Bread making is a practice that has long been discovered and has been the subject of much progress. In more recent years, developments in bread making have been increasingly focused on the enhancement and diversification of the sensory pleasures of taste, texture, and appearance of the final product
[19]. The degradation of the dough carbohydrates (namely fructose, glucose, sucrose and maltose) present in the flour, or even wittingly added, is carried out by yeasts, resulting in the release of carbon dioxide and ethanol, produced through glycolysis and posterior pyruvate decarboxylation and reduction
[17][19][30]. Carbon dioxide is responsible for the dough leavening, while most of the ethanol evaporates during the baking process. However, the latter also plays an important role in the properties of the dough
[17]. The choice of the appropriate yeast is usually based on (i) good fermentative power which could be translated into its ability to leaven the dough; (ii) capacity to use different carbon sources; and (iii) tolerance to stressful conditions, namely, osmotic, and freezing stresses
[30][47][48].
S. cerevisiae strains have been domesticated and optimized for baking applications and are usually the manufacturer’s required yeast for the baking industry. This species efficiently uses maltose as a source of energy, as opposed to
Candida humilis and
Kazachstania exigua which, according to de Vuyst et al.
[17], are sourdough-specific maltose-negative yeasts.
S. cerevisiae is commonly implemented as a leavening agent, becoming an alternative to sourdough (extensively used for years) particularly in rapid and industrial-scale bread productions
[17]. However,
T. delbrueckii is being pointed out as an alternative to
S. cerevisiae in this industry, mainly due to its high osmotic and freeze-thawing resistance, showing improvement of the quality of the bakery products
[29][30]. Experiments conducted by Almeida and Pais
[29] demonstrated greater leavening activity in lean and frozen dough for
T. delbrueckii strains, comparing to
S. cerevisiae, as the traditional yeast was more prone to suffer from freeze damage during the storage of the doughs. Apart from this feature,
T. delbrueckii strains displayed rapid growth, a more rapid response when exposed to hyperosmotic conditions, and high biomass production accompanied with sweet properties (associated with the release of aromatic compounds). These observations were later confirmed by Hernandez-Lopez, Prieto and Randez-Gil
[49]. Due to its osmotolerant properties,
T. delbrueckii has already been used in the bakery industry in Japan, for the production of sweet breads and pastries
[50].
Co-cultures using
S. cerevisiae and
T. delbrueckii species enhanced bread quality with superior aroma and improved sensorial attributes, with 47 volatile compounds—predominately alcohols, aldehydes, and esters—being identified in the bread crumb leavened with both yeasts
[19]. Wahyono et al.
[19] highlighted some properties of the resulting mixed bread which, using a radar plot, rated within a range of 4.73–5.57 from a total of 7 points, such as acceptability, enhanced flavor, mouthfeel, and color, in comparison with
S. cerevisiae single cultures, which recorded within 4.07–5.71 range in the same radar plot.
5.2. Production of Fermented Beverages
In recent years, researchers worldwide have been paying particular attention to
T. delbrueckii exploitation to improve wines organoleptic final profile and quality. As referred above, its physiological and metabolic properties revealed positive effects in wines characteristics towards acids and sugar consumption, but also an enhancement of the aroma complexity through the production of important metabolites
[2][3][4][23][51][52][53][54]. During wine fermentation, higher alcohols (also termed fused alcohols) and esters contribute 30 to 80% to the aroma profiles of wine, being the two most relevant groups of metabolites
[54]. Isobutanol, phenyl ethanol and isoamyl alcohol are the main fusel alcohols reported to contribute to the wine’s scent in concentrations ranging from 1.41 mg/L to 9.2 mg/L
[55]. According to Ebeler
[56], yields of this type of metabolites can achieve 140–420 mg/L, but concentrations over 300 mg/L contribute negatively to the aroma quality. Besides fusel alcohols, the aromatic matrix of wine is composed of esters, which are by-products of yeasts metabolism during malolactic fermentation, ageing and, most relevant in this context, alcoholic fermentation. These molecules reach maximum values when yeasts achieve the stationary growth phase
[57], as its production by
T. delbrueckii is a strain-dependent feature
[55]. Two main esters classes are present in wine: the ethyl esters and the acetate esters. The contribution of the latter encompasses desirable floral and fruity sensory properties in wine, contributing about 75% to the flavor profile
[55][56][57]. However, as stated in Belda et al.
[57], wines holding concentrations of ethyl acetate higher than 90 mg/L are considered to be faulty. Other important metabolites are fatty acids, which are detected in alcoholic beverages as mainly straight-chain and saturated molecules, with palmitoleic acid considered the most relevant unsaturated fatty acid. Besides these, fatty acids with different chain lengths are part of the wine’s matrix but prevail in small amounts, which makes them not so significant as the previous ones
[56]. The main fermented beverages in which
T. delbrueckii is employed are reviewed in
Table 2.
Table 2. Torulaspora delbrueckii’s applications in fermented beverages.
w |
/ |
v |
) of oleuropein |
|
| [ | 73 | ]
|
Coffee
|
Coffee cherries
|
Improve coffee’s sensorial quality
|
Pronounced astringency depending on the coffee variety
|
[74][75]
|
Bio-protection
|
–
|
Reduction in the use of chemical preservatives to control food spoilage
|
–
|
[76][77]
|
Beverages Applications
|
Used Substrate
|
Advantages
|
Disadvantages
|
References
|
Beer
|
Wort
|
High tolerance to hop compounds; good flavor-forming properties
|
Low sugar utilization
|
[35][58][59][60]
|
Mezcal
|
Agave juice †
|
Rich in volatile compounds; acceptable in sensory tests
|
Low performance
|
[61][62][63]
|
Tequila
|
Agave juice *
|
Positive influence on the final sensory profile
|
–
|
[64]
|
Cider
|
Apple juice †
|
Great production of ethyl decanoate and ethyl hexanoate
|
Low performance; negligible amounts of acetate esters
|
[65][66]
|
Mead
|
Honey sugar
|
Good fermentation ability; Good sensory features
|
Grassy flavor
|
[7]
|
Soy alcoholic beverage
|
Soy whey
|
Enrich aroma profiles: high levels of ethyl decanoate and ethyl hexanoate; metabolize hexanal;
|
–
|
[67]
|
* Specifically from Agave tequilana; † sterile.
5.3. Other Food Applications
The reported versatility of
T. delbrueckii makes it a remarkable asset to be explored, not only for bread and fermented beverages purposes, but also in other diverse food products (
Table 3). One example is the production of chocolate in which yeasts play a key role in flavour development, as the quality of chocolate is reduced if the cocoa fermentation process is conducted without these microorganisms
[68]. This importance is reinforced by Visitin et al.
[69] by showing the involvement of
T. delbrueckii in the fermentation of cocoa beans (
Theobroma cacao [68]) to produce chocolate, despite not yet being standard in this industry. Authors showed that through a combination with
S. cerevisiae, modifications on the analytical profile of the chocolate are obtained. Moreover, differences in the samples obtained from
S. cerevisiae and
T. delbrueckii inoculated chocolate had a significant impact on the consumers’ perception of the final product, mentioned by some as fruitier. Therefore, the use of this unconventional yeast resulted in a positive contribution to the development of the chocolate’s final aroma. In addition,
T. delbrueckii can also be explored in the cheese industry, benefiting from its tolerance to low temperatures, low pH, high salt concentrations and low water activity
[70]. Andrade et al.
[71] produced cheese from fermented milk, with the aim of evaluating the impact of
T. delbrueckii (in mixed or pure inocula) on cheese production, detecting a slow consumption of lactose which can be translated into a reduced
β-galactosidase activity, as stated by the authors.
Table 3. T. delbrueckii industrial food applications.
Food Applications
|
Used Substrate
|
Advantages
|
Disadvantages
|
References
|
Chocolate
|
Cocoa beans
|
Good flavor quality of cocoa and, therefore, the chocolate
|
Expedite in mixed fermentations with S. cerevisiae
|
[69]
|
Cheese
|
Cheese
|
Varied aromatic properties
|
Unable to inhibit pathogenic bacteria; low β-glucosidase activity
|