the shelf-life of shredded mixed lettuce. Besides volatile metabolites, organic acid metabolites and consumption of sugars were measured. Microbiological growth on the mixed-lettuce agar resulted in metabolite production and consumption of sugars. Bacteria and yeasts produced a range of volatile organic compounds both under air conditions and MA-conditions: ethanol, ethyl acetate, 2-methyl-1-propanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 2,3-butanedione, 3-methyl-1-pentanol, 1-butanol and 1-hexanol. Under MA-conditions, 2-methyl-1-butanol, 3-methyl-1-butanol and ethanol were the first compounds that were detected in the headspace as being produced by the inoculated micro-organisms. In the case of the yeast P. fermentans, production of these compounds was detected from a count of 5.0+/-0.1 log cfu/cm(2) with a fast increase when exceeding 6.0-6.5 log cfu/cm(2). Unlike P. fermentans, the yeast C. laurentii showed a slow metabolism under MA-conditions, compared to air conditions. In the case of the bacteria, production of 2-methyl-1-butanol and 3-methyl-1-butanol was detected starting from a count of 6.7+/-0.1 log cfu/cm(2) in the case of R. aquatilis and from a count of 7.1+/-0.4 log cfu/cm(2) in the case of P. agglomerans with a fast increase when exceeding 8 log cfu/cm(2). No production of ethanol by the bacteria under MA-conditions was detected in contradiction to air conditions. It could be concluded that, if these counts are reached on the cut surfaces of shredded mixed lettuce which are simulated by the mixed-lettuce agar, sensorial quality of shredded mixed lettuce could be influenced by the microbiological production of metabolites.

Effect of temperature and glucose concentration on the N-butanoyl-l-homoserine lactone production by Aeromonas hydrophila

N-butanoyl-L-homoserine lactone (C4-HSL) production by Aeromonas hydrophila 519 has been established. C4-HSL production at 22 and 30 degrees C was found after 20-24 h of incubation corresponding to a population density of ca. 10(8)-10(9) cfu/ml, respectively. Reduced C4-HSL production was noted after 72 h of incubation at 12 degrees C when the culture reached ca. 10(9) cfu/ml. No C4-HSL production was detected at 37 degrees C, although a dense population was obtained. In LB broth supplemented with 0.1% and 0.5% glucose, C4-HSL production was noted whereas with 1% glucose no C4-HSL was detected although a high colony count was obtained. In the latter culture residual levels of glucose (0.65%) were found after 43 h whereas in the 0.1% and 0.5% supplemented LB, glucose was quickly consumed which may have stimulated C4-HSL production. In conclusion, the present study shows an effect of environmental conditions (temperature, glucose concentration) on the C4-HSL production and warrants further investigation to elucidate the effect of external conditions on production of AHL signal molecules to reveal the relevance of quorum sensing in, e.g. foods.

Role of yeast proliferation in the quality degradation of strawberries during refrigerated storage

Quality changes of strawberries during storage can be caused both by microbiological and physiological processes. There is little known about the possible contribution of microbiological processes to the quality degradation of strawberries. In this study, quality of strawberries during storage was evaluated by analytical and sensorial analyses. It was the aim to investigate the influence of microbiological activity on the changes of different quality factors of strawberries during storage. During storage at 7 degrees C, quality was mainly determined by the odor and by visual defects. Regarding the odor, highly microbiologically contaminated late-season strawberries packaged in air at 7 degrees C became sensorially unacceptable due to the presence of high amounts of ethyl acetate. This could be attributed to the yeast proliferation: at yeast concentrations above 5.0 log cfu/g, an increase in ethanol was detected in the headspace of the strawberries. It was shown that ethanol was converted to ethyl acetate by strawberries resulting in an unacceptable odor. In an experiment with low microbiologically contaminated early-season strawberries, not reaching the above mentioned yeast counts, less ethyl acetate was detected which resulted in strawberries that were sensorially acceptable during the whole storage period (12 days). Strawberries packaged in modified atmosphere conditions showed a different quality pattern due to the effect of decreased O2-concentrations on both microbiological and physiological processes. This paper demonstrates that also microbiological processes on strawberries should be considered as they could play an important role in the sensorial quality when interacting with physiological processes.

The analysis of 1-propanol and 2-propanol in humid air samples using selected ion flow tube mass spectrometry

Following the observation that propanol is present in the breath samples of cystic fibrosis (CF) patients infected by Pseudomonas aeruginosa (PA), a study of the reactions of H(3)O(+), NO(+) and O(2) (+.) with 1-propanol and 2-propanol has been conducted using selected ion flow tube mass spectrometry (SIFT-MS). In this study the number and the distribution of the product ions from NO(+) reactions with the two propanol isomers under humid air conditions were able to differentiate between the two isomers. The reaction mechanisms and the structures of the product ions for these reactions, especially those with H(3)O(+) and NO(+), have been proposed. As an example, 2-propanol was shown to be present in a breath sample from one CF patient infected with PA, and also in a PA isolate from another CF patient grown on Pseudomonas-selective media. The results of this study allow an analytical procedure to be advanced for the analysis of the two propanol isomers, which can also be utilised in other applications.