Exploiting the potential of bacteria in the cheese ecosystem

Characterization of Microbial Shifts during the Production and Ripening of Raw Ewe Milk-Derived Idiazabal Cheese by High-Throughput Sequencing

Simple Summary

Idiazabal is a traditional cheese produced from raw ewe milk in the Basque Country (Southwestern Europe). The sensory properties of raw milk cheeses have been attributed, among other factors, to microbial shifts that occur during the production and ripening processes. In this study, we used high-throughput sequencing technologies to investigate the microbiota of Latxa ewe raw milk and the dynamics during cheese production and ripening processes. The microbiota of raw milk was composed of lactic acid bacteria (LAB), environmental bacteria and non-desirable bacteria. Throughout the cheese making and ripening processes, the growth of LAB was promoted, whereas that of non-desirable and environmental bacteria was inhibited. Moreover, some genera not reported previously in raw ewe milk were detected and clear differences were observed in the bacterial composition of raw milk and cheese among producers, in relation to LAB and environmental or non-desirable bacteria, some of which could be attributed to the production of flavour related compounds.

Abstract

In this study, we used high-throughput sequencing technologies (sequencing of V3–V4 hypervariable regions of 16S rRNA gene) to investigate for the first time the microbiota of Latxa ewe raw milk and the bacterial shifts that occur during the production and ripening of Idiazabal cheese. Results revealed several bacterial genera not reported previously in raw ewe milk and cheese, such as Buttiauxella and Obesumbacterium. Both the cheese making and ripening processes had a significant impact on bacterial communities. Overall, the growth of lactic acid bacteria (LAB) (Lactococcus, Lactobacillus, Leuconostoc, Enterococcus, Streptococcus and Carnobacterium) was promoted, whereas that of non-desirable and environmental bacteria was inhibited (such as Pseudomonas and Clostridium). However, considerable differences were observed among producers. It is noteworthy that the starter LAB (Lactococcus) predominated up to 30 or 60 days of ripening and then, the growth of non-starter LAB (Lactobacillus, Leuconostoc, Enterococcus and Streptococcus) was promoted. Moreover, in some cases, bacteria related to the production of volatile compounds (such as Hafnia, Brevibacterium and Psychrobacter) also showed notable abundance during the first few weeks of ripening. Overall, the results of this study enhance our understanding of microbial shifts that occur during the production and ripening of a raw ewe milk-derived cheese (Idiazabal), and could indicate that the practices adopted by producers have a great impact on the microbiota and final quality of this cheese.

A versatile and straightforward process to turn plastics into antibacterial materials

Antibacterial activity without cell cytotoxicity is conferred to common plastic materials by dispersion of amphiphilic cationic methacrylate-based block copolymers (0.5–2 wt%), while maintaining the mechanical properties of the materials.

Bacteria and yeasts associated to Colonial cheese production chain and assessment of their hydrolytic potential

Abstract Different types of microorganisms are important in cheese-making because of the contributions their metabolism offers during the process. Few microorganisms present in Colonial cheese are known, in addition to the ones that are introduced to kick-start the processes or the ones that are associated with infections or poisonings. This study aimed to identify, by MALDI-TOF and/or DNA sequencing, the bacteria and yeasts isolated from samples collected in the main stages of Colonial cheese production, i.e., a type of cheese produced in the southern region of Brazil. The lytic capacity of these microorganisms at 5 °C and 30 °C was also evaluated. The 58 bacterial strains were distributed in 10 species among the genera Bacillus, Citrobacter, Klebsiella, Lactococcus, Paenibacillus, Staphylococcus and Raoutella. From the 13 yeasts strains analyzed, three species were identified as following: Candida pararugosa; Meyerozyma guilliermondii; and Rhodotorula mucilaginosa. In three yeasts isolates it was possible to identify only the genus Candida sp. and Trichosporon sp. The species L. lactis (48%) and M. guilliermondii (46%) were, respectively, the predominant bacteria and yeasts species isolated. The highest microbial lytic activity observed was at 30 °C. Lipase activity on isolates was proportionally more observed with yeasts and proteolytic activity with bacteria. Lower caseinase and lipase activity was observed at 5 °C, demonstrating the importance of refrigeration in controlling microbial activity. This research highlighted the cultivation of some microorganisms that are part of the Colonial cheese microbiota as well as that several of them can hydrolyze various compounds present in milk and that could be associated with its maturation or, in uncontrolled circumstances, could be the cause of product deterioration.

Role of lactic acid bacteria in flavor development in traditional Chinese fermented foods: A review

Traditional Chinese fermented foods are favored by consumers due to their unique flavor, texture and nutritional values. A large number of microorganisms participate in the process of fermentation, especially lactic acid bacteria (LAB), which are present in almost all fermented foods and contribute to flavor development. The formation process of flavor is complex and involves the biochemical conversion of various food components. It is very important to fully understand the conversion process to direct the flavor formation in foods. A comprehensive link between the LAB community and the flavor formation in traditional Chinese fermented foods is reviewed. The main mechanisms involved in the flavor formation dominated by LAB are carbohydrate metabolism, proteolysis and amino acid catabolism, and lipolysis and fatty acid metabolism. This review highlights some useful novel approaches for flavor enhancement, including the application of functional starter cultures and metabolic engineering, which may provide significant advances toward improving the flavor of fermented foods for a promising market.

Metagenomics-based approach for studying and selecting bioprotective strains from the bacterial community of artisanal cheeses

A metagenome-based approach was used to assess the taxonomic affiliation and functional potential for bacteriocin production of the bacterial community in cow's milk artisanal cheeses from Northwestern Argentina. Three different samples were analyzed by high-throughput sequencing of the V4 region of the 16S rRNA gene and shotgun metagenomics. Taxonomic analysis showed that cheese A and C were quite similar whereas cheese B displayed a rather different bacterial composition. Overall, two families, Streptococceae and Enterococceae, dominated the artisanal cheese microbiota, being the former family prevalent in cheese B and the later family the most important in samples A and C. Besides the usual species associated to cheeses, a number of bacterial taxa that have not been previously found in Argentinean artisanal cheeses were reported in the present work such as Macrococcus caseolyticus and Streptococcus macedonicus Functional metagenomics analysis using the bacteriocin mining software BAGEL3, identified 2 ORFs encoding antimicrobial peptides in cheese B and 42 different peptides in sample C. The bacteriocin genes found showed good correlation with taxonomy. Based on the microbial diversity and functional features found through shotgun metagenomic sequencing, a culture-dependent approach was applied aiming to isolate bacteriocin-producing bacteria able to inhibit the growth of the foodborne pathogen Listeria monocytogenes. From 151 bacterial colonies derived from the cheese samples, 10 were associated to high anti-Listeria activity. Based on partial 16S rRNA gene sequencing and RAPD-PCR analysis, all bacteriocinogenic isolates were identified as Enterococcus faecium. Finally, we carried out a pilot experiment with L. monocytogenes-contaminated cheese using one of the enterococcal isolates as a bioprotective adjunct culture. The use of E. faecium CRL1879 during artisanal cheese manufacturing did not alter the main organoleptic properties of the cheese and ensured an efficient control of the foodborne pathogen up to 30 days. This finding supports the use of E. faecium CRL1879 as an adjunct culture in the cheese-making process with a combination of both safety and minimal processing.

Microbiological Characteristics of Gouda Cheese Manufactured with Pasteurized and Raw Milk during Ripening Using Next Generation Sequencing

Gouda cheese, one of most popular cheeses in the Korea, has been produced from only pasteurized milk in Korean dairy farms. Recently, it has become legally possible to produce ripened cheese manufactured with raw milk in Korea. In the present study, we investigated the physico-chemical and microbiological characteristics of Gouda cheese manufactured with raw (R-GC) or pasteurized milk (P-GC) during manufacturing and ripening. Particularly, this study characterized the bacterial community structure of two cheese types, which are produced without pasteurization during ripening based on next generation sequencing of 16S rRNA gene amplicons. During ripening, protein and fat content increased slightly, whereas moisture content decreased in both P-GC and R-GC. At the 6 wk of ripening, R-GC became softer and smoother and hence, the values of hardness and gumminess, chewiness in R-GC was lower than that of P-GC. Metagenomic analysis revealed that the bacterial genera used a starter cultures, namely Lactococcus and Leuconostoc were predominant in both P-GC and R-GC. Moreover, in R-GC, the proportion of coliform bacteria such as Escherichia, Leclercia, Raoultella, and Pseudomonas were detected initially but not during ripening. Taken together, our finding indicates the potential of manufacturing with Gouda cheese from raw milk and the benefits of next generation sequencing for microbial community composition during cheese ripening.

S. epidermidis strains from artisanal cheese made from unpasteurized milk in Poland - Genetic characterization of antimicrobial resistance and virulence determinants

In Poland artisanal cheese production is an important local economic activity. Artisanal cheese is usually produced using raw cow's milk, animal rennet and salt, without the addition of starter cultures. Coagulase negative staphylococci (CoNS) are often present in artisanal cheeses. Pathogenic potential of some CoNS species, especially S. epidermidis, suggests that they could correspond to emerging pathogens. The identified risk factors correspond to virulence, antibiotic resistance and biofilm formation. Therefore, we aimed to characterize S. epidermidis isolated along the artisanal raw milk production chain. Seventy artisanal cheeses samples from unpasteurized cow milk purchased in Podlasie and Warmia and Mazury region in Poland, were included in this study. A total of 26 S. epidermidis isolates were obtained. Most of them were antimicrobial resistant, such as to penicillin (84,6%), clindamycin (46,2%), tetracycline (42,3%), erythromycin (42,3%) and cefoxitin (26,9%). Only one isolate was susceptible to all antibiotics used in the study. All methicillin resistant S. epidermidis strains (26,9%) harbored mecA gene. Isolates, phenotypic resistant to tetracycline, harbored at least one tetracycline resistance determinant on which tet(M) was most frequent. Moreover, all tetracycline resistant strains harbored Tn916-Tn1545-like integrase family gene. In the erythromycin resistant isolates, the macrolide resistance genes ermC, ermB or msrA/B were present. Seven strains demonstrated a strong ability to form biofilm and moderate and weak biofilm was demonstrated by 4 strains, whereas 11 of S. epidermidis isolates were found to be unable to form a biofilm. All strains producing strong biofilm harbored the icaD gene which occurred independently or in combination with the icaA. Insertion element IS256, was identified in 15,4% of S. epidermidis strains, all of which were multidrug resistant. Arginine Catabolic Mobile Element (ACME) was identified in 13 of the 26 examined strains (50%). Most common was ACME type I (26,9%), followed by type III (15,4%) and type II (7,7%). Our data indicate that S. epidermidis are widely present in artisanal cheeses from raw whole cow milk in Poland. Many isolated strains containing more virulence factors and antibiotic resistant and carry mobile genetic elements which represent a potential source of resistance transmission to bacteria in humans.

Bacterial diversity of artisanal cheese from the Amazonian region of Brazil during the dry and rainy seasons

The microbiota from artisanal cheeses produced in the Amazonian region is evaluated. Samples of artisanal cheeses were obtained from markets in Conceição do Araguaia and Redenção (Pará, Brazil) over rainy and dry seasons, and their biodiversity was assessed by culture-dependent and culture-independent methods. Mean counts of lactic acid bacteria (LAB) in cheeses ranged from 7.32 to 8.84 log CFU/g, for both seasons. Members of genera Lactobacillus, Lactococcus, Weissella, Enterococcus, Pediococcus, and Leuconostoc were predominant. The amplification of the 16S rRNA V6-V9 region, followed by a temporal temperature-gradient gel electrophoresis (TTGE) and sequencing of the TTGE bands revealed important differences in the microbial composition variability between samples from the two seasons and among cheese samples analyzed. TTGE showed the presence of microorganisms that are frequently found in cheese, such as L. lactis subsp. lactis, as well as other non-usual species, such as Macrococcus caseolyticus and Corynebacterium variabile. Moreover, TTGE analysis revealed the presence of microorganisms that have been isolated from other types of foods (Paralactobacillus selangorenses) along with some not usually found in foods, such as Exiguobacterium acetylicum, plus the presence of pathogenic microorganisms (Granulicatella elegans and Aerococcus sanguinicola). The present molecular approaches combined with culture-dependent methods provided a more detailed description of the microbial ecology of traditional cheeses from the Amazonian region in northern Brazil.

Hurdle factors minimizing growth of Listeria monocytogenes while counteracting in situ antilisterial effects of a novel nisin A-producing Lactococcus lactis subsp. cremoris costarter in thermized cheese milks

The capacity of growth, survival, and adaptive responses of an artificial contamination of a three-strain L. monocytogenes cocktail in factory-scale thermized (65 °C, 30 s) Graviera cheese milk (TGCM) was evaluated. Bulk TGCM samples for inoculation were sequentially taken from the cheese making vat before process initiation (CN-LM) and after addition of a commercial starter culture (CSC), the CSC plus the nisin A-producing (NisA+) costarter strain Lactococcus lactis subsp. cremoris M78 (CSC + M78), and all ingredients with the rennet last (CSC + M78-RT). Additional treatments included Listeria-inoculated TGCM samples coinoculated with the NisA+ costarter strain M78 in the absence of the CSC or with the CSC in previously sterilized TGCM to inactivate the background microbiota (CSC-SM). All cultures were incubated at 37 to 42 °C for 6 h, followed by additional 66 h at 22 °C, and 48 h at 12 °C after addition of 2% edible salt. L. monocytogenes failed to grow and declined in all CSC-inoculated treatments after 24 h. In contrast, the pathogen increased by 3.34 and 1.46 log units in the CN-LM and the CSC-SM treatments, respectively, indicating that the background microbiota or the CSC alone failed to suppress it, but they did so synergistically. Supplementation of the CSC with the NisA+ strain M78 did not deliver additional antilisterial effects, because the CSC Streptococcus thermophilus reduced the growth prevalence rates and counteracted the in situ NisA+ activity of the costarter. In the absence of the CSC, however, strain M78 predominated and caused the strongest in situ nisin-A mediated effects, which resulted in the highest listerial inactivation rates after 24 to 72 h at 22 °C. In all TGCM treatments, however, L. monocytogenes displayed a "tailing" survival (1.63 to 1.96 log CFU/mL), confirming that this pathogen is exceptionally tolerant to cheese-related stresses, and thus, can't be easily eliminated.

Evaluation of technological properties of Enterococcus faecium CECT 8849, a strain isolated from human milk, for the dairy industry

In this work, a variety of biochemical properties of Enterococcus faecium CECT 8849, which had been isolated from breast milk, were analyzed. Its acidifying capacity and proteolytic activity were low but, in contrast, remarkable peptidase and esterase activities were observed. Ethanol and 3-hydroxy-2-butanone were the most abundant volatile compounds found in experimental model cheese manufactured with E. faecium CECT 8849. This strain inhibited the growth of several Listeria monocytogenes and Listeria innocua strains in vitro. Enterocin A and B structural genes were detected in E. faecium CECT 8849. Model fermented milk and cheeses were manufactured from milk inoculated or not with L. innocua CECT 8848 (2.5-3 log10 colony forming units mL(-1)) using E. faecium CECT 8849 or Lactococcus lactis ESI 153 as starter cultures. Although E. faecium CECT 8849 controlled Listeria growth in both dairy models, it led to lower reduction in Listeria counts when compared with L. lactis ESI 153.

Spatial Distribution of the Metabolically Active Microbiota within Italian PDO Ewes' Milk Cheeses

Italian PDO (Protected Designation of Origin) Fiore Sardo (FS), Pecorino Siciliano (PS) and Pecorino Toscano (PT) ewes' milk cheeses were chosen as hard cheese model systems to investigate the spatial distribution of the metabolically active microbiota and the related effects on proteolysis and synthesis of volatile components (VOC). Cheese slices were divided in nine sub-blocks, each one separately subjected to analysis and compared to whole cheese slice (control). Gradients for moisture, and concentrations of salt, fat and protein distinguished sub-blocks, while the cell density of the main microbial groups did not differ. Secondary proteolysis differed between sub-blocks of each cheese, especially when the number and area of hydrophilic and hydrophobic peptide peaks were assessed. The concentration of free amino acids (FAA) agreed with these data. As determined through Purge and Trap (PT) coupled with Gas Chromatography-Mass Spectrometry (PT-GC/MS), and regardless of the cheese variety, the profile with the lowest level of VOC was restricted to the region identified by the letter E defined as core. As shown through pyrosequencing of the 16S rRNA targeting RNA, the spatial distribution of the metabolically active microbiota agreed with the VOC distribution. Differences were highlighted between core and the rest of the cheese. Top and bottom under rind sub-blocks of all three cheeses harbored the widest biodiversity. The cheese sub-block analysis revealed the presence of a microbiota statistically correlated with secondary proteolysis events and/or synthesis of VOC.

Temporal and spatial differences in microbial composition during the manufacture of a continental-type cheese

We sought to determine if the time, within a production day, that a cheese is manufactured has an influence on the microbial community present within that cheese. To facilitate this, 16S rRNA amplicon sequencing was used to elucidate the microbial community dynamics of brine-salted continental-type cheese in cheeses produced early and late in the production day. Differences in the microbial composition of the core and rind of the cheese were also investigated. Throughout ripening, it was apparent that cheeses produced late in the day had a more diverse microbial population than their early equivalents. Spatial variation between the cheese core and rind was also noted in that cheese rinds were initially found to have a more diverse microbial population but thereafter the opposite was the case. Interestingly, the genera Thermus, Pseudoalteromonas, and Bifidobacterium, not routinely associated with a continental-type cheese produced from pasteurized milk, were detected. The significance, if any, of the presence of these genera will require further attention. Ultimately, the use of high-throughput sequencing has facilitated a novel and detailed analysis of the temporal and spatial distribution of microbes in this complex cheese system and established that the period during a production cycle at which a cheese is manufactured can influence its microbial composition.

Characterization of the bacterial biodiversity in Pico cheese (an artisanal Azorean food)

This work presents the first study on the bacterial communities in Pico cheese, a traditional cheese of the Azores (Portugal), made from raw cow's milk. Pyrosequencing of tagged amplicons of the V3-V4 regions of the 16S rDNA and Operational Taxonomic Unit-based (OTU-based) analysis were applied to obtain an overall idea of the microbiota in Pico cheese and to elucidate possible differences between cheese-makers (A, B and C) and maturation times. Pyrosequencing revealed a high bacterial diversity in Pico cheese. Four phyla (Firmicutes, Proteobacteria, Actinobacteria and Bacteroidetes) and 54 genera were identified. The predominant genus was Lactococcus (77% of the sequences). Sequences belonging to major cheese-borne pathogens were not found. Staphylococcus accounted for 0.5% of the sequences. Significant differences in bacterial community composition were observed between cheese-maker B and the other two units that participated in the study. However, OTU analysis identified a set of taxa (Lactococcus, Streptococcus, Acinetobacter, Enterococcus, Lactobacillus, Staphylococcus, Rothia, Pantoea and unclassified genera belonging to the Enterobacteriaceae family) that would represent the core components of artisanal Pico cheese microbiota. A diverse bacterial community was present at early maturation, with an increase in the number of phylotypes up to 2 weeks, followed by a decrease at the end of ripening. The most remarkable trend in abundance patterns throughout ripening was an increase in the number of sequences belonging to the Lactobacillus genus, with a concomitant decrease in Acinetobacter, and Stenotrophomonas. Microbial rank abundance curves showed that Pico cheese's bacterial communities are characterized by a few dominant taxa and many low-abundance, highly diverse taxa that integrate the so-called "rare biosphere".

Traditional cheeses: rich and diverse microbiota with associated benefits

The risks and benefits of traditional cheeses, mainly raw milk cheeses, are rarely set out objectively, whence the recurrent confused debate over their pros and cons. This review starts by emphasizing the particularities of the microbiota in traditional cheeses. It then describes the sensory, hygiene, and possible health benefits associated with traditional cheeses. The microbial diversity underlying the benefits of raw milk cheese depends on both the milk microbiota and on traditional practices, including inoculation practices. Traditional know-how from farming to cheese processing helps to maintain both the richness of the microbiota in individual cheeses and the diversity between cheeses throughout processing. All in all more than 400 species of lactic acid bacteria, Gram and catalase-positive bacteria, Gram-negative bacteria, yeasts and moulds have been detected in raw milk. This biodiversity decreases in cheese cores, where a small number of lactic acid bacteria species are numerically dominant, but persists on the cheese surfaces, which harbour numerous species of bacteria, yeasts and moulds. Diversity between cheeses is due particularly to wide variations in the dynamics of the same species in different cheeses. Flavour is more intense and rich in raw milk cheeses than in processed ones. This is mainly because an abundant native microbiota can express in raw milk cheeses, which is not the case in cheeses made from pasteurized or microfiltered milk. Compared to commercial strains, indigenous lactic acid bacteria isolated from milk/cheese, and surface bacteria and yeasts isolated from traditional brines, were associated with more complex volatile profiles and higher scores for some sensorial attributes. The ability of traditional cheeses to combat pathogens is related more to native antipathogenic strains or microbial consortia than to natural non-microbial inhibitor(s) from milk. Quite different native microbiota can protect against Listeria monocytogenes in cheeses (in both core and surface) and on the wooden surfaces of traditional equipment. The inhibition seems to be associated with their qualitative and quantitative composition rather than with their degree of diversity. The inhibitory mechanisms are not well elucidated. Both cross-sectional and cohort studies have evidenced a strong association of raw-milk consumption with protection against allergic/atopic diseases; further studies are needed to determine whether such association extends to traditional raw-milk cheese consumption. In the future, the use of meta-omics methods should help to decipher how traditional cheese ecosystems form and function, opening the way to new methods of risk-benefit management from farm to ripened cheese.

Dynamics of bacterial communities during the ripening process of different Croatian cheese types derived from raw ewe's milk cheeses

Microbial communities play an important role in cheese ripening and determine the flavor and taste of different cheese types to a large extent. However, under adverse conditions human pathogens may colonize cheese samples during ripening and may thus cause severe outbreaks of diarrhoea and other diseases. Therefore in the present study we investigated the bacterial community structure of three raw ewe's milk cheese types, which are produced without the application of starter cultures during ripening from two production sites based on fingerprinting in combination with next generation sequencing of 16S rRNA gene amplicons. Overall a surprisingly high diversity was found in the analyzed samples and overall up to 213 OTU97 could be assigned. 20 of the major OTUs were present in all samples and include mostly lactic acid bacteria (LAB), mainly Lactococcus, and Enterococcus species. Abundance and diversity of these genera differed to a large extent between the 3 investigated cheese types and in response to the ripening process. Also a large number of non LAB genera could be identified based on phylogenetic alignments including mainly Enterobacteriaceae and Staphylococcacae. Some species belonging to these two families could be clearly assigned to species which are known as potential human pathogens like Staphylococcus saprophyticus or Salmonella spp. However, during cheese ripening their abundance was reduced. The bacterial genera, namely Lactobacillus, Streptococcus, Leuconostoc, Bifidobacterium, Brevibacterium, Corynebacterium, Clostridium, Staphylococcus, Thermoanerobacterium, E. coli, Hafnia, Pseudomonas, Janthinobacterium, Petrotoga, Kosmotoga, Megasphaera, Macrococcus, Mannheimia, Aerococcus, Vagococcus, Weissella and Pediococcus were identified at a relative low level and only in selected samples. Overall the microbial composition of the used milk and the management of the production units determined the bacterial community composition for all cheese types to a large extend, also at the late time points of cheese ripening.

Physicochemical and microbiological evaluation of corrientes artisanal cheese during ripening

The aim of this study was to evaluate some physical and chemical parameters (total solids, pH, acidity, fat, acid degree value of fat, salt, protein and nitrogen fractions) and their effects on the beneficial (lactic acid bacteria: LAB) and undesirable microbial populations (coliforms, Escherichia coli, Staphylococcus aureus, moulds, and yeast) during ripening of Artisanal Corrientes Cheese, an Argentinian cow's milk variety, to determine whether a longer ripening period than usual improve its hygienic-sanitary quality. The protein content was much higher than that of other cow's milk cheeses with similar values of fat. The larger peptides showed values three times higher in the 30 day-old cheese than those obtained in the beginning of the process. Staphylococcus aureus and Escherichia coli were detected (3.04 ± 1.48 log10 cfu/g of cheese, 2.21 ± 0.84 log10 MPN/g of cheese) even at 15 and 30 days of ripening, respectively. The distribution of three hundred LAB strains classified to the genus level (lactococci:lactobacilli:leuconostocs) was maintained during the ripening period. The high number of LAB in rennet may have contributed to the fermentation as a natural whey starter, unknown source of LAB for this specific cheese so far. The physicochemical changes that occur during ripening were not big enough to inhibit the growth of undesirable microorganisms.

Microcalorimetric study of the growth of bacterial colonies of Lactococcus lactis IL1403 in agar gels

Growth of Lactococcus lactis IL1403 in solid agar gels and liquid cultures at different glucose concentrations of 2, 10 and 20 g/L and different inoculation rates from 10(0) to 10(6) cfu/mL with the 10-fold increment was studied using thermal activity monitor TAM III. In parallel to calorimetric measurements the changes of glucose and lactic acid concentrations and pH of culture media were measured in order to obtain additional information for the interpretation of calorimetric power-time curves. Maximal specific growth rates of heat evolution proportional to growth rates of biomass μ(max) (W/h), heat produced during different growth stages Q(TOT) (J/mL), Q(ExP) (J/mL) and duration of lag-phases λ (h) were obtained by processing calorimetric curves. The sizes of colonies were measured also at the end of growth using microscope. The data obtained together with calculated heat yield coefficient Y(Q) (J/cfu) allowed to analyze and describe quantitatively the growth of individual colonies and develop a model of multistage growth of a typical colony of L. lactis in 1% agar gel. Microcalorimetry used in combination with other relevant methods is a very powerful and precise tool in studying solid-state fermentations.

Two strains of nonstarter lactobacilli increased the production of flavor compounds in soft cheeses

The contribution to flavor generation and secondary proteolysis of 2 strains of mesophilic lactobacilli isolated from cheese was studied. Miniature soft cheeses (200 g) were produced with or without the inclusion of a culture of Lactobacillus plantarum I91 or Lactobacillus casei I90 in the starter composed of Streptococcus thermophilus. During ripening, cheeses containing the added lactobacilli showed an increased content of total free amino acids, but this increase was only significant in cheeses with Lb. plantarum I91. In addition, free amino acid profiles were modified by selective increases of some amino acids, such as Asp, Ser, Arg, Leu, and Phe. Cheeses inoculated with Lb. plantarum I91 or Lb. casei I90 were also characterized by a significantly higher concentration of diacetyl, a key flavor compound, and an increased content of acetoin. Results suggest an increase in the catabolism of either citrate or aspartate, with the production of the derived aroma compounds. Overall, aspartate content increased in both lactobacilli-added cheeses, whereas citrate was more or less constant, suggesting that aspartate could be the source of increased diacetyl and acetoin. A triangle aroma test showed that the addition of the lactobacilli strains significantly changed the sensory attributes of cheeses. At least 11 of 12 panelists commented that the aroma of cheeses with adjuncts was more buttery than that of control cheeses, which is desirable in most soft cheeses. Both Lb. plantarum I91 and Lb. casei I90 performed well as adjunct cultures by influencing cheese aroma development and cheese proteolysis.

Microbial interactions in cheese: implications for cheese quality and safety

The cheese microbiota, whose community structure evolves through a succession of different microbial groups, plays a central role in cheese-making. The subtleties of cheese character, as well as cheese shelf-life and safety, are largely determined by the composition and evolution of this microbiota. Adjunct and surface-ripening cultures marketed today for smear cheeses are inadequate for adequately mimicking the real diversity encountered in cheese microbiota. The interactions between bacteria and fungi within these communities determine their structure and function. Yeasts play a key role in the establishment of ripening bacteria. The understanding of these interactions offers to enhance cheese flavour formation and to control and/or prevent the growth of pathogens and spoilage microorganisms in cheese.

Selection and use of autochthonous multiple strain cultures for the manufacture of high-moisture traditional Mozzarella cheese

Lactobacillus plantarum 18A, Lactobacillus helveticus 2B, Lactobacillus delbrueckii subsp. lactis 20F, Streptococcus thermophilus 22C, Enterococcus faecalis 32C and Enterococcus durans 16E were the most acidifying strains within 146 isolates for natural whey starters. The effect of media and temperature on 2 autochthonous multiple strain cultures (AMSI: 18A, 2B, 20F and 22C, 32C and 16E and AMSII: 18A, 2B, 20F and 22C) was studied. Genomic analysis showed a constant cell numbers for AMSII during 16 days of propagation in whey milk. Mozzarella cheese was made by using AMSII, commercial starter (CS) or citric acid (DA). Compared to other cheeses, the DA had a lower level of protein, ash, Ca, free amino acids and a higher level of moisture. Based on confocal laser scanning microscopy analysis, AMSII cheese showed the lowest microstructural variations during the period of storage compared to other cheeses. All the sensory attributes were scored highest for AMSII cheese. ASMII extend the shelf-life to ca. 12-15 days instead of the 5-7 days of traditional high-moisture Mozzarella cheese.

Bacteriocin-based strategies for food biopreservation

Bacteriocins are ribosomally-synthesized peptides or proteins with antimicrobial activity, produced by different groups of bacteria. Many lactic acid bacteria (LAB) produce bacteriocins with rather broad spectra of inhibition. Several LAB bacteriocins offer potential applications in food preservation, and the use of bacteriocins in the food industry can help to reduce the addition of chemical preservatives as well as the intensity of heat treatments, resulting in foods which are more naturally preserved and richer in organoleptic and nutritional properties. This can be an alternative to satisfy the increasing consumers demands for safe, fresh-tasting, ready-to-eat, minimally-processed foods and also to develop "novel" food products (e.g. less acidic, or with a lower salt content). In addition to the available commercial preparations of nisin and pediocin PA-1/AcH, other bacteriocins (like for example lacticin 3147, enterocin AS-48 or variacin) also offer promising perspectives. Broad-spectrum bacteriocins present potential wider uses, while narrow-spectrum bacteriocins can be used more specifically to selectively inhibit certain high-risk bacteria in foods like Listeria monocytogenes without affecting harmless microbiota. Bacteriocins can be added to foods in the form of concentrated preparations as food preservatives, shelf-life extenders, additives or ingredients, or they can be produced in situ by bacteriocinogenic starters, adjunct or protective cultures. Immobilized bacteriocins can also find application for development of bioactive food packaging. In recent years, application of bacteriocins as part of hurdle technology has gained great attention. Several bacteriocins show additive or synergistic effects when used in combination with other antimicrobial agents, including chemical preservatives, natural phenolic compounds, as well as other antimicrobial proteins. This, as well as the combined use of different bacteriocins may also be an attractive approach to avoid development of resistant strains. The combination of bacteriocins and physical treatments like high pressure processing or pulsed electric fields also offer good opportunities for more effective preservation of foods, providing an additional barrier to more refractile forms like bacterial endospores as well. The effectiveness of bacteriocins is often dictated by environmental factors like pH, temperature, food composition and structure, as well as the food microbiota. Foods must be considered as complex ecosystems in which microbial interactions may have a great influence on the microbial balance and proliferation of beneficial or harmful bacteria. Recent developments in molecular microbial ecology can help to better understand the global effects of bacteriocins in food ecosystems, and the study of bacterial genomes may reveal new sources of bacteriocins.