Topping Analysis

January 17, 2012

It looks like we’re finally going to get some rain on Thursday, which means we’ll have to stop pruning for awhile and bring things back inside.

Luckily, it’s time to top our barrels again, so there’s a need for some indoor labor anyhow. Topping is a good opportunity to examine the wine, both for how its’ sensory characteristics are developing and to run analysis on it. Topping analysis is generally done to ensure that the wine is not spoiling. Wine is reasonably stable because it is acidic and contains alcohol, but there are still some bugs that can grow in it, given the opportunity.

The primary analysis we do to monitor spoilage is called VA, or Volatile Acidity. There are a number of different types of acid in wine that are considered volatile, but the one we are primarily concerned with is acetic acid, also known as vinegar.

Our lab is very small, so things get a little chaotic when we run analysis. The glass chamber with boiling water and wine in it on the right side of the picture is a Cash Still, it is the device we use to measure Volatile Acidity.

Acetic acid is present in all wines to some degree, but at too high a concentration it has an unpleasant aroma, and a sudden increase in acetic acid is a sure sign of spoilage in the wine, which can lead to all kinds of problems. We check the concentration of acetic acid regularly in every lot of wine to make sure that the wines are still sound and do not require any special treatment. Happily enough, everything checked out as usual, so we will proceed with normal topping later this week.

To visit the Amapola Creek Winery main  site, please click here.

Chemistry Friday; TA

October 14, 2011

An important parameter that we measure frequently during harvest is TA, or Titratable Acidity. This is an approximate measure of the concentration of acid molecules in juice or wine. Acids are responsible for the sour components of a wine’s flavor, so this time of year we generally measure TA in vineyard samples to make sure that the acid in the grapes will not be too high at harvest and leave us with an overly tart wine. We also examine the sourness of the samples by tasting them, but sometimes the high sugar content of the grapes can make it difficult to perceive sourness accurately, so TA gives us a quantifiable way of looking at the issue.

The TA of juice at harvest is usually somewhere between 5 grams / liter and 9 grams / liter, and the TA in finished wine is usually a little bit lower because tartaric acid (the predominant type of acid in grapes) falls out of solution during fermentation.

The way we measure TA is with a potentiometric titration.  We take a known volume of the juice, and add a base with a known concentration until the pH of the sample reaches 8.2 (this is what is known as the titration endpoint, the pH at which, theoretically, all of the acid molecules have been neutralized by base). Using the strength and volume of the base it took to neutralize the acid in the sample, we can then calculate how many molecules of acid must have been present.

TA is quick and easy, but it is also only an approximate measure of the concentration of acid in juice or wine. The analysis assumes that all of the acid in the wine is tartaric acid, which has a titration endpoint close to 8.2, but in fact wine and juice have a complex mixture of acids with a variety of titration endpoints. Still, tartaric acid is the most common acid in grapes, so the results of the analysis are close enough. 

To visit the Amapola Creek Winery main site, please click here.

 

 

One of the most important parameters that is analyzed for in the winery lab during harvest is sugar content; we use it to determine ripeness and to monitor the progress of fermentation, both of which are critical to the winemaking process. At Amapola Creek (as in just about every other winery in California) we express sugar content in terms of degrees Brix.

Imagine you have one hundred milliliters of water (around half a cup). When there is no sugar in the water, we say that it is at 0 degrees Brix.

Now imagine that you have one gram of sugar (a very small spoonful), and you dissolve it in enough water that the final sugar solution has a volume of one hundred milliliters. This would be 1 degree Brix.

If there are two grams of sugar in one hundred milliliters of solution, it would be 2 degrees Brix, and so on. One gram of water (at 20 degrees C) is equivalent to one milliliter, so the Brix scale is a way of expressing what percentage of the solution (by weight) is sugar. A solution that is 10 degrees Brix would be made of ten grams of sugar and ninety milliliters of water, a solution that is 50 degrees Brix would be made of fifty grams of sugar and fifty milliliters of water, etc., etc.

Most grapes when they are ready to harvest are somewhere in the neighborhood of 25 degrees Brix (roughly 25% sugar by weight, this is only approximate because of course there are other compounds, like natural acids, that are dissolved in the juice as well). This can vary a few degrees Brix in either direction depending on the vineyard, weather, and style of wine to be made, but as a rule of thumb 25 degrees Brix is a good approximation.

When measuring the sugar content of juice that is not fermenting yet, such as vineyard samples or samples from grapes that have just been crushed at the winery, we use a device called a refractometer.

The denser a liquid is, the more it will bend the light that passes through it. A refractometer works by measuring how much light passing through the liquid is bent, and then inferring the density (and by extension the sugar content) from that measurement. Many refracs use a simple setup of lenses and a prism to make the measurement, but we happen to have this nice digital one.
Once the juice has started to ferment, however, the refractometer will no longer work, because bubbles of CO2 interfere with the path of the light as it passes through the juice. To measure the Brix of fermenting juice, we use a device called a hydrometer.
 

Hydrometers are neat. They are designed so that you float them in a container of juice, and then read the number off the scale printed on the stem at the point that it emerges from the surface. The one in the picture above is floating in water, so if you were to look at the point where the stem emerges from the surface, it would read '0'. If there were sugar in the water, the water would be denser and therefore displace more of the mass of the hydrometer, pushing more of it up out of the liquid and changing the point at which the stem and and the liquid surface meet, giving a higher reading.

 Hydrometers are quick and easy to use, but you do have to be careful of certain sources of error. If a sample is fermenting very hard, the CO2 bubles can lift the hydrometer up and give an artificially high reading. Similarly, if the sample has a lot of solids in it, it can become viscous enough that hydrometer readings can be artificially high.

 
One more interesting thing to note about degrees Brix is the impact of alcohol on measurement. Alcohol is less dense than water. So, in wine, where the sugar is gone but there is plenty of alcohol, the Brix readings will actually be negative. A negative Brix is when we usually decide to schedule pressing, because it implies that the sugar is largely used up.
 
That’s it for now, see you Monday!
 
To visit the Amapola Creek Winery main website, please click here.