The impact of ultrasound applied during pre-fermentation maceration leads to the breakage of cell walls, which facilitates the release and extraction of cell contents.
Introduction
Volatile thiols often characterise fruit-driven Sauvignon blanc wines and are known to contribute attributes such as “guava”, “passion fruit” and “citrus”. These thiols can either be released during fermentation from non-volatile precursors (mostly located in the skin of the berry) or it can be formed de novo through other mechanisms. Even though many producers perform pre-fermentation maceration to increase the extraction of non-volatile precursors from the grape skins, a recent study1 has shown that a large amount of precursors remain in the grape marc after pressing. It seems that there is untapped potential that is discarded with the grape marc that could be utilised to increase the thiol content in wine.
Researchers from Italy investigated the use of ultrasound technology on crushed grapes (a technique recently approved by the Organisation of Vine and Wine)2 to try to exploit this untapped potential. The theory is that the use of ultrasound can enhance the extraction of the non-volatile thiol precursors from Sauvignon blanc grapes, possibly also increasing the thiol content of the resulting wine. Other than the increased extraction of certain compounds, the acceleration of the extraction will significantly reduce the amount of time needed to perform pre-fermentation maceration. This will, in turn, reduce the risk of microbial spoilage and minimise energy usage (for cooling).
Ultrasound
Ultrasound technology is based on mechanical waves with frequencies ranging from 20 kHz to 10 MHz. The generated waves cause the creation of multiple series of compression and rarefaction that can lead to acoustic cavitation in a liquid medium, producing micro-bubbles (small amounts of steam). The compression cycle leads to high temperatures and pressures inside the bubbles, which can result in ionisation and the formation of radicals that can participate in various reactions.1,3
Power ultrasounds (20 – 100 kHz) have been used for many years in the food industry to generate emulsions, disrupt cells and disperse aggregated materials. More recently the application of ultrasound technology has shown potential for the modification and control of crystallisation processes, degassing of liquid foods and enzyme inactivation.4
During maceration of grape must, the ultrasonic extraction of compounds from the berry skin cell walls can be facilitated by mechanical actions, disruption of tissues and cells and an increase in temperature.5
Materials and methods
Whole berries were harvested from various sites in Trentino (Italy). The berries were crushed and 100 g aliquots subjected to continuous sonication at a frequency of 20 kHz for either three of five minutes. No sonication was applied to the control samples. Sonication was performed using an ultrasonic processor with a 13 mm titanium sonotrode probe. After sonication, the sample was pressed until a 60% (v/w) yield was obtained and the juice vinified.
Results
Ultrasound treatment of crushed berries showed an increase in catechin, total phenols as well as conductivity. This increase confirmed the efficacy of the ultrasound treatment in breaking down berry tissues. Unlike the results reported for catechin, total phenols and conductivity, there was no increase in thiol precursors. One of the precursors even showed a decrease in concentration from the beginning to the end of the ultrasound treatment. This result comes as a surprise considering the efficacy of the ultrasound to extract phenolic compounds.
In an attempt to explain the results found in the study the researchers proceeded to experiment with a model wine medium containing known amounts of glutathionylated and cysteinylated thiol precursors. Ultrasound treatment was then applied to the model wine.
Results showed a reduction in the non-volatile precursors during the sonication treatment suggesting that the sonication led to the breaking of the precursor compound. This breakage is likely due to the radical and electron-transfer processes generated by the high energy and excited-state species created in the solution following cavitation. The breakdown of the precursor could result in the cleavage of the carbon-sulfur bond, releasing the aromatic volatile thiol into the medium.
Indeed, after sonication of the model wine containing the precursors, the mean results for 3MH and 4MMP was significantly higher than those of the control samples. It seems that ultrasound treatment has the potential to release the aromatic thiols from the non-aromatic precursor without the use of yeast activity. The amount of precursors broken down during sonication, however, does not correspond to the amount of free thiols released (much less thiols are released). The thiol content of the wines made from the sonicated crushed berries were not measured. Further studies will have to investigate the effect of sonication on the free volatile thiols (possibly also inducing the breakdown of the aromatic compound), the reaction of the thiols with formed radicals and/or possible stripping effects due to the volatility of the free thiols during the ultrasound treatment.
Conclusion
Ultrasound is a relatively low cost, non-hazardous and environmentally friendly technology and could be an important technological innovation speeding up some slow reactions required in the winemaking process such as maceration. The application of ultrasound technology can significantly enhance the extraction of various grape components, however, the enhanced extraction of volatile thiol precursors from Sauvignon blanc grape berries due to ultrasound treatment remains to be determined.
Tests performed in a model wine solution showed that the ultrasound waves led to the cleavage of the non-volatile precursor, releasing the aromatic thiol. In a real wine medium, the presence of other compounds and the formation of radicals during sonication can negate the potential increase in free thiols. Further research is needed to investigate the mechanisms of interaction between ultrasound and thiol precursors and the potential degradation products during winemaking.
The prescriptions specified by the OIV regarding ultrasound treatment:2
- The treatment must be carried out on destemmed and crushed grapes to increase the performance of the treatment;
- To avoid a temperature increase in the bulk of crushed grapes, this treatment should be done with the bulk in movement;
- To boost an efficient cavitation process, the solid/liquid proportion in the bulk of crushed grapes must be adequate.
Abstract
The extraction of non-volatile thiol precursors (which is predominantly located in the skins of the berry) can possibly be enhanced by ultrasound treatment, however, the treatment can also result in the cleavage of the precursor thereby releasing the aromatic thiol. The exact interaction between ultrasound and non-volatile thiol precursors and the effect of ultrasound on the free thiol remains to be established.
References
- Roman, T., Tonidandel, L., Nicolini, G., Bellantuono, E., Barp, L., Larcher, R. & Celotti, E., 2020. Evidence of the possible interaction between ultrasound and thiol precursors. Foods 9(1). https://doi.org/10.3390/foods9010104.
- OIV, 2019. OIV Resolution OIV-OENO 616-2019. Geneva, Switzerland.
- Tao, Y. & Sun, D-W., 2015. Enhancement of food processes by ultrasound: A review. Critical Reviews in Food Science and Nutrition 55(4), 570 – 594. https://doi.org/10.1080/10408398.2012.667849.
- Celotti, E. & Ferraretto, P., 2016. Studies for the ultrasound application in winemaking for a low impact enology. In: 39th World Congress of Vine and Wine, Bento Gonçalves, Brazil, pp. 104 – 106.
- Ferraretto, P., Cacciola, V., Ferran Batlló, I. & Celotti, E., 2013. Ultrasounds application in winemaking: Grape maceration and yeast lysis. Italian Journal of Food Science 25(2), 160 – 168.
– For more information, contact Carien Coetzee at carien@basicwine.co.za.
