To: Regional Wine Producers
From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech
Subject: Aroma/Flavor and the 2002 Season, continued; Budget Reduction; Filtration: Absolute.
Aroma/Flavor and the 2002 Season, continued. As discussed in the previous edition, temperature is a primary influence of macro-, meso- and micro-climate. Vineyards at 35 degrees latitude in mid-summer can show a significant difference (up to 17%) in sunlight interception between north-south vs. east-west rows. Difference in sunlight interception can occur also based on vine side.
We have been evaluating the influence of vineyard management on glycosides, including anthocyanins. Anthocyanins are present in the fruit and subsequent wine as either monomers (unbound), small polymeric phenols (SPP) (bound with other phenols up to 5 sub-units) or larger polymeric phenols (LPP) (bound to more than 5 sub-units).
The degree of polymerization influence astringency, spectral color (co-pigmentation) and color stability of red wines.
In one study we evaluated an impact of canopy side on Cabernet Sauvignon pigments.
With a N-S row, fruit from the east side of the canopy contained 290 µg/berry of SPP's, while fruit from the west side (at the same degrees Brix) contained 220 µg/berry.
This same fruit had a higher concentration of LPP's on the east side (135 µg/berry) vs. the west side (115 µg/berry).
Total (spectral) color was 13.9% higher in fruit from the east vs. west side.
Total tannins showed the opposite trend: a higher concentration in west vs. east-side fruit. Some of these tannins are quercetin-type glycosides which are, in part, responsible for dry, dusty-like tannins in wines.
The side of the grapevine canopy can have a dramatic influence on fruit chemistry and resulting wine chemistry and quality.
With your site, stylistic and quality goals, should you be selectively harvesting based on canopy side?
Some of these issues will be discussed at the Annual meeting of the Virginia Vineyards Association to be held February 13 - 15, 2003 in Charlottesville, VA. More details to follow.
Budget Reduction . The College of Agriculture and Life Sciences at Virginia Tech has experienced a 20% reduction in operating funds.
Even though the number of wineries in Virginia has increased drastically, our budget for extension activities has not, and will be reduced further.
I will communicate regarding the direct impact on enology extension as soon as the effect of budget cuts becomes clear.
Filtration. This begins a series on the principles and practice of filtration.
In any solution, the particles in suspension have a number of properties that affect the way in which they are retained by a filter. The smaller the particle, the more difficult it is to remove it from solution. One important characteristic of suspended particles is their size distribution.
Although the range of particle size is large, a substantial number of particles are well below 1 micrometer in size. The average diameter of yeast (Saccharomyces) is about 1.2 micrometers. The particle size distribution in juice or wine is nonsymmetrical in nature. The preponderance of particle sizes is clustered rather closely toward the smallest particles in the distribution, with the population of larger particles being minimal.
Another important characteristic of suspended material in solution is their mechanical nature. Particles can be classified as either nondeformable or deformable. Nondeformable particles are those which retain their shape. In wine, the principle nondeformable particles are diatoms. Diatoms often added uniformly before and/or during filtration to increase the filtration surface area. Because of their rigid nature and geometry, they themselves act as a filtration medium. Using various grades of diatomaceous earth is a means of controlling the size and amount of particles which the filter will retain. (This will be further elaborated upon in a discussion of precoating and body feeding).
The other type of particles in juice or wine are the gelatinous or deformable materials. These include yeast, bacterial cells and many colloids such as fining agents, including gelatin, bentonite, etc. Such materials offer more problems in filtration than do rigid materials. Because of their elastic nature, they are capable of spreading over a larger surface area. Hence they are often active in blocking filtration, due to their spreading or matting effect.
A third property of suspended particles is their tendency to agglomerate or flocculate. Many suspended particles will adhere if they come in contact with a similar particle. The result is a single larger particle, where, formerly, there were two. If it has enough mechanical stability, the larger particle will possess many of the properties of a single particle of that size. This tendency can be put to practical use in that larger particles may precipitate naturally or by the addition of fining agents. It is easier to remove larger particles by filtration than smaller particles. Thus prefiltration fining is often a desirable means of increasing filterability.
Absolute Filtration. Among the more useful ways of classifying filter media are the concepts of absolute and depth. These two types of filters differ fundamentally in their mechanisms of retention.
- Absolute type filters. An absolute filter is a geometrically regular, porous matrix that retains particles on its surface primarily by a sieving mechanism. The pore size is controlled in the manufacturing process. Filtration through such a filter is inherently absolute in that anything larger than the pore size is retained on the filter surface. These are the 'membrane' type filters used in the wine industry as a final filtration just prior to bottling for the removal of wine microorganisms. The advantages and disadvantages of such a filter are summarized as follows:
- Using absolute filtration, it is possible to derive a specific rating of membrane efficiency independent of flow rate and pressure differential. Therefore a winemaker can be assured that no micro-organisms larger than the pore diameter will travel through the filter if the filter is properly functioning. (Integrity testing will be outlined in a subsequent issue.) Most winemakers attempting to remove yeast use a membrane filter with a 0.6 micrometer pore diameter, while lactic acid bacteria are generally retained by a 0.45 micrometer membrane filter.
- Owing to the homogeneous nature of the membrane, no media migration or sloughing occurs. Thus no particles larger than the membrane pore diameter or membrane material itself will travel down stream.
- Since membranes are very thin (membrane thickness = 150 micrometers), there is no possibility of microbial growth within the inner layers. Coupled with this property, there is reduced product loss.
- Successive layers of larger particles may act to prevent the passage of particles smaller than pore diameter.
The following points are disadvantages of the absolute type filter:
- Because of surface retention, membrane filters have a low "dirt-handling capacity". This is especially true of particles with diameters approximately equal to those of membrane pores. Therefore only 'clean' wine should be filtered through these units for the purpose of removing microorganisms.
Not all small particles (with diameters less than pore size) will pass readily through. Many may be retained in the port passage, hence blocking the flow.
Depth Filtration. In depth filtration, the separation of solids from the liquid phase takes place inside the filtration medium only. The filtration medium consists of numerous tortuous channels of all diameters and configurations. All the channels vary in diameter from the upstream to the downstream side. The particles float at random through the channels and, at some point, impact on the walls of the channel and are retained by entrapment or adsorption.
As the particles are deposited in the depth filter, its retention capacity increases. This increases the flow resistance and the differential pressure. Eventually, this results in complete blocking.
- Media migration can occur. This refers to the tendency of filter media (fragments) to slough off during filtration. This problem is increased in cases where the wine to be filtered encounters the filter as a surge, rather than at a uniform flow.
- Microbial growth within the filter matrix may become a problem, especially in long filter runs. Under proper conditions, organisms may reproduce within the filter and successive generations will penetrate deeper into the matrix. The result is filtrate contamination.
- A certain amount of product may remain within the filter matrix after filtration. In the wine industry the filter is usually 'blown out' with nitrogen and the wine transferred back to the feed tank.
Since the depth filter can retain particles throughout its matrix, rather than solely on its surface, it will filter many times the material that the absolute type filters can process. Further, owing to its principle of adsorption, this filter will retain particles smaller than its flow passages.
Because of the nature of depth filtration, an absolute particle retention rating is difficult. These filters are assigned a "normal rating". This is usually a particle size, above which a certain percentage (usually 98%) of particulates will be retained. This is determined experimentally after the filter is produced. It is important to note that this rating is valid only under strictly defined conditions of flow, temperature, pressure, and viscosity. Change in any parameter will effect particle retention. The so-called sterilizing pads are depth filters especially made to have a uniform porosity. These pads, however, can only remove yeast under a specific set of conditions, such as flow rate and differential pressure across the pad.
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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293