Sandy Paetz
just sent me an update about what he is doing at the MSU Viticulture
& Enology Program.
This program was brought to our attention last year when a
MSU student, Bernard Call, who is taking this course, purchased some
grapes at our farm and was carefully measuring the brix of the grapes
with a refractometer. The following year he returned to our farm after
being in the program for one year and he was very enthusiastic, interesting
and knowledgeable about the course.
Sandy Paetz, one of our past Sales Stand Associates, had
also enrolled in the MSU program. When the two of them found
out that they both were taking the courses, the conversations became
very stimulating and they agreed to forward some of their program
notes to us for our newsletter.
We have some inside stories of what Bernard and Sandy are
doing at MSU.
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.
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