Microbial Resources and Enological Significance: Opportunities and Benefits

Abstract

Among the innovative trends in the wine sector, the continuous exploration of enological properties associated with wine microbial resources represents a cornerstone driver of quality improvement. Since the advent of starter cultures technology, the attention has been focused on intraspecific biodiversity within the primary species responsible for alcoholic fermentation (Saccharomyces cerevisiae) and, subsequently, for the so-called ‘malolactic fermentation’ (Oenococcus oeni). However, in the last decade, a relevant number of studies proposed the enological exploitation of an increasing number of species (e.g., non-Saccharomyces yeasts) associated with spontaneous fermentation in wine. These new species/strains may provide technological solutions to specific problems and/or improve sensory characteristics, such as complexity, mouth-feel and flavors. This review offers an overview of the available information on the enological/protechnological significance of microbial resources associated with winemaking, summarizing the opportunities and the benefits associated with the enological exploitation of this microbial potential. We discuss proposed solutions to improve quality and safety of wines (e.g., alternative starter cultures, multistrains starter cultures) and future perspectives.

Introduction

Wine has been consumed by humans for thousands of years and produced by crushing grapes and allowing them to ferment using the organisms present on the grapes and in the surrounding environment (Whitener et al., 2016). The microbiology of wines involves two main phases, alcoholic fermentation (AF) and malolactic fermentations (MLF) that rely on a heterogeneous microbiota composed by different indigenous microorganisms (e.g., yeast, bacteria and filamentous fungi). Considerable possible cellular/biochemical interactions can take place among these microbial resources in wine (Liu J. et al., 2017). Although of the entire wine microflora contribute to the wine chemistry, yeasts detain a predominant role (Capozzi et al., 2015). Yeast species are usually classified in two groups: Saccharomyces and non-Saccharomyces (Taillandier et al., 2014). Saccharomyces species play the key role, with Saccharomyces cerevisiae as the dominant species (Masneuf-Pomarède et al., 2010). On the other side, non-Saccharomyces yeast include different genera such as Hanseniaspora, Issatchenkia, Pichia and, Schizosaccharomyces, Brettanomyces, Zygosaccharomyces, Kluyveromyces, Candida, Torulaspora (Taillandier et al., 2014). These yeasts can experience anaerobic or aerobic growth and may persist during the fermentation, competing with Saccharomyces for nutrients, producing secondary compounds or modifying the metabolism of S. cerevisiae (Ciani et al., 2016). Several genera of lactic acid bacteria (LAB) have been reported in association to the wine prokaryotic consortia: Enterococcus, Lactobacillus, Leuconostoc, Oenococcus, Pediococcus, and Weissella (Capozzi et al., 2011; Berbegal et al., 2016; Salvetti et al., 2016; Cappello et al., 2017). In particular, Oenococcusoeni is the main species associated with MLF because of its tolerance to the harsh wine conditions (high ethanol concentration, low pH and nutrient content) (Cappello et al., 2017), even if increasing studies confirmed the relevance of Lactobacillus plantarum strains associated with specific physico-chemical conditions (du Toit et al., 2011).

Inoculation of selected starter cultures in wine must is an established enological practice in order to mitigate product losses or the production of off-flavors (García-Ríos et al., 2014; Whitener et al., 2015). Currently, a debate is still open about the use of commercial starters able to mimic some advantageous enological traits, which are present when the spontaneous fermentation is ruled by indigenous populations (Capozzi et al., 2015). For example, wines produced using single inocula are thought to lack the sensory complexity and rounded palate structure (Bellon et al., 2013). In this respect, mixed fermentation/multi-strains starter cultures including non-Saccharomyces yeasts/malolactic bacteria are generally regarded as having improved characteristics, such as complexity, mouth-feel and flavors (Tronchoni et al., 2017).

This review offers an overview of the available information on the enological/protechnological significance of microbial resources associated with winemaking, with a special focus on non-Saccharomyces yeast genera/species. A summary of the current state of knowledge about the microbial strategies to solve some technological and safety problems in winemaking is given in Table 1.

Saccharomyces cerevisiae Model for Winemaking Workhorse

For the last two decades, the natural variation of the yeast S. cerevisiae has been massively exploited with the aim of understanding ecological and evolutionary processes (Cubillos, 2016). It has been used as a model to study aging, regulation of gene expression, signal transduction, cell cycle, metabolism, apoptosis, neurodegenerative disorders, and many other biological processes (Karathia et al., 2011). Genome sequence information is now available for >80 strains of S. cerevisiae in some form (complete, draft, or raw data) (Borneman and Pretorius, 2015). In addition, it is a key player in many industrial fermentation of food matrices such as wine, bread and sake (Cubillos, 2016). The first reported use of a selected yeast starter for wine production dates from 1890, when Müller-Thurgau introduced this technology adapting the techniques developed by Christian Hansen for the Carlsberg Brewery (Padilla et al., 2016). Since then, the role of S. cerevisiae as starter culture in the wine industry has received the most attention. This yeast is not only responsible for the metabolism of grape sugar to alcohol and CO₂ but has other important side-roles, including the conversion of grape aroma precursors to sensory active molecules (Jolly et al., 2014; Belda et al., 2017). The metabolism of fermenting S. cerevisiae can be divided into primary and secondary. Primary metabolism is essential for growth, cell division and survival, producing metabolites such as ethanol, glycerol, acetaldehyde, and acetic acid. Secondary metabolites include the fusel alcohols, esters, carbonyls, sulfur compounds, thiols and terpenoids (Romano et al., 2015; Hirst and Richter, 2016). The dominance of S. cerevisiae in the fermentation is expected and desired (Jolly et al., 2014; Capece et al., 2016). One of the main features that allow S. cerevisiae to overcome is its remarkable sugar consumption rate and ethanol production coupled with a high alcohol tolerance. Through this quick proliferation in grape must S. cerevisiae efficiently depletes nitrogen sources and other nutrients required for yeast biomass production from the medium (Tronchoni et al., 2017). In addition, several studies raised evidence that microbial interactions play an important role in the early death of non-Saccharomyces yeasts (Albergaria and Arneborg, 2016). Therefore, S. cerevisiae has been the primary choice for producing wine starters (Albergaria and Arneborg, 2016). In modern winemaking, fermentations are driven largely by single-strain inoculations; pure cultures of selected strains of S. cerevisiae are added to grape must as soon as possible after crushing. This practice ensures the control of vinification, leads to outcomes that are more predictable and decreases the risk of spoilage by other microorganisms (Chambers and Pretorius, 2010). Several studies addressed the genetics underlying these unique properties and tried to unravel the evolutionary path Saccharomyces strains have undergone to become the specialized fermentation organisms they are today. It was shown that duplication of several key genes, such as those encoding alcohol dehydrogenase, hexose transporters, and enzymes linked to glycolysis, as well as global rewiring of the transcriptional network after whole genome duplication, might contribute to the suitability of S. cerevisiae as a driver of industrial fermentations (Steensels and Verstrepen, 2014). There are many-probably hundreds of-different yeast strains available, and the winemaker’s choice could have a significant effect on the quality of the wine (Chambers and Pretorius, 2010). Table 2 shows the main enological properties of some exemplificative commercially available S. cerevisiae wine yeasts. However, while this practice may reduce sources of microbial spoilage, some winemakers feel that the exclusive use of S. cerevisiae has resulted in a lack of organoleptic complexity when compared with successful spontaneous fermentations (Whitener et al., 2015), thus contributing to an increased interest on the role of non-Saccharomyces yeasts in winemaking (Whitener et al., 2016).

Non-Saccharomyces Yeasts

The world wine market is experiencing increasing interest in new yeast strains that can produce unique wines with novel properties (Mylona et al., 2016). Non-Saccharomyces yeasts have garnered interest in winemaking due to their beneficial effects and because consumers are demanding new wine styles (Lleixà et al., 2016). Recently, some commercial yeast manufacturers have already included non-Saccharomyces yeasts as part of their product portfolio (Albergaria and Arneborg, 2016). The main enological properties of some commercially available non-Saccharomyces wine yeasts are outlined in Table 3. The main benefit of using complementary yeast starters in winemaking is the improved aromatic complexity (Comitini et al., 2011; Tronchoni et al., 2017). Non-Saccharomyces yeasts possess enzymatic activities, which can catalyze the release of volatile aroma compounds from non-volatile bound precursors. These yeasts can also affect aroma production directly by their own metabolic activity (production of alcohols and esters) or by the release of extracellular enzymes which transform S. cerevisiae-derived metabolites. Strains of non-Saccharomyces yeasts have also shown potential for producing aroma compounds not associated with fermentation by many strains of S. cerevisiae, such as various monoterpenes and other terpenoid compounds (Rossouw and Bauer, 2016). In addition, the use of non-Saccharomyces yeasts has been proposed to improve glycerol or mannoprotein content, volatile acidity, or color stability (Tronchoni et al., 2017) or to reduce the ethanol levels of wines (Rossouw and Bauer, 2016). On the other hand, there have been a large number of researches on the isolation and characterization of yeast strains degrading malic acid, as alternative to malolactic bacteria-fermentation starters (Kim et al., 2008). Positive features of non-Saccharomyces yeasts have been highlighted regarding to the production of metabolites beneficial for wine quality and stability. This is the case of yeasts producers of active extracellular molecules, able to counteract the development of wild spoilage microorganisms (Comitini et al., 2017).

However, most non-Saccharomyces yeasts cannot ferment to dryness, thus S. cerevisiae should be also inoculated. Two modes of inoculation have been proposed and used (Whitener et al., 2016): the first is known as co-inoculation and some studies have showed that the inoculation of selected non-Saccharomyces yeasts at high cell concentration together with S. cerevisiae might produce wines with distinct characteristics while avoiding stuck fermentations. Others researchers have explored the use of non-Saccharomyces yeasts in sequential inoculations; non-Saccharomyces yeasts are first inoculated at high levels and allowed to ferment on their own for a given amount of time before S. cerevisiae is added to take over the fermentation. This practice gives the non-Saccharomyces yeast more time to express their unique metabolic footprint uninhibited by the stress of Saccharomyces competition (Whitener et al., 2015). Moreover, the effects of the non-Saccharomyces yeasts on fermentation and wine quality were strictly dependent on the Saccharomyces/non-Saccharomyces inoculum ratio (Comitini et al., 2011).

The principal outcomes of fermentations conducted with the aid of non-Saccharomyces yeast genera/species have been documented in the following subsections and summarized in Supplementary Table S1.

Torulaspora delbrueckii

Torulaspora delbrueckii was one of the first commercial non-Saccharomyces yeast to be released (Jolly et al., 2014). This species has been previously suggested for the vinification of low sugar and acidity musts, and it has been used for the production of red and rosé wines in Italy and for Sauvignon Blanc in South Africa (González-Royo et al., 2015). T. delbrueckii is characterized by high purity fermentation, low production of glycerol, acetaldehyde, acetic acid, and ethyl acetate (Loira et al., 2015). When used in sequential or mixed fermentations with S. cerevisiae, it can contribute to correct certain defects such as the volatile acidity (Loira et al., 2015). In Semillon wine, a mixed T. delbrueckii/S. cerevisiae culture at a 20:1 ratio produced 53% less in volatile acidity and 60% less acetaldehyde than a pure culture of S. cerevisiae (Bely et al., 2008). Some authors showed that the strong β-glucosidase activity of this species enhanced wine aroma by modulating the levels of nor-isoprenoids, terpenols, and lactones by hydrolysing their respective precursors (Renault et al., 2015). Maturano et al. (2012) confirmed the high production of extracellular enzymes of enological relevance by this species. Amarone wines produced by co-inoculation/sequential inoculation with T. delbrueckii and S. cerevisiae were judged to have increased aroma intensity, including ‘ripe red fruit’ aroma, increased sweetness and astringency and decreased intensity for vegetal attributes (Azzolini et al., 2012). Gewurztraminer wines produced by sequential inoculation showed increased concentration of terpenes α-terpineol and linalool (Cus and Jenko, 2013). T. delbrueckii/S. cerevisiae multi starters have been proposed to modulate wine flavor in Sauvignon Blanc and Merlot (Renault et al., 2015), as well as Shiraz wines (Loira et al., 2015).

The growth of T. delbrueckii is often negatively affected by the presence of S. cerevisiae. Indirect interaction, i.e., interaction between strains via components of the medium have been demonstrated. Substrate competition or amensalism (production by S. cerevisiae of metabolites that inhibit T. delbrueckii growth) are examples of the indirect interactions already suggested (Taillandier et al., 2014).

Recently, new killer toxins from T. delbrueckii with potential biocontrol activity of Brettanomyces bruxellensis, Pichiaguilliermondii, P. manshurica and P. membranifaciens wine spoilage were identified and characterized (Comitini et al., 2017).

Lachancea thermotolerans

Lachancea thermotolerans has been investigated for its ability to enhance wine acidity and improve the overall quality (Whitener et al., 2016). L. thermotolerans produces high concentration of L-lactic acid from glucose and fructose as well as low levels of volatile acidity and undesirable flavor compounds (Erten and Tanguler, 2010). This attribute could be of concern to address the problems of increased alcohol content and a reduction in the total acidity of wines associated with global climate change and variations in viticulture and wine-making practices (Balikci et al., 2016). L. thermotolerans was used as starter culture in Airén wine (increase of lactic acid of 3.18 g/L and pH reduction of 0.22) (Benito et al., 2016a), Emir wines (the use of L. thermotolerans in mixed and sequential cultures led to an increase in final total acidity of 5.40–6.28 g/L) (Balikci et al., 2016), Riesling wine (peach/apricot character) (Benito S. et al., 2015), Sangiovese and Cabernet-Sauvignon (spicy attributes) (Gobbi et al., 2013).

Metschnikowia pulcherrima

Metschnikowia pulcherrima is commonly associated with grapes and wine (Whitener, 2016). It is a high producer of β-glucosidase and its presence in mixed cultures can decrease the volatile acidity and increase the production of medium-chain fatty acids, higher alcohol, esters, terpenols and glycerol (González-Royo et al., 2015). Wines of the grape varieties Chardonnay and Shiraz obtained by sequential fermentation with Caesalpinia pulcherrima and S showed ca. 1% v/v lower ethanol concentration (Varela et al., 2016). In addition, M. pulcherrima has been reported to increase: (1) the levels of methyl butyl-, methyl propyl-, and phenethyl esters production in Sauvignon Blanc (Whitener et al., 2016); (2) the ‘overall impression,’ ‘citrus/grape fruit’ and ‘pear’ attributes of Riesling (Benito S. et al., 2015); (3) foam persistence and ‘smoky’ and ‘flowery’ attributes of Macabeo wine (González-Royo et al., 2015). It has also been reported that M. pulcherrima might have an antagonistic effect toward several yeast. cerevisiaes including S. cerevisiae which leads to delays in fermentation. This phenomenon was the result of a pulcherrimin pigment (Jolly et al., 2014).

Candida Species

The genus Candida is large and extremely diverse with over 50 different identified species several of which have been associated with winemaking (Whitener, 2016). Among the non-Saccharomyces wine yeasts involved in grape juice fermentation, C. stellata has been frequently isolated during the course of must fermentations in different countries (fermentation of botrytized wines and other wines produced by overripe grapes, in cooked musts, and in traditional balsamic vinegars) (Tofalo et al., 2012). Ciani and Maccarelli (1998) suggested that it might be used as a starter culture to increase glycerol levels in Trebbiano Toscano wine, where it 11.76 g/L of glycerol, which is higher than the sensory threshold level for glycerol sweetness, i.e., 5.2 g/L (Jolly et al., 2006). Similarly, wines of the grape variety Syrah obtained by mixed and sequential fermentation with C. cantarellii and S. cerevisiae showed 44.3 to 52.8% glycerol content higher than the control wines (Toro and Vazquez, 2002). C. sake increased concentrations of terpenes and higher alcohols in Pedro Gimenez wines (Maturano et al., 2015).

C. zemplinina (synonym Starmerella bacillaris; Duarte et al., 2012) is a non-Saccharomyces yeast, isolated for the first time in Napa Valley (California, United States) in 2002, under the name EJ1 (Mills et al., 2002). This yeast differs from the other common non-Saccharomyces yeasts, since it can survive and resist until the end of the AF due to its ability to tolerate high concentrations of ethanol (Englezos et al., 2016a).

Candida zemplinina showed some interesting characteristics, such as: (1) high glycerol production (Tofalo et al., 2012; Zara et al., 2014; Englezos et al., 2016b); (2) reduced ethanol yield (Bely et al., 2013; Englezos et al., 2016b); (3) increase of aroma complexity (Tofalo et al., 2016; Whitener et al., 2016); (4) ability to metabolize malic acid (Tofalo et al., 2012); (5) reduction of the acetic acid production in combination with S. cerevisiae (Rantsiou et al., 2012). These applications support the use of C. zemplinina, which could be a good choice to achieve various desired results, mainly due to its fructophilic character and the poor ethanol yield from sugar (Masneuf-Pomarede et al., 2015; Englezos et al., 2016a).

Recently, Mehlomakulu et al. (2014) found two novel killer toxins, CpKT1 and CpKT2, from the wine isolated strains of C. pyralidae, able to control the development of spoilage yeast B. bruxellensis. In addition, these killer toxins inhibited neither the Saccharomyces cerevisiae nor the LAB strains tested.

Pichia Species

Pichia fermentans was investigated by Clemente-Jimenez et al. (2005) in lab-scale vinifications of Macabeo wine. Sequential fermentations with P. fermentans and S. cerevisiae produced wines with increased concentrations of acetaldehyde, ethyl acetate, 1-propanol, n-butanol, 1-hexanol, ethyl caprilate, 2,3-butanediol and glycerol.

Sequential inoculum of Riesling must with P. kluyveri and S. cerevisiae increased the ‘overall impression’ and ‘peach/apricot’ character (Benito S. et al., 2015). Co-fermentation of Sauvignon Blanc grape juice with P. kluyveri has been reported to lead to higher levels of 3-mercaptohexyl acetate (Anfang et al., 2009); however, it has also been reported to produce many off odor compounds (Whitener et al., 2016).

Malic acid is sometimes detrimental to the quality of wines when present at high concentrations in some varieties. Several grape varieties contain considerable amounts of malic acid and grapes grown in the cooler regions contain higher amounts of malic acid than those grown in the warmer regions. Excessive amounts of malic acid (15–16 mg/ml) were detected in grapes grown during exceptionally cold summer in cool regions (Kim et al., 2008). In this respect, co-inoculation of P. kudriavzevii enhance the catalysis of malic acid in grape juice fermentation (Kim et al., 2008).

It is reported that P. membranifaciens increased esters production in Muscat wine (Viana et al., 2008).

Schizosaccharomyces pombe

Schizosaccharomyces pombe was initially considered as a spoilage yeast because of the production of undesirable metabolites with a negative sensory impact; on the other hand, it could be successfully used at an industrial level in cane sugar fermentation during rum making, palm wine production and cocoa fermentation (Mylona et al., 2016). However, Schizosaccharomyces pombe species is highly appreciated in colder regions because of its ability to completely transform the malic acid of the must into ethanol, thanks to its particular metabolism of maloalcoholic fermentation (Loira et al., 2015). In this respect, S. pombe perform effective malic acid deacidification and significantly reduces the levels of biogenic amines and ethyl carbamate precursors without the need for any secondary bacterial MLF (Benito et al., 2014, 2016b). Mylona et al. (2016) confirmed the ability of yeast to produce safe wines.

One new application exploits Schizosaccharomyces ability to increase the formation of vitisins and vinylphenolic pyranoanthocyanin (Loira et al., 2015). In addition, the rapid autolytic release of cell wall polysaccharides after death could reduce the time required to complete aging over lees (Benito et al., 2014).

Hanseniaspora Species

The apiculate yeast Hanseniaspora uvarum is the non-Saccharomyces found at highest levels in grape must, in fact it is among the relevant contributors to wine quality (Jolly et al., 2014). In single fermentation, H. uvarum produced low volatile acidity and high levels of glycerol (Tofalo et al., 2016). H. uvarum has also been used in mixed fermentations with S. cerevisiae for wine production. Tristezza et al. (2016) assessed the oenological potential of H. uvarum in co-inoculation and in a sequential inoculation with S. cerevisiae for industrial wine production. The mixed starter was able to successfully dominate the different stages of the fermentation process and the H. uvarum strain ITEM8795 contributed to increase the wine organoleptic quality and reduce the volatile acidity. In Solaris wine, mixed fermentations produced higher levels of glycerol, heptyl acetate, and 2-phenylethyl acetate (Liu et al., 2016).

Hanseniaspora vineae has been demonstrated to increase fruity aromas and produce high amounts of acetate esters, such as 2-phenylethyl acetate and ethyl acetate, in wines produced by a sequential fermentation with S. cerevisiae (Lleixà et al., 2016). Co-fermentation of Bobal grape must with H. vineae and S. cerevisiae produced wines that not only showed an increased concentration of 2-phenylethyl acetate (approximately three–ninefold higher) but also exhibited higher ‘fruity’ sensory scores than wines produced with S. cerevisiae pure culture (Viana et al., 2009). Chardonnay wines produced with H. vineae and S. cerevisiae have shown increased aroma and flavor diversity and reduced biogenic amines content (Medina et al., 2013).

Hanseniaspora guilliermondii can contribute to the overall quality of wines. In single-culture this yeast produced high amounts of 2-phenylethyl acetate (Viana et al., 2008). Tinta Roriz grape must inoculated with H. guilliermondii led to the production of wine with higher concentrations of 1-propanol, 2-phenylethyl acetate and 3-(methylthio)propionic acid, and lower amounts of ethyl hexanoate, pentanoic acid, free fatty acids, 2-methyltetrahydrothiophen-3-one and acetic acid-3-(methylthio)propyl ester (Moreira et al., 2011).

Zygosaccharomyces Species

The genus Zygosaccharomyces is known, together with Brettanomyces species (Capozzi et al., 2016a), for its ability to spoil wine, namely sweet and sparkling wines (Whitener et al., 2016). Zygosaccharomyces bailii and Zygosaccharomyces rouxii are often the source of spoilage in acidic and shelf-stable foods as well as sweet wines due to their ability to tolerate high acid, salt and sugar conditions (Whitener, 2016). However, selected strains of Zygosaccharomyces spp. might be useful (Garavaglia et al., 2015), because they can yield high levels of ethyl esters in Chardonnay wine (Garavaglia et al., 2015). Ethyl esters are highly interesting because they have a pleasant fruity and floral aromatic note, and are responsible for beer and wine aroma (Garavaglia et al., 2015).

Zygosaccharomyces bailii is fructophilic, and metabolizes fructose more easily than glucose (Garavaglia et al., 2015). This triat could be beneficial in grape musts from over-ripened grapes (Jolly et al., 2006). Zuehlke et al. (2015) used inoculation with Z. bailii to remove residual sugar from Cabernet Sauvignon and Syrah sluggish fermentations.

Zygosaccharomyces kombuchaensis is a newly discovered yeast. It can contribute to increase flavor intensity’ and several ‘fruity’ attributes in Ribolla Gialla (Dashko et al., 2015), as well as benzaldehyde in Sauvignon Blanc and Syrah (Whitener et al., 2015) wines.

Debaryomyces Species

Debaryomyces may contribute to the maturation, aroma, and flavor of foods, such as cheese and meat products, but it also might spoil fermented food products (Wrent et al., 2014). In winemaking, two species are of interest: Debaryomyces pseudopolymorphus and Debaryomyces vanriji. Co-fermentation of Chardonnay grape juice with D. pseudopolymorphus and S. cerevisiae resulted in an increased concentration of the terpenols (citronellol, nerol, and geraniol) (Cordero Otero et al., 2003). Sequential fermentation of Muscat of Frontignan grape juice with D. vanriji and S. cerevisiae produced wines with increased concentration of geraniol (Garcia et al., 2002). Sequential fermentation of Pedro Gimenez must with D. vanriji and S. cerevisiae increased the concentrations of esters and fatty acids (Maturano et al., 2015).

Kazachstania Species

The Kazachstania genus as a whole is fairly new; it was first mentioned in literature in 2003. Kazachstania aerobia was first identified in 2004 from corn silage while Kazachstania gamospora was discovered as a species in 2007. Being relatively new yeasts and closely related to S. cerevisiae, the genus is worthy of investigation (Whitener, 2016).

Dashko et al. (2015) used K. gamospora in Ribolla Gialla fermentation, resulting in increased acetate and ethyl ester amounts. In Sauvignon blanc and Syrah musts, this yeast caused a 200-fold increase of phenethyl propionate (Whitener et al., 2015). Sauvignon Blanc wines produced with K. aerobia and S. cerevisiae have shown increased ethyl acetate production (Whitener et al., 2016).

Wickerhamomyces anomalus

Wickerhamomyces anomalus is a constituent of the normal grape flora in the early phases of fermentation. It could spoil wine by excessive production of acetic acid and ethyl acetate, but also contributes to wine aroma by the production of volatile compounds. This species has gained considerable biotechnological interest due to its tolerance toward environmental stress factors (e.g., low pH, high osmolarity), metabolic versatility and production of exoenzymes (Sabel et al., 2014). Mazuela wines produced by sequential inoculation of W. anomalus and S. cerevisiae showed increased concentration of acetate- and ethyl- esters, along with a significant and panel preference (Cañas et al., 2014).

Williopsis saturnus

Williopsis saturnus has been reported to increase the levels of main terpenols (linalool, citronellol, and α-terpineol), produce some terpenoid esters (citronellyl and neryl acetate) and retain the concentration of cis-rose oxide in mango wine with ethanol levels of 2–4% (v/v) (Chen et al., 2015). This specie is not generally found from the natural environment of surfaces of grapes and winery equipments: however, it might potentially enhance the fruity flavor in wines obtained from neutral cultivar characteristics (Erten and Tanguler, 2010).

Emir wines produced by co-inoculation with W. saturnus and S. cerevisiae showed higher concentrations of acetic acid, ethyl acetate and isoamyl acetate (Erten and Tanguler, 2010).

Zygotorulaspora florentina

Zygotorulaspora florentina was used in mixed fermentations at different inoculum ratio with S. cerevisiae, and caused an increase of the production of polysaccharides and a modulation of the final concentrations of the various volatile compounds (Domizio et al., 2011). Recently, Sangiovese wines produced by co-inoculation with Zygotorulaspora florentina and S. cerevisiae showed an enhancement of polysaccharides and 2-phenylethanol content, a reduction of volatile acidity, and high concentration of glycerol and esters (Lencioni et al., 2016).

Bacteria

Malolactic fermentations is a secondary microbial-based biochemism that usually takes place in wine during or at the end of AF; MLF is mainly carried out by one or more species of LAB (Lerm et al., 2011; Cappello et al., 2017). During this phase, the biological conversion of interest in enology is the L-malic acid decarboxylation to produce L-lactic acid and carbon dioxide. Aside from impacts on acidity, LAB can also metabolize other precursors present in wine during fermentation and, therefore, affect the chemical composition of the wine resulting in an increased complexity of wine aroma and flavor (Sumby et al., 2014; Campbell-Sills et al., 2016).

Spontaneous MLF implies several risks, such as a considerable increase in volatile acidity, the consumption of residual sugars, and the formation of undesirable metabolites, such as biogenic amines, that can affect human health and lead to low quality wines (Spano et al., 2010). In recent years, wine industries have moved toward using pure starter cultures of selected LAB to promote a reliable and rapid malic acid bioconversion (Spano et al., 2010). Table 4 shows the main enological properties of exemplificative commercially available wine malolactic bacteria.

Actually, the production of efficient malolactic starter cultures has become one of the main challenges for oenological research (Lerm et al., 2011; Berbegal et al., 2016; Cappello et al., 2017). There are various important criteria to address when selecting LAB for possible use in a starter culture, like the ability to tolerate low pH, high ethanol and SO₂ concentrations, good growth characteristics under winemaking conditions, compatibility with the selected yeast strain, the inability to produce biogenic amines and the lack of off-flavor or off-odor production (Capozzi et al., 2010; Lerm et al., 2011). A minor but also important aspect to be considered is the susceptibility of LAB to polyphenols. O. oeni is the major LAB used in commercial starter cultures for MLF (Lerm et al., 2011). O. oeni has been the only species within the Oenococcus genus until the mid-2000s when O. kitaharae was identified in composting distilled shochu residue. Over centuries of selective pressure, O. oeni has honed and perfected various adaptive strategies that enable it to outcompete with other potential MLF bacteria, during the later stages of vinification and thus to dominate in wine (Campbell-Sills et al., 2015). Recently a third Oenococcus species has been identified, O. alcoholitolerans isolated from Brazilian cachaça (Cappello et al., 2017).

In the selection of MLF starters, a challenge is the time of inoculation. Starter cultures can be co-inoculated with yeast (at the beginning or toward the end of AF), or sequentially (after AF) (Bartowsky et al., 2015). Generally, it has been demonstrated that bacteria inoculated in must performed better than those inoculated after AF, especially when cell growth conditions are not favorable (Azzolini et al., 2010). Some Lactobacillus species have also showed the ability to survive the harsh wine conditions; the species Lactobacillus plantarum has shown the most potential as a starter culture (Lerm et al., 2011). This versatile bacterium tolerates ethanol up to 14% v/v and has similar SO₂ tolerance of O. oeni (Cappello et al., 2017). The introduction of some L. plantarum strains to the fermenting musts could significantly modify the wine aroma profile due to a different enzymatic profile. In addition, L. plantarum could synthesize antimicrobial peptides which might help preventing the production of undesired compounds, or inhibiting the indigenous LAB microflora (Sun et al., 2016). Due to these characteristics, selected strains of L. plantarum are currently being commercialized to induce MLF in wine (Table 4).

Concluding Remarks

Wine fermentation is generally performed through inoculated or spontaneous fermentation (Martiniuk et al., 2016). One of the main trends in the industry of starter cultures for enology relies on the survey of the microbial resources associated with spontaneous fermentation in order to design products able to maximize wine quality (Bokulich et al., 2016; Corbo et al., 2017; Romano and Capece, 2017; Russo et al., 2017). In this light, our contribute proposes an overview of the opportunities and benefits associated with the exploitation of this microbial potential in winemaking. Considering future perspectives, the increasing number of species/strains used, often associated to new isolations from spontaneous fermentations (e.g., Garofalo et al., 2015; Garofalo et al., 2016), introduces a relevant change in terms of interspecific interactions (Ciani et al., 2016; Liu Y. et al., 2017; Tronchoni et al., 2017). A field of particular interest if we consider that different grape juices and batch volumes could influence the growth and final biomass of yeasts in mixed fermentations (Gobbi et al., 2013) and that most studies have been performed at laboratory scale without an effective validation at industrial or semi-industrial scale, questioning their applicability at cellar (Belda et al., 2016). Moreover, it is relevant to underline how this protechnological potential is often a reservoir of interesting biotechnological applications in the food sector (Capozzi et al., 2011, 2016b; Russo et al., 2016; Petruzzi et al., 2017).

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