Introduction

Oak barrels have long been used in winemaking. There exists a natural harmony among oak wood compounds, wine constituents, and the oxygen transferred into barrels that, in general, results in greater wines compared to their non-oak-aged counterparts. The main wine constituents of interest here are polyphenols, and more specifically, tannins, the substances responsible for the drying, puckery sensation in the mouth, and anthocyanins, the red-color pigment molecules.

Aside from the plethora of aromas and flavors that oak and particularly toasted oak impart to wine, the infinitesimally small amount of oxygen exchange in barrels from the outside environment improves and stabilizes color, smoothens tannins for a softer “mouthfeel,” i.e. the combined sensation of tannins, acids, ethanol, and polysaccharides, and improves aging potential. This oxygen exchange phenomenon is known as micro-oxidation; the term micro-oxygenation is also used interchangeably. Those aromas and flavors also become more concentrated due the slow evaporative loss of ethanol and water — the “angel’s share.”

But oak barrels are a significant investment; acquisition costs are high, they require care and maintenance, and they have a limited lifespan in that they become “neutral” after several uses and no longer impart those much-desired aromas and flavors.

Oak powder, chips, staves, cubes, balls, and spirals — collectively referred to as oak adjuncts — are inexpensive alternatives to oak barrels, driving the cost of “oak-aged” wines down significantly.

Oak adjuncts can be used either in inert vessels, such as stainless steel tanks, or neutral oak barrels. There is no oxygen transfer into inert vessels and therefore oak adjuncts are only used for imparting oak aromas and flavors. In neutral barrels, micro-oxidation still occurs, albeit at a slower rate, and can therefore better replicate newer barrels when used with adjuncts.

There are several other technologies and products that attempt to replicate the benefits of oak-barrel aging. One such technology is micro-oxygenation, known as MOX, which involves injecting minuscule amounts of oxygen into tanks fitted with oak adjuncts, typically long oak staves. Vessels manufactured from HDPE (high density polyethylene) have also been developed to replicate the physics and chemistry of barrels if used with adjuncts. Flextank vessels, manufactured by Smak Plastics, Inc., is one such line of products and the focus of this study.

NOTE: I use the term micro-oxidation to refer to the passive transfer of oxygen through a material and into wine, and micro-oxygenation to refer to the deliberate, active injection of oxygen into wine using specialized equipment in conjunction with oak adjuncts.

Flextanks

Flextanks are manufactured from resin into extremely durable polyethylene approved by the USFDA (US Food and Drug Administration) and EFSA (European Food Safety Authority) that, according to Flextank, can last up to 20 years, significantly much longer than the useful life of an oak barrel before it becomes neutral. Flextank also claims that these vessels are designed to have a similar oxygen permeation rate, or oxygen transfer rate (OTR), to that of a typical second-year barrel when used at a nominal cellar temperature of approximately 13°C (55°F). To my knowledge, there is no published OTR data for Flextank products.

Flextanks are available in various volumes from cylindrically shaped 55- to 2000-liter (15- to 500-gallon) tanks to egg-shaped maturation tanks and stacking bins. For a complete list of Flextank products, please visit Flextank’s website.

Objectives of this Study

The objective of this study is to compare the performance of a Flextank vessel to that of a second-year barrel, both equipped with an oak adjunct. Performance will be assessed both quantitatively by measuring and analyzing pertinent enological parameters and qualitatively by tasting and evaluating the wine throughout the duration of the study. This study is expected to last a minimum of 12 months and up to 24 months.

Specifically, a 55-liter (15-gal) Flextank (ECO15/EM15) and a similarly sized two-year-old French oak Boutes barrel will be used in this study and each equipped with a demijohn-sized WineStix oak stave made from Allier French oak with a medium-plus toast level. The barrel had previously and continuously held two batches of the same wine for approximately 24 months. The wine is a Touriga Nacional sourced from Lodi, California from the 2019 vintage acquired from Musto Wine Grape Company. The chemistry of the wine is outlined further down.

NOTE: The WineStix were both trimmed by 5 cm (2 in.) so that the one for the barrel could be completely inserted.

How the Study Will Be Carried Out

A Touriga Nacional wine was vinified from frozen must as a single batch up to the end of both the alcoholic and malolactic fermentations and stabilization with potassium metabisulfite (K₂S₂O₅). Both the barrel and Flextank were filled with the same wine and WineStix product. To compensate for possibly different oxygen uptake and free sulfur dioxide (FSO2) consumption during the vessel-filling operations, FSO2 and dissolved oxygen (DO) levels were re-measured after the transfers, and FSO2 adjusted back to identical levels and based on pH and DO of the now separate batches.

NOTE: The dry airlock supplied with the Flextank will be used instead of fitting a silicone bung with a wet airlock on the threaded opening on the lid. Teflon tape has been applied to all threaded parts to minimize air ingress and the possibility of leakage through the top lid should wine expansion occur. The Flextank is equipped with a threaded ½” ball valve and hose barb at the bottom.

Both batches will be held at 13°C (55°F) in a temperature-controlled cellar with relative humidity (RH) between 55% and 75%.

The barrel will be topped up every two weeks with the same wine reserved in a separate vessel. The Flextank should not require any topping beyond possibly a topping in 7−10 days after the initial filling and insertion of the WineStix.

At one-month intervals, pH, FSO2 and DO will be measured in each batch to determine FSO2 consumption and oxygen uptake, and then FSO2 will be adjusted back to the proper level based on pH and DO.

At three-month intervals, in addition to the parameters measured at one-month intervals, total acidity (TA), volatile acidity (VA), total SO₂ (TSO2), ethanol concentration, turbidity, color and phenol index will be measured. Sensory evaluations will also be performed to assess evolution of each wines using qualifiers of appearance, aroma and bouquet, taste, aftertaste and overall impression. The tastings will be conducted blind. Both samples will be compared to the base wine aging in a glass carboy, albeit of different volume than the barrel and Flextank, and so, the comparison will not be completely valid from a scientific perspective.

The significance of the relevance of the enological parameters is outlined below.

Regular updates will be posted in this blog and a final report will be published upon conclusion of the study.

Other Studies

At least two studies have examined the effects of oxygen transfer through HDPE material. del Alamo‐Sanza et al. (2015) and Nguyen et al. (2010) demonstrated the positive effects of micro-oxidation on color parameters. Although del Alamo‐Sanza et al. (2015) concluded that HDPE tanks in combination with oak adjuncts are a viable alternative for storing wines, their study compared HDPE tanks to new oak barrels and of slightly different volumes (190-L/50-gal tanks vs. 225-L/59-gal barrels) for a duration of four months in vessels and two in bottles. And the study by Nguyen et al. (2010) did not consider oak barrels; it only examined HDPE and stainless steel tanks and without the use of oak adjuncts.

Enological Parameters

Following is an overview of enological parameters that will be monitored and which could provide clues as to any potential differences in wine evolution between the barrel and the Flextank batches.

Total Acidity (TA), expressed as tartaric acid equivalents in g/L, measures the concentration of all fixed and volatile acids. Changes in TA would primarily be due to changes in volatile acidity (VA) although a drop can occur due to potassium bitartrate formation and precipitation. Potassium ion (K⁺) and tartaric acid concentrations will be monitored to confirm any changes in TA due to potassium bitartrate.

Volatile Acidity (VA), expressed as acetic acid equivalents in g/L or mg/L given its relatively smaller concentration, measures the concentration of all volatile (steam-distillable) acids. Increasing VA levels would point to increasing acetic acid amounts resulting from the activity of acetic acid bacteria, which thrive in the presence of oxygen, or from chemical oxidation of ethanol into acetaldehyde then into acetic acid, which would point to excessive oxygen ingress and exposure. My lab is not equipped to measure acetaldehyde concentrations.

pH will be monitored as changes in TA could trigger changes in pH. pH measurements are also used to determine the optimum FSO2 level when needing to make adjustments at the three-month intervals.

Ethanol content, expressed as a percentage of ethanol volume, or %ABV, is a measure of the amount of ethanol in wine. A decrease in %ABV can result from evaporative loss through the barrel and Flextank materials and from microbial or chemical transformations as described above.

Free Sulfur Dioxide (FSO2), expressed in mg/L, measures the concentration of molecular SO₂ and bisulfite ions collectively protecting wine against microbial and chemical spoilages. Differences in FSO2 measurements between batches could point to differences in oxygen consumption, which would mean vessels are exchanging oxygen at different rates. As FSO2 changes can also be due to binding reactions, Dissolved Oxygen (DO), expressed in mg/L, will also be measured.

To understand the extent of binding, Total Sulfur Dioxide (TSO2), expressed in mg/L, will be measured to see if the oak wood from the barrel is still contributing tannins (tannins are SO₂ binders). If there is oxidative spoilage with the formation of acetaldehyde, a strong SO₂ binder, there will be a decrease in FSO2 and an increase in Bound Sulfur Dioxide (BSO2). BSO2, expressed in mg/L, is simply calculated from the difference between measured TSO2 and FSO2.

Color evolution in reds is used to assess the age of wines. Young wines are characterized by a reddish, somewhat purple color. As wine ages, polyphenols oxidize into their brown-colored forms, known as o-quinones, which can be seen by an orange-hued tint at the rim, then to brownish hues as the wine ages and exceeds its ability to preserve red color. A number of parameters are used to gauge color evolution and to assess a potential browning problem. A very low FSO2 is the first sign of impending oxidative spoilage.

Color intensity and hue are the two very important color parameters for assessing reds.

Color Intensity (IC), expressed in absorbance units (a.u.), is a measure of the intensity of color of wine and is calculated as the sum of measured spectral absorbances (A_λ) at wavelengths (λ) of 420 nm, 520 and 620 nm. These wavelengths correspond to the yellow, red and purple/blue components of color, respectively. Light-colored reds will have IC values between 3 and 5; medium-colored reds will have IC values between 5 and 8; and deep-colored reds will have IC values between 8 and 12. As wine ages and oxidation reactions slowly take foot, the red component will decrease and the yellow component will increase.

Hue (H) is a calculation of the ratio of the yellow component of color to the red component, i.e. A₄₂₀/A₅₂₀. Young wines will have H values below 0.8, and increasing H values, especially beyond 0.8, is indicative of oxidation and increasing browning. The purple-bluish component visible in very young wines almost disappears as browning occurs during aging. However, oxidative reactions can give rise to blue-colored compounds that are not usually visible but which will be detected by spectral analysis.

The changes in purple/bluish color will be monitored using a Blue Index, calculated as the ratio of the purple component of color to the red component, i.e. A₆₂₀/A₅₂₀, while the loss of red color intensity and brilliance will be monitored via a Brilliance of Red (DA) parameter, expressed as a percentage, calculated from the ratio of yellow and purple components to (twice) the red component.

The Total Phenol Index (TPI) calculates an index based on the amount of polyphenols measured in wine, and can point to differences in tannin reactions, and possibly extraction from oak wood of the barrel. Light-bodied, low-tannin wines will have TPI values between 25 and 30; medium-bodied wines with good tannic structure will have TPI values between 30 and 50; and full-bodied, tannin-loaded wines will have TPI values over 50. The Folin-Ciocalteu method with absorbance readings at 750 nm is used to calculate TPI.

Turbidity, expressed in Nephelometric Turbidity Units (NTU), is a measure of the degree of turbidity, or clarity, of a wine. Since the barrel and Flextank have different physical properties—a “bulging” cylindrical shape on the horizontal axis versus a completely cylindrical shape on the vertical axis—it is conceivable that precipitable matter can precipitate at different rates.

Test Wine

The Touriga Nacional wine used in this study has the following basic chemistry: 13.90% ABV, pH of 3.65 with a TA of 6.86 g/L, of which 0.227 g/L (226.9 mg/L) is VA, FSO2 of 25.3 mg/L, TSO2 of 54.0 mg/L (the first sulfite addition was adjusted by 100% to account for binding), turbidity of 13.0 NTUs, residual sugar (RS) of 3.75 g/L, IC of 5.96 with H of 0.73, and TPI of 44.10. The wine has undergone complete malic acid conversion, which was confirmed by enzymatic analysis—residual malic acid concentration was below detection. Each batch was then adjusted with potassium metabisulfite (K₂S₂O₅) to a FSO2 of approximately 56 mg/L based on DO of approximately 1.80 mg/L. The target FSO2 is based on 0.5 mg/L of molecular SO₂ with an adjustment factor of 33% to account for FSO2 “lost” to binding.

Acknowledgements

I would like to thank everyone in my Home Winemaking Facebook group who have taken the time to review and comment on my testplan for this study.

References

del Alamo‐Sanza, M., V.F. Laurie and I. Nevares. 2015. Wine evolution and spatial distribution of oxygen during storage in high‐density polyethylene tanks. J. Sci. Food Agric. 95:1313-1320.

Nguyen, D., L. Nicolau, S.I. Dykes and P.A. Kilmartin. 2010. Influence of Microoxygenation on Reductive Sulfur Off-Odors and Color Development in a Cabernet Sauvignon Wine. Am. J. Enol. Vitic. 61:457-464.