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w i n e M A K IN G OAK INSERTS The effect of grain orientation on extraction of oak flavor compounds BY Kevin Crawford, Almon Benton, Alyssa Dreger, Department of Chemistry, University of WisconsinOshkosh; David Plumb, Alfred Grigg, Fine Northern Oak, Minneapolis, MN; Corresponding author email: crawfork@ uwosh.edu P roperly aged and toasted oak, sourced from the proper species and growing climate, imparts desirable flavor characteristics to wine when used in the traditional fire-toasted barrels. The chemical composition of oak is variable depending on species as well as aging and toasting conditions.1 In modern winemaking, barrel alternatives provide a form that can be toasted in convection ovens, a reproducible method for imparting oak flavor compounds to a wine compared to expensive and relatively small-capacity barrels. Barrel alternatives also provide an opportunity for higher surface area exposure to the wine, resulting in more rapid extraction of oak flavor. Extraction of flavor compounds from oak into wine is a time-dependent process. In a traditional 60-gallon barrel, the wine is exposed only to the parallel (vertical) grain of the oak (one side of the stave exposed to wine). All flavor extraction must occur from this surface. A wine might spend several months in barrel, and the barrel will still retain enough flavor compounds to be extracted during/ from additional fills. In contrast, barrel alternatives are submerged in the wine, providing exposure on both the parallel and cross-cut axes on all sides of the oak. Not only is more surface area exposed to the wine, but more importantly, two of these surfaces will consist of the cross-cut axis of the wood grain. When barrel alternatives are used, the entire piece of wood is exposed to the This text edited and expanded by the authors from first publication: November/December 2010. 20 p racti c al w i ne ry & v i ne yard JANUARY 20 14 wine, allowing penetration and extraction along the cross-cut and parallel axes. In order to determine the relative extraction rates of oak flavor compounds based on grain orientation, we prepared blocks of wood with epoxy coating, preventing wine exposure on either the cross-cut or parallel axis. These blocks were soaked in a model wine solution that was sampled weekly for SPME-GCMS quantification of furfural and vanillin Our goal was to determine the relative contribution of cross-cut grain exposure to the rate of extraction of furfural and vanillin from toasted oak. (first experiment) or more frequently sampled and analyzed by HPLC for a wider variety of molecules (second experiment). Our results indicate that furfural is rapidly extracted from the cross-cut axis relative to the parallel axis and that vanillin extraction was only measurable in the cross-cut axis exposed wood during the 12-week exposure (first experiment). Our second experiment demonstrates that cross-cut grain exposure in oak impacts both rate of extraction and total extractable amount of oak flavor molecules. As expected, access to the natural pores of the wood increases the rate of extraction of all molecules studied. More significantly, molecules with limited water solubility and larger size were extracted in higher amounts from crosscut oak. These results demonstrate that maximum exposure of the cross-cut axis of oak pieces will result in a much more rapid extraction of flavor compounds into wine. Solid Phase MicroExtraction–Gas Chromat ography–Mass Spectrometry (SPME-GC-MS) is a preferred method for Nylon mesh bags containing 36 Fine Northern Oak cross-cut staves hang from a chain around the circumference inside a wine tank to allow oak flavor extraction into red wine. quantifying desirable and undesirable flavor and odor compounds in wine.2,3,4 This method extracts and concentrates the flavor compounds (SPME) followed by separation (GC) and identification/ quantification of the compounds (MS). It also involves much less manual sample preparation than the traditional liquidliquid extraction and concentration techniques. High-Performance Liquid Chroma tography (HPLC) is an alternative to gas chromatography that is suitable for molecules that may break down in a gas chromatography (high-temperature) system. This includes polar molecules such as gallic and vanillic acids. The method relies on a liquid phase to conduct the separation with detection of the molecules based on their ability to absorb ultraviolet light. Minimal sample preparation is required, but it cannot detect molecules at the same low levels as SPME-GC-MS. Vanillin and furfural are two desirable flavor compounds present in toasted oak (see structures in Figure 1a on page 22.) Furfural is the smaller of the two compounds and is more volatile than most other flavor compounds present in oak. It also has a low octanol-water partition coefficient (log P) indicating high water solubility. Furfural and similar compounds are responsible for a sweet and butterscotch flavor in wine. Vanillin has a higher molecular weight and is less volatile than furfural and many of the other flavor compounds extracted from oak. It also has a much larger log P value, indicating less water solubility.