Winegrape growers and winemakers have long sought a method for selecting the optimum time to harvest winegrapes.  Measurements of sugars (Brix, or soluble solids), pH, acidity, anthocyanins, berry weight, glutathione, arginine, protein levels, and terpenes, among others, were made, but they did not correlate very well with optimum wine quality.  Some researchers looked for combinations of factors, and found that ºBrix x pH, ºBrix x pH², and especially ºBrix/acidity were much more useful in making harvest decisions than the individual factors alone.^1-6 These factors continue to be recommended by major reference books.^7-11 When using the ºBrix/acidity ratio, values in the range of 30 – 32 have proven optimal, although best time to harvest varies slightly between varieties and between general climate.

Some additional techniques have been developed, such as gas chromatographic analysis of aroma components, spectrophotometric measurement of anthocyanin levels, and near infrared analysis of specific phenols.  For now, however, these techniques are at best research oriented, off line methods.

There is one method that has recently gained favor among some winemakers as a means of judging time to harvest:  sensory analysis of grapes, otherwise known as tasting.  A September 5, 2006, Wine Industry Symposium on this technique was recently described in “Wine Business Monthly.”¹² There are two schools of thought on this technique.  First, at an earlier conference discussing the technique of using taste to judge optimum time to harvest, Dr, Richard Smart stated "Winemakers go into the vineyards and they chew on some grapes, and they look at the seeds and the stems, and they mumble a few things, and then they proclaim the harvest date. And curiously, that date is always in the future, never in the past. I never heard a winemaker taste the grapes and say, 'Darn, we got here too late’."¹³ Also, in a recent study expert wine tasters showed considerable variation in their assessments of wine quality.¹⁴ 

What do I suggest?  Perhaps we should follow the advice of the late renowned French enology researcher Emile Peynaud:¹⁵  “Attempts have been made to express the state of maturity of the grape using certain ratios between different ingredients.  The sugar/acidity ratio is the simplest and most meaningful.  Since sugar levels rise while acidity diminishes during ripening, the sugar/acidity ratio rises sharply and develops more quickly than its two factors.  Although the accumulation of sugars and the combustion of acids not being subject to the same factors, take place independently in the grape, this index is a fairly accurate expression of the state of ripeness.  Its value depends on the variety.”  And if there has been a change in weather conditions such as the very cool spring this year in Oregon, perhaps doing a little extra canopy manipulation (e.g., leaf removal) will produce much better grapes than simply letting them hang for an extended period of time.

What do you think?

1.   C. S. Ough, V. L. Singleton, “Wine quality prediction from juice Brix/acid ratio,” Am. J. Enol. Vitic., 19 (3): 129 - 138 1968.

2.   C. S. Ough, C. J. Alley, “Effect of Thompson seedless grape maturity on wine composition and quality,” Am. J. Enol. Vitic., 21 (2): 78 - 84 1970.

3.   J. Ribereau-Gayon, E. Peynaud, P. Ribereau-Gayon, P. Sudraud, Traite d’oenologie sciences et techniques du vin, vol. 2, Dunod, Paris, 1975.

4.   B. G. Coombe, R. J. Dundon, A. S. Short, “Indices of sugar-acidity as ripeness criteria for winegrapes,” J. Sci. Food Agr., 31: 495 - 502 1980.

5.   C. S. DuPlessis, P. C. Van Rooyen, “Grape maturity and wine quality,” S. Afr. J. Enol. Vitic., 3 (2): 41 -45 1982.

6.   P. C. Van Rooyen, P. C. Ellis, C. S. Du Plessis, “Interactions between grape maturity indices and quality for Pinotage and Cabernet sauvignon wines from four localities,” S. Afr. J. Enol. Vitic., 5 (1): 29 - 34 1984.

7.   B. W. Zoecklein, K. C. Fugelsang, B. H. Gump, F. S. Nury, “Wine Analysis and Production,” Chapman and Hall, 1995.

8.   R. B. Boulton, V. L. Singleton, L. F. Bisson, R. E. Kunkee, “Principles and Practices of Winemaking,” Chapman and Hall, 1996.

9.   P. Iland, A. Ewart, J. Sitters, A. Markides, N. Bruer, “Techniques for Chemical Analysis and Quality Monitoring During Winemaking,” Patrick Iland Wine Promotions, 2000.

10.  J. L. Jacobson, “Introduction to Wine Laboratory Practices and Procedures,” Springer 2006.

11.  P. Ribereau-Gayon, D. Dubourdieu, B. Donneche, A. Lonvaud, “Handbook of Enology,” Vol. 1, 2006

12.  M. Greenspan, “Assessing Ripeness through Sensory Evaluation,” Wine Business Monthly, Nov. 2006.

13.  D. Berger, “”Water into Wine: the smoking gun,” Wines & Vines. 86 (3): 52 – 56 2005.

14.  R. Gawel, P. W. Godden, “Evaluation of the consistency of wine quality assessments from expert wine tasters,” Aust. J. Grape & Wine Res., 14: 1 - 8 2008.

15.  E. Peynaud, “Knowing and Making Wine,” Wiley-Interscience, 1982.

About me:

I am an analytical/clinical chemist by training, and have spent 30 years developing and manufacturing diagnostic testing products.  I switched from laboratory-based testing to point of care testing for humans, and most recently adapted the human diagnostic testing techniques to wine analysis.  I am the founder and chief of product development for Accuvin, LLC.

My current interests are wine and food analysis, winemaking, and simplifying the science of winemaking.

Thanks for checking out my blog.  I hope to offer much food for thought. 

 
Accuvin History:

 The idea for Accuvin came with the first harvest from my vineyard.  I had about a half a ton of premium Paso Robles west side Cabernet sauvignon, and I needed to do something with it.  I talked with one of the local commercial winemakers, and he offered to help me make my first wine. In return I offered to do his lab testing for him.  When shown his “laboratory” – really, a small counter in the corner of his office – I was surprised.  The collection of cylinders, burettes, beakers, and a pH meter he had were not what I expected.  In the medical testing area where I had been working, laboratory work involved either fairly expensive equipment for detail work, or simplified test devices for bedside and other point-of-care analyses.  The beaker and burette had been pushed to the side ten to twenty years earlier.

As I worked on the needed testing for the crush, I became curious and contacted other wineries to see if they had moved to more modern techniques.  Well, they hadn’t.  Simple test devices were just not available for wine analysis.  So, I thought maybe I could simplify my life by applying some of the technologies of medical testing to wine testing.

 The first step was interference removal.  Nobody wants to be bothered with some pretreatment step requiring careful measurement and timing.  In medical testing the main culprit was red-colored hemoglobin; in wine testing it was red-colored anthocyanins.  I found a way to do this in a test strip format with a technique similar to that found in the products diabetics use to self-monitor their blood sugar levels.

Following this I adapted some chemical methods to the test strip technology.  The result was a family of tests that included pH, residual sugar for monitoring the end of primary fermentation, and L-Lactic acid and Malic acid for monitoring the start and completion of malolactic fermentation.

Some analyses were not readily transferable to the test strip format.  For these a tube format was best.  For example, I formulated a mixture of color indicators and premeasured titrant to develop a one step test for titratable acidity.

Using these tests I was now able to monitor grapes for optimum time to harvest, and follow key parameters of the winemaking process, in each case getting answers in less than five minutes without cumbersome titrations with a burette and without having to purchase a spectrophotometer or send samples for expensive commercial analysis.