Enumeration and rapid identification of yeasts during extraction processes of extra virgin olive oil in Tuscany

Monitoring the yeasts ecology and volatiles profile throughout the spontaneous fermentation of Taggiasca cv. table olives through culture-dependent and independent methods

Taggiasca table olives are typical of Liguria, a Northwestern Italian region, produced with a spontaneous fermentation carried out by placing the raw drupes directly into brine with a salt concentration of 8-12 % w/v. Such concentrations limit the development of unwanted microbes and favor the growth of yeasts. This process usually lasts up to 8 months. Yeasts are found throughout the entire fermentation process and they are mainly involved in the production of volatile organic compounds, which strongly impact the quality of the final product. The aim of this study was to evaluate the dynamics of autochthonous yeasts in brines and olives in a spontaneous process with no lye pre-treatment or addition of acids in the fermenting brine with 10 % NaCl (w/v) in two batches during 2021 harvest. Three hundred seventy-three yeast colonies were isolated, characterized by rep-PCR and identified by the D1/D2 region of the 26S rRNA gene sequencing. Mycobiota was also studied by 26S rRNA gene metataxonomics, while metabolome was assessed through GC-MS analysis. Traditional culture-dependent methods showed the dominance of Candida diddensiae, Wickerhamomyces anomalus, Pichia membranifaciens and Aureobasidium pullulans, with differences in species distribution between batches, sampling time and type of sample (olives/brines). Amplicon-based sequencing confirmed the dominance of W. anomalus in batch 1 throughout the entire fermentation, while Cyteromyces nyonsensis and Aureobasidium spp. were most abundant in the fermentation in batch 2. Volatilome results were analyzed and correlated to the mycobiota data, confirming differences between fermentation stages. Given the high appreciation for this traditional food, this study helps elucidate the mycobiota associated to Taggiasca cv. table olives and its relationship with the quality of the final product.

Exploring the Connection between the Occurrence and Intensity of "Grubby" Defect and Volatile Composition of Olive Oil

In order to investigate the relationship between the occurrence of the "grubby" sensory defect caused by olive fruit fly (Bactrocera oleae (Rossi)) infestation and the resulting volatile composition, virgin olive oils were extracted from olives of the Leccino cultivar with 0%, 50%, and 100% olive fly infestations and subjected to analysis of the basic chemical quality parameters, fatty acids and volatiles, and sensory analysis by the Panel test. A 100% olive fly infestation reduced the basic chemical quality of the oil, while the fatty acid composition was not affected in any case. The overall sensory quality score and intensity of the positive sensory attributes decreased, while the intensity of the "grubby" defect increased proportionally to the degree of infestation. The occurrence and intensity of this defect were clearly causally related to the concentrations of 3-methylbutanal, 2-methylbutanal, β-ocimene, ethyl 2-methylbutyrate, dimethyl sulfoxide, 4-methyl-5H-furan-2-one, α-farnesene, 6-methyl-5-hepten-2-one, 1-octanol, E-2-nonen-1-ol, benzeneacetaldehyde, heptanal, and octanal, implying that the perception of "grubby" comes from their joint contribution to the overall olive oil flavour. In addition to contributing to the understanding of the chemical origin of "grubby", the results obtained could potentially be used to develop strategies to support sensory analysis in the classification of olive oil quality and the confirmation of the presence of this sensory defect in oil samples.

Effect of oil-born yeasts on the quality of extra-virgin olive oils of Basilicata region

The olive oil microbiota mainly consists of yeasts, which may positively or negatively affect the physicochemical and sensory features of product. In this study, 17 yeast strains belonging to Candida boidinii, Lachancea fermentati, Nakazawaea molendinolei, N. wickerhamii and Schwanniomyces polymorphus species were collected during olive oil production, identified and tested for the ability to ferment sugars, to grow at low temperatures, for the occurrence of different enzymatic activities, for the tolerance and degradation of phenolic compounds, radical scavenging activities, biofilm formation, survival to simulated gastro-intestinal (GIT) tract. Yeasts were also inoculated in extra virgin olive oils (EVOO; from Leccino and Coratina cultivar) to evaluate their survival and their effect on EVOO quality (changes in analytical indices) during 6-months of storage. Most of strains were able to grow at 15°C, while the ability to ferment different sugars was strain-specific. All strains had β-glucosidase activity, while none exhibited lipolytic activity; peroxidase was widespread among the strains, while protease activity was strain-dependent. Esterase and the ability to hydrolyse oleuropein and form hydroxytyrosol was present only in N. wickerhamii strains. All strains were able to survive in olive mill wastewater, used as a model of phenolic compounds-rich matrix. A potential biofilm formation was observed only in N. wickerhamii, while the ability to scavenge radical and to cope with GIT-associated stresses were strain-dependent. High levels of survival were observed for almost strains (except S. polymorphus), in both Leccino and Coratina samples. Yeasts limited the acidity rise in olive oils, but overtime they contributed to increase the parameters related to oxidative phenomena (i.e. peroxides, K₂₃₂, K₂₇₀), resulting in a declassification of EVOOs. The total phenolic content (TPC) was correlated to the presence of yeasts and, at the end of storage period (6 months) inoculated samples had significantly lower concentrations compared to the control oils. This study confirms that yeasts are able to survive in olive oils and, therefore, the control of their occurrence during extraction process and storage conditions is needed to obtain high-quality products and to maintain the standards of EVOO classification.

Selection of Yeast and Lactic Acid Bacteria Strains, Isolated from Spontaneous Raw Milk Fermentation, for the Production of a Potential Probiotic Fermented Milk

Probiotic milk is a class of fermented milk that possesses health-promoting effects, not only due to the lactic acid bacteria (LAB) presence but potentially also to yeast activity. Hence, the aim of this work was to isolate and select yeasts from spontaneous milk fermentations to be used as inoculum, together with LAB, for manufacturing a potentially probiotic acidic low-alcohol fermented milk. Six yeast species were detected from the spontaneous milk fermentation. A screening of 13 yeast strains and 14 previously isolated LAB strains, based on the resistance to bile salts and to acidic conditions, was carried out. The best performing strains were successively tested for in vitro gastrointestinal tolerance. A strain of Kluyveromyces marxianus and a strain of Lactococcus lactis were selected for the manufacturing of two different fermented milk. The values of the main technological and microbiological parameters (pH, organic acids, ethanol, and microbial concentrations) of the experimental milk were in the range of those reported for this category of products. The evaluation of microorganism survival in fermented milk samples subjected to simulated gastrointestinal conditions highlighted a high resistance of both strains. In conclusion, the selected microbial starter culture enabled the setting up of potential probiotic fermented milk.

Role of yeasts in the qualitative structuring of extra virgin olive oil

This review sought to describe the role played by some components of the microbiota of extra virgin olive oil (EVOO), particularly yeasts, in structuring the physicochemical and sensorial quality of freshly produced olive oil. Yeasts can survive during the entire storage period of the product. To date, approximately 25 yeast species isolated from oil produced in more than six countries have been identified, eight of which are classified as new species. Some yeast species with probiotic traits improve the health qualities of oil, whereas many others improve the chemical composition and sensory characteristics based on β-glucosidase and esterase enzymes, which are involved in the hydrolysis of the bitter glucoside known as oleuropein. However, some species, which are typically favored by the high water content in the oily matrix, such as lipase-producing yeasts, can worsen the initial chemical characteristics of EVOO oil during storage. Some physical treatments that are compatible with the EVOO production specification affect the biotic component of the oil by reducing the concentration of yeasts. The possibility of minimizing the invasive action on the biotic component of the oil by appropriately selecting the physical treatment for each oil is discussed.

Effects of the Filtration on the Biotic Fraction of Extra Virgin Olive Oil

Filtration is a widely used process in the production of extra virgin olive oil. We studied the influence of filtration performed with cotton filters and cellulose filter press on the biotic components of the oily mass containing probiotic traits in two freshly produced monocultivar extra virgin olive oils. The concentration of bacteria was reduced from 100% to 28%, while that of fungi was reduced from 100% to 44% after filtration, according to the filtration system and the initial contamination of the original monocultivar extra virgin olive oil. Compared with the control, the yeast content in the oil samples filtered with cotton filters was reduced from 37% to 11% depending on the cultivar. In the oil filtered with cellulose filter press, the yeast content reduced from 42% to 16%. The viable yeast that passed through the oily mass during the filtration process with cellulose filter press, unlike all the other samples, were unable to survive in the oil after a month of storage. The possible health benefits of compounds from both the biotic and abiotic fraction of the oil, compared to the control, were significantly low when filtered with the cellulose filter press.

Virgin Olive Oil Quality Is Affected by the Microbiota that Comprise the Biotic Fraction of the Oil

This review summarizes the current knowledge on the effects of oil-borne yeasts on the physicochemical, sensorial, and health-related characteristics of virgin olive oil (VOO) during storage. Bacteria, yeasts, and molds constitute the biotic fraction of freshly produced VOO. During storage, the bacteria and molds often die after a short period, while the yeasts survive and condition the quality of VOO. To date, approximately twenty-four yeast species have been isolated from different types of olive oil and its by-products, and seven of these species have been identified as new species. The activity of some yeasts of the biotic fraction of olive oil improves the sensorial characteristics of VOO. Some yeasts can also worsen the quality of the product by allowing the appearance of defects, oxidation of polar phenols, and triacylglycerol hydrolysis. Some yeast species of VOO show in vitro beneficial health effects, such as probiotic and antioxidant activities.

Differential Microbial Composition of Monovarietal and Blended Extra Virgin Olive Oils Determines Oil Quality During Storage

Extra virgin olive oil (EVOO) contains a biotic fraction, which is characterized by various microorganisms, including yeasts. The colonization of microorganisms in the freshly produced EVOO is determined by the physicochemical characteristics of the product. The production of blended EVOO with balanced taste, which is obtained by blending several monovarietal EVOOs, modifies the original microbiota of each oil due to the differential physico-chemical characteristics of the blended oil. This study aimed to evaluate the effect of microbial composition on the stability of the quality indices of the monovarietal and blended EVOOs derived from Leccino, Peranzana, Coratina, and Ravece olive varieties after six months of storage. The yeasts survived only in the monovarietal EVOOs during six months of storage. Barnettozyma californica, Candida adriatica, Candida diddensiae, and Yamadazyma terventina were the predominant yeast species, whose abundance varied in the four monovarietal EVOOs. However, the number of yeasts markedly decreased during the first three months of storage in all blended EVOOs. Thus, all blended EVOOs were more stable than the monovarietal EVOOs as the abundance and activity of microorganisms were limited during storage.

In vitro potential antioxidant activity of indigenous yeasts isolated from virgin olive oil

AIM

The aim of the present study was to investigate the in vitro antioxidant activity of yeast strains isolated from virgin olive oil.

METHODS AND RESULTS

The 2,2-diphenyl-1-picryl-hydrazyl (DPPH) free-radical scavenging activity of single cultures of 24 yeast strains belonging to eight species isolated from virgin olive oil was evaluated and compared with that of the reference yeast Saccharomyces boulardii. All the yeasts studied in vitro showed antioxidant activity similar to or superior to that of the reference yeast. The highest antioxidant activity was observed in Nakazawaea wickerhamii, which exceeded the value reached by the reference strain S. boulardii, while the significantly lowest values were observed in the Candida adriatica, Candida diddensiae and Barnettozyma californica strains. Tests performed with virgin olive oil enriched with Wickerhamomyces anomalus and S. boulardii yeast biomasses showed a positive correlation between the microbial biomass used and the percentage of antioxidant activity observed during 60 days of storage. Survival in virgin olive oil was also significantly higher for W. anomalus compared to S. boulardii.

CONCLUSIONS

All the oil-borne yeasts studied showed DPPH free-radical scavenging activity in both aqueous and oily media.

SIGNIFICANCE AND IMPACT OF THE STUDY

For the first time, the antioxidant activity of the microbiota of virgin olive oil is reported. This activity may indicate the probiotic characteristics of the microbiota.

Extra Virgin Olive Oil Quality as Affected by Yeast Species Occurring in the Extraction Process

In extra virgin olive oil (EVOO) extraction process, the occurrence of yeasts that could affect the quality of olive oil was demonstrated. Therefore, in this work, at first, the yeasts occurring during different extractive processes carried out in a Tuscany oil mill, at the beginning, in the middle, and the end of the harvesting in the same crop season, were quantified. Then, possible effects on quality of EVOO caused by the predominant yeast species, possessing specific enzymatic activities, were evaluated. Yeast concentrations were higher in extraction processes at the end of the harvesting. Twelve yeast species showing different isolation frequencies during olive oil extractive process and according to the harvesting date were identified by molecular methods. The yeast species dominating olive oil samples from decanter displayed enzymatic activities, potentially affecting EVOO quality according to zymogram analysis. HS-SPME-GC-MS analysis of the volatile compounds in commercial EVOO, inoculated with three yeast species (Nakazawaea molendini-olei, Nakazawaea wickerhamii, Yamadazyma terventina), pointed out significant differences depending on the strain inoculated. In conclusion, during the olive oil extractive processes, some yeast species colonize the extraction plant and may influence the chemical and sensory characteristics of EVOO depending on the cell concentrations and their enzymatic capabilities.

Isolation and Characterization of Live Yeast Cells from Ancient Vessels as a Tool in Bio-Archaeology

So far, most of the study of ancient organisms has been based mainly on the analysis of ancient DNA. Here we show that it is possible to isolate and study microorganisms—yeast in this case—from ancient pottery vessels used for fermentation. We demonstrate that it is highly likely that these cells are descendants of the original yeast strains that participated in the fermentation process and were absorbed into the clay matrix of the pottery vessels. Moreover, we characterized the isolated yeast strains, their genomes, and the beer they produced. These results open new and exciting avenues in the study of domesticated microorganisms and contribute significantly to the fields of bio- and experimental archaeology that aim to reconstruct ancient artifacts and products.

Ancient fermented food has been studied based on recipes, residue analysis, and ancient-DNA techniques and reconstructed using modern domesticated yeast. Here, we present a novel approach based on our hypothesis that enriched yeast populations in fermented beverages could have become the dominant species in storage vessels and their descendants could be isolated and studied today. We developed a pipeline of yeast isolation from clay vessels and screened for yeast cells in beverage-related and non-beverage-related ancient vessels and sediments from several archaeological sites. We found that yeast cells could be successfully isolated specifically from clay containers of fermented beverages. The findings that genotypically the isolated yeasts are similar to those found in traditional African beverages and phenotypically they grow similar to modern beer-producing yeast strongly suggest that they are descendants of the original fermenting yeast. These results demonstrate that modern microorganisms can serve as a new tool in bio-archaeology research.

Spoilage yeasts: What are the sources of contamination of foods and beverages?

Foods and beverages are nutrient-rich ecosystems in which most microorganisms are able to grow. Moreover, several factors, such as physicochemical characteristics, storage temperature, culinary practices, and application of technologies for storage, also define the microbial population of foods and beverages. The yeast population has been well-characterised in fresh and processed fruit and vegetables, dairy products, dry-cured meat products, and beverages, among others. Some species are agents of alteration in different foods and beverages. Since the most comprehensive studies of spoilage yeasts have been performed in the winemaking process, hence, these studies form the thread of the discussion in this review. The natural yeast populations in raw ingredients and environmental contamination in the manufacturing facilities are the main modes by which food contamination occurs. After contamination, yeasts play a significant role in food and beverage spoilage, particularly in the alteration of fermented foods. Several mechanisms contribute to spoilage by yeasts, such as the production of lytic enzymes (lipases, proteases, and cellulases) and gas, utilisation of organic acids, discolouration, and off-flavours. This review addresses the role of yeasts in foods and beverages degradation by considering the modes of contamination and colonisation by yeasts, the yeast population diversity, mechanisms involved, and the analytical techniques for their identification, primarily molecular methods.

Evolution of Yeast Consortia during the Fermentation of Kalamata Natural Black Olives upon Two Initial Acidification Treatments

The objective of this study was to elucidate the yeast consortia structure and dynamics during Greek-style processing of Kalamata natural black olives in different brine solutions. Olives were subjected to spontaneous fermentation in 7% (w/v) NaCl brine solution (control treatment) or brine acidified with (a) 0.5% (v/v) vinegar, and (b) 0.1% (v/v) lactic acid at the onset of fermentation. Changes in microbial counts, pH, acidity, organic acids, sugars, and alcohols were analyzed for a period of 187 days. Yeast consortia diversity was evaluated at days 4, 34, 90, 140, and 187 of fermentation. A total of 260 isolates were characterized at sub-species level by rep-PCR genomic fingerprinting with the oligo-nucleotide primer (GTG)5. The characterization of yeast isolates at species level was performed by sequencing of the D1/D2 domain of 26S rRNA gene. Results showed that yeasts dominated the process presenting a relatively broad range of biodiversity composed of 11 genera and 21 species. No lactic acid bacteria (LAB) or Enterobacteriaceae could be enumerated after 20 and 10 days of fermentation, respectively. The dominant yeast species at the beginning were Aureobasidium pullulans for control and vinegar acidification treatments, and Candida naeodendra for lactic acid treatment. Between 34 and 140 days the dominant species were Candida boidinii, Candida molendinolei and Saccharomyces cerevisiae. In the end of fermentation the dominant species in all processes were C. boidinii and C. molendinolei, followed by Pichia manshurica and S. cerevisiae in lactic acid acidification treatment, P. manshurica in vinegar acidification treatment, and Pichia membranifaciens in control fermentation.

Pigments in Extra-Virgin Olive Oils Produced in Tuscany (Italy) in Different Years

Pigments are responsible for the color of olive oils, and are an important ingredient that is directly related to the quality of this food. However, the concentration of pigments can vary significantly depending on the climate conditions, harvesting time, and olive cultivars. In this work, we quantified the main pigments in several extra-virgin olive oils produced from a blend of three cultivars (Moraiolo, Frantoio, and Leccino) typical of Tuscany (Italy) harvested in three different years: 2012, 2013, and 2014. Pigments—namely, β-carotene, lutein, pheophytin A, and pheophytin B—were quantified by a method based on the mathematical analysis of the near ultraviolet-visible absorption spectra of the oils. Data were analyzed by a multivariate statistical approach. The results show that the pigments’ content of extra-virgin olive oils produced in 2014 can be well distinguished with respect to previous years. This can be explained by the anomalous climate conditions, which strongly affected Italy and, in particular, Tuscany, where the olives were harvested. This study represents an interesting example of how pigment content can be significant in characterizing olive oils. Moreover, this is the first report of pigment quantification in extra-virgin olive oils produced in Tuscany.