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Review

Pre-Fermentative Cryogenic Treatments: The Effect on Aroma Compounds and Sensory Properties of Sauvignon Blanc and Chenin Blanc Wine—A Review

by
Valmary M. van Breda
1,2,*,
Francois P. van Jaarsveld
2 and
Jessy van Wyk
1
1
Department of Food Science and Technology, Cape Peninsula University of Technology, Bellville 7535, South Africa
2
Post-Harvest and Agro-Processing Technologies Division, Agricultural Research Council Infruitec-Nietvoorbij, Stellenbosch 7599, South Africa
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(4), 1483; https://doi.org/10.3390/app14041483
Submission received: 16 October 2023 / Revised: 3 February 2024 / Accepted: 6 February 2024 / Published: 12 February 2024
(This article belongs to the Special Issue Novel Approaches for Food Processing and Preservation)
Versions Notes

Abstract

:

Highlights

  • Pre-fermentative cryogenic treatments as a tool to increase the varietal thiol levels in Sauvignon blanc and Chenin blanc wines.
  • Cryogenic pre treatment technologies as an innovative method for wine production.

Abstract

Low-temperature treatments preceding alcoholic fermentation are becoming increasingly popular and have been used in winemaking as a tool to improve wine colour, aroma, and quality. Additionally, the pre-fermentative treatment of grapes with cryogenic agents protects the grape juice (must) from oxidation by reducing the diffusion of atmospheric oxygen into the liquid phase during the winemaking process. Resultant wines were reported to have enhanced varietal aromas, increased complexity, and higher thiol levels. Indications are that increased contact time between skin and juice improves the extraction of the compounds and/or precursors. Recently, there has been considerable interest in the production of wines with enhanced varietal aromas and improved quality by applying innovative winemaking technologies. This review aims to provide an overview of the aroma and organoleptic quality of Sauvignon blanc and Chenin blanc wines produced from grapes that were subjected to pre-fermentative cryogenic treatments including the impact aroma compounds, i.e., volatile thiols and methoxypyrazines.

1. Introduction

Wine complexity and sensory quality can be attributed to many flavour compounds from several sources and wine production practices, i.e., grape variety, viticulture, oenology, and climatic conditions [1,2,3]. These practices are known to influence the development and concentration of flavour compounds within the grape berry during and after ripening and throughout processing and organoleptic quality of the wine [4,5,6,7]. The grapevine species Vitis vinifera has between 5000 and 10,000 different varieties; however, only a fraction of the varieties have commercial significance for wine and table grape production [8,9]. Though each grape variety has its unique characteristics, the wine producer has the ability to utilise agricultural practices and winemaking technologies to influence the chemical composition of the grape berry and the organoleptic profiles of the wines [1,2,10].
Viticultural and oenological practices are of great importance to the levels of flavour compounds in the grape berry whilst yeast strains used for fermentation influence the final wine organoleptic quality [11,12,13,14]. Parr et al. [11] demonstrated the association between viticultural and oenological factors and their effect on the final expression of Sauvignon blanc wines from New Zealand, particularly related to vineyard location, vineyard row orientation, and grape processing operation (hand vs. machine harvesting). Cerreti et al. [15] found that the S-cysteinylated and S-glutathionylated thiol precursors increased with total soluble solids during grape ripening, in high-flavour cultivars, such as Sauvignon blanc, and in a moderate-flavour cultivar, such as Grechetto. No differences were observed in the precursor compounds related to berry ripening in a low-flavour cultivar such as Malvasia del Lazio. Moreover, they were the first to identify and quantify S-3-(4-methyl-4-sulfanyl-pentan2-ol)-glutathione in all three cultivars as a glutathione-conjugated precursor of 4-methyl-4-sulfanylpentan-2-ol (4MSPOH). Rice et al. [16] reported on the differences found in the aroma profiles of Brianna and Frontenac Gris (white grape cultivars) wines when harvested at different degrees of ripeness, which directly influenced the variation in key odour-active compounds for both cultivars.
Fungal diseases, i.e., downy mildew and powdery mildew, are known to negatively affect the grapevine and the grape berry composition [17]. Therefore, fungicides are used as standard viticultural management practices to control such fungal growth but could influence the grape aroma compounds and wine quality [17,18]. Research conducted by Dzedze et al. [18] on Chenin blanc grapes treated with different chemical sprays, i.e., Methyl-1H-pyrazole carboxylic acid phenylethyl amide, boscalid, and penconazole, with an extract of Gelania Africana highlighted that the different sprays affected wine yeast protein expression. This is important because yeast-derived proteins are needed to release certain wine-aroma-enhancing compounds (metabolites), particularly volatile thiols, during fermentation, therefore influencing the aroma profile of the wine [14,19]. Differences in metabolite levels meant that the fungicide treatments affected the yeast’s ability to express proteins, which influenced the wine’s aroma and flavour [18]. Although fungi are generally associated with spoilage of wine grapes, Botrytis cinerea (grey mould) causes noble rot under favourable conditions to produce botrytised wines [20,21,22,23]. Previous studies found that wines made from botrytised grapes had significant amounts of volatile thiols, especially 3-SH, largely due to the concentration effect that Botrytis has on the grape berries [21,22,23].
Moreover, grape growers and winemakers are continuously confronted with new challenges as climate change affects grape physiology and viticultural practices. These, in addition to advances in winemaking technologies, influence the development of varietal aroma and flavour metabolites in wines [24,25]. Numerous volatile aroma compounds are formed during the processing of grapes and grape juice (must), which contribute to the aroma of the wine together with the primary aromas to form the typical varietal aromas [10,24,25]. Therefore, winemakers should consider interactions between viticultural (climate, soil, water, cultivar, and grape-growing practices) and oenological (pre-fermentation treatments, and post-fermentation treatments) practices to produce wines with sought-after organoleptic quality, as opposed to exclusively focusing on the contribution of the cultivar type [26,27,28,29,30]. Of particular interest are studies where low-temperature, pre-fermentative maceration was applied to white wine grapes to increase contact between grape skins and juice. This allowed for an increased extraction of aroma compounds and their precursors that are mainly located in the skin of the grape berries [29,30,31,32,33,34,35].
The cold maceration process is used during white wine production to enhance the varietal character of the wines [32,33,36,37,38]. Although cold maceration increases the concentration of aromatic compounds, it also extracts components that can cause undesirable characteristics [33,35]. A modification of this technique, known as cryofreezing or cryoextraction, involves the freezing of grapes [39,40,41]. Wines produced using this method had a higher aroma intensity and better stability of taste properties than wines prepared using traditional maceration [31,41]. The perceived enhanced varietal taste and aromas of the wines resulted from the cryogenically frozen grapes, which were buffered from oxidation as well as undesirable odour compounds. Wines were acceptable, well balanced, and better-rounded, with a fuller mouthfeel [31,32,33,34,35].
Most studies produced inconclusive results as no direct correlation between the levels of precursors in grapes and the levels of varietal thiols in wine was found [26,30,34,36]. Consequently, vineyard and/or winemaking technologies for enhancing such aroma compounds in wines are required. Moreover, the use of low-temperature treatment of grapes using cryogenic technologies has the potential to be a useful option for the wine industry, based on the use of cryogenic processing technologies applied in other beverage industries, i.e., fruit juice, coffee, tea extract, and aroma extract industries [42,43,44,45,46,47,48]. Therefore, this review aims to provide an overview of wine technological processes, with an emphasis on pre-fermentative cryogenic treatment and its influence on grape aroma compounds linked to the aroma and organoleptic quality of Sauvignon blanc and Chenin blanc wine.

2. Cryogenic Technologies

Pre-fermentative cold maceration is a technique used during white wine production, to enhance the varietal character of the wine [25,31,35]. The treatment involves subjecting whole grapes and macerated grapes to rapid cooling (5 °C and below) and holding it at the desired temperature for hours or days, thus improving the extraction of compounds (phenolic compounds and primary aroma) contained within the grape skins [35,36,37,38,39,40]. Cryoextraction or cryofreezing is a modification of this conventional cold maceration process and involves the freezing of grape berries [41]. During the freezing process, grapes are frozen and ice crystals are formed, which tear the pectocellulose walls, disorganizing the tissues, thus facilitating the extraction of compounds from the grape skin [32]. Cryogenic treatments have been applied using various methods, i.e., large-capacity refrigerators/chambers, freeze-concentration technology, blast-freezing, or by adding cryogenic agents in a solid or liquid state, i.e., nitrogen (N2) or carbon dioxide (CO2) [32,40] (Table 1).
Conventional freezing involves the use of large refrigerators/chambers for cooling/freezing. However, this leads to increased costs and energy consumption due to cooling taking place at a much slower rate, ranging from hours to days [32,41]. Consequently, this increases the risk of the development of undesirable flavours due to continuous biochemical changes taking place within the grape berry as well as the risk of oxidation [42]. In addition, during conventional or slow freezing, large ice crystals are formed, which damage cells in the berry; therefore, the speed of freezing should be considered when applying this technique. Nonetheless, conditions can be controlled when using large freezing chambers, which reduces the extraction of bitter tannins transferred to the wine [42].
Freeze-concentration is a technology involving solid–liquid phase separation at low temperatures (<0 °C) for recovering a food solute from a solution, based on the separation of pure ice crystals from a freeze-concentrated liquid phase [43,44,48]. The process takes place at low temperatures, therefore resulting in high-quality concentrates since heat-sensitive volatile compounds are protected. The process involves three steps, ice crystal formation, ice crystal growth, and the separation of ice crystals from the solution. Based on ice crystal formation, two methods of freeze-concentration exist, i.e., suspension freeze-concentration and progressive freeze-concentration [47]. Following suspension freeze-concentration, fewer soluble solids remain in the ice crystal, making this technology easier to use on an industrial scale [49]. This technology can be valuable for the wine industry because the process is conducted at a low temperature where no vapour/liquid interface exists, therefore resulting in a minimal loss of volatiles [44,48].
Table 1. Summary of the cryogenic technologies used in winemaking and the effect on targeted aroma compounds.
Table 1. Summary of the cryogenic technologies used in winemaking and the effect on targeted aroma compounds.
Cryogenic
Technology		Temperature (°C)/TimeAdvantagesDisadvantagesAroma Compounds
Affected		Resultant
Increase/
Decrease		References
Large capacity refrigerators−8 to −20/9 h to 4 monthsCool large quantities of grapes/must/juice
Conditions can be
controlled		Increased costs and energy consumptionSlower rate of cooling
Increased risk of undesirable flavour development		Bitter tanninsDecreases[32,34,41,42,50]
Freeze-concentration<0/10 min to 12 hWines produced from concentrated grape juice were superior in terms of chemical and sensory profiles
Wineries with fermentation tanks with cooling systems have the potential to perform freeze-concentration without purchasing new equipment		High energy
consumption, depending
on the
technology
applied		VolatilesIncrease[40,42,43,44,45,46,47,48,49]
Blast-freezing−10 to −120/<25 min to 20 hAllows for quick freezing of grape berries, <25 minHigh energy
consumption		ProanthocyanidinsIncrease[42,51,52]
Cryogenic agents, i.e., carbon dioxide (CO2) and nitrogen (N2)−20/8 s to 24 hCreates an inert atmosphere, protecting the grapes from oxidation
Final wines were more aromatic than control wines		Increased
cost		Terpenoids, hydroxylic compounds, fatty acids, anthocyaninsIncrease[32,36,42,53,54]
Most wineries have fermentation tanks with cooling systems in which ice can be crystallised and, therefore, have the potential to perform freeze-concentration without purchasing new equipment [47]. Zhang et al. [39] used suspension freeze technology to concentrate grape juice and discovered that wines produced from the concentrated grape juice were superior to control wines in terms of chemical and sensory profiles.
Blast-freezing is a rapid freezing technique used to preserve food before transportation and has also been used as a pre-fermentation treatment in the wine industry [42]. During the conventional freezing process, water within the food structure crystallises and forms large ice crystals, which cause the cell walls of food materials to burst, affecting the food quality and flavour [51]. The speed of freezing is, therefore, an important aspect to consider, since it is directly related to the degree of the disorganisation of the food and in the case of grapes, the berry structure [42,51]. Rapid freezing within a blast-freezer causes the formation of very small ice crystals, which causes less damage and preserves food at a higher quality [53]. Blast-freezing used as a pre-fermentative cryogenic treatment for wine grapes is conducted by blasting cold air with temperatures ranging between −10 °C and −120 °C directly onto whole grape bunches. This freezing method allows all individual berries to be frozen within 1–2 h [42,51].
Cryogenic agents, i.e., carbon dioxide (CO2) and nitrogen (N2), are commonly added to whole grapes before crushing or during crushing in a solid state (dry ice or solid CO2) or using the direct injection of the cryogenic agent in liquid form [32]. The use of both these cryogens, solid CO2 and liquid N2, induces a thermal shock, leading to an increase in the degradation of the cell structure of grape berries [29,32,42]. This is advantageous as it prolongs the contact time between the grape pulp and must, thus protecting the grapes from oxidation [42]. If dry ice is used for the pre-fermentation skin contact, the cell walls are disrupted and “disorganised”, which enables the extraction of compounds located within the grape skin [32]. The freezing process causes an increase in the intracellular liquids, therefore disrupting the membranes, allowing for the release of aromatic and phenolic compounds. Cryomaceration with cryogens was found to be the most effective method because they are heavier than oxygen, they displace the air, and they create an inert atmosphere, protecting the grapes from oxidation [32,42].

3. Effect of Cryogenic Technologies on Physicochemical and Aroma Compounds of Grape Must and Wine

3.1. Physicochemical Parameters

Total soluble solids (TSSs) of must were shown to be the least affected by freezing, as no difference was observed for storage periods less than six months [25,35,42,50,51]. However, in a study conducted by the authors of [32], different freezing techniques, i.e., involving liquid nitrogen as opposed to ultra-fast mechanical freezing, resulted in significant differences in TSS values. Total acidity (TA) of grape must was lower in cryogenic-treated samples, which validated the earlier findings of Olarte Mantilla et al. [50], Santesteban et al. [42], and Zhang et al. [25]. This was related to the precipitation of potassium (K+) salts during the freezing process and the lower solubility of acidic salts during defrosting of the grapes. Additionally, higher pH levels observed in must obtained after freezing and defrosting complemented previous studies [25,32,35,42,51]. Moreover, the chemical parameter most affected by freeze storage was found to be titratable acidity [35].
Research conducted by Naviglio et al. [51] and Naranjo et al. [52] involved the rapid cooling of Bianchello del Metauro grapes by sparging the grapes with liquid CO2 before crushing and destemming. The white wines produced from these treated grapes were not statistically different from the control wines in terms of alcohol, pH, titratable acidity, and volatile acidity [55,56]. This was similar to findings observed in earlier research. However, significant differences were observed for malic acid [42,51]. Previous research showed that the physicochemical parameters of wine produced from fresh and frozen grapes as well as grape juice had significant differences in tartaric acid, but not alcohol content [42,51].
Moreover, studies conducted by Zhang et al. [25] and Pedrosa-López et al. [34] found differences in alcohol levels in final wines produced from previously frozen whole grapes and macerated grapes. These differences were attributed to the extraction of compounds, which affected the fermentation process. Zhang et al. [25] further found that wines produced from cold-macerated grapes had higher pH and glycerol levels compared to the control and skin-macerated treatments. This is similar to previous findings [42,51], although it differed from the findings of Carillo et al. [36] and other research, which showed that wines produced from grapes subjected to cold maceration did not show significant differences in chemical parameters when compared to the control [32,35,38,51].
It should, however, be noted that these effects were not necessarily only due to the freezing treatment but resulted from the storage time, duration of skin contact, and grape cultivar [25,32,42]. Furthermore, the type of cryomaceration treatment applied as well as type of berries used, i.e., whole berries or macerated grapes or juice, influence must and wine physicochemical parameters as conflicting results were observed between studies [25,32]. Furthermore, this was not the focus of this review and will not be further discussed.

3.2. Effect of Cryogenic Technologies on Grape Aroma Compounds

Aroma compounds or odourants can be classified according to the different stages of wine grape and/or juice processing that they originated from (Table 2), namely varietal (cultivar), pre-fermentative (processing), fermentation (yeast and bacteria during alcoholic and malo-lactic fermentation), and post-fermentation (aging and maturation in wood, wine bottle storage, and preservation) aromas [6,7,57]. These compounds are often present in the grape as odourless or non-volatile precursors and are released during winemaking, specifically during the alcoholic fermentation process [57,58].
The three major classes of odourants that contribute significantly to varietal characteristics in wines are monoterpenes, methoxypyrazines, and volatile thiols [28,59]. Volatile thiol compounds, also considered impact odourants, can either have a positive (tropical-, passionfruit-, guava-like) or negative influence (rotten egg, cooked vegetables, onion, cabbage) depending on their concentration in wines [60]. Other compounds also present in wine and found to play a significant role in its aroma are esters, fatty acids, higher alcohols, and aldehydes [28,57,61].
Table 2. Aroma development stage, compounds, and their origin.
Table 2. Aroma development stage, compounds, and their origin.
Aroma Development StageCompoundOrigin
VarietalPrecursors (free or bound)Grape berry (skin and pulp)
Pre-fermentativeC6 compoundsEnzymatic/catalytic reactions due to processing (crushing of berries)
FermentationEthyl esters, fusel alcohols, fatty acids, thiolsMicroorganism metabolism (yeast and bacterial)
Post-fermentationOxidation of volatile aroma compounds; increase in fatty acids, esters, aldehydes, ketones, and polyphenolsWine aging (bottle, barrel, storage, aging on lees)
Adapted from [28,62].
Over the past three decades, there has been considerable interest and research into the volatile thiol aroma compounds and their precursors [57,60,62]. Grape aroma compounds are predominantly located in the grape skin and require an extraction process to be released. The extraction of a compound is dependent on the nature of the compound, the concentration in the berry, the location within the berry, and the method used during processing [63,64,65]. Winemakers usually achieve this by using a maceration step whereby the compounds are transferred from the solid components to the juice. Pre-fermentative cold maceration is another technique that has gained popularity during the white wine production process and was shown to enhance the varietal character of the wines produced [29,31,34].
Research conducted by Carillo et al. [36] showed significant differences in phenolic acid (gallic, caftaric, coutaric, caffeic, and syringic acid) and polyphenol concentrations between wines produced from frozen and fresh grapes. Zhang et al. [25], Naviglio et al. [51], and Ruiz-Rodríguez et al. [31] had similar observations regarding polyphenol concentrations, which confirms the earlier findings. Ruiz-Rodríguez et al. [31] found significant differences in acetaldehyde concentrations between their wines, which supported earlier research. Ouellet and Pedneault [63] investigated the impact of frozen storage on 22 free volatile compounds of two table grape varieties, i.e., Thompson Seedless and Flame Seedless. The free volatile compound profile of the frozen grapes and juice was significantly different from the fresh juice [63]. It was further shown that the different freezing treatments affected the volatile profiles of juice differently and that the grape variety played a role. When analysing the volatile profiles of grape juice, fresh juice is preferable to frozen juice [63]. However, should long-term storage be required, storage under liquid N2 or at a temperature of −80 °C is advised to reduce biochemical reactions from altering the free volatile compounds [63]. Alti Palacious et al. [29] and Pedrosa-López et al. [34] found that extractable anthocyanins, total phenolics, and terpenic alcohols were among the compounds most affected by the freezing process. Alti Palacious et al. [29] further found that the differences observed in the concentration of aroma compounds in their research between vintages could be more related to climatic and geographical factors.

3.3. Effect of Cryogenic Technologies on Varietal Thiols

Varietal thiols are sulphur compounds found in grapes in a bound form that originate from fatty acids [6,19]. These sulphur-containing compounds are originally associated with off-odours resulting from hydrogen sulphide (H2S), methylmercaptan (methanethiol), ethanethiol, and methionol [58]. They are considered the main compounds involved in the aroma of wine and responsible for its archetypal flavour [30]. These volatile sulphur compounds are typically divided into two categories, i.e., highly volatile compounds, most of which are associated with aroma defects (carbon sulphide, ethanethiol, methanethiol, hydrogen sulphide), and low-volatile compounds, including the main desirable sulphur compounds that contribute to the enhancement of the sensorial quality of wines. Thiols are “bound” with glutathione or cysteine and released by the yeast during the fermentation process via the carbon–sulphur lyase (C-S) enzyme. Therefore, the quantification of their natural precursors in the must is important and can help the wine producer determine the aromatic potential of the grapes. An accurate quantification of these natural and deuterated compounds, i.e., 4-sulfanyl-4-methylpentan-2-one precursors (S-4-(4-methylpentan-2-one)-L-cysteine and S-4-(4-methylpentan-2-one)-glutathione), is achieved using SIDA (stable isotope dilution assay) that involves labelled analogues [6,64]. Key thiols present in Sauvignon blanc and responsible for its varietal aromas are 4-methyl-4-sulfanylpentan-2-one (4-MSP), 3-sulfanylhexan-1-ol (3-SH), and 3-sulfanylhexyl acetate (3-SHA) with perception thresholds of 0.8 ng L−1, 60 ng L−1, and 4 ng L−1, respectively (Table 3). They are predominantly responsible for the “tropical” (gooseberry, grapefruit, and passion fruit) characteristics associated with Sauvignon blanc [56]. However, it is interesting to note that when present in excessive concentrations, they often impart less desirable strong, sweaty aromas resembling “cat urine” [54,56]. Furthermore, research conducted on South African (SA) Chenin blanc revealed the presence of the varietal thiols 3-SH and 3-SHA in concentrations above their aroma thresholds, indicating that these two compounds also contribute significantly to the aroma of Chenin blanc wines [65,66,67].
Varietal thiols are present in grape juice in the form of aroma-inactive, non-volatile precursors and are released by yeast enzymes during the fermentation process [12,38,56,68,69,70]. Pinu et al. [58] showed that the production of varietal thiols and other aroma compounds in Sauvignon blanc wines is not necessarily only dependent on nitrogenous and sulphur compounds but is also influenced by other juice metabolites such as carboxylic and fatty acids. Their research demonstrated that concentrations of wine aroma compounds can be modified using pre-fermentative treatments to produce different wine styles from the same grape varietal based on the metabolic profile of the juice, thus altering the metabolite levels. In addition, juice modulation through new winemaking practices, i.e., metabolite supplementation or blending, could be seen as a useful tool to create new wine styles [58].
Pre-fermentative cold maceration has gained popularity in white wine production as it was shown to enhance the varietal characteristic of the wines [30,34,54]. Two volatile precursors of 3-SH (3-S-cysteinylhexan-1-ol (Cys-3-SH) and 3-S-glutathionylhexan-1-ol (Glut-3-SH) were significantly higher in frozen thawed berries than in the juice of fresh berries as well as in the frozen juice of fresh berries [30,34]. Glut-3-SH was fourfold higher in frozen grapes stored at −20 °C for two months compared to that found in frozen or fresh juices [30,34]. Capone et al. [71,72] made similar observations where a fivefold difference was found in precursor concentrations between freezing whole grapes and freezing juice, especially for the Glut-3-SH precursor, whilst no significant difference was found in the concentrations of the Cys-3-SH precursor. These results revealed that berry damage was the primary cause of the differences, and the major contributor was the glutathione conjugate formation rather than the extraction process resulting from the freezing and thawing processes [71,72]. The results suggested that the Cys-3-SH precursor was already present in the grape berry whereas the Glut-3-SH precursor was formed because of berry damage [71,72]. Although numerous studies have been conducted using cryogenic pre-treatment techniques on whole grapes and grape juice, their focus was on the overall aroma compounds and precursor formation in the grape juice. However, the effect of such treatments shows no direct relationship between the levels of precursors in the grape juice and the levels of varietal thiols in the wine. Therefore, this warrants further investigation and understanding [30,34,53,71,72,73].
Table 3. Varietal thiols present in Sauvignon blanc and Chenin blanc wines: aroma description, perception, and range in wine.
Table 3. Varietal thiols present in Sauvignon blanc and Chenin blanc wines: aroma description, perception, and range in wine.
CultivarCompound &
Chemical
Structure		Aroma
Description		Aroma
Perception
in Wine
(ng L–1)		Range in
Wine
(ng L–1)		Range in
SA 1 Wine
(ng L–1)
Sauvignon blanc4–methyl–4–sulfanylpentan–2–one (4MSP)
Applsci 14 01483 i001 		Boxwood, blackcurrant 0.80–880–21.9
Chenin blanc0–23n.d. *
Sauvignon blanc3–sulfanylhexyl acetate
(3SHA)
Applsci 14 01483 i002		Passionfruit, tropical, boxwood 40–10623–151
Chenin blanc0–1005–253
Sauvignon blanc3–sulfanylhexan–1–ol
(3SH)
Applsci 14 01483 i003		Grapefruit, tropical, passionfruit 60350–5664178–904
Chenin blanc10–136899–1124
Sauvignon blancbenzyl mercaptan (BM)
Applsci 14 01483 i004		Smoke, toasty, struck flint 0.30.6–5.5n.d. *
Chenin Blanc30–40n.d. *
Sauvignon blanc2–furfurylthiol (FFT)
Applsci 14 01483 i005		Roasted Coffee 0.41–36n.d. *
Chenin blanc14n.d. *
1 South Africa * Not detected; adapted from [53,57,70].

3.4. Effect of Cryogenic Technologies on Methoxypyrazines

Methoxypyrazines (MPs) are volatile nitrogen-containing heterocyclic compounds found in plants, insects, fungi, and bacteria [74,75]. They are primarily responsible for the vegetative, grassy, green pepper, capsicum, and asparagus aromas present in Sauvignon blanc [6,28,75,76]. The perception of green attributes is seen as positive and adds complexity to Sauvignon blanc [28,76,77]. The most essential MP found in grapes and wines is 2-methoxy-3-isobutylpyrazine (ibMP), the main contributor to the vegetative, grassy, green pepper, capsicum, and asparagus aromas in Sauvignon blanc [28,74,75,76,77]. It is typically present in wine as free volatile compounds in concentrations ranging from 2 to 30 ng L−1 (Table 4).
Moreover, two additional MPs present in must and wine at lower concentrations are 2-methoxy-3-isopropylpyrazine (ipMP) and 2-methoxy-3-sec-butylpyrazine (sbMP), which contribute to the earthy, asparagus aromas [28,74,75]. Sensory detection thresholds for ibMP, ipMP, and sbMP are typically very low, i.e., 1–2 ng L−1 in water and 2–16 ng L−1 in wine [28,74,75]. Interestingly, during literature searches, the authors noted no publications investigating the effect of cryogenic treatments on MPs, which warrants further investigation and understanding as MPs are major contributors to the aroma profile of Sauvignon blanc wines. Previous research incorporating cryogenic practices focused mainly on their effect on major aroma compounds, i.e., polyphenols, terpenes, esters, higher alcohols, fatty acids, as well as the varietal thiols, i.e., 3-SH, 3-SHA, and 4-MSP [28,31,59,77,78].

4. Effect of Cryogenic Technologies on Sensory Properties of Wine

Wine quality and consumer acceptance of wine are frequently determined by organoleptic properties (aroma, colour, and taste), particularly the aroma profile [26,28,78,79,80,81]. In most cases, wines produced from grapes subjected to pre-fermentative cryomaceration treatments had a higher aroma intensity, improved mouthfeel, improved colour, oxygen stability, as well as enhanced aroma characteristics related to the cultivar [29,32,33,34,35,56,81]. This is due to an increase in the extraction of the aroma and flavour compounds, i.e., terpenes, thiols, esters, phenols, etc., present in the grape skin [29,32,34,35,36,37,78]. Alti Palacious et al. [29] demonstrated that cold pre-fermentation maceration treatments prior to vinification were capable of modifying the nutrient composition of the grape must, therefore enhancing the formation of aroma compounds, resulting in wines with an enhanced final quality.
In addition to the discussion on how cryogenic pre-treatment techniques affect the sensory profiles of the final wines [36], it was shown that wines produced from whole grape bunches sprayed with liquid CO2 (inertized wine (IW)) whilst passing through a cooling tunnel had a better colour than the untreated wine. This was confirmed by analysing the total phenol concentration and the lowest value of gallic acid. Moreover, non-trained judges (preference test) preferred the IW wines. Inertized wines were found to achieve a good quality standard, capable of satisfying consumer preferences [36]. Furthermore, the research conducted by Alti Palacious et al. [29] further found that treating grape must with dry ice (solid CO2) assisted in modulating the aroma compounds, therefore enhancing the aromatic quality and complexity of the final wines. Overall, the general trend observed from the literature shows that freezing techniques produced wines of a more intense aroma when compared to wines obtained using traditional methods. Moreover, the cryogenic method affected the overall quality of the wines [31].

5. Concluding Remarks and Future Prospects

Various scientific and wine technological developments have occurred in the wine industry to improve the varietal aroma and flavour profiles as well as the quality of wines. These advances have given rise to innovative wine styles being produced from the same grape varieties through either the direct manipulation of the grape berry before the winemaking process, or a combination of processes during wine production. However, the ability to manage the development of flavour compounds, i.e., varietal thiols, and their association with the sensory quality of wine using viticultural or oenological practices is yet to be demonstrated. The literature reported that a combination of these practices could influence varietal thiols and their precursors. Subsequently, most procedures demonstrated varied results, with the number of precursors in grapes not directly reflected in the varietal thiols quantified in the final wine. Therefore, specific vineyard and winemaking practices for increasing thiol and aroma compound concentrations in wines are still needed.
Moreover, the cryogenic treatment of grapes as a pre-fermentation treatment could be a useful alternative based on cryogenic processing technologies already applied in the beverage industry. Previous studies demonstrated its potential for thiol management in Sauvignon blanc, with thiol precursors increasing by four times in frozen grapes stored at −20 °C for two months compared to that found in frozen or fresh juices. Subsequently, studies using dry ice for the pre-fermentative cryomaceration of Sauvignon blanc grapes must also show an increase in 3-SH and 3-SHA concentrations in the resulting wine. However, it should be noted that although these new winemaking technologies greatly contribute to the increase in the extraction of aroma compounds, the grape variety used is still largely responsible for the aroma profile of the final wine due to differences in their skin cell layers and sizes. Moreover, there is a lack of information on the effect of these treatments on methoxypyrazines, which needs to be addressed as they are major compounds contributing to the varietal style of Sauvignon blanc and other wine grape varieties.
Furthermore, it is recommended that future research applications combine ultrasound technology, microwave-assisted extraction, and pulsed electric field technology with cryogenic pre-treatment technologies to further improve the extraction of aroma compounds to produce innovative wine styles. These technologies could add value to the wine industry as they showed promise in the acceleration of aroma extraction from oak chips, for the reduction in microbiological populations, reducing SO2 usage, as well as the acceleration of aging on the lees. However, current studies are only being conducted on a small or laboratory scale and further optimisation on an industrial scale is needed to verify the reliability of results.

Author Contributions

Writing—review and editing, V.M.v.B.; visualization, V.M.v.B.; supervision, F.P.v.J. and J.v.W.; project administration, F.P.v.J.; funding acquisition, F.P.v.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by South Africa Wine grant number [P04000266].

Acknowledgments

Thanks to the Agricultural Research Council (ARC), Infruitec-Nietvoorbij for providing the infrastructure and financial support; the industry partner, i.e., South Africa Wine, for financial support; Rodney Hart (Post-Harvest and Agro-processing Technologies Division, Agricultural Research Council Infruitec-Nietvoorbij) for continuous technical support, mentorship, and motivation; Carien Coetzee (Basic Wine) for consultation services and technical support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Table 4. Methoxypyrazines present in Sauvignon blanc wines: aroma description, perception, and range in wine.
Table 4. Methoxypyrazines present in Sauvignon blanc wines: aroma description, perception, and range in wine.
Compound & Chemical StructureAroma DescriptionAroma Perception in Water
(ng L–1)		Aroma Perception in Wine
(ng L–1)		Range in
Wine
(ng L–1)
2–methoxy–3–isobutylpyrazine (ibMP)
Applsci 14 01483 i006		vegetative, green pepper 1–22–163 2–30
2–methoxy–3–isopropylpyrazine (ipMP)
Applsci 14 01483 i007		earthy, mushroom, cooked, or canned asparagus, green beans 1–22–16 <10
2–methoxy–3–sec–butylpyrazine (sbMP)
Applsci 14 01483 i008		green (peas, bell pepper, galbanum), ivy leaves, bell pepper 1–22–16 <10
Adapted from [28,74,75,76].
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van Breda, V.M.; van Jaarsveld, F.P.; van Wyk, J. Pre-Fermentative Cryogenic Treatments: The Effect on Aroma Compounds and Sensory Properties of Sauvignon Blanc and Chenin Blanc Wine—A Review. Appl. Sci. 2024, 14, 1483. https://doi.org/10.3390/app14041483

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van Breda VM, van Jaarsveld FP, van Wyk J. Pre-Fermentative Cryogenic Treatments: The Effect on Aroma Compounds and Sensory Properties of Sauvignon Blanc and Chenin Blanc Wine—A Review. Applied Sciences. 2024; 14(4):1483. https://doi.org/10.3390/app14041483

Chicago/Turabian Style

van Breda, Valmary M., Francois P. van Jaarsveld, and Jessy van Wyk. 2024. "Pre-Fermentative Cryogenic Treatments: The Effect on Aroma Compounds and Sensory Properties of Sauvignon Blanc and Chenin Blanc Wine—A Review" Applied Sciences 14, no. 4: 1483. https://doi.org/10.3390/app14041483

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