Vol. 15, No. 2 March - April, 2000
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061, E-mail: firstname.lastname@example.org
Table of Contents
I. Establishing a HACCP Plan (continued) 1
II. Research Review
Aroma/Flavor Trapping 3
Thermal Vinification 3
Nitrogen Status of Grape Juice 4
I. Establishing a HACCP Plan (continued)
The principles and importance of a HACCP (Hazard Analysis and Critical Control Point) plan were reviewed in the previous edition. A HACCP plan should involve the features listed below and the creation of a flow diagram as demonstrated in Figure 1.
VINEYARD SAMPLING Juice aroma assessment, skin tannins, seed tannins,
| color, fruit condition,Brix, rot, rot type, sugar per berry,
| pH, TA, temperature, others
CRUSHING Degree of crushing, seed breakage,
| skin contact time and temp., phenol extraction
MUST Sugar, pH, non-soluble solids, assimilable nitrogen,
| TA, SO2, temp., oxygen
FERMENTATION Temperature, rate
| Yeast (strain, innoc vol., % budding, % viability, purity)
| MLF (Strain, innoc. vol, purity)
| Wood, wood type, seasoning, age
/ \ pH, sensory evaluation, tannin and color evaluation
FREE RUN PRESSINGS Sensory, others
FERMENTATION Reducing sugar, pH,
| TA, SO2, VA, alcohol, MLF,
| sensory evaluation, protein stability, bitartrate stability
CLARIFICATION Clarity, fining trials, sensory evaluation, oxygen
MATURATION /AGING Sensory evaluation, oxygen, SO2
| Wood, wood type, wood age, seasoning, fill period
| Physical stability (protein, bitratrate oxidative), micro.
| stability, clarity/filterability, sensory evaluation, bottling line
| sanitation, CO2 , level, fill level
BOTTLING SO2, oxygen, wine temperature, materials QC
AGING Sensory evaluation, SO2, optimum wine release date
Wine storage conditions (temp., relative humidity)
II. Research Review
Much of the research of the Enology - Grape Chemistry Laboratory has involved the evaluation of grape glycosides, in part as aroma/flavor precursors. We have observed that as fruit sugar increases there are increases in the pool of grape glycosides. This increase does not occur with a parallel increase in Brix because the two operate by different mechanisms. Aroma/flavor compounds are synthesized from surplus sugars in excess of those required for respiration and tissue growth. Therefore, changes in Brix and aroma/flavor would not necessarily be parallel. As reported previously, there is a significant increase in the pool of glycosides which occurs with little change in Brix. This upswing in aroma/flavor at the later stages of fruit maturation (termed engustment) occurs with all varieties but is acutely evident in varieties like Viognier. For this reason, Brix should not be used as the primary harvest gauge for stylistic wine production.
This research has helped to answer an important question: Does crop load influence grape composition simply as a function of delayed maturity or is there a more direct physiological influence? We have found that grape vines over cropped beyond a certain fruit to leaf area ratio do not produce the same quantity or quality of grape volatiles regardless of how long the fruit is on the vine.
Thermal vinification and flavor trapping are two research areas designed to take advantage of vineyard management practices which increase grape glycosides. These include methods of maximizing aroma/flavor production from the products of glycoside breakdown, or hydrolysis, and retention of aroma/flavor compounds typically lost during fermentation.
Aroma/Flavor Trapping. Wine aroma compounds are fairly volatile with little solubility in must or wine. Consequently, these compounds will partially transfer into the CO2 during fermentation and, as a result, will be swept away or entrained with the carbon dioxide. The extent of this transfer of the aroma/flavor compounds into the CO2 is temperature dependent. Greater loss occurs at higher temperatures. Essentially, the greatest losses are with the most volatile and sensorily important compounds (the esters) while most of the long chained alcohols are retained.
We have been evaluating a simple system for selective capture and return. Those that attended our preharvest workshop titled Practical Issues in Primary and Secondary Fermentation had an opportunity to review several wines produced by selective retention. Sensory analysis has demonstrated a strong preference for wines produced with selective retention. While there can be several reasons for the preferences, these wines had a lower concentration of higher alcohols and a higher concentration of esters. Wines produced in 1999 by the new technology are currently under review.
Practical benefit of flavor trapping based on our preliminary research:
Thermal Vinification. We have also been investigating the effects of thermal vinification, or the use of heat, to enhance wine quality. Heat induces the release of aroma, flavor, and color compounds possibly as a result of glycoside hydrolysis.
This research involves the evaluation of hydrolysis products during post-fermentation thermal vinification of Cabernet Sauvignon wines. In addition, concentrations of color, total phenolics, total anthocyanins, total hydroxycinnamates, and aroma volatiles have been quantified.
Practical benefits of thermal vinification based on our preliminary research:
Microoxygenation. This research is being conducted at Horton Cellars. Traditional aging of red table wines utilizes storage in wood containers (generally oak) of varying capacities for periods of time ranging from several weeks to a year or more. Oak aging achieves two important goals. First, interaction of wine and wood extracts unique flavor and odor-active components that are generally perceived by consumers as beneficial. The second component of wood aging is slow and selective oxidation. Incursion of oxygen through the staves and slow interaction with the phenolic components of wine bring about the process referred to as aging. During aging, the monomeric phenolic and anthocyanin components interact to produce dimers and larger species. This decreases the bitterness and astringency that characterize young red wines and creates soft tannin structure associated with aged wines. Oak aging also causes the wine's color to shift to brick-red.
Although the process of barrel aging has traditionally been the method-of-choice for winemakers, several significant issues must be addressed. Initially the winemaker is faced with the daunting issue of cost. American oak barrels range from $250-300 each while French cooperage may cost well over $650 depending upon monetary exchange. Secondly, the process of barrel aging is slow. Depending upon wine variety and style, size of the barrel, and temperature, the time frame may easily be a year or more. These two issues alone make barrel utilization, in some instances, impossible. A third important consideration is the useful life of cooperage. Even under ideal conditions, the potential for flavor and odor extraction from barrels is largely exhausted by the fourth season. Subsequently, they may be reconditioned, used solely as storage containers or sold to home vintners or others. This best case scenario is dramatically altered if microbiological spoilage has occurred at some stage.
Alternatives to the use of oak barrels exist. Oak cube addition during primary fermentation or to wines aging in stainless steel has been effectively used for flavor/odor extraction. Success depends upon timing of cube addition as well as the source and processing history of the cubes. Other alternatives include utilization of oak planks or strips in stainless steel tanks or barrels. Still other producers utilize oak flavor extractives as post-fermentation blending tools. However, in each case where adjuncts or alternatives are used in a stainless steel container, the process accomplishes only flavor extraction. There is little opportunity for oxidative aging to occur.
Winemakers recognize the beneficial aspects of limited oxygen exposure of young red wines immediately after fermentation and during the early stages of aging. These issues have been extensively covered in previous editions. The process of micro-oxygenation is an extension of this observation and of current understanding of the process of phenolic polymerization as it relates to aging .
Micro-oxygenation is a process whereby young red wines are continuously exposed to oxygen. Oxygen is supplied in the form of the compressed gas via a micron-size diffuser positioned close to the bottom of a stainless steel tank. Oxygen catalyzes oxidation of ethanol to acetaldehyde which then serves as a bridging agent, linking together phenolics/phenolic-anthocyanins via C4-C8 reactive sites. This is the same process that would occur during barrel aging except that the time frame is significantly shortened.
Aims and benefits of micro-oxygenation:
Our thanks to Dennis Horton (Horton Cellars) for helping to sponsor this research.
Nitrogen Status of Grape Juice. The study, done in conjunction with colleagues at Fresno State and Cal Poly, relates grape juice nutritional (nitrogen) status, as determined by several analytical methodologies, to vineyard practices and wine quality. This research continues efforts to develop and optimize the Formol method for determining assimilable nitrogen levels and to determine grape nitrogen levels related to vineyard location and management practices.
Poor nutrition can have dramatic sensory effects on fermented wines including:
A low concentration of assimable nitrogen, therefore, can cause fermentation sticking or, more commonly, a reduction of the floral intensity of wines. Excessive amounts of nitrogen compounds in juice and wines can impact wine by the formation of ethyl carbamate. Ethyl carbamate, or urethane, is a carcinogen that occurs naturally in fermented foods, including wine, as a result of the fermentative and assimilative activities of microorganisms. Toxicity studies will be completed by the U.S. Food and Drug Administration by year 2000 prior to recommending maximum acceptable levels (Gahagan, 1999 personal communication). At present, the U.S. wine industry has established a voluntary target level of <15 ug/L (ppb) for table wines and <60 ug/L for dessert wines. Because of the possible production of ethyl carbamate, it is essential that the industry understand optimum levels of fermentable nitrogen required for successful fermentation.
Nitrogen compounds in grapes play important roles as nutrients for microorganisms involved in the winemaking process. All of the 20 commonly occurring amino acids are found in grapes and wine. Of these, only the free alpha-amino acid (FAN) fraction is directly assimilable by yeasts. This fraction includes arginine, serine, threonine, alpha-amino butyric, aspartic and glutamic acids. Collectively, this group comprises 35-40% of the total N and 75-85% of the total amino acids. Arginine is typically present at levels ranging from 5-10 times that of the other amino acids and represents 30-50% of the total nitrogen utilization.
Vineyard management has a direct influence on FAN and fermentation problems are often vineyard-specific. Nitrogen deficiency in apparently healthy grapes can be severe and has been demonstrated in Virginia. Drought, grapevine nutrient deficiencies, high incidences of fungal degradation and level of fruit maturity all influence must nitrogen.
Cultivar, rootstock, crop load and growing season may also influence must nitrogen. Some varieties, such as Chardonnay, have a greater tendency towards deficiency. Higher total nitrogen may also be associated with certain rootstocks. For example, grapes grown on St. George are higher in total nitrogen than those on AXR1. Therefore, it is important to look at vineyard management at various sites and evaluate methodology of nitrogen availability prediction.
The Formol titration is a simple and rapid method for determination of the quantity of assimilable nitrogen in juice. Overall, the Formol method can provide a very useful index of the nutritional status of a juice or must. As has been discussed in the column and at two short course meetings, the simplicity of this procedure and its general ability to correctly describe the amount of assimilable nitrogen make it ideal for use in a winery production laboratory. Work by our group in conjunction with California State University, Fresno suggests the analysis is effected by the ratio of proline to arginine - two abundant amino acids.
Vineyard management practices influence the proline/arginine ratio and therefore need to be evaluated. Samples collected under specific growing conditions from various vineyards and regions of the State and country will help provide this information.