**2.1 Sensory analysis**

The sensory analysis includes wine tasting, its sensory estimation and appreciation, and its description. In this process, the subjectivity of the individual towards the sensations, emotions and memories that can awaken on him a certain smell (affective aspect) is very involved. Subjectivity is present because two stages coexist in the process: chemistry (biochemical reactions between the aroma/taste molecules and our papillae) and psychology (electrical signals processed in the brain that assigns a description) [36].

To avoid subjectivity in the obtained results, it is necessary to standardize the process and establish a series of standards. Various levels such as CEE, OIV, ISO, etc., have described regulations to carry out the tasting [37].

Even though sensory analysis does not provide information on the chemical composition of the aroma, it is useful for the search of certain sensory alterations, detection of adulterations or the typification of the product according to its origin and variety. However, the incorporation of instrumental techniques (olfactometric detection, electronic nose, etc.) requires sensory analysis for calibration and interpretation [37].

#### **2.2 Instrumental analysis**

Despite the complexity of the human smell and the study of wine's aroma, various techniques have provided valuable and complementary information on different aspects of aromas. Those include gas chromatography (qualitative and quantitative chemical composition of the aroma), olfactometric detection (individual aromatic characterization of compounds) and electronic nose (chemometric modeling of the global aroma). This analysis allows the quantitative determination and the qualitative evaluation of the aromatic compounds, as well as an instrumental interpretation of the overall aroma. It is necessary for the winemaker to know these techniques when seeking answers to their needs [35].

One of these techniques is gas chromatography (GC), which allows the identification and quantification of the volatile compounds of the wine. This system has been improved through stages of extraction and concentration of the volatile compounds, chromatographic separation and detection systems, such as mass spectrometry (MS) or flame ionization (FID) [38].

There is another technique coupled to the chromatographic analysis called olfactometric detection or sniffing (gas chromatography-olfactometry, GCO), which allows to determine the aromatic profile of a wine. In this assay, a person smells the compounds after they have been separated and eluted from the column. Thus, each peak is associated with an olfactory descriptor. Aromas are classified as active or inactive based on the value of aromatic activity (OAV) (This value allows measuring the aromatic activity of a compound (C) in a specific matrix as far as its activity is above the detection threshold (DT)). Thus, active aromas possess numerical OAV greater than one [39].

$$OAV = \frac{C}{DT} \tag{1}$$

This assay presents some limitations, for example, the evaluation of the compound β-damascenone, which is overestimating due to its very low detection threshold with GCO, its wide range and its dependence on the composition of the medium [34]. Nevertheless, this technique has been one of the most used in the last decade for overall identification of odor-active compounds in all types of wine

numerous regulations and protocols were elaborated to standardize the methodology and aromatic descriptors. The obtained results were significant but never distinguishable by the subjectivity of the taster. In this regard, technological advances that allow the investigation of the compounds responsible for the aroma and aromatic profile of a wine have been carried out. Nonetheless, there is no analytical method that allows interpreting the perceptions received during the tasting [33]. As mentioned before, the aroma of wines is complex and more than 800 volatile compounds have been identified, some of them at the ng/L level. Because of that complexity, the determination of aromatic compounds is not simple and it is difficult to isolate a specific aroma character. However, the development of techniques such as gas chromatography coupled to mass spectrophotometry or electric nose has

Sensory methods and gas chromatography are analytical methodologies often slow and expensive. Developing new analysis techniques to solve these limitations is of great interest. This could be the case of the electronic nose (e-nose), characterized by its high sensitivity, speed, low cost and minimal, or no sample preparation [35]. Therefore, there are complementary tools that in combination will provide a

allowed to identify hundreds of them [34].

*Different techniques that can be applied to wine aroma analyses.*

**Advantages Disadvantages**

*Advantages and disadvantages of each type of analysis methodology.*

**Sensory Instrumental Sensory Instrumental** Direct Objective Subjective Indirect

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

Repeatable Non repeatable Can be toxic or no innocuous Innocuous

Few material Low quantity of sample High quantity of sample Expensive equipment More realistic One person Several persons Need of sensory analysis for

realistic results

**Figure 2.**

**Table 1.**

**52**

lot of information regarding the evaluation of aroma [33].

(white and red musts, young and aged wines, sweet wines) made from different varieties of grapes such as Chardonnay, Riesling, Gewürztraminer, Merlot, Cabernet Sauvignon, Grenache, Tempranillo, Zalema, Palomino Fino, Touriga Nacional, Aragonez or Trincadeira. It can also be applied to study the sensory profiles of wines produced with sound and sour rot affected grapes and compare them to understand the role of sour rot in the odor nuances of wines [40].

Recently, the electronic nose has been introduced in the wine industry. It consists of an instrument equipped with chemical sensors and a chemometric model recognition program, capable of identifying and comparing individual or complex odors. As its main objective is to obtain results comparable to those from the human olfactory system, the aim of this method is to relate the perceived aroma with a response that, after being stored in memory, will serve as a model in further analysis. It has been displayed as a useful tool due to the advantages it offers: short analysis time in chromatography position (5–10 min), continuous control, it is a non-destructive method and it does not require qualified personnel. However, it is limited by the effectiveness of the detectors [41]. Most of its applications are related to the discrimination of wines to prevent their adulteration or detection of disagreeable odors, but only a few of them consider the identification of the quality of wine aromas. Despite all, this system allows a good classification of typical red and white wine aromas [42].

Finally, along the scientific literature, it has been described as an innovative technique of aroma determination. It consists of an array of conducting polymer sensors coupled to a selective solid-phase micro-extraction (SPME) fiber. This assay allows carrying an analysis of the principal components, differentiating the aromas of the sample, even for wines with very similar sensory characteristics. Moreover, the response is fast and consistent. The selective adsorption of the fiber provides a better distinction, increasing the concentration of the minor compounds of an aroma [43, 44].
