Sour Grapes

Derek B. Zahl, Ph.D., P.E.  & Thomas P. Jur, P.E.

Sour Grapes?

Here in Northern California, we find ourselves fortunate to live in the middle of Wine Country. Within a few miles of our homes, we find ourselves among rolling hillside vineyards and a multitude of wineries and tasting rooms. This proximity has resulted in our involvement with losses in the wine industry. While the losses we have investigated cover the gamut from injury in the vineyard or winery, to fires, to machinery breakdowns, to packaging, what we’d like to touch on here are losses involving potential contamination of wine products.

The winemaking process is a blend of science and art, with the winemakers acting as the maestro in this symphony. It all starts in the vineyard, with choosing the optimum time to harvest when the grape sugar content (Brix) is just right. During the crushing season, the harvested grapes get delivered to the wineries around the clock, with crews typically working long and hectic shifts.  

The grapes are dumped into a hopper and then transferred through various equipment including destemmers, sorters, crushers, and into primary fermentation tanks. Throughout this process, as the grape juice is converted to wine, the juice passes through a variety of pumps, hoses, filters, tanks, sometimes barrels, and finally packaged (most often in glass bottles with cork seals). Some fermentation tanks have open tops to facilitate venting of evolved fermentation gases, and some tanks have temperature-controlled jackets, typically using food-grade propylene glycol solution as the coolant.

Based on all these process steps, there are many opportunities and a variety of equipment where contaminants could be introduced into the wine that adversely affect the resulting wine quality. Contaminants can include naturally occurring species of yeasts, molds and bacteria common to vineyards, atmospheric contaminates such as smoke taint prevalent during the recent California wildfires, or the inadvertent introduction of chemical products used in cleaning and maintenance, or the glycol coolant itself. However, there are also many other factors normal to winemaking that influence the resulting wine quality including winemaker decisions on fermentation timing, temperatures, additives, and barrel aging.

During the investigation of reported wine contamination losses, we often find it appropriate to assemble a team to supplement our engineering expertise, with experts in viticulture and winemaking (the art), analytic chemistry (to identify potential contaminants), and forensic accounting (even bad wine has commercial value). By methodically conducting an objective investigation of the winemaking process, we can determine whether the cause of a bad batch of wine is due to winemaker decision making, winery operational practices, equipment failure, or atmospheric conditions.

The multitude of wine contamination investigations we have conducted are typical of EDT’s general approach to investigating potential contamination of many types of consumer products such as pharmaceuticals, dairy, bottled water, beverages, cosmetics and foods.

About the Authors

Derek B. Zahl, Ph.D., P.E. and Thomas P. Jur, P.E. are consulting engineers with our Oakland, CA office

Mr. Zahl provides consulting services in the areas of mechanical systems, materials and industrial/commercial losses. 

Mr. Jur provides consulting in the areas of boiler and machinery investigations, scope of damage and value of loss estimates, business interruption timeline schedules, project management, construction defects evaluations, fire causation, and product contamination investigations.

You may contact either Derek at or Tom at for your forensic engineering needs or (925) 674-8010

Learn about how EDT Forensic Engineering & Consulting approaches cause of damage, and forensic engineering by assigning a file today.