December 20, 2011
As a result of the relatively high cost of oak barrels, winemakers have been searching for a comparable alternative at a fraction of the cost. Oak chips, while somewhat taboo in their earlier days, are gaining favor among winemakers as being a good alternative to the traditional oak barrel. In blind taste tests, consumers showed no preference for wine aged in oak barrels versus wine aged with oak chips. For the details of that particular study, click here to read a past review by The Academic Wino.
Producing staves for barrel or oak chips is not a fast process. One of the lengthiest steps is the outdoor seasoning process, which takes anywhere from 24 to 36 months. It is during this time that the wood undergoes many biochemical transformations of biopolymers and other compounds by fungi and bacteria. Studies have found the fungi population present during this seasoning process consists ofAureobasidium pullulans (83%), Trichoderma harzianum, andTrichoderma konigii (the latter two making up 15% of the population). These fungi function to hydrolyze wood heterosides (including ellagitannins, coumarins, and polysaccharides) which result in a decrease in bitterness and astringency.
Over the 24-36 month seasoning time, the fungal community changes. Over this time, fungi belonging to the genera Penicillum, Geomyces, and Geotrichum, with the species Penicillum purpurogenum the most represented. In the internal layers of the staves, studies have found that the species Candida sp., Paecilomyces variotii, andPhialemonium sp. were the most represented molds.
Some scientists have hypothesized that by inoculating oak staves with certain fungi, they may be able to better control the metabolic reactions and therefore which wood compounds are hydrolyzed. This could potentially lead to new seasoning places and greater customization of the desired flavors in the wine that is aged in a particular barrel or with particular oak chips. Some studies have reported that by inoculating oak staves with fungi have increased the seasoning rate, thereby dropping the wait time for a finished staves or oak chips from 12-36 months to just one month.
The goal of the paper reviewed today, which was published last year, was to treat oak chips with certain combinations of fungi in order to potentially improve the impact of oak chips in red wine maceration, and to obtain effects comparable to wine aged in oak barrels.
The following fungi were used in this experiment: Ph. chrysosporium Burds. (MUT 2660), P. purpurogenum Stoll (MUT 3316), A. pullulans (de Bary) G. Arnaud (MUT 3237), and Phi. obovatum W. Gams & McGinnis (MUT 2702). The fungi used were all non-mycotoxinogenic. After an incubation period, an agar plug (8mm in diameter from along the edge of an actively growing colony) of each fungus was used to prepare and inoculate the oak chips. Each preparation used a different combination of fungi and growth medium.
Oak chips were toasted at a low degree and were of medium size. A 3mL aliquot of fungi and growth medium preparations was added to a flask containing 4g of oak chips and either 12mL of laboratory medium or 12mL of saline solution. Oak chips were sterilized either with the laboratory medium or the saline solution.
These cultures were incubated for 12 weeks in the dark under static conditions at temperatures optimized for each type of fungus. In addition to each fungus by itself, a combination of A. pullulans and Ph. chrysosporium was studied. For this combination, oak chips were first inoculated with A. pullulans for 6 weeks, then sterilized, then inoculated with Ph. chrysosporium for another 6 weeks.
Following the incubation period, oak chips were removed and brushed off to remove visible pieces of debris. The chips were then used for aging of red wine using traditional winemaking processes. The two grape varieties used were Montepulciano d’Abruzzo (70%) and Merlot (30%) from the 2006 vintage and originating from vineyards in San Severo, Apuila (southern Italy). Artificial aging was done by placing 1g of oak chips in 500mL bottles containing the wine and storing them in a 20oC room (82% relatively humidity) for 17 days.
The following volatiles were measured and analyzed: furfural, furfuryl alcohol, guaiacol, syringol, cis-β-methyl-γ-octalactone, 2-phenylethanol, 4-vinylguaiacol, benzyl alcohol, 2,3-butanediol, γ-butyrolactone, benzylaldehyde, and 4-ethylguaiacol. Gallic acid and ellagic acid were also analyzed.
o For the saline solution, this increase was found in the treatments inoculated with P. purpurogenum, A. pullulans, and Phi. obovatum.
o For the laboratory medium, this increase was found in the treatments inoculated with P. purpurogenum and A. pullulans.
o For treatments with A. pullulans, guaiacol was above the perception threshold.
Ellagic and Gallic Acids
What do these results means?
The results of this study show that the fungal treatment of the oak chips significantly affected the chemical profile of the wine. Based on principle components statistical analysis, the results showed two distinct groups that affected wines in specific ways. Oak chips treated with Phi. obovatum, A. pullulans, and the combination of A. pullulans and Ph. chrysosporium (“Group A”)affected the chemical profile of wines in one particular way, and oak chips treated with P. purpurogenum and Ph. chrysosporiumaffected the chemical profile of wines in another way (“Group B”).
Group A treated wines showed increases in guaiacol and syringol concentrations, whereas Group B treated wines showed increases in furfural and benzylaldehyde concentrations.
Even though the effect of the fungi were variable depending upon what kind of medium was used for the chips (laboratory medium versus saline solution), Group B treated wines showed increases in furfural and benzylaldehyde regardless of the medium the chips were treated with.
Based on these results, the authors claim that the microfungal treatment of oak chips increases the concentrations of some volatile components in red wine during aging. It may be possible, that with fungal treatment of oak chips for the aging of red wines, to tailor the flavors and aromas to those desired for a particular style of finished wine. If the goal is to have a wine with greater “toasty” character, the use of fungi from Group A may be useful. If the goal is to have a wine with more “dried fruit” or “almond” character, then a fungi of Group B may be better.
Of course, this research is in its infancy, and more work would need to be done, particularly in regard to examining the biology and enzymatic profile of the fungi, and any potential positive or negative health consequences of using it in the aging of wines.
I’d love to hear what you all think of the use of fungi in extracting more oak character from oak chips in the aging of wine. Please feel free to leave your comments below!
Source: Petruzzi, L., Bevilacqua, A., Ciccarone, C., Gambacorta, G., Irlante, G., Pati, S., and Sinigaglia, M. 2010. Use of microfungi in the treatment of oak chips: possible effects on wine. Journal of the Science of Food and Agriculture 90: 2617-2626.