Spanish-style table olives are consumed in many countries. During their elaboration process, olives are first treated with alkali, and then passed through a stage of washing and fermentation in brine that gives them unique, organoleptic characteristics. However, this process of fermentation and its subsequent conservation can bring about deterioration defects associated with abnormal fermentation or environmental conditions (de Castro et al., 2022). The olive fermentation is mainly carried out by lactic acid bacteria (IOC, 2020); however, some yeasts have positive effects, producing volatile compounds and desirable metabolites that improve the sensorial characteristics (Hernández et al., 2007; Nisiotou et al., 2009; Arroyo-López et al., 2012; Hurtado et al., 2012; Zago et al., 2013; Schaide et al., 2019).

On the other hand, the fermentation process can cause the growth of filamentous fungi whenever environmental conditions are favorable, contaminating the olives and producing harmful mycotoxins such as those derived from Aspergillus and Penicilium species (Ghitakou et al., 2006). In other cases, the alteration occurs because the spores reach the brines and cause fungal growth to develop during the storage of the olives, affecting their appearance, flavor, taste, and texture (Franzetti et al., 2011). For this reason, process control precautions, such as good handling, storage temperature, salinity, and packaging processes should be recommended to avoid the development of fungi and mycotoxin throughout the entire table olive process (Bavaro et al., 2017). In fact, the regulation of International Olive Council (IOC) (2021) classified table olives according to sensory evaluation. Different types of spoilage may appear in table olives, such as putrid, zapateria, butyric, musty, rancid, or vinegary sensations (Lanza and Amoruso, 2020; Martín-Vertedor et al., 2021). Therefore, uncontrolled industrial practices could provoke the development of certain table olives alterations.

The early detection of defects during the fermentation process currently requires complex and/or time-consuming analytical techniques based on microbiological analysis, gas chromatography, or spectroscopic methods, and sensory analysis. The latter is a regularly used method to quickly detect and identify odors and spoilage microorganisms; on the contrary, the detection of volatile organic compounds is limited due to its expensive instruments, and slow, complex and voluminous analysis processes (Seesaerd et al., 2022).

Defects in food matrices are therefore determined by means of electronic devices such as the E-nose, which is used as an olfactory system through a series of sensors (Duan et al., 2021). These sensors mimic the human nose to recognize, classify, and evaluate the different volatile compounds (Siadat et al., 2014). Authors such as Sánchez et al. (2021b) used an E-nose to discriminate anomalous fermentations in Spanish-style table olives. Therefore, the objective of this work was to discriminate altered, synthetic brines inoculated with different mold strains according to their sensory attributes and volatile compound profile with the use of an E-nose.

According to the PPD established by the International Olive Council [International Olive Council (IOC), 2021], the metabolism of mold strains could have caused sensory defects in the synthetic brines. In contrast, when evaluated by the tasting panel, the non-inoculated synthetic brines presented positive sensory attributes that could be due to the yeast extract added to the brine to improve the microbial growth. It should be noted that these positive attributes were also present in inoculated brines but in all cases to a lesser extent. The decrease in their concentration in brines with molds may be due to the microbial metabolism caused during their development.

Tasters detected many negative attributes in different concentrations in the inoculated brines. In this sense, most of the molds (G.G.2, P.E.3, P.E.4, P.E.20, A.F.9, and A.F.18) provoked some alteration in the brine and were classified into the first category as the DPP had a score of 4 (3 < DPP ≤ 4.5). The rest of the mold strains (A.F.21, F.S.11, and P.G.19) were the only mold strains classified into the extra category (PPD ≤ 3) although they presented sensory defects at low concentrations. Furthermore, the PPD in the synthetic brines inoculated was less than 6, thus it could be marketed.

In the Spanish-style table olive process, the sensorial properties can be modified by the formation of volatile and non-volatile aromatic compounds (Mas et al., 2014). These include peptides, amino acids, vitamins, minerals, and polyphenols, all of which can contribute to the sensory aroma. The non-inoculated brines (C) presented VOCs responsible for positive aromas. This is the case of 3-methyl-1-butanol, a product formed in alcoholic fermentations by microorganisms (Pietruska et al., 2010), and with a fruity aroma such as apples and bananas (Shen et al., 2022). The carbonyl compounds found in the non-inoculated samples are thought to be related to the commercial yeast extract added during the preparation of the brines. The prominent compound was Nonanal whose concentration varied depending on the microorganism inoculated in the brine; P.E.4, P.E.20, A.F.21, and F.S.11 presented similar concentrations to C; in the rest of the mold strains, the concentration of nonanal was reduced by half compared to C. This compound, like other aromatic aldehydes found in essential oils of various plants, has antifungal properties such as the inhibition of A. flavus (Li et al., 2021). A further compound that provides positive attributes is 2,4-dimethyl-benzaldehyde, related to aroma of cherry, almond, and vanilla (Zhang et al., 2021). Overall, the concentrations in inoculated brines were lower compared to the control, except for P.G.19, which increased. In the case of aromatic ketones such as 2-nonanone, only its concentration in F.S.11 increased in the brines in relation to C. This aromatic carbonyl compound also exhibits antimicrobial properties (Mahmound et al., 2020).

On the other hand, VOCs associated with unpleasant aromas were found in the brines inoculated by different mold strains. The microorganisms produced two short-chain carboxylic acids, namely propanoic acid and butanoic acid. The first has a cheesy odor and the second produces a buttery and cheesy odor (Liu et al., 2022). A.F.9 and A.F.18 were the main contributors to the formation of these carboxylic acids. In general, molds generate undesirable products during fermentation processes (Sánchez et al., 2021b), and at the same time cause serious problems to food safety and human health (Gong et al., 2019). Two derivatives of butanoic acid found in our study were 2-methyl-butanoic acid and butyl ester-butanoic acid. These are associated with unpleasant odors in brines; the first derivative has a cheesy, sweaty, and sharp odor (Tachihara et al., 2006). The butyl ester-butanoic acid is a compound synthesized by microorganisms through enzymes acyl transferases and lipases that play an important role in butanol esterification (Noh et al., 2019). In our study, these compounds were synthesized by the strain of G. galactomyces G.G.2. In other studies (Eliskases-Lechner et al., 2011), Geotrichum was heavily involved in the flavor and aroma of cheese, and was responsible for the deterioration and reduction of the shelf-life of fermented products derived from milk. The dodecanal aldehyde compound was found in different concentrations in all the inoculated brines. G.G.2 and P.E.3 were the microorganisms capable of synthesizing the highest amounts of this compound. Microorganisms could be used to synthesize aldehydes as an alternative to traditional synthetic routes (Kunjapur and Prather, 2015).

In this way, the different mold strains used to inoculate synthetic brines were discriminated with the E-nose. This discrimination could be due to the different aroma profiles present in the inoculated brine. This electronic tool was even able to discriminate brine samples inoculated with several strains of A. flavus and P. expansum. Few references exist on the discrimination of olives altered by mold using an E-nose. Sánchez et al. (2021b) discriminated table olive defects produced by different microorganisms, such as zapateria, butyric, putrid, and mold during the fermentation period of Spanish-style table olives. There are studies that demonstrated the usefulness of sensory-based E-nose systems to the fungal detection and identification of associated species (Mota et al., 2021); and to develop a detection system to differentiate fungus using an E-nose device (Loulier et al., 2020). Therefore, the results obtained in this study prove that an E-nose, combined with chemometric analysis, is a powerful tool with an analytical capacity to discriminate VOCs produced by different mold strains. Moreover, it is a fast, precise and low-cost method compared to classic analysis techniques since it is non-destructive and does not require highly qualified personnel (Nishi et al., 2015).

The inoculated mold strains produced negative sensory defects in the synthetic brines. The main defects detected were mold, rancid, and leather, most of which were classified into the first category established by the IOC. The inoculated brines showed a different profile of volatile compounds associated with unpleasant aromas that depended on the inoculated mold strain; dodecanal was the VOC with the highest content, followed by butanoic acid and propanoic acid. Therefore, taking into account the sensory analysis and VOC, the mold strains that caused the most unpleasant effects in the synthetic brines were G.G.2, P.E.3, A.F.9, and A.F.18. These defects were also discriminated by using an E-nose, a powerful tool capable of differentiating the aroma of the synthetic brines inoculated with different mold strains. Combined with other chemometric tools, this easy-to-use, low-cost device can discriminate samples according to incipient alterations caused by molds, thereby confirming its suitability to detect different alterations during the fermentation process in Spanish-style table olives at industrial level, although further studies would be required to prove this.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

RS, FP-N, IM-F, and DM-V contributed to the conceptualization of the study. RS, JL, and FM contributed to data curation and methodology. Formal analysis was performed by RS, IM-F, and DM-V. DM-V and FP-N acquired the funding. DM-V carried out the project administration. RS, FP-N, and DM-V performed the investigation and organized the resources. JL and DM-V carried out the supervision. JL, RS, FM, and DM-V contributed to the validation. RS and DM-V contributed to the visualization. RS, IM-F, FM, JL, and DM-V wrote the original draft. RS, IM-F, JL, and FP-N contributed to the writing—review and editing of the final manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by the Junta de Extremadura (Spain) and the European Regional Development Fund (grant number: GR21121).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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