Product Quality

Fruit and product quality are important for growers in all grape sectors. However, the metrics that define quality are different for each market.

  • For juice grapes, high yield and sugar content (°Brix) are the measures of quality. Raisin and juice grapes producers desire high yield and sugar content.
  • For table grapes yield and fruit composition are also important, but physical appearance of berries including size, weight, color, and firmness are considered. Postharvest quality in table grapes is also important, particularly rachis and berry appearance and wholesomeness (lack of rot). Maintenance of berry firmness is also important, as is retention of berries on the clusters
  • For wine grapes, quality measures are more difficult to quantify and are impacted by wine processing, aging, and storage. Some quantifiable characteristics that are generally accepted to influence wine composition include yeast assimilable nitrogen (YAN) concentrations, potassium concentration, acidity, berry size, sugar content, anthocyanins, tannins, and flavors that can impact color and wine sensory perception.

Some of these fruit quality parameters are influenced by vine nutrient status. For example, nitrogen may impact yield and nitrogen concentration in the berries in the form of amino acids and ammonia. This berry nitrogen (YAN) is used by wine yeast for fermentation, and if there are not sufficient levels, there can be issues with the fermentation. Potassium can impact fruit and wine pH that then need to be amended in the winery. The goal of the Product Quality Team is to relate vineyard nutrient status to parameters of product composition and quality. The team is evaluating the impacts of the Plant Nutrition Team’s vineyard trials by making wines and conducting composition and sensory analyses. The specific projects are outlined below.

  • In Washington and Oregon, the effect of nitrogen (N), potassium (K), and magnesium (Mg) application rates on wine quality parameters is being evaluated in four different varieties at three different locations.  The Oregon N trial and Washington N and K trials are being processed by the Washington State University team for wine composition and sensory.
  • In Virginia, wines from the soil and foliar N trials are being evaluated in Chardonnay and Chardonnel.
  • In California, fruit yield and quality measures of table grapes are being evaluated from field N level trials and two potted studies with N and Mg treatments. Yield and quality measures of raisin grapes are being measured on three N treatments.

Team members

Researchers (see details here)

  • Matthew Fidelibus, University of California – Davis
  • Jim Harbertson, Washington State University
  • Amanda Stewart, Virginia Tech


Research Lab Staff

Megan Mershon, PhD student, Virginia Tech

Megan Mershon, MS student, Virginia Tech

Megan is pursuing a MSLFS in Food Science and Technology at Virginia Tech and works in Dr. Amanda Stewart’s lab. Her research investigates the ways in which nitrogen nutrition in the vineyard impacts wine fermentation and quality, as determined by chemical and sensory outcomes. Megan is producing experimental wines from grapes involved in the Virginia Tech vineyard trials and conducting parallel experiments evaluating effects of vineyard nutrient status on wine fermentation and quality.

Juliana Pazos, PhD student, Washington State University

Juliana is a PhD student in the Food Science at Washington State University where she is working in the lab of Dr. Jim Harbertson. Her research involves evaluating the effect of vineyard nutrition on phenolic compounds and proteins in the grapes and wines. She is also working on evaluating whether different vineyard nutrition treatments could affect wine sensory characteristics.

Dr. Marlon F. Ac-Pangan

Dr. Marlon F. Ac-Pangan, Postdoctoral Associate, Virginia Tech

Dr. Ac-Pangan recently joined the Department of Food Science and Technology at Virginia Polytechnic Institute and State University (Virginia Tech). Dr. Ac-Pangan’s research interest includes the use of analytical chemistry and sensory science to understand the impact of growing conditions (terroir) and processing practices (e.g., fermentation, yeast inoculation, drying, roasting) in the quality of food products and fermented beverages oriented to high-value niche markets, such as craft chocolate, cider, wine, and specialty coffee. Dr. Ac-Pangan is part of the team at Virginia Tech that is evaluating how different levels of nitrogen fertilization in the vineyard impact yeast assimilable nitrogen, fermentation dynamics, and wine quality.

Past researchers

  • Ryan Doyle, MSc, Washington State University – graduated summer 2023
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