Harvest Series 9 --- Laboratory Analysis

Perhaps you’ve heard winemaking described as part art, part chemistry.  That’s because although a winemaker has the freedom to create wine in different styles, they are also bound by science in order to end up with a sound product.  This installment is for the chemistry lovers out there, as I will go through the basic lab testing required at harvest time and why it’s done.

Before harvesting, you need to make sure the fruit is at optimal ripeness.  As they say, it all starts in the vineyard, and it’s very hard to make great wine with mediocre fruit.  After gathering a grape sample, it is sent to a lab for brix, pH, and TA testing.  

Brix is the measurement of sugars in the grape and is expressed in degrees.  For example, an optimally ripe red grape will have around 24 or 25 degrees brix, and a white will have around 22 or 23 degrees brix.  The brix level also corresponds to the finished alcohol content, so harvesting in the mid-twenties will produce a wine that is in the 13-15% alcohol by volume range.  

pH is measured to give an idea of the acidity of the juice.  Grape juice is on the acidic side when harvested, around 3-4 pH (water is 7).  Optimal pH at harvest is 3.5-3.7 for reds and 3.1-3.3 for whites.  If the pH is too low at harvest, the wine will end up too tart and is tough to adjust up to a higher pH.  

TA is titratable acidity.  This is the measurement of acids in the grape (mostly tartaric, malic, and citric) that can be recovered by titrating with a strong base solution and an indicator solution or endpoint pH of 8.2.  This is different from the total acidity of the grape.  Titration is a laboratory method where small amounts of a strong base solution is added to a sample until an endpoint is reached. In this case, the amount of base solution used is calculated to find the TA measurement.  Optimal TA is around 0.6-0.7g/100ml for red grapes and around 0.65-0.75g/100ml for white grapes.

Once the juice has been fermented, a sample of the finished wine is sent to the lab for testing, this time for pH, ABV, RS, and free SO2.  pH tends to go up during fermentation, so it needs to be checked right afterwards in case tartaric acid needs to be added to bring it back down.  A pH that is too high will make the wine unstable and more likely to spoil, not to mention give it a flat or flabby taste.


ABV is alcohol by volume, and is pretty self-explanatory.  If the alcohol content is too high, it can taste out of balance, give a slight burning sensation in the mouth and throat, and can make the wine unsuitable for aging.  Knowing the alcohol content gives the winemaker the option to blend it down with a lower-alcohol wine, make it into a fortified dessert wine, or remove some of the alcohol using reverse osmosis filtration.

RS is residual sugar.  This is the amount, if any, of sugars left in the wine after the yeast have died off.  For dry wines, it is usually under 1.0%, but can be up to 20% for sweet wines.  Knowing the RS at the end of fermentation gives the winemaker the choice to either sweeten it with grape concentrate, or lower it by blending in a drier wine, if it is not in the range they desire.  A higher amount of RS in a wine can also potentially re-ferment in the bottle if yeasts or bacteria still present are not filtered out at bottling.

Free SO2 is the amount of sulfur dioxide in the wine that is not bound.  Free SO2 is what protects the wine from spoilage and the color from becoming dull or browned.  A tiny amount of SO2 is produced during fermentation, but it is not enough to protect the wine, so knowing the amount present allows the winemaker to the add the right amount needed.


For the final installment of the Harvest Series, I will discuss the close of harvest, and what is happening in the vineyard and the cellar to button it all up until the next year.