2. Fungi in Dry-Fermented Sausages
The surface of dry-fermented sausages is naturally colonised by yeasts and moulds during their ripening. Some yeasts and moulds can provoke a wide variety of human diseases, such as allergic reactions and fungemia. However, these infections due to the consumption of food with these microorganisms are infrequent
[21]. On the other hand, both their positive and negative effects on the sensory and safety attributes have been widely described and will be summarised below.
2.1. Yeasts
Dry-fermented sausages constitute an optimal substrate for the growth of yeasts due to their physicochemical characteristics, such as their pH as well as salt and carbon-rich compound content
[22], being
Debaryomyces hansenii (teleomorph of
Candida famata) the predominant species
[22][23][24][25][26][27][28][29].
Candida zeylanoides and
Yarrowia lipolytica have also been isolated to a lesser extent
[22]. Apart from those previously mentioned, other genera have been isolated from different types of dry-fermented sausages, such as
Pichia,
Rhodotorula,
Cryptococcus,
Saccharomyces, and
Trichosporon [24][25]. Normally, the amount of these naturally occurring yeasts in dry-fermented sausages is around 10
6 cfu/g
[25][30][31][32][33].
A positive effect on the organoleptic characteristics of dry-fermented sausages has been associated with some of their native yeasts
[22][23][25][34]. For this reason, several studies have been focused on the intended inoculation of
D. hansenii, promoting colour development through oxygen absorption and proteolytic activity (
Table 1), which in turn results in the production of positive volatile compounds
[23][35][36][37][38][39][40]. These volatile compounds are also the result of lipid oxidation and carbohydrate fermentation
[35], producing compounds, such as 3-methylbutanol, 3-methylbutanal, and 2-propanone, which give dry-fermented sausages their highly appreciated characteristic aroma.
Table 1. Effects of yeasts as starter cultures on dry-fermented sausages.
Another interesting technological yeast property with positive effects on the sensory characteristics of dry-fermented sausages is its lipolytic activity
[25] and its ability to degrade peroxides
[22][41] (
Table 1).
Another important property of
D. hansenii, as a dominant species in dry-fermented sausages, is its antimicrobial capacity
[24][36]. Currently, the use of protective cultures in the meat industry is mostly limited to the antifungal capacity of bacteria, such as LAB
[43]. However, yeasts isolated from meat products are also being evaluated for their potential as inhibitors of toxigenic microorganisms with a possible application in the meat industry
[44][45][46][47][48]. One of the main hazards to be addressed using
D. hansenii as a protective culture is that posed by toxigenic moulds because this yeast has been shown to have an impact on the metabolism of
Penicillium verrucosum [48][49],
Penicillium nordicum [44][45][46][50],
Penicillium expansum [36],
Aspergillus niger [36],
Aspergillus parasiticus [51][52],
Fusarium graminearium and
Penicillium crustosum [24],
Penicillium griseofulvum [53], and
Aspergillus westerdijkiae [54] (
Table 2). Additionally,
D. hansenii has displayed activity against some pathogenic yeasts, such as
Candida albicans [36]. Different mechanisms of its antagonistic action have been established for
D. hansenii (
Table 2). In addition,
C. zeylanoides has also been shown to have antifungal activity against
P. nordicum and
A. westerdijkiae in simulant media of fermented sausages
[55] (
Table 2).
Table 2. Effect of yeasts as protective cultures on dry-cured meat products.
2.2. Moulds
The moulds that most frequently colonise the surface of dry-fermented sausages belong to the genera
Penicillium and
Aspergillus [58][59][60]. Some of these moulds have shown a positive impact on their sensory characteristics (
Table 3). The mycelial layer gives a characteristic external appearance to dry-fermented sausages, creating a microclimate by regulating moisture loss and protecting them from light and oxygen, thus reducing fat oxidation and subsequent rancidity
[61]. This antioxidant effect is enhanced by the production of catalase
[62]. On the other hand, moulds contribute to the proteolytic and lipolytic changes that occur during ripening
[63], causing an increase in the concentration of amino acids and free fatty acids, which are precursors of desirable volatile compounds in dry-fermented sausages
[10]. As a consequence, these activities contribute to the development of their characteristic flavour
[64]. In this sense,
Penicillium aurantiogriseum and
Penicillium camemberti are some of the moulds that positively impact on the organoleptic characteristics in the final products (
Table 3), both being used as starter cultures in sausage ripening
[10]. Furthermore,
Penicillium chrysogenum and
Penicillium nalgiovense are responsible for the covering and ripening of sausages, resulting in a more consistent taste and flavour as well as in a more uniform appearance
[10][65]. In addition,
P. aurantiogriseum, despite not being native, has also been shown to improve the odour and flavour of dry-fermented sausages due to their increase in the production of desirable volatile compounds and other compounds derived from mould metabolism
[64] (
Table 3). Moreover, less lipid oxidation was also achieved due to its antioxidant activity
[64].
Although
P. camemberti coats the rind of some cheeses
[66], it has also been proposed as a potential starter culture for dry-fermented sausage production
[67]. Concretely, this mould provokes intense proteolysis and lipolysis, which results in an increase in the concentration of volatile compounds, free amino acids, and free fatty acids with subsequent flavour improvement (
Table 3).
Furthermore, Hierro et al.
[68] tested the effect of
Mucor racemosus,
P. aurantiogriseum, and
P. camemberti isolated from Spanish fermented sausages and showed that their applications as starter cultures in dry-fermented sausages provoked a different flavour in the final product and protected against lipid oxidation (
Table 3).
Despite the positive impact of mould populations on dry-fermented sausages, some of them can provoke unwanted effects, such as the alteration of their sensory characteristics. Thus,
Cladosporium oxysporum has been described as being responsible for the formation of undesirable black spots on dry-fermented sausages
[69]. In addition, some species belonging to the genus
Mucor can produce a strong ammonia odour due to their high proteolytic activity
[61]. Other undesirable moulds are those having the ability to produce secondary metabolites, such as mycotoxins
[63][69]. In this sense, the most worrying mycotoxin in dry-fermented sausages is ochratoxin A (OTA)
[70][71], with
P. nordicum being the most frequent ochratoxigenic species isolated from them
[60]. Additionally,
A. westerdijkiae has also been described as a potential OTA producer in dry-fermented sausages
[65].
On the other hand, autochthonous moulds can be used as protective cultures to control the growth of undesirable moulds. Therefore, certain strains of the former are capable of producing antifungal proteins against mycotoxin-producing moulds
[52][72][73]. Specifically,
Penicillium allii-sativi CECT 20922 (formerly
P. chrysogenum CECT 20922) has been shown to produce the PgAFP protein, which confers to the mould the ability to reduce the growth of toxigenic moulds as well as OTA production
[57] (
Table 3). Apart from being used as a starter culture on the casings of dry-fermented sausages,
P. nalgiovense may also be applied as a protective culture. Bernáldez et al.
[8] thus reported the efficacy of
P. nalgiovense to prevent the presence of toxigenic moulds and their mycotoxins in these kinds of products (
Table 3).
Table 3. Main positive effects described for moulds in dry-fermented sausages.
3. Probiotic Properties of Yeasts from Dry-Fermented Sausages
Although LAB are the main probiotic microorganisms selected by food industries as previously mentioned, yeasts have been reported as also having promising functional properties. Apart from improving the intestinal microbial balance, yeasts may synthesise metabolites in the fermented food with potential dietary and therapeutic importance. Within the functional abilities of yeasts, health-promoting and biotherapeutic properties (prevention and treatment of gastrointestinal infections, type II diabetes, and several cancers and adjusting intestinal microbiota and supporting healthy mucosal and immune system functions), nutraceutical abilities (cholesterol assimilation, degradation of unwanted/antinutrient compounds, and production of antioxidants or increase in their bioavailability), and improvement in food quality and safety (biological control of foodborne bacteria and fungi, degradation and removal of mycotoxins and bacterial toxins, food bio-fortification, and increase in the sensory scores) have been stated
[11]. Furthermore, it is also noticeable that yeasts from food are rarely (if ever) associated with outbreaks or cases of foodborne illness
[74].
Despite these features, only
Saccharomyces boulardii and
Kluyveromyces fragilis B0399 are currently used as probiotics for humans
[11][75]. Perhaps the detrimental effects of these eukaryotic microorganisms on the shelf life and sensory properties due to their spoilage of many foodstuffs are responsible for their scarce use as probiotics despite their pivotal potential. Nonetheless, there is an increasing interest to characterise the probiotic properties of yeasts and their applications in foods
[11].
Data about the probiotic potential of non-
Saccharomyces and non-
Kluyveromyces yeasts from fermented food are also available. Such yeasts include the species
D. hansenii and
Y. lipolytica, both found in dry-fermented sausages
[22], with their high colonisation by the former, as mentioned in
Section 2.1, being noteworthy. While the safety status of
D. hansenii has been determined,
Y. lipolytica can only be applied for production purposes when this implies the absence of its viable cells in the final product and food and feed products based on microbial biomass
[5]. The usefulness of native
D. hansenii strains as both starter and protective cultures has been reported for fermented meat products
[23][46][56], but its probiotic effect has not been evaluated yet. However, the probiotic potential of
D. hansenii isolated from cheeses has been confirmed, although it was strictly strain-dependent, which could lead to different effects
[76]. Regarding its survival in gastrointestinal tract conditions, some of the strains were able to tolerate some of the
in vitro stresses under microaerobic and/or anaerobic conditions and could have a potential to reach the gut alive (
Table 4). Their adhesion to the surfaces of three models of intestinal epithelial surfaces (differentiated Caco-2 cells, undifferentiated Caco-2 cells, and mucin) was also checked with that of undifferentiated Caco-2 cells showing the best results. Even some strains adhered more strongly to the Caco-2 cells and mucin than the tested
S. boulardii strains. In addition, some
D. hansenii strains triggered a higher production of cytokine secretions in human dendritic cells than the established probiotic yeast
S. boulardii [76]. Differences at the strain level were also established by Merchán et al.
[77] when evaluating the functional properties of
D. hansenii isolated from two Spanish soft paste cheeses (
Table 4). The
D. hansenii strain 414 isolated from Feta cheese, which is able to resist the stress imposed by bile salts and to adhere to Caco-2 cells, has also been reported
[78], suggesting its ability to colonise the intestine. However, such a dairy isolate did not display an appropriate capacity to survive at a low pH and modulate the immune responses, apart from exhibiting a low potential to reduce blood cholesterol and no antimicrobial activity. Interestingly, most of the
D. hansenii isolates from Pecorino Abruzzese cheese had the potential ability to express antigenotoxic and antioxidative activities, together with acid–bile tolerance, being expected to reach the gut in a viable form and thus reducing the potential genotoxic risk due to mutagenic and genotoxic compounds
[79] (
Table 4). Furthermore, valuable functional traits were found in
D. hansenii isolated from cheeses in the Italian Marche region
[80] (
Table 4). In addition to cheeses, the probiotic features of
D. hansenii isolated from other fermented foodstuffs have been described (
Table 4). Accordingly, all the
D. hansenii strains (15) isolated from Greek-style black olives were able to grow at a low pH, and a vast majority of them tolerated the presence of bile salts (
Table 4), which indicates their probiotic potential
[81]. Additionally, some of them showed antimicrobial action against foodborne pathogens (
Table 4). When Jeong et al.
[82] evaluated the probiotic potential of two strains of
D. hansenii isolated from Korean soy sauce, both displayed viability in the presence of bile salt and at a low pH and beneficial immunomodulatory activity, with the potential to suppress overactivated inflammatory responses. Additionally, they showed neither virulence nor acute oral toxicity (
Table 4).
Although knowledge is scarce about the probiotic features of
D. hansenii from food origins, its use as a feed additive for animals has shown beneficial health effects. Specifically, there is a broad range of data on
D. hansenii as a probiotic in fish farming
[83][84]. Some of the therapeutic traits exerted by
D. hansenii include positive modulation of the immunomodulation, antioxidant activity, and growth enhancement of beneficial intestinal bacteria (
Table 4).
Due to the production of compounds with functional properties,
D. hansenii could also be used for fortifying foods with postbiotics. Therefore,
D. hansenii-derived β-glucans modulate the innate immune response in goat leukocytes
[85].
Table 4. Beneficial characteristics described for Debaryomyces hansenii as potential probiotic.
4. Probiotic Properties of Moulds from Dry-Fermented Sausages
There are few studies about the probiotic capacity of foodborne moulds mainly due to the few species considered GRAS
[6] and their lack of survival in the gastrointestinal tract, as they are aerobic microorganisms. Although some studies have detected
Penicillium spp.,
Aspergillus spp., and
Cladosporium spp. within the human gut microbiota
[92][93], their presence in this tract cannot be assumed as the exertion of a given desired activity herein.
While there are no studies about the probiotic potential of moulds from dry-fermented sausages, some recent works in other fermented products indicate the beneficial effects of moulds, in some cases isolated from food matrices, such as rice and tea, in animals and humans (
Table 5). Nonetheless, such positive properties linked to the consumption of foods manufactured with moulds seem to be related to their postbiotic ability instead of their probiotic one, because there are some bioactive compounds generated by these fungi that have demonstrated functional attributes
[94]. Some of these compounds are antimicrobial substances as well as proteolytic and lipolytic enzymes that may help to digest the food, increasing the nutrient availability
[94]. However, because to the best of our knowledge there are no studies focused on the postbiotic metabolites from moulds, the mechanisms involved in their beneficial activity are not exactly known.
Regarding the functional attributes of moulds, species belonging to the
Monascus genus are frequently used in traditional Chinese medicine to ferment rice to obtain red mould rice, also known as red koji. In mice, the ingestion of monascin from red mould rice improved intestinal bacterial dysbiosis and repaired liver damage through the regulation of the transcription of genes involved in the lipid metabolism, antioxidative status, and inflammatory response, as well as the production of metabolites, such as taurine and riboflavin
[15] (
Table 5). Wistar rats fed with
Monascus-fermented products decreased their glucose level due to the increase in insulin secretion, and thus these products are considered a useful tool for diabetes treatment
[15] (
Table 5). Similarly,
Aspergillus cristatus isolated from Fuzhuan brick tea has been described as a potential probiotic due to its antibacterial properties against intestinal pathogens and its adhesion to human colon cells and gastrointestinal survival in mice
[95] (
Table 5). The mould
Chrysonilia crassa decreased the serum low-density lipoprotein (LDL) concentration and alanine aminotransferase, enhancing the health of broilers supplemented with fermented rice by the mould
[16] (
Table 5).
Aspergillus awamori added to the diet of broilers decreased the abdominal fat, plasma cholesterol, lipid peroxidation, and glutamic–oxaloacetic transaminase activity
[96] (
Table 5).
Aspergillus oryzae has also been tested as a probiotic in poultry, although it is known that the mould cannot survive in anaerobic conditions, and consequently its activity is due to its postbiotic action
[97]. This species is able to improve the digestibility and intestinal health in broilers and inhibits cholesterol biosynthesis in human hepatic T9A4 cells
[94][98] (
Table 5). The diet of Nile tilapia fishes supplemented with
A. oryzae improved the defence against hypoxia stress by increasing the antioxidative enzymes superoxide dismutase and glutathione peroxidase and the immunomodulation by the decrease in glucose and cortisol
[17] (
Table 5). Fermented brown rice and rice brands with such mould species also showed anti-inflammatory and antimutagenic activities in mice
[99] (
Table 5). The fermented soybeans “tempeh” elaborated with
Rhizopus oligosporus decreased the glycated haemoglobin A1 and the triglyceride concentrations of type II diabetic patients when the “tempeh” was encapsulated
[100] (
Table 5).
Table 5. Moulds with potential postbiotic activity in animals or humans.
Mould |
Food |
Model |
Main Findings |
References |
Monascus spp. |
Red koji |
Mice |
Improvement in intestinal dysbiosis Repair of liver damage Reduction in lipids Antioxidative activity Anti-inflammatory activity |
[15] |
Monascus spp. |
Feed |
Wistar rats |
Decrease in glucose |
[15] |
Aspergillus cristatus |
Fuuzhuan brick tea |
Human colon cells and mice |
Antimicrobial activity |
[95] |
Chrysonilia crassa |
Rice |
Broilers |
Decrease in low-density lipoprotein Decrease in alanine aminotransferase |
[16] |
Aspergillus awamori |
Feed |
Broilers |
Decrease in fat and cholesterol Decrease in lipid peroxidation Decrease in glutamic–oxaloacetic transaminase activity |
[96] |
Aspergillus oryzae |
Feed |
Broilers |
Improvement in digestibility |
[94] |
A. oryzae |
Feed |
Broilers |
Decrease in cholesterol |
[98] |
A. oryzae |
Feed |
Nile tilapia |
Antioxidant activity Increase in immunomodulation |
[17] |
A. oryzae |
Brown rice and rice brands |
Mice |
Anti-inflammatory activity Antimutagenic activity |
[99] |
Rhizopus oligosporus |
“Tempeh” |
Humans |
Decrease in glycated hemoglobin Decrease in triglycerides |
[100] |