Modernizing Microbial Stability in Wine

Updated: Aug 26, 2019


In winemaking, microbial stability refers to the potential spoilage caused by microbes in finished wine. To control this, winemakers commonly sterilize surfaces before exposing them to the juice by adding sulfur (in various forms) or using filters pre-bottling. These conventional methods may be easy to use, however these methods are not entirely effective and many pose risk to the quality of the finished wine. In order to avoid the negative effects of these methods, I believe that using radiation could be a more sustainable, effective and quality-preserving alternative.


Quick Disclaimer on Irradiation

Many people are hesitant to use radiation, despite the fact that it is used in many industries including the international food, spice and enzyme industry to eliminate spoilage and pathogenic microbes (Glaess. 2002). This technique has been approved by the JECFI, IAEA, WHO and FAO and used in the food industry for the 60 years in 40 countries (Jayathilakan et al. 2015). Do you realize that you are consuming products that have been sterilized using this technique everyday? Gamma is used in at least 30% of spices and this percentage was increased in 2013 after encouragement from the FDA. Further, it has been shown that relevant levels of gamma do not significantly impact the nutritional content or composition of the irradiated material (EFSA. 2011)(Grandison. 2001) (Siddhuraju et al. 2002). For wine, special consideration should be given to phenolic integrity which is responsible for the flavor and aroma of wine. It has been shown that radiation does not impact those phenolics, but has in some cases resulted in increased aging capacity (Bögl et al. 1985).


How does Gamma Irradiation actually work?

Gamma radiation is a form of high intensity, high penetrance ionizing radiation. Ionizing radiation is the emittance of energy or particles from an unstable particle that results in the ionization of exposed material. In this case, it is microbial DNA. There are three main types of ionizing radiation: alpha, beta and gamma. Without getting into all the details, gamma radiation optimizes the penetrance and intensity required to disrupt microbial DNA to sterilize an area or material.


How do you use Gamma Radiation?

As this technique has been around for over 60 years, there are plenty of commericial equipment manufacturers. Companies like Nordion in Canada, Sterigenic in Illinois or Steris in San Diego, offer irradiation services, however on a commercial scale this is unlikely to be economical or time effective. Commercial gamma sources can be purchased and fitted to existing equipment. The most common gamma source is Co-60 and Cs-137. The sources available for sale are designed to reach 3.7-10GyA, which is effective for sterilization. The current designs are generally stationary, do not require intensive training or extensive ongoing maintenance. I will have a blog post in the near future that includes information about a commercial facility and their source that I am visiting as well as my adjustments to the design for use in the wine industry. The typical processing cycle (courtesy of Steris) would be the following:

  1. At the wine or cork production facility, material intended to be sterilized (MITBS) would be brought to the source rack.

  2. The dosimeters (meter that measures radiation dose) would be placed with the MITBS

  3. The MITBS would be exposed to the source for a given time/distance (I will provide calculations in the next blog post)

  4. The dosimeters will be read to ensure that the proper dose is received.


Comparison to Conventional Methods

Firstly, I want to explain some of the deficiencies of the current conventional methods in comparison to using radiation.

  • Caustic and Citric are used to sterilize fermentation vessels before exposing them to juice or wine. Although the chemical species may differ between wineries, this technique is highly reagent intensive. To clean one 5 tonne tank once, expect at least 400g of each reagent to be used. Significantly, it is extremely water intensive, which is of special consideration for drought affected regions like Napa. Theses reagents are applied in several rounds of washes in wineries. If one were to use radiation emitted by a gamma source, there would be no reagent exhaustion. A gamma source could be purchased once and used indefinitely. By using a gamma source to emit microbe-killing radiation over a tank, less time and reagents would be used. There would still have to be a single wash step, as radiation does not remove particulate matter such as dust.

  • Sulfur is used in various forms as an antimicrobial. It can be used at any stage of the process from berry intake, fermentation and barreling. It is generally not used pre-bottling, because it can have a negative sensory impact in low quantities (i.e. the aroma of burnt eggs) (Sweigers & Pretorius. 2002). It can also be produced by yeast in stuck fermentations, often a result of nitrogen deficiency and microbial competition (Henschke and Jiranek. 2001). The trade-off of using sulfur to kill spoilage microbes has been discussed at length, and while effective in most cases, the sensory impact cannot be overlooked in any quality level of wine production. *Understand that my qualm with sulfites is due to the sensory impact. I am not touching upon allergies or supposed sulfite sensitivity. A latter idea, which originated from a paper in 2009 discussing how crap weather at large is correlated with incidence headaches, which was grossly generalized. This was also combined with the 1988 inflammatory law mandating "contains sulfites" to printed on the label.

  • Gamma radiation offers a sustainable and effective alternative to conventional methods. Now, I want to show you a couple of examples of applications and points along the winemaking process where radiation could be used to enhance microbial stability.


Potential Applications


  1. Elimination of Cork Taint. Cork taint has been estimated to affect between 10-30% of wines. This results in huge product loss and opportunity cost. A soil-dwelling bacterium is responsible for cork taint, which is caused by the production of trichloroanisole (TCA, a reaction byproduct of lignin degradation). This bacterium infects the cork bark that is exposed to dirt either from being low to or placed to the ground, or from dirt kicked up during the process. Cork producers offer a service where someone will smell each individual cork to "guarantee" that it has no TCA, but this process is not entirely effective. Alternatives of course are canned wine, screw caps and synthetic corks, however all of these processes inhibit micro-oxygenation associated with aging as well as not being well-received by consumers. For this application of gamma, cork sheets would be exposed before entering the production facility. This would eliminate cork taint by killing the microbes responsible. By exposing cork before entering the production facility, contamination is minimized. Although there does not yet exist a specialized irradiation design for cork production (I am in the process of making one), current designs are similar enough that they could be easily adapted for the first phase of cork intake.

  2. Reducing sulfur additions. Currently, sulfur is ubiquitously used to eradicate microbes from fruit pre-fermentation as well as in wines in barrel. Sulfur additions have two significant negative effects: bleaches red white and can result in off smells. Gamma radiation avoids these two problems and can be used in all of the applications that sulfur is currently used in. For instance, the source could be placed over the sorting table or augur to irradiate incoming fruit. This would destroy all of the endemic yeasts (whose competition is associated with stuck fermentations) as well as spoilage microbes (such as ethyl acetate producing bacteria). In the case of barrel aging wines, barrels could be placed under the source to ensure continued stability. By using this technique, less or no sulfur could be used. In these ways, gamma offers a sustainable and less sensorily impactful alternative.

  3. Final wine stability. If it is further accepted that irradiation does not impact the flavor or aromas of wine, gamma sources could be co-opted for use on the bottle line. Currently, wineries only option to ensure microbial stability is to rely on filters, however there is much hesitation throughout the industry about using filters as they may impact the sensory profile. There is mixed literature on the veracity of this. Instead, the end of the bottling line could be fitted with a gamma source that could irradiate an entire pallet. This would kill microbes in the wine, ensuring microbial stability. Incidentally, there is already room in the FDA regulations that would allow the wine to be exposed to doses of 3.7GyA. The point of irradiating at the end of the bottling line is that generally these lines move so quickly (>1 bottle a sec) that they would likely not be exposed for long enough (calculations in the next blog post).

  4. Reducing water use when cleaning tanks. As an additional thought, gamma could be used to sterilize tanks by the mechanisms mentioned before. As tanks require multiple rounds of washes with chemicals and rinses, it is extremely water intensive. Using gamma radiation to sterilize combined with a simple rinse for particulates would significantly decrease water consumption at wineries. This of particular interest for areas such as California, Baja, South Africa and some parts of Australia, that are currently experiencing drought. The limitations of this technique is that the gamma source is high penetrance, and therefore would require small VC1000s or 3T tanks that can be moved and placed under the source so as to avoid irradiating the adjacent areas.




Suggestions for future research

  1. Demonstration of sterlization: Importantly, gamma radiation should be tested for all potential applications for its effectiveness in eliminating pathogens. For instance, cork sheets should be sampled before and after radiation to determine .

  2. Impact on organoleptics: Of course, given the molecular complexity of wine, future studies such look into the sensory impact of gamma radiation on wines.

  3. Impact on aging: Another major area of interest, in the claim in one study that the potential aging of the wine could be improved (by a means other than just killing off the microbes). I would be very curious to see the chemical basis for this claim.

  4. Sulfur removal: Another interesting thing to consider is that gamma radiation causes water radiolysis (click here for the decomposition pathway). In a wine, this means that chemical species such as hydrogen peroxide would be produced (Dezaugus et al. 2015). Interestingly, hydrogen peroxide is one means of "removing' sulfites from wines (Hoffman & Edwards. 1975). I would be curious to see how using gamma radiation could be used in problematic wines that have high levels of microbes and sulfur. Potentially kill two birds with one stone, so speak? No idea, but it could be interesting.

I hope you enjoyed reading this article, and I would love to hear more opinions on this matter. I understand the reason for hesitation given many's inexperience, but the potential benefits out weigh the reason to let this hesitation keep us uniformed.



Citations

  1. Bögl, W. (1985). Radiation sterilization of pharmaceuticals-chemical changes and consequences. Radiation Physics and Chemistry (1977), 25(1-3), 425-435.

  2. Dzaugis, M. E., Spivack, A. J., & D'Hondt, S. (2015). A quantitative model of water radiolysis and chemical production rates near radionuclide-containing solids. Radiation Physics and Chemistry, 115, 127-134.

  3. EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids (CEF). (2011). Scientific opinion on the chemical safety of irradiation of food. EFSA Journal, 9(4), 1930.

  4. Grandison, A. (2001). High Dose Irradiation: Wholesomeness of Food Irradiated with Doses Above 10kgy (WHO Technical Report Series No. 890). International Journal of Food Science & Technology, 36(3), 338-339.

  5. Henschke, P. A., & Jiranek, V. (1991). Hydrogen sulfide formation during fermentation: effect of nitrogen composition in model grape must. In Proceedings of the International Symposium on Nitrogen in Grapes and Wine: Seattle, Washington, Usa 18-19 june 1991 (pp. 172-184). American Society for Enology and Viticulture, ASEV.

  6. Hoffmann, M. R., & Edwards, J. O. (1975). Kinetics of the oxidation of sulfite by hydrogen peroxide in acidic solution. The Journal of Physical Chemistry, 79(20), 2096-2098..

  7. Jayathilakan, K., Sultana, K., Reddy, K. J., & Pandey, M. C. Radiation Processing of Meat and Meat Products–An Overview.

  8. Siddhuraju, P., Makkar, H. P. S., & Becker, K. (2002). The effect of ionising radiation on antinutritional factors and the nutritional value of plant materials with reference to human and animal food. Food Chemistry, 78(2), 187-205.

  9. Swiegers, J. H., & Pretorius, I. S. (2007). Modulation of volatile sulfur compounds by wine yeast. Applied Microbiology and Biotechnology, 74(5), 954-960.

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