Steam-Powered Precision: Mastering Volatile Acid Determination in Port Wine

Madalena stands on the balcony of her family's estate in the Douro Valley in Portugal. Terraced vineyards stretch along the riverbanks, where her family produces their renowned ports. She looks on with a mix of pride and anxiety, as a legacy spanning generations now rests on her shoulders. 

In large letters above the entrance to the cellar is the Portuguese proverb and family motto “Águas passadas não movem moinhos,” which translates to “Past waters do not move mills.” Madelena’s family is adamant that their success lies in adapting and changing their methods with each new generation, and today’s the day Madelena will assess her first batch. 

She enters the old cellar, opens a bottle of port, and her worst fears are realized. 

Something is wrong.

The familiar notes of ripe fruits and spice mingle with an unmistakable tang. As she leans closer, a scent of vinegar and nail polish remover stings her nose. Her heart races as she tastes it. The acid strikes her pallet like a sharp knife. Not knowing the cause, Madelena seeks the help of the detectives.

Back in the office, Shallot Holmes explains the details of the case. Eggcule Poirot, who has experience performing various types of analysis of beer, spirits, and wine, thinks he can help. Holmes agrees that Eggcule should go so long as he stays out of any nearby forests!

Eggcule Poirot arrives at the vineyard and, to his delight, is taken straight to the cellar to assess the port. On entering the cellar, Eggcule sees a line of stainless-steel containers. “Ah, this must be where the fermentation and fortification occur,” he says. “Umm, well, that’s what my parents used, but I have tried to change things to keep with my family’s tradition,” says Madalena as she walks past the fermentation tanks into a back room. 

As they enter the back room, Eggcule is greeted by the site of a wide, shallow, open stone vessel. “This is a lagar. It’s where the initial fermentation occurs; it hasn’t been used for decades, as my parents changed to using the steel vats and more automated processes,” says Madalena. “Oh, so you are using a more traditional process. Fascinating! I know a lot about modern methods and science but very little about traditional methods,” says Eggcule.

“Last year, I heard an old man saying to a child, "Para compreenderes o presente, tens de conhecer o passado" – To understand the present, look to the past.  I was immediately inspired and thought that my change to the family process could be to combine the traditions of the past with modern techniques,” explains Madalena. “What a fantastic idea,” says Eggcule. “Well, that’s what I thought, but when I tasted the wine, it was like vinegar!” says Madalena.

Eggcule Poirot thinks for a moment and asks to try the port. As soon as he holds the glass to his nose, he begins to mutter, “Hmm, volatile acids… acetobacter aceti… ethyl acetate.” Madelena looks a little confused. " Acetobacter aceti? Is that one of the volatile acids?” she asks. Eggcule explains that it converts ethanol into the acetic acid responsible for the vinegar aroma. Madalena is keen to know more and asks Eggcule if he would teach her the science if she taught him about traditional methods. “Great idea,” says Eggcule, who begins questioning Madalena about traditional methods.

What is the purpose of lagares in port production?

The wide, shallow, open-topped lagares allow grapes to be trodden for maceration. This traditional method gently crushes the grapes without breaking the seeds, leading to a less bitter port. The whole family gets together to speed up the process. The mixture of grape juice, seeds, and skins is called grape must, which begins to ferment in the lagar. 

How do you know when to stop the fermentation and begin fortification?

The fermentation should be stopped when about half of the sugar has fermented. The process is stopped by adding a grape spirit known as aguardente that stops fermentation by killing the yeast due to the high alcohol content it introduces. 

“I know that a hydrometer or a refractometer can determine the sugar levels in wine, but how would your grandparents have known when to add the aguardente?” asks Eggcule. “My grandmother would have popped you into the grape must,” says Madelena, chuckling. Eggcule looks confused. Madelena explains that before floating hydrometers that track a drop in gravity indicating sugar conversion, traditionally, people would use an egg. The egg would act as a buoyancy indicator and float higher in must with a high sugar content. “This would have been one of many indicators used by my relatives, along with taste, bubble activity, color, time, and environmental conditions, says Madelena. 

It was now time for Eggcule to explain the science of volatile acids and help Madelena remedy her process. 

What is volatile acidity?

Volatile acidity encompasses a range of volatile acids, particularly acetic acid (responsible for the vinegar-like aroma), and secondary compounds like ethyl acetate (which smells like nail polish remover). They are present in all wines and ports in trace amounts and can add complexity to their profile; however, too much can lead to spoilage. 

What causes volatile acidity?

The acetobacter aceti bacteria convert sugars and alcohol into acetic acid, especially when exposed to oxygen. Microorganisms thrive and produce acetic acid when wine is in contact with air.

“Using the lagares increases the exposure to oxygen, but port is generally more tolerant of volatile acids due to the fortification process. You said the lagares hadn’t been used for decades. The porous surfaces are a breeding ground for microorganisms, and meticulous cleaning could prevent contamination and solve the problem,” says Eggcule.

How can you reduce or prevent the production of volatile acids?

Adding sulfur dioxide SO₂ can help mitigate the issue by inhibiting acetic acid bacteria. SO₂ also prevents excessive oxidation by neutralizing oxygen, which is crucial in an open fermentation vessel like a lagar. Like volatile acids, there are regulations that determine limits on the amount of SO₂ allowed in port. 

The addition of SO₂ can also help with another problem faced by port producers: the sufficient extraction of anthocyanins to provide the intense red color. Due to the short fermentation period, sufficient pigment extraction is difficult to achieve. Modern auto fermenters assist with this, but traditional methods require considerable human effort. The addition of SO₂ helps the release of anthocyanins.

How can you determine volatile acids and SO₂ in port?

Measuring volatile acidity and sulfur dioxide in port involves steam distillation followed by titration. This method separates the volatile acids from the port, allowing for accurate quantification. Since SO₂ and sorbic acid also distill over and affect the results, their presence must be corrected by subtracting their respective values. Eggcule provides Madalena with a reliable procedure using modern steam distillation equipment.

Sample Preparation:

• Storage: Samples must be stored in sealed containers in a dark, room-temperature environment.
• Preparation: On the analysis day, the sample is shaken to release CO₂ before processing. 

Steam Distillation Process:

• Instrument Setup: The instrument is preheated with an empty sample tube, and a vessel is placed under the condenser to collect the distillate. 
• Blank, reference, and sample preparation: 20 mL of deionized H₂O (blank), acetic acid standard solution (reference), and port sample are placed in a 300 mL sample tube. Add 0.5 g of tartaric acid to each tube and connect to the distillation unit. Before distillation, place an 800 mL beaker with 80 mL of deionized H₂O below the condenser outlet tube to collect the distillate.

Titration

• Acetic acid quantification: 3 drops of phenolphthalein are added to the distillate, which is then titrated with 0.1 M NaOH until a pink color remains stable for 10 seconds.
• SO₂ correction: Perform a second iodometric titration. Acidify the distillate with a 5 mL sulfuric acid solution (1:4). Then, titrate it with a 1/64 N iodide-iodate solution using a starch indicator to quantify SO₂. Starch and iodine form an intense blue, indicating the end of the titration. 

Calculation:

• The recovery rate is calculated based on the difference between the blank and reference titration values.
• Volatile acidity is determined using a factor of 0.3 (as per the OIV method), subtracting the influence of SO₂ and sorbic acid. 

Eggcule explains that this method will provide accuracy and reproducibility, though slight discrepancies may arise due to the method’s sensitivity to non-volatile acids. Madalena is blown away by the methods and wisdom imparted to her. She is more confident than ever that she will successfully bring about the changes required to continue her family’s legacy of progress through change. “How about we try some of the port my family has made over the years?” asks Madalena. “What a great idea, says Eggcule. “Para se compreender o presente, tem de se conhecer o passado!”says Madalena as they toast to a job well done.