Sunday, July 8, 2012

Successful Fermentation - Red Varietals (Part II)

Read previous posts on Successful Fermentation - Introduction, Selecting the Right Yeast (Part I, Part II), Preparing Juice/Must for Inoculation, Indigenous Yeast, Inoculation, Yeast Propagation (Part I, Part II, Part III), Fermentation Monitoring, Red Varietals (Part I).

As discussed previously, winemakers take a different approach when fermenting red varietals. Though these practices are not strictly limited to red varietals due to increasing experimentation, here are a few things to think about during red varietal winemaking.

Cold Soaking - The practice of cold soaking grapes has become more prevalent in the modern wine industry, primarily deemed useful for light red varietals such as Pinot Noir, St. Laurent, and Monestrall. Cold soaking, or cold maceration, is the practice of allowing grapes after processing to sit for a period of time prior to fermentation (usually just a few days, but some winemakers allow up to 2 weeks). The aim of cold soaking is to allow extraction of water-soluble color and flavor compounds without the extraction of harsher phenolic compounds (tannins) that are more soluble with the presence of alcohol. Additives to increase color extraction and stability are commonly completed (discussed below). Cold soaking proponents contend finished wine will have increased color, fruitiness, flavor intensity, and mouthfeel with lower astringency and bitterness. Cold soaking is also desirable because it promotes mixing, allowing more accurate chemical analysis (i.e. accurate brix and acid levels). The main requirements of cold soaking are:
  • Hold grape temperature cold; ideally below 10° C (50° F).
  • Ensure minimal oxygen level to limit microbial activity and oxidation; this is done by using inert gas coverage, usually CO2.
  • Keep sufficient SO2 to limit microbial activity and oxidation (disscussed below).
  • Conduct cap management on a daily basis; 1-2 times daily is sufficient.

Cap Management - During fermentation in the presence of skins, CO2 released pushes skins to the surface forming a 'fermentation cap'. Without cap management, both CO2 and heat will be trapped in the fermentation vessel, oxygen will not be able to enter the fermenting must, and extraction of skin components will be minimal. The cap will also dry out, promoting negative microbial activity. Different cap management regimes have a different effect on extraction of skin and seed components, and are often used in conjunction with one another:
  • Punchdown - Depending on the fermentation vessel, a handheld or piston-like device is used to push the cap down into the center of the tank. Punchdowns are usually used when higher extraction is desired, and completed 2-4 times daily during active fermentation.
  • Pumpover - Juice from the bottom of the tank is pumped over the top of the fermenting vessel, wetting the cap but not fully submerging it. Pumpovers can be completed in a gentle or more rigorous fashion depending on the desired rate of extraction, and can also be used to oxygenate the must if desired. These are usually completed 2-3 times daily during active fermentation.
  • Rack and Return - Juice is drained from the bottom of the fermentation vessel, leaving behind only the grape solids (primarily skins). It is then pumped back over the top of the tank, either immediately or several hours later. This is believed by many to be the gentlest way for mixing an entire tank, and is also used for seed removal (seeds, particularly less-than-ripe seeds, are believed to have more astringent/bitter tannins and many wineries try to remove them once fermentation is underway). Rack and returns are not usually completed everyday during active fermentation, used in conjunction with a punchdown or pumpover regime.
  • Rotary Tank - Rotary fermentation tanks can be rotated on their axis, mixing the tank thoroughly and promoting a more uniform temperature throughout the must and faster extraction. Often, they are only used for the first half of fermentation before juice is drained/pressed off and allowed to complete the remainder in tank or barrel.

Additives - Winemaking additives are in no way exclusive to red varietals or considered necessary during winemaking activities. Though often bolstered by winemakers, all the additives discussed below are naturally occurring in wine at some levels (excluding non-ellagic tannins). Besides additions of acid (discussed in later post) and fermentation nutrients, tannins and enzymes are becoming increasingly common:
  • Sulfur dioxide (SO2) - Considered by many as essential to the production of quality, age-worthy wines in today's industry, SO2 is usually added to red varietals at the destemmer/crusher stage when fermentation will not be immediately induced (i.e. cold soaking desired). This protects the must from microbial activity and oxidation; common additions prior to cold soaking are at 30-50 ppm.
  • Enzymes - Enzymes are now commonly used during red varietal winemaking to increase extraction of stable phenolic compounds and help stabilize color. There are numerous proprietary enzymes available on the market today that are designed for creating different wine styles, from cellar-worthy to early-release wines. These pectolytic enzymes have higher levels of hemicellulase side activity (increased breakdown of cell walls, thus increasing extraction), while limiting anthocyanase activity (releases sugar-bound anthocyanins and thus promotes color loss) and cinnamyl esterase activity (leads to formation of vinyl phenols, which have positive flavor and color stability properties in low concentrations, but negative flavor effects in higher concentrations). Enzymes should be added prior to fermentation (usually adding at the beginning of cold soaking), while ensuring SO2 levels are not too high to inhibit their activity.
  • Tannins - There are several different types of tannins naturally occurring in grape stems, skins, and seeds; these are classed as polyphenolic compounds and attribute to a wine's astringency, mouthfeel, aroma, and color stability. A broad range of proprietary tannin products are available to winemakers today,  sourced not only from grapes but oak wood, nuts, and other fruits; they are used for varying reasons, including increasing color stability, increasing phenolic structure, inhibiting specific microbials, and decreasing oxidation; most are designed for addition prior to or during fermentation.

6 comments:

  1. I was always confused by the definition of cold saking or cold maceration (even the use of the two names rises confusion). This is just a part of what I found:During cold-soak treatment the must is held at low temperatures, usually 10 to 15°C, for several days before fermentation (Sacchi et al., 2005) but Heredia et al. (2010) report that “ This technique
    consists in maintaining the crushed grapes at low temperatures (5–10 °C) for a variable period from one to several weeks”. Notwithstanding Sacchi et al (2005) concluded “While cold soak has been reported to have little effect on the phenolic composition of the resulting wines,…” this technique is still used for red grape must without any scientific sense to improve extraction of anthocyanins or other phenolic substances while, in contrast, the use of must freezing as Sacchi et al. (2005) said “….freezing the must before fermentation potentially has a much greater effect.”. I was wrtng a manuscript on the use of this technique and the confusion about it. Who is interested to the Introduction, I can send. The problem for the extraction should be first of all the cell maturity, which means knowing perfectly the stage of harvest when the cell wall is ready to release compounds of enological interest. It is not easy to catch but it is more important of phenolic ripening. If we get this index we can save energy in winery.
    Fabio

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  2. So, you recommend punching down the cap during cold soaking, while trying to minimize exposure to O2?

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  3. Thanks Fabio, I would be interested in reading your article if you want to e-mail it through to me. Like plenty of practices in the industry, there isn't scientific proof to the effectiveness of cold soaking and thus a lot of different beliefs; I have seen very variable results from it personally, and was trying to use phrases like 'cold soaking proponents contend..." in an effort to leave it open.

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  4. Bob, I am recommending exactly that. Once-a-day cap management during cold soaking will help keep microbial activity on the surface down, and (theoretically at least) encourage more extraction. For me, a major reason cold soaking is desirable is the mixing of must for more accurate analysis.

    If one is using invert gas coverage (i.e. CO2 from dry ice), then there should be little oxygen added when completing light punchdown or pumpover of must. Of course, I said minimal oxygen because it's pretty much impossible to keep it all out, but trying your best I guess is the suggestion.

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  5. For cold soaking, it depends somewhat on fruit quality, winery sanitation, etc. I usually recommend 30-50 ppm SO2. At this level, the must will be protected but it will diminish sufficiently prior to inoculation.

    Copied from above:

    Sulfur dioxide (SO2) - Considered by many as essential to the production of quality, age-worthy wines in today's industry, SO2 is usually added to red varietals at the destemmer/crusher stage when fermentation will not be immediately induced (i.e. cold soaking desired). This protects the must from microbial activity and oxidation; common additions prior to cold soaking are at 30-50 ppm.

    ReplyDelete