The contribution of rays and tyloses to the "impermeability" of oak cooperage

Oak's active contribution to the finished wine has been documented extensively and has even received some coverage in this forum. What I have not covered previously is the counterpoint to oak's active contribution -- what I call its passive contribution -- the elements of which are captured in the chart below. I will expand on those elements in this post.

Before getting into the meat of the topic, lets revisit the path from oak tree to wine barrel (shown in the chart below).

In order to make its active, positive contributions to the wine, the oak barrel has to first present as an impervious airtight container and that function is enabled in no small part by oak structures called Rays and Tyloses.

Rays
Rays are illustrated in the wood images in the first chart on this page. They are "collections of elongated cells positioned radially along the trunk axis" which serve as the vehicles through which water and nutrients flow between the wood and bark. Compared to other tree families, oak has a high percentage of medullary rays -- between 19% and 32% of oak wood volume is made up of rays -- and this makes it relatively stronger than many other hardwoods.

Staves are split or sawed along the radius causing the wood's rays to run parallel to its broad surface. Wine diffusing into a ray is shunted along that ray and comes to a complete stop because there is no continuation of that path in the neighboring stave. Further, there is no contiguous path of rays from the inside surface to the outside surface of the stave. It is suggested that wine seeking to seep through a barrel wall would have to negotiate a "tortuous" path through at least five large rays. According to Dharmadikari, "Large rays provide a formidable barrier to the diffusion of liquid and make wood relatively impermeable."

The rays provide dimensional stability by allowing minimal shrinkage when the wood dries. The radius of oak shrinks only about 4% from wet to air-dry, making it easier to keep the barrel liquid tight when stored under alternate wet and dry conditions.

According to Jackson, the high proportion of rays gives oak much of its "resiliency and flexibility." Without that resilience, the staves would crack rather than bend, as it does, to form the curved sides of the barrel. The curved sides allow full barrels to be easily rolled.

Tyloses
Each annual ring of the oak tree consists of spring and summer growth with the xylem vessels formed during the spring being larger than those formed during the summer. The pores facilitate the flow of water and nutrients up the tree during the growing season. As the sapwood differentiates into heartwood, these pores become plugged with cellular inner growths called tyloses. "Tyloses development is so extensive in white oak that it effectively renders the wood impervious to the movement of liquids or gases" (Jackson).

Only heartwood is used in the construction of oak staves. So while sapwood is ring porous, allowing liquids to move vertically along its length, tyloses growth cuts off that avenue once the wood transitions to heartwood. Thus, wine in the barrel cannot drain out either the top end or the bottom end of the staves.

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According to Jackson, "the combined effects of rays, tyloses, and the placement of the radial plane of the stave tangential to the circumference severely limit the diffusion of air and wine through the wood. With proper construction and presoaking, oak cooperage is essentially an impervious, airtight container."

Bibliography
Maria del Alamo-Sanza & Ignacio Nevares (2018) Oak wine barrel as an active vessel: A critical review of past and current knowledge, Critical Reviews in Food Science and Nutrition, 58:16, 2711-2726, DOI: 10.1080/10408398.2017.1330250.
Murli Dharmadikari, Wooden Cooperage, extension.iastate.edu
Ronald S. Jackson, Wine Science: Principles and Applications, 3rd Ed.

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