Grape Sampling and Fruit Maturity Evaluation

MSU HRT 434 L: Laboratory 2 Grape Sampling and Fruit Maturity Evaluation - W.C. Paetz September 4, 2003

Abstract:
One of the most important decisions regarding wine grapes is knowing, when to harvest? The grower may desire to pick as early as possible. This may preclude rot, rain cracking, bird harvest or frost which may decimate the crop. The wine maker on the other hand desires fully ripe well- balanced fruit. This may fulfill the enologist’s desire for a great wine.

As berries mature the color develops, sugar levels increase, acids are respired and pH rises. Desirable characteristics of the grape cultivar are developed. To determine the optimally perfect harvest time is as much an art as it is a science. Experience over a wide variation of seasons can help predict when is most propitious. The improvement of wine quality that results from properly matured fruit is well documented even legendary.

Our goal of a selection of a representative sampling was assured by a protocol of culling berries from every part of the canopy of the grape-vines we were assigned. We selected from interior and exterior of the vine canopy. We selected basal as well as apical berries. We picked grapes from the interior and exterior of the vine. After a selection of fifty berries we carried our sample to the lab and conducted tests which disclosed the following results: Brix = 15.3, pH = 2.99 and TA of 16.475 g/L. My initial assessment is that this is too early to harvest.

Introduction:
In a climate that is relatively consistent from year to year the characteristics of the various cultivars develop with predictable regularity. The sugar / acid ratio may be enough to predict when to harvest. While the acid level and aroma are temperature moderated the sugar is dependent upon photosynthesis. The measurement of total acid, titratable acid and pH of the fruit is a better index of maturity. Although the French may disagree, scientific approaches give repeatable accurate results and they take some of the guesswork out of enology.

Equipment and experiments conducted:

We used a hand held refractometer to determine Brix or sugar level. After squeezing fifty berries, selected to assure a random representative sample a large drop of juice was placed upon the refractometer glass. After closing the lid the sugar level or Brix was read directly from the scale on the instrument. No adjustments were made for temperature. My reading was 15.3 or 15.3% sugar. (Brix also correlates to soluble solids within the must or grape juice)

Our second measurement was done with a pH-meter. First we calibrated the meter using a buffer solution to assure the correct reading on the scale. We then washed the electrodes with distilled water and assumed that the temperature of the wine must and pH sample was the same. We poured a small 50-ml sample into a beaker and inserted the electrode. The blue pippin was removed from the probe and kept open during our testing. After a brief moment we were able to record the pH level from the meter. My pH reading was 2.99. This is the potenz Hydrogen or percentage of Hydrogen. This hydrogen ion concentration is an inverse measure of acid. A measure of 7 would be neutral, 14 would be extremely basic in this case 2.99 high acid is indicated.

Our third measurement was to determine the total acidity. 100 mills of water were added to 10mills of juice. Water was at 8.2 = our zero point prior to adding the sample. It required 23.3 mills of NaOH sodium hydroxide to neutralize the solution back to the set point of 8.2. The probe was washed and dried then inserted into the solution. A magnetic stir bar was used to make a solution of the must and water. Slowly sodium hydroxide was added until 8.2 read on the scale. The amount of NaOH required was 23.3 mills this factor multiplied by .75 resulted in a reading of 16.475 g/L of total acid. This is a very high reading for making a dry red wine. Perhaps it may be closer to the desired characteristics of a sparkling wine must.

Results:
Too often a green-grape herbaceous taste is present in wines and can not be over come by the enologist. This component may well be attributed to the early harvest of under-ripe fruit. I believe in Michigan sugar levels of over 21% and acid of not over 1% are desirable for optimum wine quality. In general Michigan grapes should be harvested as late as possible (before a killing frost) to foster higher sugar content and a corresponding decrease in acidity. Although the experienced enologist may be able to judge grapes for potential harvest by tasting in the vineyard, even assuming the most developed organoliptic skills pale in comparison to a scientific analysis.

Conclusions:
The most significant criteria of wine grape maturity are sugar, acidity, pH, and Brix-acid ratio of fresh pressed juice. The criterion most commonly employed is sugar content. To a limited extent pH has been used as the sole basis for determination that wine grapes should be harvested. It is true that pH is important to taste and color especially in table grapes, but with wine grapes the need for full development of the berry and higher sugar content is essential.

The must of our Marechal Foch indicates that it is not yet ready to be harvested. In spite of the fact that this is an early cultivar, the grapes we sampled should be left to hang until the phenols; sugars, acids and anthocyanins are more desirable. I would guess within the next two weeks a great harvest could be depended upon especially because of the hot sunny days we have recently experienced.

Back to MSU Page