Enology Notes #45, May 9, 2002
To: Regional Vintners
From: Bruce Zoecklein
Subject: Gelatin Fining, Virginia Wine Guide, Student Award Winner
Gelatin is prepared from collagen, the major structural protein in skin and bones. Hydrolysis from the multistranded polypeptide in solutions of acid and base causes strand separation, producing gelatin. The peptide chain ranges in size from 15,000 to over 140,000 daltons, and contains high levels of the amino acids glycine, proline, and hydroxyproline compared with most proteins.
The isoelectric point of gelatin is pH 4.7. Therefore, it occurs in juice or wine as a positively charged entity, capable of reaction with negatively charged species, such as tannins, via hydrogen bond formation. It finds principal application in clarification, as well as in modification of astringency. It is also employed to reduce harshness (astringency) and improve clarity in juice before fermentation.
Gelatin preferentially binds with larger molecules having more phenolic groups and potentially more hydrogen bonding sites. Thus gelatin has less dramatic effect on color and tannin reduction in younger wines than in older products; since the latter generally have a greater percentage of larger polymeric phenolics. Gelatin additions may result in color shifts in red wines from tawny (brown) to a more ruby red, perceived visually as a shift in hue (the ratio of absorbance at 420 and 530 nm). Gelatin helps reduce the phenol level and brown color in press juice before fermentation. Such applications usually occur in conjunction with silica dioxide fining.
Gelatin may strip wine of its character; therefore laboratory trials should precede any cellar additions. Only good quality gelatin, free from undesirable flavors and odors, should be used. Commercial gelatin is available in several forms and grades, and is usually rated according to purity as well as "bloom." Bloom refers to gelatins' ability to absorb water, usually 6 to 10 times its weight. Thus the higher the bloom rating, the greater is its absorbing capability. Gelatin recommended for wine treatment ranges from 80 to 150 bloom. Using a higher bloom gelatin in conjunction with silica dioxide may result in unreacted gelatin being left in solution.
In that the number of potential bonding site determines its effectiveness, the size of the gelatin molecule is also an important consideration. Lower molecular weight gelatin reduces the rate of precipitation but enhances clarification and lees compaction.
The quantities of gelatin needed to achieve clarification may reduce wine astringency to undesirably low levels. Most white wines have such a low perceptible phenolic content that an exogenous source of tannic acid or silica gel is needed for reaction with the excess gelatin. Tannic acid is often added 24 hours before the gelatin fining. The ratio of tannin to gelatin is usually 1:1 (wt/wt), but will vary depending on the individual wine.
Most winemakers prefer to counterfine with silica dioxide rather than tannin. The replacement of tannin with silica dioxide moderates the activity of gelatin on wine flavor. Gelatin is occasionally used as a counterfining agent after bentonite additions to remove residual haze. In this case, the negatively charged planar surface of the bentonite platelets react with positively charged gelatin and the two precipitate from solution. This technique may additionally help in compaction of troublesome bentonite lees. Overfining with gelatin, as with most protein fining agents, may render the wine unstable with respect to heat labile proteins, as well as potential biological activity. Additionally, where copper levels exceed "safe limits", residual gelatin may increase the possibility of casse formation.
Gelatin is commonly available as a powder but also may be purchased as a liquid concentrate at 30 to 46% or in sheets. Liquid gelatins are produced by hydrolysis which lowers the molecular weight and prevents gelling at high concentration. Liquid gelatin concentrations are typically stabilized with benzoates and/or sulfur dioxide.
Before use, dry gelatin must be hydrated in warm water (44ºC/ 112ºF), at 60 g/L (1 lb gelatin/2 gal water). Prolonged or excessive heating may result in denaturation of the protein and reduce activity. Due to potential biological deterioration, gelatin solutions should not be stored over extended periods.
Addition levels generally range upward from 0.75 g/hL (1/16 lb/1,000 gal). In red wines, levels often range from 4.8 to 10 g/hL (0.4-0.8 lb/1,000 gal). Much larger doses are used in juice fining, especially press juice. Heavy press juice may require up to 48 g/hL (4 lb/1,000 gal) to reduce astringency and oxidized color. Counterfining with silica dioxide or bentonite is recommended.
The Virginia Wine Guide is now available on-line at www.virginiawineguide.com. This publication evaluates VA wine, discusses vintages, food and wine pairing, etc. Check it out; it is well done.
The Italian Food Packaging Association recently selected Bridget M. Archibald to receive an award. Bridget is a masters candidate in the Enology-Grape Chemistry Group at Virginia Tech.
Fourteen U.S. colleges and universities participated in the competition. Students were required to write a paper on Advances in Food Packaging. One winning paper from each school was then sent to the Italian Food Packaging Association for final selection. Six winners were chosen nationwide. The award provides a two-week, all expenses paid trip to Italy to tour packaging facilities. Archibald’s paper, Advances in Wine Packaging, was one of the six chosen. The paper focused on the advantages and disadvantages of natural cork, synthetic cork, plastic and rubber stoppers, and screw-caps used to close wine bottles around the world.
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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu