Ed Hellman, Texas AgriLife Extension
Harvesting wine grapes at optimal fruit maturity, or ripeness, presents many challenges, not the least of which is accurate assessment of fruit ripening. Much of the difficulty with discussions of grape ripeness is that there is often an implied standard, but in reality, ripeness is subjective. There are two issues to address: 1) how do we define grape maturity, and 2) how is maturity measured.
What is Grape Maturity?
Numerous winegrape ripeness indices have been investigated (summarized by Bisson, 2001) and a few analytical laboratories are attempting to quantify grape ripeness through complex chemical analyses of flavor and aroma constituents, phenolics, color compounds, sugars, acids, and pH. But there will never be a single set of numbers that defines ripeness for a particular grape variety under all circumstances and for all purposes. Ripeness is defined by the individual and is primarily a function of the intended use for the grapes. Often, an individual’s definition of ripeness is also influenced by what is “typical” for that variety in his or her growing region. Some benchmark of ripeness is achieved in one or more seasons and all subsequent crops are compared to that benchmark.
Winemakers commonly have a target for grape ripeness they would like the fruit to achieve for the wine they plan to produce. That target can vary, even within the same grape variety, depending on the type or style of wine that will be made. For example, one winery may prefer to produce a wine emphasizing red fruit characteristics while another winery would prefer riper black fruit characteristics. Grape ripening is a continuous process and the progression of aroma and flavor characteristics for red grapes is shown in Figure 1. Timing of harvest, therefore, is a matter of determining that point along the ripening continuum that best fits the winemaker’s objective for the wine.
The ability to harvest grapes at the desired fruit ripeness is dependent upon one’s current knowledge of the progression in fruit maturity occurring in the vineyard. Weather conditions will cause seasonal differences in the rate and characteristics of grape ripening. Varieties and even blocks of the same variety are likely to have different patterns of ripening. The only way to know where the fruit is on the ripening continuum is to collect samples of the fruit periodically and assess ripeness. An excellent discussion of how to monitor fruit ripening can be found in the book chapter ‘Monitoring Fruit Maturity’ (Watson, 2003). Much of the forthcoming discussion is adapted from this chapter.
Fruit maturity of grapes is commonly monitored by periodically measuring soluble solids content of ripening berries with a handheld refractometer. But sugar content is not necessarily related to accumulation of flavor and aroma compounds. Tasting fruit for a subjective assessment of flavor development typically augments the quantitative measure of sugar content. Such simple techniques can be very useful indicators of grape maturity, but only if the sample tested is appropriate. Too often however, conclusions about grape ripening status are drawn from very small, nonrandom and unrepresentative fruit samples. The key to a good estimate of fruit maturity is to collect berry samples that are truly representative of the vineyard block to be harvested.
Fruit samples should be taken weekly beginning about three weeks before harvest is anticipated. More frequent sampling should be done as the anticipated harvest date becomes closer, particularly if there are changes in the weather that could affect ripening or condition of the fruit.
Sample Preparation and Analysis (adapted from Watson, 2003)
Accurate assessment of fruit ripeness also depends on proper sample preparation and analytical procedures. Fruit samples should be processed quickly, preferably within a few hours of collection, and processing procedures should simulate winery conditions as closely as possible. The fruit can be crushed and pressed by hand, taking care to crush each berry thoroughly. Large samples are more easily crushed with a small roller-crusher and pressed with a small bench-top press. Crushing should be accomplished without breaking the seeds. The crushed fruit can be hand-squeezed tightly through cheesecloth to obtain both the free run and the pressed juice. Fruit constituents are not evenly distributed in the pulp of the berry so a thorough pressing or squeezing is necessary with all of the juice combined. A common mistake is to use only the free run juice for analysis, which tends to have higher sugar and titratable acidity, lower pH, and lower potassium than fully expressed juice. Juice yields from commercial processing can be approximated by pressing hard enough to obtain approximately 300 ml of juice per pound of fruit. This corresponds to about 160 gallons/ton.
Red winegrape samples are best prepared by crushing, de-stemming, and macerating the skins for 1-2 hours at room temperature before pressing. Ripe red grapes rapidly release the anthocyanin pigments from the skin upon crushing and pressing.
Juice samples should be temporarily stored in sealed, full containers and allowed to settle to remove suspended solids. Refrigeration aids settling and delays enzymatic browning. Browning can be reduced by the addition of 25 mg/liter each of sulfur dioxide and ascorbic acid (vitamin C), which also helps maintain sample freshness for sensory evaluation. Pectolytic enzymes can be added to enhance juice clarity, if necessary. A sensory evaluation of aromas and flavors should also be conducted. Samples can be held refrigerated in full containers for up to 1-2 weeks for comparison with later samples.
Soluble solids are measured as degrees Brix using either a refractometer or a hydrometer. Refractometers should be calibrated following the manufacturer’s instructions. Accurate hydrometers are calibrated to narrow ranges of 5 to 10 degrees and are subdivided to 0.1 degree units. Inexpensive hydrometers typically have a large range such as 0-30 degrees and have other scales such as ‘potential alcohol’. These hydrometers are not very accurate. Both hydrometer and refractometer readings are usually calibrated at 20ºC (68ºF) so if the juice sample is at a different temperature, a correction must be made.
Digital refractometer (L), and a pH meter with calibration solutions. Photos by Lane Greer, Oklahoma State University, and Ed Hellman, Texas AgriLife Extension, respectively.
Laboratory procedures for determining soluble solids, titratable acidity, and juice pH are found in several books (Iland et al., 2000; Ough and Amerine, 1988; Watson, 2003; Zoecklein et al., 1995). The accuracy of a chemical analysis is highly dependent upon following appropriate procedures and maintaining properly calibrated equipment. Common errors with refractometer measurements include failing to calibrate with distilled water and not making the necessary temperature corrections. Titratable acidity measurements can be inaccurate because of careless pipetting of the sample, failure to neutralize the acidity in the water before adding the juice sample, over-titration, and failure to calibrate the pH meter properly. Common errors in pH measurement include failure to standardize the pH meter, disregarding temperature correction, and the use of worn or insensitive electrodes. An article by Weeks (2002) provides excellent advice on pH analysis and troubleshooting.
Sensory evaluation should be conducted on the juice sample collected using the processing procedures described above. Crushing and pressing extracts aroma, flavor, and color from the grape skins. The juice sample should be evaluated for both intensity and quality of aroma and flavor, acidity and taste balance, and color.
References and Resources
Bisson, L. 2001. In search of optimal grape maturity. Practical Winery & Vineyard. July/August. Pp. 32-43.
Iland, P., A. Ewart, J. Sitters, A. Markides, and N. Bruer. 2000. Techniques for Accurate Chemical Analysis and Quality Monitoring During Winemaking. Wine Promotions, Campbell Town, Australia.
Ough C.S. and M.A. Amerine.1988. Methods for analysis of musts and wines. 2nd Edition. John Wiley & Sons.
Watson, B. 2003. Evaluation of Winegrape Maturity. In: E.W. Hellman (ed.) Oregon Viticulture. Oregon State University Press. Corvallis, Oregon. Pp. 235-245.
Weeks, S. 2002. Measuring, maintaining, and trouble-shooting pH. Practical Winery & Vineyard. September/October. Pp. 74-77.
Zoecklein, B.W., K.C. Fugelsang, B.H. Gump, and F.S. Nury. 1995. Wine Analysis and Production. Chapman & Hall Enology Library.
Reviewed by Jodi Creasap Gee, Cornell University and William McGlynn, Oklahoma State University