Madeline Wimmer, Extension Educator and Soon Li Teh, Extension Specialist
Grapes are a fruit crop with distinct, measurable ways to determine fruit ripeness. Harvested too early, and the fruit will be unbalanced with higher acidity and lower sugar levels. Using taste and visual cues can be informative for fresh grapes, but sweetness can often mask acidity and be deceptive.
With knowledge of the fruit development process and by using the right tools and methods to track berry ripening, a vineyard can be confident in its ability to harvest a quality crop.
Information in this article:
Grape berry ripening phases
How to sample berries
Analyzing juice for sugar levels, pH, and titratable acidity
Additional ripeness indicators
Grape berry ripening phases
Grape berries generally go through three developmental stages for ripening:
Stage I:
This is the earliest phase that happens right after fruit set. Cells within the berry begin to divide and multiply, which leads to an increase in berry size.
Stage II:
Much energy is focused on embryo development within the grape seeds (keep in mind seed production is the goal for fruit crops) during this phase. Stage II ends when berries begin to change color at veraison.
Stage III:
At veraison, stage III is characterized in the fruit by a loss of chlorophyll, increase in sugar levels, reduction in acid levels, and berry softening. Additionally, the seeds continue to mature and eventually have a “nutty” taste when sampled.
How to sample berries
One grape berry, or even one cluster can offer a peak into fruit ripening for a quick check when time is short, but a representative sample using multiple berries from multiple clusters is the best practice while determining when to harvest.
This is because there is natural variation based on where a fruit cluster is located throughout a grape canopy. If only exposed clusters are sampled, for example, this can bias the sample and mislead the vineyard into thinking the crop is more ripe than it actually is, which could lead to harvesting too early.
Consistent canopy management practices (shoot thinning, basal leaf removal, hedging) also impact grape development and how consistent fruit quality is throughout the vineyard by altering how exposed clusters are to sunlight and changing the crop load. Grapes grown without any canopy management experience more shade and can have higher acid levels and lower sugar levels than ones that are fully exposed to sunlight.
Steps for sampling and preparing samples for analysis
Grape berries can be sampled weekly starting around 3 weeks before the anticipated harvest.
Choose the number of berries needed to represent the area being sampled (usually between 15-30 berries).
Collect a representative sample of berries for the variety or region being sampled .
Place berries into the plastic zip lock bag.
Either refrigerate berries if processing samples at a later time, or crush berries within the zip lock bag to extract the juice.
Choosing which berries to sample
The following steps for sampling are guidelines that can give some method to creating a representative sample.
Image: Grape clusters can be divided into four regions: left shoulder, right shoulder, center, and tail.
Start with sampling 1-2 berries from each cluster. Rotate the location where berries are sampled by picking berries on the front and backside, and selecting berries from the four cluster regions: left shoulder, right shoulder, center, and cluster tail (see diagram above).
Additional considerations for berry sampling
Outlier berries: Avoid selecting berries that are not representative of the majority of grapes within a vineyard. These berries could be diseased, or damaged from insect pests, birds, hail, etc.
Individual block sampling: Beyond canopy density and the cluster position, it may be desirable to separate samples by vineyard block, or location of vines are located at different sites, or have varied topographies, soils, training systems, etc.
Rain: Avoid sampling during or immediately after a rain event, as rainwater can dilute the sugar concentration and cause berry swelling.
Tools and supplies for fruit sampling
Plastic sandwich bags labeled with the grape cultivar name, or region being sampled. (Large cups or containers may also be used.)
Handheld refractometer for testing sugar levels in the field
Water bottle or spray bottle and a towel or paper towels to rinse and dry the refractometer as needed if measuring sugar levels in the vineyard.
Juice collection containers can be labeled and used to bring juice samples to a location where samples will be processed and analyzed.
Additionally, a cooler with ice is helpful when transporting samples a long distance.
Notebook and pencil to record results. Notes can be kept each growing season to compare to last year’s ripening—this is where the magic starts to happen as you’ll start to understand the trends over time!
Video tutorial: Vineyard/fruit sampling for quality and maturity, Part I: In the field
In this video, University of Minnesota Wine Maker, Drew Horton, discusses how to monitor grapes in the field as harvest approaches using the flying blind technique. This technique encourages samplers to pay less attention to which clusters are being selected for berry sampling.
Link: Vineyard/fruit sampling for quality and maturity, Part I: In the field.
3. Analyzing juice for sugar levels, pH, and titratable acidity
After the berries have been collected and juiced, sugar levels (i.e., soluble solids concentration), pH, and titratable acidity can be measured to determine how ripe the berries are.
Sugars/soluble solids concentration (SSC)
Refractomers:
Grape juice sugar levels are estimated by measuring the soluble solids concentration (SSC; measured in degrees Brix) using a refractometer as the tool for measurement.
The amount of sugar that eventually becomes alcohol in the final wine product translates to about one degree Brix becoming about 0.55% alcohol. For instance, juice that measures out to 22° Brix could have an estimated final alcohol content around 12%, depending on the yeast used, along with other conditions present during fermentation.
Optimal ranges for juice Brix levels can vary between wine grape cultivars and the desired wine style and method used. Having an idea about what type of wine will be made with any single or blend of cultivars can set the goal for the desired Brix level during harvest.
Hydrometers:
Using a hydrometer to measure specific gravity is another method for estimating sugar in a juice, but a larger sample is usually required making it more practical to use than a hand-held refractometer. Measuring the specific gravity can be most helpful after harvest, before fermentation begins.
Additional reading:
How to use a refractometer (University of California Davis)
Estimating grape maturity by total soluble solids (Iowa State University Extension)
Where does the term Brix originate?
The Brix unit (i.e., degrees Brix) is the last name of the person who developed methods to measure sugar content in juice. One degree brix represents one gram of sucrose in one hundred grams of a solution. Refractometers measure brix by measuring the amount of light that bends when passing through a liquid.
pH
Having an optimum juice pH is important to the final wine stability and prevents unwanted microbial growth, which can lead to wine faults. The pH technically measures how acidic or basic a solution is based on the concentration of hydrogen ions and values range from 0-14 with 0 values being the most acidic and 14 being the most basic.
Measuring juice pH is helpful for the reasons listed above, but knowing the titratable acidity is still important. Not all acids are the same strength, and titratable acidity indicates the total amount of acids present in the juice, regardless of their strength. Cold climate grape juice pH typically ranges from 3.0 to 3.5, depending on variety, growing conditions and intended wine styles.
Measuring juice pH can be done using a pH meter; today, a wide range of pH meters exist on the market for this purpose, and the key to maintaining an accurate meter depends on how well it is calibrated and cared for over time.
Can pH strips be used to measure juice pH?
pH strips are usually inexpensive, easy to use, convenient for in-field measurements, and can indicate juice pH levels. Most brands can measure juice pH values between 2.7 and 4.3.Their downfall is that strips are less precise, and the reading itself can be challenging to determine for darker pigmented wine grapes.
Additional reading:
pH Analysis (University of California Davis)
Estimating grape maturity with potential power of hydrogen (pH) (Iowa State University Extension)
Titratable acidity (TA)
The final element to measuring grape ripeness is to measure juice acid levels using a process known as titration*. Titratable acidity (TA) is not the same as total acidity, although these two traits are sometimes used synonymously in winemaking literature.
Total acidity represents the total amount of fixed, or stable acids (e.g., tartaric, malic, citric) and volatile, or aromatic acids (e.g., acetic, butyric, sulfurous) in a solution, whereas TA reflects the amount of acid that can be neutralized by an alkaline solution—it is literally measuring how many protons react with the solution***. While total acidity can be measured using methods like spectrometry or chromatography, it’s not practical for grape growers and winemakers. Thus TA is the industry standard. There are a wide range of different titration methods, some of which are less mechanized than others. Check out the additional resource below to learn more about the process of measuring TA.
TA goals for cold climate grapes: Titratable acidity of cold climate grape juice typically ranges from 8 to 15 g/L depending on varieties, growing conditions, and intended wine styles.
Additional resource:
Measuring grape maturity by titratable acidity (Iowa State University Extension)
*What is titration?
Titration is a type of chemical analysis that determines the concentration of a dissolved substance within a liquid solution. The titrant is (usually something basic like sodium hydroxide; NaOH) slowly added to the solution until it reaches the point of equivalence, which could be indicated by a change in color, or other other property (example indicators include phenolphthalein or a fixed end point pH like 8.2).
*** Titratable acidity only captures 70-80% of what is anticipated to be the actual concentrations of the acid being measured.
Video tutorial: Vineyard/fruit sampling for quality and maturity, Part II: In the lab
In this video, University of Minnesota Enologist Drew Horton discusses how to process and analyze grape samples to measure sugar levels/soluble solids, pH, and titratable acidity to determine fruit ripeness for harvest.
Link: Vineyard/fruit sampling for quality and maturity, Part II
4. Additional ripeness indicators
Image: a close-up photo showing the variation in color of ripe Frontenac Gris clusters at harvest.
While measuring soluble solids, pH, and titratable acidity tend to be the most common ways to quantify fruit ripeness, qualitative indicators are still important.
Remember to taste what is out there and visually evaluate how your crop is doing as a whole. Even the taste of the grape seeds can indicate how close harvest is, as riper grapes develop a nutty, or toast-like flavor. With a mixture of quantitative and qualitative approaches, year after year, it becomes easier to know when your crop is at its optimum ripeness for harvesting.
It is important to note that optimal fruit ripeness is not always achievable. Extreme weather conditions, like hail, which can damage a whole crop, disease, or even labor availability can lead to an earlier harvest than anticipated.
Finally, for those who are new to measuring fruit ripeness and for whom this information feels too technical or too complex, the best thing to do is take it slow and take it one step at a time.
Adopt practices as you can as they are relevant to your situation and goals. Learning is a part of the journey and any effort you make will be helpful.
This article may be shared for educational purposes with attribution to the University of Minnesota Extension. For other uses, please contact UMN Extension for permission.
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